EP4723136A1

EP4723136A1 - Method for producing astatine-211, apparatus for producing same, and quantum line output window apparatus

Info

Publication number: EP4723136A1
Application number: EP24815557.4A
Authority: EP
Inventors: Hiroshi Arata; Shingo Nakamura; Takashi Kurihara; Hiromitsu HABA
Original assignee: Metal Technology Co Ltd; RIKEN
Current assignee: Metal Technology Co Ltd; RIKEN
Priority date: 2023-05-30
Filing date: 2024-05-30
Publication date: 2026-04-08
Also published as: WO2024248076A1; JPWO2024248076A1

Abstract

In one embodiment of the present disclosure, in order to increase production efficiency of astatine (At)-211, provided is a method for producing At by alpha particle beam irradiation, and the method includes: an installation step S32 for receiving and arranging, in a housing part of a target material container (3), a target material (4) containing bismuth (Bi)-209; a rotation drive irradiation step S40 for irradiating at least a portion of the target material (4) with an alpha particle beam (2) while rotating the target material container (3) around a rotation axis; a transportation step S46, after said step S40, for supplying a carrier gas and sending, through an exhaust pipe, an atmospheric gas around the target material; and a collection step S46 for collecting, from the atmospheric gas, astatine-211 generated from Bi by the irradiating with the alpha particle beam. In a preferred embodiment of the present disclosure, the rotation drive irradiation step is performed with Bi of the target material in a solid state. Further, in a preferred embodiment of the present disclosure, the target material container is heated to a temperature higher than the melting point of Bi and lower than the boiling point of Bi in either or both of the transportation step or the collection step.

Description

Technical Field

The present disclosure relates to a method for producing astatine-211, an apparatus for producing same, and a quantum beam output window apparatus. More specifically, the present disclosure relates to a method for producing astatine-211 with enhanced production efficiency, an apparatus for producing it, and a quantum beam output window apparatus suitable therefor.

Background Art

Various radioactive isotopes (radionuclides) are employed for radiation therapy and as tracers. One alpha radioactive nucleus receiving significant attention is astatine-211 (²¹¹At). ²¹¹At is a halogen element with a melting point of 302°C and a boiling point of 337°C, and is a sublimable solid at room temperature (20°C). ²¹¹At can be artificially produced using an alpha particle beam produced by an accelerator or similar device. By irradiating a target made of bismuth-209 (²⁰⁹Bi, melting point 271.5°C, boiling point 1564°C) with an alpha particle beam, the produced ²¹¹At, generated by nuclear transmutation, can be separated and collected using a chemical separation apparatus. Production of ²¹¹At for research purposes has already been carried out using solid ²⁰⁹Bi as a target (Non-Patent Document 1). Regarding methods for producing ²¹¹At, another radioactive isotope production method is also disclosed (Patent Document 1).

Citation List

Patent Document(s)

Patent Document 1: JP 2021-96147 A1

Non-Patent Document(s)

Hiromitsu Haba, "Production of Radioisotopes for Targeted Radionuclide Therapy at RIKEN", [in Japanese] Drug Delivery System, Vol.35, No.2, p.114-120 (2020) DOI: 10.2745/dds.35.114

Summary of Disclosure

Technical Problem

There is a problem that the production yield of ²¹¹At generated by irradiating an alpha-particle beam onto ²⁰⁹Bi as the target material is difficult to increase. Practical methods for its production and supply are being sought (e.g., Non-Patent Document 1). Although the present inventors have also developed methods for producing radionuclides by utilizing the properties of gases during alpha-particle beam irradiation and techniques employing rotation to apply centrifugal force to the target material, methods for increasing production quantities remain under development. The present disclosure contributes to the widespread use of ²¹¹At by solving the aforementioned problem.

Solution to Problem

The present inventors have created a solution to increase production yield in a method utilizing the properties of gases when irradiating an alpha-particle beam to produce ²¹¹At.
Specifically, in an embodiment of the present disclosure, provided is a method for producing astatine-211 (²¹¹At) by alpha-particle beam irradiation, comprising an installation step wherein a target material containing bismuth-209 (²⁰⁹Bi) is placed and positioned in a target container, the target container capable of receiving irradiation with an alpha-particle beam from an alpha-particle beam generator inside of an interior of a target chamber, and wherein the target material container has a containing part and an opening for allowing the containing part to communicate with the exterior, and is rotatable about an axis of rotation passing through the containing part and the opening, and is placed within the interior of the target chamber, a rotational drive irradiation step wherein an alpha-particle beam is irradiated onto at least a portion of the target material while rotating the target material container about the axis of rotation, a transport step, after the rotational drive irradiation step, wherein a carrier gas is supplied into the target chamber and an ambient gas surrounding the target material is discharged outside the target chamber through an exhaust pipe, and a trapping step, after the rotational drive irradiation step, wherein a trap device connected to the exhaust pipe is configured to trap ²¹¹At from the ambient gas, ²¹¹At being a radionuclide generated from ²⁰⁹Bi through irradiation with the alpha-particle beam.
Furthermore, in an embodiment of the present disclosure, provided is a target holding device for alpha-particle beam irradiation, comprising: a target material container made of a material capable of being induction heated, the target material container having a containing part for the target material to be irradiated with the alpha-particle beam, an opening for allowing the containing part to communicate with the exterior, a base, and a surrounding wall extending from the base, and the target material container being rotatable about an axis of rotation passing through the containing part, the opening, and the base, wherein the base and the surrounding wall demarcate at least a portion of the containing part from the exterior, a rotational drive mechanism generating driving force for rotating the target material container, a container heater including an induction heating coil capable of heating at least a portion of the surrounding wall of the target material container, and a container heater controller for controlling heating operation by the container heater based on a temperature of the target material container or the target material, wherein the axis of rotation is inclined relative to the horizontal direction, and the base and the surrounding wall are configured such that an inner surface thereof that is in contact with a bottommost portion of the containing part of the inclined target material container is positioned close to a heated range of the induction heating coil.
In a further embodiment of the present disclosure, provided is a target holding device for alpha-particle beam irradiation, comprising: a target material container made of a material capable of being induction heated, the target material container having a containing part for the target material to be irradiated with the alpha-particle beam, an opening for allowing the containing part to communicate with the exterior, a base, and a surrounding wall extending from the base, and the target material container being rotatable about an axis of rotation passing through the containing part, the opening, and the base, wherein the base and the surrounding wall demarcate at least a portion of the containing part from the exterior, a rotational drive mechanism generating driving force for rotating the target material container, a container heater including an induction heating coil capable of heating at least a portion of the surrounding wall of the target material container, and a container heater controller for controlling heating operation by the container heater based on a temperature of the target material container or the target material, wherein the target holding device comprises a first target material container and a second target material container, wherein the target material in the second target material container can be heated by the container heater when the alpha-particle beam is irradiated onto the target material in the first target material container while the first target material container is rotated by the rotational drive mechanism, and wherein the target material in the first target material container can be heated by the container heater when the alpha-particle beam is irradiated onto the target material in the second target material container while the second target material container is rotated by the rotational drive mechanism.
The present disclosure further provides an output window device for quantum beam irradiation and a system for alpha-particle beam irradiation.
In the present disclosure, an alpha-particle beam generator means any type of equipment to generate an alpha-particle beam, such as a particle accelerator. The target chamber generally refers to a box or enclosure for keeping the target material environment under controlled conditions, capable of maintaining an airtight interior as required. Carrier gas refers to any kind of a gas that can function to transport substances (radioactive materials) that may contain the generated radionuclides. Ambient gas generally refers to the gas surrounding the target material, which may contain both the carrier gas and the radioactive material transported by that carrier gas. The carrier gas may be a single gas or a mixture of gases. The ambient gas may contain substances other than the carrier gas and the radioactive material transported by it.
Trapping and trap devices refer to the following phenomena and the apparatus for achieving them: They can collect, trap, or separate the target substance itself or substances containing the target substance from the ambient gas according to any mechanism, such as absorption, chemical adsorption, physical adsorption, filtering, chemical reaction, centrifugation, condensation or distillation, recrystallization, precipitation, dissolution, or dispersion. Furthermore, since various phenomena may occur when the target substance is mixed with the carrier gas to constitute the ambient gas, the phenomena for collection in the present disclosure are not particularly limited. A typical trap device that may be used when the produced radionuclide is ²¹¹At includes a gold foil trap maintained at an appropriate temperature or an activated carbon column (activated carbon filter), and may also be a cold trap. At trapped in the cold trap is recovered by washing with chloroform, methanol, pure water, etc. Therefore, the gold foil trap and activated carbon column also serve as traps for testing whether At has been successfully removed from the chamber. The trap device of the present disclosure includes any type of device other than these, and is intended not only for collecting radionuclides such as ²¹¹At as the product, but also for suppressing the release of radionuclides to the external environment.
The definition of "multiple steps being performed in parallel" shall mean that there exists at least an instant during which all of the multiple steps are performed simultaneously. Multiple steps that are not explicitly stated to be performed in parallel, or multiple steps for which only the possibility of being performed in parallel is described, are assumed to be performed either simultaneously or non-simultaneously.
The axis of rotation is an infinitely long straight line that provides the central axis for the rotational motion imparted to the target material container. A physical structure such as a spindle may or may not exist along this axis of rotation. Therefore, for an object to be traversed by the axis of rotation simply means that the geometric straight line forming the axis of rotation passes through the object. The same applies to the window rotation axis. The target material is the object irradiated by the alpha-particle beam, where alpha particles (helium-4 nuclei) collide with it at a velocity and flux depending on the beam intensity. The target material includes metals, organic substances, inorganic substances, and mixtures thereof, existing in any state of solid, liquid, or solid-liquid mixture. There are no restrictions on the material type or elements. When the target material contains ²⁰⁹Bi, it need not necessarily be pure ²⁰⁹Bi. The target material container is employed to accommodate the target material and ensure it is properly held during alpha-particle beam irradiation. The containing part of the target material container is a spatial region formed in any shape for accommodating the target material. An opening is a surface area through which the containing part can communicate with the outside. The target collectively indicates the target material and the target material container, meaning an object in which the target material is housed within the target material container. In this case, it is not particularly important whether the target material is integrated with the target material container or fixed to the target material container.

Advantageous Effects of Disclosure

Any of the embodiments of the present disclosure is capable of increasing the intensity of the alpha-particle beam, thereby increasing the production capacity of ²¹¹At or improving its yield, which enhances the practicality of the method for producing ²¹¹At.

Brief Description of Drawings

FIG. 1 is a schematic diagram showing the general structure of the ²¹¹At production system according to an embodiment of the present disclosure.
FIG. 2 is an enlarged cross-sectional view of the vicinity of the target holding device in the ²¹¹At production system according to an embodiment of the present disclosure.
FIGS. 3A-C are a configuration diagram (FIG. 3A) of the trap device in the ²¹¹At production system of an embodiment of the present disclosure and explanatory diagrams (FIGS. 3B, 3C) illustrating its operation.
FIG. 4 is a schematic diagram showing the structure of an exhaust pipe assembly employing a double-pipe structure in an embodiment of the present disclosure.
FIGS. 5A-B are an explanatory diagram (FIG. 5A) illustrating the specific arrangement of the ²¹¹At production system in an embodiment of the present disclosure, and an enlarged view (FIG. 5B) showing an example structure of a particulate filter employing quartz wool.
FIG. 6 is a flowchart showing online collection, a typical method for producing ²¹¹At using the ²¹¹At production system of an embodiment of the present disclosure.
FIG. 7 is a flowchart illustrating the irradiation processing method where the target material container is rotated during irradiation in the production method of the ²¹¹At according to an embodiment of the present disclosure.
FIGS. 8A-B are schematic diagrams showing the specific structure of a container cover used with a typical target material container in the alpha-particle beam irradiation system of an embodiment of the present disclosure.
FIG. 9 is a flowchart illustrating a preferred processing method for producing ²¹¹At in an embodiment of the present disclosure.
FIG. 10 is a chart showing the interrelationship of target-specific processing sequences in an embodiment where multiple targets are used.
FIG. 11 is an explanatory diagram schematically depicting the processing of targets in an embodiment of the present disclosure where multiple targets are used.
FIGS. 12A-B are schematic cross-sectional views illustrating the shape of the target material container modified in embodiments of the present disclosure, showing the pre-modified state (FIG. 12A) and the post-modified state (FIG. 12B).
FIGS. 13A-B are partially sectional side views illustrating the structure of the target material container and container holding part, which utilize a gap adopted in an embodiment of the present disclosure to switch thermal contact. FIG. 13A shows a thermally disconnected state, and FIG. 13B shows a thermally connected state.
FIGS. 14A-B are partial sectional views illustrating the modified target material container and its operation in an embodiment of the present disclosure, showing a stationary state (FIG. 14A) and a rotating state (FIG. 14B).
FIGS. 14C-D are partial sectional views illustrating further improvements to the target material container and its operation modified in embodiments of the present disclosure, showing a stationary state (FIG. 14C) and a rotating state (FIG. 14D).
FIGS. 15A-B are a schematic diagram (FIG. 15A) showing the structure of a trap device for an embodiment of the present disclosure when Bi particles are generated, and an explanatory diagram (FIG. 15B) illustrating its operation.
FIGS. 16A-B are explanatory diagrams of an example employing an automatic supply mechanism for ²⁰⁹Bi (FIG. 16A) and an example employing a discharge mechanism for ²⁰⁹Bi (FIG. 16B), respectively, in the ²¹¹At production system of an embodiment of the present disclosure.
FIGS. 17A-D are a conceptual diagram, a cross-sectional view, a plan view, and a detailed cross-sectional view showing the structure of the main parts of the output window device of an embodiment of the present disclosure.
FIG. 18 is a cross-sectional view showing another structure example of the output window device according to an embodiment of the present disclosure.
FIG. 19 is a cross-sectional view showing yet another configuration example of the output window device according to an embodiment of the present disclosure.
FIG. 20 is a schematic diagram showing the structure of an²¹¹At production system employing differential pumping.
FIG. 21 is a graph showing the relationship between the power input to the induction heating coil and the measured temperature of the target material container in a preliminary experiment according to an embodiment of the present disclosure.
FIG. 22 is a graph showing the time variation of the measured values obtained by the radiation thermometer for the target material container and the measured values obtained by the radioactivity rate meter indicating the At amount in Experiment 1 of an embodiment of the present disclosure.
FIG. 23 is a graph showing the time variation of the measured values obtained by the radiation thermometer for the target material container and the measured values obtained by the radioactivity rate meter indicating the At amount in Experiment 2 of an embodiment of the present disclosure.
FIG. 24 is a graph showing the time variation of the measured values obtained by a radiation thermometer for the target material container in Experiments 3-1, 3-2, and 3-3 of an embodiment of the present disclosure, and the measured values obtained by a radioactivity rate meter indicating the amount of At.
FIG. 25 is a graph showing the measured values obtained by a radiation thermometer for the target material container in Experiments 4-1 and 4-2 of an embodiment of the present disclosure.
FIG. 26 is a graph showing the measurements obtained by a radiation thermometer for the target material container in Experiment 5 of an embodiment of the present disclosure.

Description of Embodiments

The following describes the structure of a system for alpha-particle beam irradiation, a method for irradiation, and a method for producing ²¹¹At. It also describes the target holding device and target used for alpha-particle beam irradiation. Unless otherwise specified in the following description, common parts or elements are indicated by common reference numerals. In addition, each element in the drawing should be understood as not being drawn to scale. Furthermore, in the figures, the elements of each embodiment are not necessarily shown maintaining their relative scale. Hereinafter, astatine-211 (²¹¹At) may be denoted simply as At, and bismuth-209 (²⁰⁹Bi) may be denoted simply as Bi.

1. Embodiment (Overview of At Production Using a Rotating Target)

FIG. 1 is a schematic diagram showing the general structure of the At production system of the present embodiment, and FIG. 2 is an enlarged cross-sectional view of the area near the target holding device in the At production system of the present embodiment. The At production system 1000 is equipped with an alpha-particle beam generator 200 and a target holding device 100. The alpha-particle beam 2 from the alpha-particle beam generator 200 irradiates the target material 4 held in the target holding device 100. The alpha-particle beam generator 200 may employ, for example, a linear accelerator, a cyclotron, or a nuclear reactor. In addition to beryllium foil, materials suitable for vacuum window 22 include Ti foil and Havar foil (cobalt alloy foil). At-production system 1000 is equipped with an At-trap system 6. The At-trap system 6 roughly consists of a carrier gas supply system 61 and an ambient gas exhaust system 65. The carrier gas supply system 61 is any means for introducing carrier gas into the target chamber 7. In the case where target material 4 is Bi and the generated radionuclide is At, helium gas is typically used as the carrier gas. It is supplied to the target chamber 7 via the carrier gas supply system 61, with flow rate controlled by a regulator (not shown) from a cylinder (not shown). The ambient gas exhaust system 65 is used to exhaust the gas (ambient gas) that constitutes the atmosphere of the target material 4 from inside the target chamber 7 and to trap At from that ambient gas. The ambient gas may contain, in addition to the carrier gas, gases or aerosols generated and released or remaining in the atmosphere of the target material 4 for some reason. The ambient gas exhaust system 65 includes an exhaust pipe assembly 66a and an exhaust pipe heater 68. The end of exhaust pipe 66 in the exhaust pipe assembly 66a, on the side facing target chamber 7, is opened to allow communication with the target material container 3's containing part 30 at a position suitable for exhausting the ambient gas. The other end of exhaust pipe 66 in exhaust pipe assembly 66a is connected to trap device 69. The target chamber 7 is typically made airtight, except for carrier gas a gas supply system 61 and exhaust pipe 66.The target chamber 7 is pre-treated with appropriate purging and other preparatory processes prior to use. In typical operation, carrier gas is supplied from the carrier gas supply system 61 during a step where the alpha-particle beam 2 irradiates the target material 4, and the ambient gas is evacuated from the target chamber 7 via the ambient gas exhaust system 65. After irradiation of the target material 4 with the alpha-particle beam 2 is stopped, the supply of carrier gas and the exhaust of ambient gas may continue or be restarted. The generated radionuclide At can be incorporated into the carrier gas to form ambient gas and transported via the ambient gas exhaust system 65. However, the amount and concentration of radionuclide At transported as part of the ambient gas depend on various conditions. Furthermore, Bi may vaporize for some reason, becoming a gas or fume, and be incorporated into the ambient gas.
As shown in detail in FIG. 2, a typical target holding device 100 of the present embodiment comprises a target material container 3 and a rotational drive mechanism 5. The target material container 3 includes a containing part 30. The target material 4 is accommodated in the containing part 30 and placed within the target chamber 7. That is, the containing part 30 in the target material container 3 is used to accommodate the target material 4. Furthermore, the target 1 is the target material 4 loaded into the target material container 3. The target chamber 7 is an enclosure ("chamber") for maintaining an environment suitable for subjecting the target material 4 to irradiation processing by the alpha-particle beam 2. The interior of the target chamber 7, where irradiation of the target material 4 by the alpha-particle beam 2 is performed, is connected to the output side of the alpha-particle beam generator 200 and can maintain its airtightness as necessary via the vacuum window 22. In a typical structure, the target material container 3 is rotatable about the axis of rotation 11, which lies within the plane of the paper surfaces of FIGS. 1 and 2. The general shape of the target material container 3 shown in cross-section in FIGS. 1 and 2 may be any container having an opening 31, and in a non-limiting example, it is a bottomed cylinder whose axis coincides with the axis of rotation 11. The rotational drive mechanism 5 is a mechanism capable of rotating the target material container 3 together with the target material 4 about the axis of rotation 11, appropriately maintaining that rotation and stopping it. As shown in FIG. 2, the rotational drive mechanism 5 typically includes a rotation feedthrough unit 55, a rotary coupling 56, a spindle 57, a container holding part 51, a rotation transmission mechanism 58, and a motor 59. By operating this rotational drive mechanism 5, the target material 4 can be centrifugally forced from the inside against the inner surface 33 of the surrounding wall 32 of the target material container 3. For example, by rotating the target material container 3 while heating it, the molten target material 4 can be pressed against the inner surface 33 of the surrounding wall 32 by the rotation. Stopping the heating while maintaining rotation allows a thin layer of solid target material 4 to be formed on the inner surface 33 of the surrounding wall 32. Furthermore, rotating the target material container 3 during the irradiation step of the alpha-particle beam 2 emitted from the alpha-particle beam generator 200 enables to increase the beam intensity of the alpha-particle beam 2. The opening 31 serves to allow the containing part 30 to communicate with the external environment. Here, the external environment refers to the spatial region outside the target material container 3. Opening 31 is used to receive target material 4 into the containing part 30, to discharge or remove products generated by nuclear reactions, and also serves as the irradiation path for the alpha-particle beam 2 to the target material 4. Opening 31 also serves as the path for carrier gas supplied from the carrier gas supply system 61 into the target chamber 7 to flow into the containing part 30. The rotation feedthrough unit 55 is designed to transmit rotation while maintaining an airtight seal, for example, using a magnetic fluid seal. The spindle 57 can be cooled with cooling water or the like to maintain the magnetic fluid seal at a suitable temperature against heat generated according to the intensity of the alpha-particle beam 2.
FIGS. 3A-C are a structure diagram (FIG. 3A) of the trap device 69 shown in FIG. 1 and explanatory diagrams (FIGS. 3B, 3C) illustrating its operation. The trap device 69 is equipped within the At production system 1000 to trap At from the ambient gas. Specifically, the role of the trap device 69 is at least one of removing or recovering the generated At from the ambient gas. The structure of trap device 69, as shown in FIG. 3A, utilizes a cold trap 696, a trap system 698, and an exhaust pump 699 to enable At collection by the cold trap 696. Simultaneously, it employs three-way valves 692a, b, and fittings 693a to d to provide a bypass path as needed. The exhaust pump 699 is employed both to ensure the gas flow required for trap and to maintain negative pressure inside the target chamber 7, thereby suppressing At leakage to the outside. The trap system 698 prevents At leakage to the outside and has its structure consisting of a series connection of three components: a charcoal filter, an empty glass bottle, and a bottle containing Na₂S₂O₅. The airflow indicated by the connection structure and arrows in FIG. 3A is that for the trap operation of the trap device 69. Fittings 693a and 693c, and fittings 693b and 693d are connected respectively. Three-way valves 692a and 692b are configured such that ambient gas from the ambient gas exhaust system 65 passes through the cold trap 696 located between fittings 693c and 693d. In cold trap 696, the tube is cooled, for example, by liquid nitrogen, trapping At from the ambient gas onto the inner wall of the tube. Once At is trapped in cold trap 696, three-way valves 692a and 692b are switched toward their bypass path, disconnecting fittings 693a and 693c from each other, and disconnecting fittings 693d and 693b from each other. As shown in FIG. 3B, the cold trap 696 is removed. This cold trap 696 is connected to a solvent elution system 69a, separate from the ambient gas exhaust system 65, as shown in FIG. 3C. When an appropriate solvent, such as chloroform, is passed through the cold trap 696 by the liquid transfer pump 695, the At is recovered into the vial 697 together with the solvent.
FIG. 4 is a schematic diagram showing the structure of the exhaust pipe assembly 66a employing a double-pipe structure in the present embodiment. The thermal conductive sheath 67 preferably consists of a tube made of a metal such as copper. In the exhaust pipe assembly 66a employing a double-pipe structure, the thermal conductive sheath 67 is directly heated by the exhaust pipe heater controlled by the exhaust pipe heater controller 682, while the exhaust pipe 66 is indirectly heated via the thermal conductive sheath 67. This allows spatially uniform heating to be performed. Therefore, an exhaust pipe 66 made of a material capable of withstanding direct contact with the ambient gas can be used, allowing the thermal conductive sheath 67 to serve the function of suppressing temperature variations according to position. Furthermore, in the exhaust pipe assembly 66a employing a double-pipe structure, both the exhaust pipe 66 and the thermal conductive sheath 67 can also serve the role of sealing any accidental leakage of the ambient gas. An exhaust pipe heater controller 682 can regulate the heating operation based on the temperature of the thermal conductive sheath 67. Furthermore, although the exhaust pipe heater 68 is depicted as cylindrical in FIG, heating elements (heaters) of various shapes can be adopted. This is because the exhaust pipe 66 is heated uniformly via the thermal conductive sheath 67, making it difficult for uneven heating, such as localized temperature increases, to occur.
This section describes the typical operation of the At production system 1000. Figure 5A is an explanatory diagram illustrating the specific configuration of the At production system of the present embodiment. One typical operation of the At production system 1000 is online collection. "Online collection" in At production refers to the operation of transporting and trapping At using a carrier gas (helium gas) in a situation where At can be continuously generated by actually irradiating the target material 4 (Bi) with the alpha-particle beam 2. This operation is performed, for example as shown in FIG. 5A, by irradiating with the alpha-particle beam 2 in the irradiation room R1 while extending the ambient gas exhaust system 65 to an adjacent laboratory room (laboratory room R2) shielded by the shield S. The exhaust pipe heater 68 adopted for the ambient gas exhaust system 65 maintains the exhaust pipe assembly 66a at 100 to 110°C. This structure enables the trap of At in the trap device 69. The trap device 69 is installed inside the hood 691. Although not shown in FIG. 5A, the trap system 698 and exhaust pump 699 (FIG. 3A) are also installed inside the hood 691. During online collection, for example, the gaseous helium flow rate is set to 1.0 L/min during irradiation by the alpha-particle beam 2, and the container covers 8A, 8B (described later, FIG 8A, FIG 8B) are used as necessary. The target material container 3 is maintained at a temperature of 650°C to vaporize At, and is rotated when irradiating the target material 4 with the alpha-particle beam 2.
When the At production system 1000 shown in FIG. 5A is operated appropriately, the At production system 1000 can be made to perform various operations. In a typical At production system of the present embodiment, as described above for online collection, the transport of ambient gas and the trap of generated At can be performed simultaneously with any of the above-mentioned steps or appropriately after the completion of any of the steps. Referring to FIGS. 6 and 7, the online collection method, a typical production method capable of producing At using the At production system of the present embodiment, is described in further detail. For the sake of illustrative explanation, reference will continue to be made to the At production system 1000 employing a structure with a target material container 3 and a rotational drive mechanism 5. Furthermore, while FIGS. 6 and 7 and the explanations referring thereto illustrate a typical At production method, further modifications within the scope of the present disclosure may be made to increase the At production yield.
FIG. 6 is a flowchart showing online collection, a typical method for producing ²¹¹At using the ²¹¹At production system of the present embodiment. As shown in FIG. 6, in the step of producing At using an alpha-particle beam 2, during the installation step S02, the target material 4, namely Bi, which has been received in the containing part 30 of the target material container 3 for alpha-particle beam irradiation, is placed inside the target chamber 7. Subsequently, necessary conditions, such as gas replacement inside the target chamber 7, are established (not shown). Then, in step S04, helium gas is supplied as carrier gas into the interior of the target chamber 7 via the carrier gas supply system 61. The supply of carrier gas continues until step S16, where it is stopped immediately before the end of the production process. Then, in step S06, which initiates the ambient gas transport stage, the ambient gas surrounding target material 4 is started to be discharged from exhaust pipe assembly 66a of ambient gas exhaust system 65. At this time, the temperature of exhaust pipe 66 is maintained, for example, by exhaust pipe heater 68. In step S08, the trap operation by the trap device 69 is enabled. To achieve this, for example, the three-way valves 692a, b are switched. At this step, a state is realized where, if the radionuclide At is subsequently generated, it can be trapped. After having been achieved in this state, irradiation of the target material 4 by the alpha-particle beam 2 begins (step S10). This irradiation is performed until a pre-determined point in time, such as when the required irradiation dose of the alpha-particle beam 2 reaches a predetermined value, and then ends (S12). Afterwards, the trap operation by trap device 69 is disabled(S14), and the supply of carrier gas and the exhaust of ambient gas are stopped (S16). It should be noted that the production method for At of the present embodiment can be performed without necessarily following the chronological order of the steps described here.
FIG. 7 is a flowchart illustrating the irradiation processing method, wherein the target material container 3 is rotated during irradiation, in the production method of the ²¹¹At of the present embodiment. Upon starting the process, in the installation step S22, the target material container 3, which holds the Bi target material 4 as the target material container for alpha-particle beam irradiation in the containing part 30, is placed inside the target chamber 7. Subsequently, necessary conditions, such as gas replacement inside the target chamber 7, are prepared. In the spin-up step S24, the target material container 3 is spun up and maintained at the desired rotational speed. When the target material container 3 rotates, if the target material 4 is solid, friction acts to cause it to rotate at the same speed. Including cases where the target material 4 is liquid, the target material container 3 and the target material 4 eventually reach the same rotational speed. Furthermore, to form a meltable target material 4 into a thin layer supported on the inner surface 33, the target material container 33 can be heated to a temperature exceeding the melting point of the target material 4 (271.5°C for Bi) before or during the spin-up step S24. The temperature of the target material container 33 can then be cooled below the melting point while maintaining rotation. Through this operation, the melted target material 4 forms a thin layer due to centrifugal force and solidifies in that state. During the irradiation step S26, an alpha-particle beam 2 is irradiated while maintaining the rotation of the target material container 3. Depending on the irradiation conditions and the material of the target material 4, the target material 4 may melt even if it was initially solid. Upon completion of irradiation step S26, irradiation stops and spin-down step S28 is performed, halting the rotation of target material container 3. After completing spin-down, the process terminates. Beyond the processes described here, this series of operations may incorporate any processing steps compatible with the At production system of the present embodiment, as well as installation step S22, spin-up step S24, and irradiation step S26. After completing spin-down step S28, as indicated by the dotted line in the figure, it is possible to return to installation step S22 or rotation start step S24 as needed and continue executing the alpha-particle beam 2 irradiation process. When utilizing the At production system of the present embodiment for At production, if online collection is employed, ambient gas transport can be executed simultaneously with rotation start step S24, for example.
Next, we describe the improvements adopted to enhance yield when producing At in the present embodiment. Note that "yield" refers to the ratio of the amount of At trapped as available material to the total amount of At produced in the nuclear reaction. FIGS. 8A-B are configuration diagrams showing the specific structure of the container cover used with the target material container described in relation to FIG. 1. The target material container 3D comprises a base 34, a surrounding wall 32 extending from the base 34, and an inner flange 36 extending from the surrounding wall 32. It is used similarly to the target material container 3 shown in FIG. 1. The target material container 3D is manufactured, for example, by machining a graphite block.
An induction heating coil (container heater) 302, to form a ring, is installed such that its position encircling the outer surface of the surrounding wall 32 of the target material container 3D aligns the axis of rotation of the target material container 3D with its central axis. As shown in FIG. 1, the induction heating coil 302 is electrically connected to a high-frequency power supply 306 via a matching device 304. The induction heating coil 302 generates a magnetic field that induces an electric current in the target material container 3D. The output of the high-frequency power supply 306 can be adjusted under the control of the container heater controller 322. This allows the target material container 3D to be easily controlled to the required temperature, even while being rotationally driven.
When employing a method that uses an ambient gas to transport the At generated by irradiating the target material (not shown in FIGS. 8A and 8B) received in the target material container 3D with the alpha-particle beam 2, it is preferable to position the container cover 8A (FIG. 8A). The container cover 8A generally covers the opening 31 of the target material container 3D, except for the opening for irradiation 82 of the alpha-particle beam 2. The container cover 8A is typically separated from the target material container 3D by a narrow gap and does not rotate when the target material container 3D rotates. In one example structure, the container cover 8A is mounted relative to the induction heating coil 302. During irradiation with the alpha-particle beam 2, as described in relation to FIGS. 1, 2, and 4, the carrier gas can be introduced from the carrier gas supply system 61 and the ambient gas can be discharged through the exhaust pipe 66. At this time, At generated within the target material 4 (Bi) by the alpha-particle beam 2 can be released into the atmosphere within the containing part 30. It is preferable to guide the ambient gas from the containing part 30 side of the container cover 8A to the exhaust pipe 66. The container cover 8A is manufactured to provide an opening for irradiation 82 of the minimum necessary size to substantially cover the opening 31, i.e., without blocking the irradiation of the target material 4 by the alpha-particle beam 2. Therefore, the structure using the container cover 8A is useful for trapping At with high yield. Furthermore, this opening for irradiation 82 also serves as the primary path for carrier gas supplied into the target chamber 7 to enter the containing part 30. The container cover 8A is preferably fabricated from a material (e.g., quartz glass) that is highly heat-resistant, does not contaminate the generated At, and is not prone to adsorbing At. Figure 8B shows another embodiment of the container cover 8B in the present embodiment. The container cover 8B, like the container cover 8A (FIG 8A), substantially covers the opening 31 and further suppresses leakage of ambient gas from the target material container 3D by providing a side cover section 86 extending between the induction heating coil 302 and the surrounding wall 32. Note that if the container cover 8B is made of a non-conductive material, there is no particular difficulty in heating the target material container 3D using the induction heating coil 302. Furthermore, for container covers 8A and 8B, besides structures like those shown in FIGS. 8A and 8B where the exhaust pipe 66 directly reaches the containing part 30, other structures can also be employed. For example, a nozzle (not shown) made of the same material as container covers 8A and 8B can be formed in the Exhaust Tube Port 84 of container covers 8A and 8B, with the exhaust pipe 66 connected thereto.

2. Technical Problems in Increasing Production Volume, and Solutions

The present inventor conducted repeated experiments using the aforementioned system and operation method to irradiate Bi with an alpha-particle beam to produce At. Through this, we have clarified the factors hindering the increase in At production volume and devised solutions.

2-1. Unexpected Bi Transport

Under the operating conditions of online collection, where collection is performed while irradiating with the alpha-particle beam 2, when the temperature of the target material containing Bi is raised to a high, such as 650°C, the Bi reaches the cold trap (cold trap 696 in FIG. 3A) installed in the trap device 69. This phenomenon can be confirmed by the presence of particulate Bi in the solvent (chloroform) of vial 697 (FIG. 3C). This Bi is a substance that should not be present in the produced At.

2-1-1. Particulate Filter

The present inventor conceived of connecting a particulate filter in series within the exhaust pipe 66 upstream of the trap device 69 as one solution against this technical obstacle. When the particulate filter 66f was actually placed upstream of the cold trap 696 (for example, immediately before the trap device 69 shown in FIG. 5A), black substances adhered to the white filter material. However, downstream of this point, for example from the cold trap 696, no Bi was observed to reach as described above. FIG. 5B is an enlarged view showing an example structure of the particulate filter 66f employing quartz wool. The particulate filter 66f is inserted into exhaust pipe 66 via a suitable connector and can be fabricated by packing the pipe with a filter material that is noncontaminating and heat-resistant. Quartz wool is a typical example of such filter material. Although no thermal conductive sheath 67 is present at this location, it can be heated or maintained at a constant temperature by the exhaust pipe heater 68. The temperature of the particulate filter 66f is preferably selected from temperatures that will vaporize At without melting Bi, as this allows the particulate filter 66f to remove Bi without hindering At trap. The particulate filter 66f was actually able to filter out Bi at 110°C. The black substance visible near the upstream side within the quartz wool of the particulate filter 66f is Bi that was vaporized from the target material 4 or subsequently became fumes, and it may potentially contain At. However, when the radioactivity near the Bi-adhered filter material of the particulate filter 66f was measured, it appeared that almost no At remained. In other words, it was confirmed that if the particulate filter is properly installed and maintained at an appropriate temperature by means such as the exhaust pipe heater 68, it is possible to filter out Bi without adversely affecting the At trap function. Note that if the temperature of the particulate filter 66f is low, there is concern that At may remain on Bi or become trapped in the quartz wool. Furthermore, quartz wool is a material with high selectivity as a filter, as it can block Bi passage at 100 to 110°C and also suppress At adhesion. The position of the particulate filter may be appropriate not only immediately before the trap device 69 shown in FIG. 5A. Without the particulate filter, Bi could potentially reach any position along the entire length of the exhaust pipe 66. This is because, while the exhaust pipe 66 is maintained at a temperature where At is unlikely to adhere due to the exhaust pipe heater 68 and thermal conductive sheath 67, Bi could still adhere. In practice, the amount of Bi adhering is greatest near the target material container 3 along the entire length of the exhaust pipe 66. Therefore, it is also useful to install the particulate filter, for example, at the end of the exhaust pipe 66 near the target chamber 7 or near the target material container 3, or to install it at multiple locations along the exhaust pipe 66.

2-1-2. Change in Operating Mode (Temporal Separation of Irradiation and Ambient Gas Transport)

As a measure against the unexpected transport of Bi described above, the present inventor also conceived a method to suppress the transport of Bi itself. The transport of Bi is caused by its vaporization (volatilization). As shown in FIG. 6, in online collection, the exhaust and transport of the ambient gas (S06) continue until the exhaust of the ambient gas is stopped (S16), during which time irradiation (S10 to S12) occurs. Meanwhile, during the irradiation with the alpha-particle beam, if the temperature of Bi is maintained at a high level (above Bi's melting point of 271.5°C, e.g., 400°C) and the intensity of the alpha-particle beam is strong, Bi appears to vaporize. This vaporization can occur when the temperature of the target material container 3 or the target material containing Bi housed within it reaches or exceeds the melting point of Bi, even if the temperature is sufficiently below the boiling point of Bi, provided the alpha-particle beam intensity is strong.
As one of the solution against this technical difficulty, the present inventor conceived of changing the operating mode from online collection to reduce the possibility of Bi vaporization. Specifically, to suppress Bi vaporization during alpha-particle beam irradiation processing, the target material container 3 is maintained at a temperature below the melting point of Bi (271.5°C) prior to the irradiation step. In this state, the alpha-particle beam is irradiated without heating the target material container 3. This enables irradiation processing with an alpha-particle beam while keeping Bi in a solid state, thereby suppressing vaporization compared with the case where Bi is melted. Furthermore, to transport At using the ambient gas, the target material container 3 is reheated. That is, if the target material container 3 is heated during either or both the transport step or the trapping step, reaching a temperature higher than the melting point of Bi, it is preferable because the At generated in the target material Bi can be vaporized and easily mixed into the ambient gas. However, if the heating of the target material container 3 causes the temperature to exceed the boiling point of Bi, Bi will be transported. Therefore, the temperature of the target material container 3 is maintained below the boiling point of Bi. Furthermore, when irradiating the solid Bi target material 4 with an alpha-particle beam, it is preferable that a thin layer of Bi is formed such that it is supported on the inner surface 33 (Fig. 8A, B). For this purpose, an operation for thinning the target material (target thinning step) can be performed.
FIG. 9 is a flowchart illustrating a preferred process for producing At in the present embodiment. In this preferred process, as shown in FIG. 9, during the installation step S32, the target material 4, namely Bi, is placed inside the target chamber 7. Subsequently, the required conditions, such as atmosphere purging, are prepared. Note that even after purging with helium gas, helium gas is supplied as a carrier gas into the target chamber 7 via the carrier gas supply system 61. Subsequently, during the heating and melting step S34, the target material container 3 is heated. The temperature is selected to be above the melting point of Bi but below its boiling point (e.g., 400°C), suitable for subsequent thin-film deposition. While maintaining this temperature, the target material container 3 is rotated during the spin-up step S36. This thin-film deposition may involve thicknesses of, for example, approximately 10 µm or less, or approximately 0.5 mm or more. This thickness may vary due to the centrifugal force from the rotational motion and the wettability of the target material 4 on the inner surface 33 of the target material container 3. This causes the target material 4 (Bi) to melt, coating the inner surface 33 and forming a thin layer. Subsequently, in the cooling and solidification step S38, the temperature of the target material container 3 is dropped. Depending on the conditions, this can be performed simply by stopping the direct heating of the target material container 3. This is because the target material container 3 is also being cooled. At this stage, the Bi solidifies as a thin layer supported on the inner surface 33, while the target material container 3 continues to rotate. Subsequently, irradiation of the target material 4 with the alpha-particle beam 2 begins in the irradiation step S40. This irradiation is performed until a pre-determined point in time, such as when the required irradiation dose of the alpha-particle beam 2 reaches a specified value, and then terminates.
During this period, the temperature of the target material 4 (Bi) irradiated by the alpha-particle beam 2 is maintained at a temperature at which it would be solid under conditions where the alpha-particle beam 2 is not irradiated (e.g., a temperature below its melting point). Therefore, the inventor is concerned that if there were a means to microscopically and instantaneously observe the surface of the target material 4, the portion of the target material 4 irradiated by the alpha-particle beam 2 might be melted in response to the influence of the alpha-particle beam 2. This is because the use of an alpha-particle beam 2 of such intensity is also expected. When the overall temperature of the target material 4 is maintained at a level where the target material 4 (Bi) is solid, additional factors that contribute to the local temperature of the Bi in the target material 4, such as the thickness of the Bi, could also contribute as to whether the irradiated surface of the target material 4 actually maintains a solid state or is melting. According to its own thermal conductivity, even if the target material container 3 is sufficiently cooled, melting cannot be avoided for strong alpha-particle beam 2 depending on the thickness of the Bi. However, even if the target material 4 might be locally melted, such as at its surface, irradiating the alpha-particle beam 2 while maintaining the target material 4 at a temperature where it remains solid under conditions where the alpha-particle beam 2 is not irradiated can at least suppress Bi vaporization. This creates more room to increase the intensity of the alpha-particle beam 2. After that, the rotation of the target material container 3 terminates and it comes to rest (spin-down S42).
Next, the target material container 3 is heated again (heating and melting step S44). The temperature is selected to be above the melting point of Bi but below its boiling point (e.g., 650°C), and to be suitable for subsequent ambient gas transport. Then, the ambient gas transport is increased, and main process of At trapping begins in the trap device 69 (ambient gas transport and trapping step S46). Furthermore, the carrier gas is supplied even during the alpha-particle beam irradiation step. This is to ensure the transport of the generated At without leakage, as it is possible for Bi to melt as described above. Accordingly, the trap device 69 is maintained in a state ready for trap. For example, the temperature of the exhaust pipe 66 is maintained by the exhaust pipe heater 68. During the ambient gas transport and trapping step S46, At can be efficiently transported via ambient gas transport due to the heating and melting step S44. Therefore, during the atmosphere gas transport and trapping step S46, helium gas is supplied as the carrier gas into the target chamber 7 via the carrier gas supply system 61, and the ambient gas surrounding the target material 4 is evacuated from the exhaust pipe assembly 66a of the ambient gas exhaust system 65. To perform re-irradiation, after completing the atmosphere gas transport and trapping step S46, the process returns to the rotation start stage S36. That is, since the temperature of the target material 4 in the target material container 3 during the ambient gas transport and trapping step S46 exceeds the melting point of Bi, heating for melting (the heating and melting step S34) is not necessarily required. Temperature control to match the desired temperature is sufficient, allowing the rotation start stage S36 to resume immediately. In the exemplary process shown in FIG. 9, the heating and melting step S34 and the rotation start step S36 serve as the target material thinning step. When the irradiation step S40 is performed again after the ambient gas transport and trapping step S46, the heating and melting step S44, the ambient gas transport and trapping step S46, and the rotation start step S36 serve as the target material thinning step. Through the target material thin-film formation step operation, the inner surface 33 can be coated such that the target material 4 containing Bi forms a thin film supported on the inner surface 33. In such operation, the rotational drive irradiation step in the present disclosure includes at least the irradiation step S40. Furthermore, the transport step in the present disclosure includes the ambient gas transport and trapping step S46, and the trapping step in the present disclosure includes the ambient gas transport and trapping step S46.
It is also advantageous from another viewpoint that the target material container 3 remains stationary during the ambient gas transport and trapping step S46, while the ambient gas is transported and At trap is performed. If the target material container 3 is stationary or rotates only very slowly, turbulence in the gas flow during the atmosphere gas transport step is minimized. Consequently, the ambient gas within the target material container 3 is less likely to mix with the carrier gas due to turbulence, enabling efficient collection of the At-containing ambient gas.
Furthermore, during irradiation step S40, the target material container 3 is rotated. This is because, even though the target material 4 is solid, Bi may vaporize due to vaporization or similar effects when the alpha-particle beam is intensified to boost At production. Therefore, irradiation using a solid target material and the atmosphere gas transport that melts the target material can be separated temporally. This means the time Bi is maintained at high temperatures is shortened, which is advantageous as it can also reduce the adverse effects on trap caused by Bi deposition in exhaust pipe 66.

2-1-3. Multiple Targets

However, the temporal separation between irradiation and atmosphere gas transport introduces other technical difficulties. One such difficulty is the reduced equipment utilization efficiency of the alpha-particle beam generator 200. During the atmosphere gas transport and trapping step S46 (Fig. 9), the alpha-particle beam capacity is not being utilized, resulting in a lower At production rate per hour that the alpha-particle beam can operate. Therefore, one measure to produce At more efficiently involves techniques such as irradiating a single beam through two or more paths, each with its own target, or preparing multiple targets for a single beam and swapping them. These embodiments employing multiple targets will be described in further detail.
To explain processing for multiple targets, we will refer to the settings for irradiating each target as "positions." Adopting multiple targets corresponds to establishing a number of positions equal to the number of targets and switching between these positions over time. The purpose of adopting multiple targets is to enhance the equipment utilization efficiency of the alpha-particle beam generation device 200 by sequentially performing the processing shown in FIG. 9 on individual targets while minimizing the time the alpha-particle beam generation device's irradiation is stopped. FIG. 10 is a chart showing the interrelationship of the processing sequences for each target in an embodiment using multiple targets. FIG. 11 is an explanatory diagram schematically depicting the processing of targets in an embodiment using multiple targets. The present embodiment is not necessarily limited to a structure employing two targets; however, it illustrates an example where two targets, Target 1A and Target 1B, similar to Target 1, are processed alternately. In FIGS. 10 and 11, two positions, P1 and P2, are prepared where the irradiation processing targets are Target 1A and Target 1B, respectively.
FIG. 10 shows the steps for targets 1A and 1B using the representation from FIG. 9, and also indicates their positions. Note that time t progresses from top to bottom on the page, and steps occurring at the same vertical position on the page happen simultaneously. The patterns are specifically distinguished for irradiation (S40) and other steps. The transport of the ambient gas and the At trap (S46, labeled "Transport/Collection" in FIG. 10) are shown with the same pattern as each step except irradiation. FIG. 11 shows the state of targets 1A and 1B. As the process in FIG. 9 progresses for each target, position P1 is selected, and an alpha-particle beam 2 is irradiated onto target 1A, as shown in the upper part of FIG. 11. This typical timing corresponds to time t = T1 (FIG. 10). At this time, the target material 4 of target 1A has solidified into a thin layer supported on the inner surface 33, while simultaneously, the target material 4 of target 1B is melted. Irradiation is thus performed on target 1A. As the process in FIG. 9 progresses further at each target, position P2 is selected, as shown in the lower section of FIG. 11. This typical timing corresponds to time t = T2 (FIG. 10). Now, the alpha-particle beam 2 irradiates target 1B, where the target material 4 is solid, while simultaneously, the target material 4 at target 1A is melted. In this way, ambient gas transport and At trap proceed for target 1A while irradiation occurs for target 1B. Subsequently, as the process in FIG 9 further progresses, position P1 is selected again as needed. The typical timing for this is time t = T3. Processing proceeds in this alternating manner (FIG. 10), and the process ends based on conditions such as the number of times irradiation (S40) or ambient gas transport and At trap (S46) has been performed as desired, the irradiation dose or trap amount reaching a desired value, or the remaining amount of target material 4 falling below a reference value. It is advantageous that such operation achieves temporal separation between irradiation and atmosphere gas transport while enabling the most effective utilization of the irradiation time of the alpha-particle beam 2.1 The difference between positions P1 and P2 is illustrated in FIG. 11 by moving targets 1A and 1B without moving the alpha-particle beam 2. This can also be performed by fixing the positions of targets 1A and 1B and irradiating each target with the alpha-particle beam 2. Furthermore, in situations where the time required for trap is longer compared to the time required for irradiation, it is also useful to have three or more targets and positions.
Another technical difficulty arising from measures against Bi vaporization is the tendency for insufficient heating of the target material 4 during the atmosphere gas transport and At trap step (S46, FIG. 9). This relates to the fact that the target material container 3 is cooled by means of water cooling or similar means. During the alpha-particle beam 2 irradiation timing, the target material 4 and the target material container 3 generate heat. The rotation feedthrough unit 55 (FIG. 2) employs a magnetic fluid seal (not shown) to achieve both hermeticity and rotation. Due to its ceiling of operating temperature, cooling of the spindle 57 may be necessary during the alpha-particle beam 2 irradiation step, and this cooling becomes essential when the alpha-particle beam 2 is intensified. However, this cooling is not consistent with the heating required to melt Bi and transfer At to the ambient gas during the atmosphere gas transport and At trap steps. The temporal separation between irradiation step and atmosphere gas transport described above is in itself one solution to this problem. During the atmosphere gas transport and At collection steps, no heat generation attributable to alpha-particle beam irradiation of target material 4 or the target material container 3 occurs, and rotation of the target material container 3 is unnecessary. Therefore, cooling to protect the magnetic fluid seal is either unnecessary or can be reduced. Thus, the temporal separation between irradiation and atmosphere gas transport is also advantageous in avoiding the inconsistency between heating and cooling.

2-1-4. Container Design

From the perspective of heating the target material 4 during the atmosphere gas transport step and the At trap step, another potential solution to further enhance the efficiency of atmosphere gas transport and At trap lies in modifying the shape of the target material container 3. FIGS. 12A-B are schematic cross-sectional views illustrating the modified shape of the target material container in the present embodiment, showing the pre-modification state (FIG. 12A) and the post-modification state (FIG. 12B). Specifically, the shape is modified from the target material container 3 shown in FIG. 12A to the target material container 3A shown in FIG. 12B. The typical target material container 3 of the present embodiment is inclined such that the axis of rotation 11 is tilted from the horizontal direction. Heating is performed by self-heating the surrounding wall 32 using an induction heating coil 302 positioned on the outer periphery of the surrounding wall 32. Furthermore, in the target material container 3, the outer surface of the base 34 is cooled because it contacts the container holding part 51, spindle 57, etc. Although heating the target material 4 (Bi) to, for example, 600 to 700°C or higher allows efficient incorporation of At into the ambient gas, the relative arrangement between the heated area (shown in a sand pattern) of the target material container 3 and the target material 4 causes cooling to inhibit heating. Therefore, modification of the shape of the target material container 3 is useful. For example, as shown in FIG. 12B, increasing the thickness of the base 34, as in target material container 3A, is useful. The base 34 and surrounding wall 32 of target material container 3A are shaped such that the inner surface of the containing part 30 (in Figure 12B, part of the inner circumferential surface 33 of surrounding wall 32) where the bottommost portion 30B of the inclined containing part 30 of target material container 3A contacts it is positioned closer to the range (indicated by the sand pattern) heated by induction heating coil 302. Here, 'positioned closer' can mean that the distance is shorter compared to the thickness of the surrounding wall 32, can mean that the distance is shorter compared to the distance to the aforementioned cooled position, or can mean that the distance is such that heating of the target material 4 is substantially effective. The bottommost portion 30B of the containing part 30 becomes the deepest part of the stored melted Bi. In the target material container 3A, the base 34 is thicker compared to the target material container 3, making it less susceptible to cooling effects on the target material 4. Furthermore, the deepest part of the target material 4 is the position most suitable for heating a liquid where convection can be expected. The fact that this deepest part is positioned close to the range heated by the induction heating coil 302 is another reason why the specially shaped target material container 3A is useful for ambient gas transport and At trap.

2-1-5. Switching Operation on Thermal Contact

From the perspective of temperature control for the target material container, it is also useful to make improvements including the structure supporting the target material container. FIGS. 13A and 13B are partially sectional side views illustrating the structure of the target material container 3 and the container holding part 51, which utilize a gap adopted in the present embodiment to switch thermal contact. They show the state when thermally separated (FIG. 13A) and the state when thermally in contact (FIG. 13B). The typical target material container 3 of the present embodiment is inclined so that the axis of rotation 11 is tilted from the horizontal direction, as shown in FIGS. 1 and 2. However, here, the axis of rotation is described as being oriented in the vertical direction on the paper. The target material container 3 is precisely temperature-controlled, primarily requiring heating and cooling conditions. During heating, the induction heating coil 302 positioned on the outer periphery of the container's surrounding wall 32 causes the wall 32 to self-heat. Conversely, cooling situations arise to prevent overheating of the target material 4 and to ensure the heat resistance of components like the magnetic fluid seal of the rotation feedthrough unit 55 (Fig. 2). Here, efficient release of At into the ambient gas can be achieved if heating to, for example, 600 to 700°C or higher is possible. At this timing, to protect the rotation feedthrough unit 55, which cannot withstand that temperature, the spindle 57 must be cooled, for example, by water cooling. This conflicting situation can be mitigated to some extent by ensuring the container holding part 51 possesses appropriate thermal capacity and thermal conductivity. However, in the present embodiment, another approach capable of simultaneously achieving heating and cooling is also useful. The present inventor conceived of switching the thermal contact itself, as shown in FIGS. 13A and 13B. Specifically, the concept involves creating a gap between the base 34 of the target material container 3 and the container holding part 51, enabling thermal separation between the outer surface of the base 34 and the container holding part 51 (Fig. 13A). Here, during the timing when the target material container 3 rotates, it is preferable to cool the target material container 3 to prevent overheating of the target material 4. Therefore, it is preferable to close the gap mentioned above when the target material container 3 rotates, thereby bringing the target material container 3 and the container holding part 51 into thermal contact with each other (Fig. 13B). By controlling the thermal contact between the target material container 3 and the container holding part 51 via the gap in this manner, it is possible to: - Enhance the cooling performance of the target material container 3 by maintaining thermal contact between the target material container 3 and the container holding part 51 while rotating the target material container 3 during beam irradiation; and when beam irradiation ends and the rotation of the target material container 3 stops, the gap G between the target material container 3 and the container holding part 51 opens, thermally separating them, making it easy to heat the target material container 3. Furthermore, as a preferred structure of the container holding part 51, it is also useful to provide a heat conduction member 512, made of a highly thermally conductive material such as a copper plate, on the surface of the container holding part 51 facing the target material container 3, as shown in FIG. 13B. Switching between the structures shown in FIGS. 13A and 13B, whether stationary or rotating, can be performed through various structures.
In the present embodiment, a mechanism utilizing centrifugal force may be employed to control the gap described above. FIGS. 14A-B are partially sectional side views illustrating the target material container and its operation in this embodiment, showing the state at rest (FIG. 14A) and during rotation (FIG. 14B). A spring 54S is disposed between the container holding part 51 and the target material container 3, biased to separate the container holding part 51 and the target material container 3, thereby opening the gap G. Guide pins 54G are provided at, for example, four locations around the base periphery of the target material container 3, along with the spring 54S, to restrict excessive opening of the gap G and hold the target material container 3 against the container holding part 51. Multiple arms 52 are arranged around the container holding part 51. Each arm 52 is positioned to hold the target material container 3 from, for example, four directions and is attached to the container holding part 51 via an arm mounting section 53. Arm 52 comprises a weight section 52W, on which the centrifugal force acts during rotation of the target material container; a claw section 52N, which contacts the target material container to close the gap G when centrifugal force acts on the weight section; and a pivot pin 52P, located between the weight section and the claw section, fixed to the container holding part and serving as a pivot axis. When the target material container 3 is stationary, the gap G is opened by the actuation of spring 54S, as shown in FIG. 14A. Conversely, during rotation of the target material container 3, as shown in FIG. 14B, centrifugal force acting on the weight section 52W causes each arm 52 to pivot about the pivot pin 52P, causing the claw section 52N to close the gap G against the biasing force of the spring 54S. In a design example verified by the inventors of the present invention, when the target material container 3 was rotated from a stationary state to approximately 1000 to 2000 rpm, the movement of closing the gap G, as shown in FIG. 14B, was confirmed.
In the structure shown in FIG. 1, when the alpha-particle beam 2 from the alpha-particle beam generator 200 is contained within a horizontal plane, the axis of rotation 11 is inclined from the vertical direction, which is the normal to the horizontal plane. As a result, the operation of the arm 52 may be affected by gravity. FIGS. 14C-D are partially sectional side views illustrating further improvements to the target material container and its operation in the present embodiment, showing the state at rest (FIG. 14C) and during rotation (FIG. 14D). FIG. 14C depicts, via imaginary lines formed by the two-dot dashed lines, how arm 52 is affected by gravity and how this causes the target material container 3 to shift, resulting in an uneven gap G. Because gravity acts on the weight section 52W of arm 52, causing it to hang down, the arm 52 positioned at the periphery of target material container 3, for example the one located lower down, ends up in the same or a similar state as during rotation due to the influence of gravity. This influence causes target material container 3 to shift, resulting in an uneven gap G. This raises concerns that planned heating during At collection, for example, could be adversely affected. Furthermore, while the degree of target material container 3 displacement can vary depending on the tilt angle of axis of rotation 11 and the biasing force of spring 54S, these conditions are determined by considering various factors and cannot necessarily be set solely based on the target material container 3 displacement perspective. In such cases, the preferred structure is to provide a means to apply a force that suppresses the rotation of arm 52 due to gravity. As shown in FIG. 14C, the plate spring, or arm spring 52S, is attached to container holding part 51 and arm 52 by appropriate fasteners (not shown). The arm spring 52S prevents arm 52 from pivoting unexpectedly due to gravity when stationary, while allowing arm 52 to pivot around the pivot pin 52P due to centrifugal force during rotation, as shown in FIG. 14D. Note that while the arm spring 52S is depicted as a leaf spring structure in FIGS. 14C and D, other types of springs, such as compression coil springs, tension coil springs, torsion coil springs, or torsion bars, may also be employed.
By adjusting the gap G appropriately during both heating and cooling phases, including structures beyond those shown in FIGS. 14A and 14B, it becomes possible to easily control the temperatures for both heating and cooling conditions. Furthermore, controlling gap G reduces the thermal conduction and heat capacity requirements for container holding part 51. This allows container holding part 51 to be simplified or designed with high thermal conductivity. In structures employing thermal conduction member 512, temperature uniformity and control are improved when target material container 3 is in thermal contact with container holding part 51. By adopting the heat conduction member 512 with the expectation that gap G will expand, the temperature uniformity of the target material container 3 can be improved during the heating process to recover At, thereby increasing the At collection efficiency. Furthermore, when the target material container 3, which is heated due to the expansion of gap G, is disconnected from the cooling system, the matching device 304 and the high-frequency power supply 306 connected to the induction heating coil 302 can be of smaller capacity. This point becomes even more significant with the adoption of the heat conduction member 512, as smaller capacity components become sufficient. The reduction in capacity of the matching device 304 and high-frequency power supply 306 contributes to the low cost and miniaturization of the apparatus.

2-1-6. In Situations Where Bi Vaporization is an Issue

When Bi vaporization cannot be fully suppressed, it has been confirmed that Bi adhering inside the capillary reduces the At yield. Furthermore, for At intended for pharmaceutical use, it is necessary to remove Bi particles. Furthermore, as mentioned above, while the amount of Bi adhesion is highest near the target material container 3 along the entire length of the exhaust pipe 66, it may be operationally difficult to place a particulate filter in that location. In such cases, Bi adhesion inside the capillary is somewhat unavoidable. Therefore, when Bi vaporization cannot be avoided, it is also useful to implement a cleaning operation for the capillary or to provide a mechanism (cleaning mechanism) to assist with this as an alternative solution. Here, the capillary refers to the tubular inner wall surface through which vaporized Bi can pass, serving as the transport path for atmosphere gas that may contain At. For example, it is the inner surface of the entire exhaust pipe 66 within the ambient gas exhaust system 65 shown in FIG. 5A. Specifically, the capillary is cleaned using nitric acid, water, or ethanol. This cleaning operation can be performed as appropriate. Furthermore, the capillary cleaning mechanism is a mechanism capable of assisting the cleaning operation, performed by an assembly of any components (all not shown) for fluid movement, such as appropriate piping, cocks, valves, etc.
Furthermore, the following two methods are useful for removing Bi particles. FIGS. 15A-B are a structure diagram (FIG. 15A) of a suitable trap device and an explanatory diagram (FIG. 15B) illustrating its operation when Bi particles are generated in the At production system of the present embodiment.
In the first method for removing Bi particles, as shown in FIG. 15A, a particulate filter 694 is connected in series upstream of the cold trap 696 between the fittings 693a and 693b shown in FIG. 3A. The particulate filter 694 comprises a quartz tube 694a packed with quartz wool 694b. This quartz wool 694b traps the particles. This operation constitutes part of the trapping step. Subsequently, as shown in FIG. 3B, the system is isolated using a valve, and At is released from the Bi particles using the At release line in FIG. 15B (sublimation step). Specifically, while flowing a mixed gas containing oxygen in gaseous helium or oxygen gas using exhaust pump 699, the particulate filter 694 is heated to a high temperature (e.g., 850°C) by a heater 694c, such as an electric furnace. This causes At to sublimate from the Bi particles contained in the quartz wool 694b and be trapped in the cold trap 696 downstream of the quartz tube 694a. The reason for adding oxygen is to facilitate the release of At from the solid Bi and to equalize the valence (chemical species) of the At, making it easier to trap in the cold trap 696. Subsequently, At can be recovered from cold trap 696 using the solvent elution system 69a shown in FIG. 3C. Additionally, a charcoal trap 6982 (FIG. 15B) and trap system 698 are connected as needed. These steps prevent Bi contamination of the At solution and enable efficient collection of At adsorbed onto Bi particles. In this first method, the sublimation step described above is performed after the trapping step.
The second method for removing Bi particles involves performing the operations shown in FIGS. 15A and 15B for the first method using a single system. This allows the sublimation step to be performed not only after the trapping step but also in parallel with the trapping step or in parallel with the irradiation step. Specifically, this method connects the particulate filter 694 in series upstream of the cold trap 696, similar to FIG. 15A. It adds an inflow system (not shown) that introduces additional oxygen upstream of the particulate filter 694. Furthermore, it adds heaters around the quartz tube 694a, similar to the heater 694c arrangement in FIG. 15B. In this arrangement, the particulate filter 694 connected in series upstream of the cold trap 696 may be, for example, the particulate filter 66f shown in FIG. 5A and FIG. 5B. In this structure, while using the exhaust pump 699 to pass the ambient gas from the target chamber 7 through the cold trap 696, oxygen is mixed into the ambient gas upstream of the particulate filter 694. Simultaneously, the particulate filter 694 is heated to a high temperature (e.g., 850°C) by the heater 694c. This is the sublimation step. Performing this operation in parallel with the trapping step facilitates the trap of At in the cold trap 696. Subsequently, as in the first method, At can be recovered from the cold trap 696 using the solvent elution system 69a shown in FIG. 3C. In this second method, since steps shown separately in FIGS. 15A and 15B are performed in a single system, the above sublimation step can be performed in parallel with the trapping step. Furthermore, since the trapping step can also be performed in parallel with the irradiation step, the above sublimation step can also be performed in parallel with the irradiation step.
In both the first and second methods described above, the sublimation step is performed by introducing oxygen into the ambient gas or by introducing oxygen into the particulate filter.

2-2. Suppression of Changes in Chemical Species

Under ideal operating conditions, the trapped ambient gas consists of helium gas mixed with elemental At. However, in practice, besides elemental At mixed with helium gas, At may exist in other chemical species, such as compounds. Such unintended chemical species of At affect the At collection rate and also impact the synthesis of pharmaceutical drugs utilizing At. In the At production system of the present disclosure, any means for preventing contamination is effective. Specifically, a gas purifier is connected at an appropriate location, such as the inlet on the supply side of the carrier gas supply system 61 shown in FIG. 2. If this can increase the purity of the helium gas introduced into the target chamber 7, it is useful for suppressing unintended chemical species.

2-3. Bi Supply/Discharge Mechanism

To supply Bi to the target material container, it is typically necessary to open the target chamber 7. Since opening the target chamber 7 disturbs At production, it is preferable to provide an automatic Bi supply mechanism as a workaround. Furthermore, it is also preferable to trap At from Bi within the target material container after discharging the target material 4 from the target material container. FIGS. 16A and 16B are explanatory diagrams of the present embodiment's At production system, showing an example employing an automatic Bi supply mechanism (FIG. 16A) and an example employing a Bi discharge mechanism (FIG. 16B), respectively. The typical supply form of Bi, the raw material for target material 4, is solid particles. When supplied in this form, it is preferable to feed a fixed quantity of these Bi pellets via the supply tube 42. A predetermined amount of Bi is conveyed through this supply tube 42, for example, from an appropriate hopper into the containing part 30 of the target material container 3. It is preferable that this supply tube 42 is inserted into the containing part 30 at the required timing and is placed in a retracted position at other times.
As shown in FIG. 16B, when Bi is melted, it is also easy to aspirate and discharge it. Therefore, the discharge mechanism of the present embodiment preferably utilizes an suction nozzle 44 capable of sucking up the melted Bi. For this suction, the suction nozzle 44 and the piping beyond it are also heated. Since the discharged Bi contains generated At, the Bi sucked up by the suction nozzle 44 can undergo processing for At trap in a storage container (collection container) separate from the target material container 3. It is also preferable that this suction nozzle 44 is inserted into the containing part 30 at the necessary timing and is retracted at other times. By discharging Bi from the target material container 3 and performing At trap, the irradiation with the alpha-particle beam and the At trap can be spatially separated from each other.

2-4. Efficient Transport of Ambient Gas

Another factor reducing At production is leakage of the ambient gas transporting At outside the trap route. Indeed, as At production tests using the At production system of the present embodiment were repeated, deposits were observed around the opening for irradiation 82 on the container covers 8A and 8B. The deposits are currently thought to be bismuth oxide (Bi₂O₃). This provides direct evidence that Bi, in a different chemical form from the Bi vaporization described above, is leaking through the opening for irradiation 82 within the target chamber 7. While this finding does not directly prove the behavior of the ambient gas, the inventors believe it suggests the possibility that ambient gas containing At could spread via a route similar to that of Bi. To address this, measures to enhance the efficiency of introducing ambient gas into the ambient gas exhaust system 65 during the ambient gas transport step are useful. One such measure is the temporal separation of irradiation and atmosphere gas transport described above. This allows the rotation of the target material container 3 to be stopped, enabling the trap of At by the atmosphere gas, and avoids leakage of atmosphere gas from the opening for irradiation 82 due to turbulence in the gas flow. Another alternative approach focuses on the cover that seals the opening 31 of the target material container 3. The container covers 8A and 8B shown in FIGS. 8A and 8B include an opening for irradiation 82. However, due to the temporal separation of irradiation and ambient gas transport, it is no longer necessary to leave the opening for irradiation 82 open during At trap. Therefore, it is preferable to provide a movable cap (not shown) to close the opening for irradiation 82 during At trap.
As a yet another improvement, it is also a preferred structure to provide an irradiation opening window (not shown) that blocks the irradiation openings 82 of the container covers 8A and 8B shown in FIGS. 8A and 8B. The irradiation opening window is made of a metal foil that transmits the alpha-particle beam over the thickness direction and is attached to the container covers 8A and 8B so as to block the irradiation openings 82. The irradiation opening window allows the pressure on both sides of the container covers 8A and 8B to be nearly equal. Consequently, a material capable of transmitting the alpha-particle beam, such as thinner beryllium foil or titanium foil, can be used for the irradiation opening window instead of the vacuum window 22, which must withstand pressure difference. This design allows both irradiation of the alpha-particle beam through the opening for irradiation 82 and avoidance of ambient gas leakage from the opening for irradiation 82. Furthermore, the thinner the material of the irradiation opening window, the more advantageous it is as it suppresses heat generation caused by the alpha-particle beam.

2-5. Stabilization of Ambient Gas Discharge Operation

As At production tests using the At production system of the present embodiment were repeated, situations were encountered where the ratio of collected At to the At production amount calculated from the irradiation dose (collection rate) was unstable. One cause of this is excessive fluctuation in the internal pressure of the target chamber 7. Furthermore, if the chamber were to become pressurized, At leakage would also be a concern. As a workaround, the internal pressure of the target chamber 7 was previously adjusted to approximately 90 to 95 kPa using a manual needle valve. It is beneficial to further stabilize the suction operation of the exhaust pump 699 (FIG. 3A, FIG. 3B). Therefore, additional solutions are desirable. Specifically, it is beneficial to install an automatic needle valve at the upstream position immediately before the exhaust pump 699 (the position between the trap system 698 and the exhaust pump 699 in FIG. 3A and FIG. 3B) to maintain a constant chamber pressure.

2-6. Multiple Collection Systems

Adopting an operation (online collection) that performs At trap while irradiating with the alpha-particle beam, without implementing the temporal separation of irradiation and ambient gas transport described above, is useful for maximizing the equipment utilization efficiency of the alpha-particle beam generator 200. Furthermore, utilizing multiple targets to enhance the equipment utilization efficiency of the alpha-particle beam generator 200 is also useful. However, even when employing online collection or multiple targets, the procedure for extracting the astatine collected in the cold trap 696 (FIG. 3A) as shown in FIG. 3C remains necessary. If the alpha-particle beam must be stopped during this time, it becomes difficult to enhance the equipment utilization efficiency of the alpha-particle beam generation device 200. To address this, it is useful to prepare multiple collection systems and switch between them. For example, a second cold trap 696 can be inserted into the bypass path shown in FIG. 3B. With two trap systems, while At is being extracted from one system (as shown in FIG. 3C), At trap can continue using the other system. This avoids impacting the utilization efficiency of the alpha-particle beam generator 200.

2-7. Technical Obstacles with Vacuum Windows

Increasing the intensity (current value) of the alpha-particle beam is useful for boosting At production. However, increasing beam intensity introduces specific problems beyond the Bi vaporization mentioned above. Typically, the durability of vacuum windows 22 (FIG 1, 2), made from materials like beryllium, becomes an issue. Furthermore, the weak penetrating power of the alpha-particle beam limits the materials suitable for the vacuum window. Currently, a structure such as a 15µm-thick beryllium foil placed in an opening with an inner diameter of 10mm is employed.

2-7-1. Rotating Window

For the durability of vacuum windows, i.e., vacuum barriers, it is useful to adopt an output window device with a rotating vacuum barrier. Rotating vacuum barriers are also used at GSI (GSI Helmholtz Centre for Heavy Ion Research, Darmstadt, Federal Republic of Germany), but in the present embodiment, further refinements have been incorporated.
FIGS. 17A-D are conceptual diagrams, cross-sectional views, plan views, and detailed cross-sectional views showing the structure of the main parts of the output window device of the present embodiment. The output window device 220 is connected to the output opening of the quantum beam generator. Here, the case where the quantum beam generator is the alpha-particle beam generator 200 (FIG. 1) is described. The output window device 220 comprises a vacuum window wheel 222 and a window rotation mechanism 242 that generates driving force for rotating the vacuum window wheel 222. The vacuum window wheel 222 is rotatable about a window rotation axis that is substantially parallel to the irradiation axis of the quantum beam, such as the alpha-particle beam 2. The window rotation mechanism 242 generates the driving force for rotating the vacuum window wheel 222. The vacuum window wheel 222 comprises an upstream disk 224 and a downstream disk 226 (not shown in Fig. 17A). The upstream disk 224 carries an upstream vacuum window array 22A to 22D, wherein multiple upstream vacuum windows 22A, 22B, 22C, 22D are arranged along the circumference of a first circle C1. The first circle C1 is centered on the window rotation axis 230 and lies within a first plane perpendicular to the window rotation axis 230. The downstream disk 226 is included in a second plane perpendicular to the window rotation axis 230 and carries a downstream vacuum window array 22E to 22H, wherein multiple downstream vacuum windows 22E, 22F, 22G, 22H are arranged on the circumference C2 of a second circle centered on the window rotation axis 230. The upstream vacuum windows 22A, 22B, 22C, 22D and the downstream vacuum windows 22E, 22F, 22G, 22H are in equal numbers. Each vacuum window incorporates a beryllium foil as its window material, which transmits the alpha-particle beam over the thickness direction. Similar to the vacuum window 22 (FIG. 2), this window material can be made from metal foils other than beryllium foil, such as Ti foil or Havar foil. The upstream disk 224 and the downstream disk 226 are connected to each other by a connecting shaft 228. A cooling gas supply system 26 is provided in the gap between the upstream disk 224 and the downstream disk 226 to supply cooling gas. As shown in FIG. 17A, the upstream vacuum window 22A of the upstream disk 224 and the upstream vacuum window 22E of the downstream disk 226 are positioned such that their rotational phases relative to the window rotation axis 230 are the same. Similarly, each of the upstream vacuum windows 22B to 22D and each of the downstream vacuum windows 22F to 22H are positioned such that they share the same rotational phase. As a result of this arrangement, even when the vacuum window wheel 222 rotates about the window rotation axis 230, the alpha-particle beam 2 can pass through both the corresponding upstream vacuum window and downstream vacuum window, thereby demarcating the spatial regions between them. As shown in FIG. 17B, a pipe axle 252 is attached to the upstream disk 224, and the pipe axle 252 is coaxial with the vacuum window wheel 222. When emitted from the alpha-particle beam generator 200, the alpha-particle beam 2 passes through the pipe axle 252. Pipe axle 252 is rotatably supported by magnetic fluid seal and bearing 282, which also serves as the upstream sealing mechanism on its outer surface. Magnetic fluid seal and bearing 282 can maintain an airtight seal between the interior of alpha-particle beam generator 200, which must be high vacuum, and the gap (intermediate chamber) between upstream disk 224 and downstream disk 226. A pulley 244 is mounted on the upstream disk 224, where a friction-engaged belt 246 transmits driving force from a motor 248. Cooling gas, such as helium, is introduced into the intermediate chamber from the cooling gas supply system 26 for cooling purposes. This gas pressure is generally maintained at approximately 1 atmosphere. The durability of the upstream vacuum windows 22A to 22D and the downstream vacuum windows 22E to 22H is ensured by the combined effects of expanding the effective area for beam transmission through the rotation of the output window device 220 and cooling by the helium gas supplied to the intermediate chamber. A spinning seal 284, which is the downstream sealing mechanism, is also positioned on the outer periphery of the downstream disk 226. The interior of the target chamber 7 is an environment of approximately 1 atm, containing helium gas as the carrier gas and potentially also an ambient gas including At. The interior of the target chamber 7 and the intermediate chamber are hermetically sealed from each other by the spinning seal 284. As shown in FIG. 17C, the window fixing frame 224f has openings corresponding in shape and number to the upstream vacuum windows 22A to D to fix these windows to the upstream disk 224. FIG. 17D is a cross-sectional view at the position passing through the upstream vacuum window 22A in FIG. 17C. As shown in FIG. 17D, the window fixing frame 224f fixes the upstream vacuum window 22A to the upstream disk 224. The openings for the upstream vacuum windows 22A-D maintain airtightness against the spaces on both sides of the upstream disk 224 via the O-ring 224p. The structure shown in FIGS. 17C and 17D for the upstream disk 224 is similarly applied to the downstream disk 226. The downstream vacuum windows 22E-H are fixed to the downstream disk 226 with airtight sealing, similarly maintained by an O-ring or the like, using the window fixing frame 226f (FIG. 17A).
Note that the rotating vacuum barrier at Germany's GSI features a single layer of beryllium foil, with an additional rotating window positioned downstream of it. A target material backed by Be is placed in this downstream rotating window, which does not function as a vacuum window (vacuum barrier). In contrast, in the present embodiment, both the upstream and downstream beryllium foils function as vacuum windows. Both the upstream and downstream discs maintain their airtightness so that the spaces before and after each disc are mutually sealed. Therefore, the output window device 220 of the present embodiment is a rotatable output window device where not only the upstream but also the downstream beryllium foil can function as a vacuum window (barrier) capable of transmitting the alpha-particle beam. As shown in FIGS. 17C and D, the rotational phase of the vacuum window wheel 222 includes a phase where the alpha-particle beam 2 collides with the spoke portion, such as between the upstream vacuum window 22A and the downstream vacuum window 22B. At this phase timing, the alpha-particle beam 2 is stopped. In the output window device 220 of the present embodiment, it is preferable that the vacuum window wheel 222 also includes a rotation detection mechanism (not shown) for detecting this rotational phase. Accordingly, it is also useful for the belt 246 and pulley 244 described above to be designed as a toothed belt and toothed pulley, or to employ non-belt-driven mechanisms such as gear drives or chain drives.

2-7-2. Spokeless Rotating Window

When the structure of a vacuum window wheel 222, which has multiple vacuum windows arranged circumferentially such as upstream vacuum windows 22A to 22D and downstream vacuum windows 22E to 22H, is adopted for a rotating window, a portion (referred to as a "spoke") exists that circumferentially divides and connects at least one of the upstream disk 224 and the window fixing frame 224f, or the downstream disk 226 and the window fixing frame 226f. In this case, to prevent the alpha-particle beam 2 from striking the spokes, a phase detection mechanism must be provided for the vacuum window wheel 222. Consequently, the duty ratio-the proportion of the beam's effective period-also decreases to approximately 50% to interrupt the alpha-particle beam 2 according to the rotational phase of the vacuum window wheel 222. To address these issues, it is also advantageous in the present embodiment to adopt a vacuum window wheel with a spoke-less structure (referred to as "spoke-less"). FIG. 18 is a cross-sectional view showing another configuration example of the output window device of the present embodiment. The vacuum window 22SA adopted for the vacuum window wheel 222A is a doughnut-shaped metal foil sheet, with only its outer and inner edges retained. The window outer frame ( outer periphery retainer ) 2202, which retains the outer margin of the vacuum window 22SA, is cylindrical in overall shape with its axis aligned with the window rotation axis 230. It is rotatably supported by a magnetic fluid seal and bearing 282. Furthermore, a belt-drivable pulley 244 is provided around the window outer frame 2202. It can be rotationally driven around the axis of rotation 230 by an appropriate drive means (not shown in FIG. 18), such as the belt 246 and motor 248 shown in FIG. 17B. The window spindle (inner periphery retainer) 2204A, which holds the inner margin of the vacuum window 22SA, is rotatably supported by ball bearings 2206. The alpha-particle beam 2 travels through the space surrounding the window spindle 2204A within the cylinder of the window outer frame 2202. The window rotation axis 230 serves as the axis of rotation for both the window outer frame 2202 and the window spindle 2204A. The magnetic fluid seal and bearing 282 are mounted on the alpha-particle beam generator 200 that delivers the alpha-particle beam 2. The space on the right side of the paper, demarcated by the vacuum window 22SA, is depressurized for the alpha-particle beam 2. In contrast, the space on the left side of the paper, demarcated by the vacuum window 22SA, is filled with carrier gas and is near atmospheric pressure because the target material container 3 (Fig. 1) is placed there during use. Consequently, a pressure difference in the thickness direction acts on the vacuum window 22SA, generating tensile stress in various parts of the sheet. When the window outer frame 2202 is rotated by an unillustrated drive means in this state, the window spindle 2204A also rotates via the vacuum window 22SA. Consequently, the window outer frame 2202, vacuum window 22SA, and window spindle 2204A rotate as a single unit, causing the entire vacuum window wheel 222A to rotate. At this time, the alpha-particle beam 2 passing through the vacuum window 22SA is irradiated into the target chamber 7 without significantly raising the temperature of the vacuum window 22SA and without being obstructed by the spokes. This is because the vacuum window 22SA is formed as a continuous, uninterrupted doughnut shape around the circumference.
The spoke-less rotating window can also be realized as the vacuum window wheel 222B shown in FIG. 19. FIG. 19 is a cross-sectional view showing yet another structure example of the output window device of the present embodiment. Described in terms of changes from the vacuum window wheel 222A of FIG. 18, the vacuum window wheel 222B shown in FIG. 19 has a different structure for the window spindle (inner periphery retainer) 2204B. Furthermore, the window outer frame 2202 and the vacuum window 22SB can also be made smaller in diameter. Window spindle 2204B, unlike window spindle 2204A, does not hold vacuum window 22SB. The metal foil forming the vacuum window 22SB extends between the window outer frame 2202 and the window spindle 2204A such that it has the window rotation axis 230 as its normal. No opening is provided inside the donut-shaped outer perimeter of the vacuum window 22SB area. The vacuum spindle 2204B has a through hole VC suitably provided to serve as a vacuum chuck. The window spindle 2204B supports the vacuum window 22SB to prevent it from being displaced excessively by pressure differences. The window spindle 2204B receives frictional force from the vacuum window 22SB and rotates in response to the rotation of the vacuum window 22SB. The vacuum window wheel 222B in FIG. 19 can adopt a vacuum window 22SB with a smaller outer size compared to the vacuum window wheel 222A in FIG. 18. This allows the vacuum window 22SB to have a continuous, uninterrupted donut-shaped configuration around its circumference while eliminating the need to shape the metal foil into a donut shape. A smaller vacuum window outer size is particularly advantageous for beryllium foil, which is harder to obtain in large areas compared to Ti foil or Havar foil.
Furthermore, it is also useful to place additional window members downstream of the alpha-particle beam 2, similar to the downstream vacuum windows 22E to 22H described in FIG. 17B, on the spoke-less rotating window shown in FIG. 18. Furthermore, although not shown, the window member added at this position may also be a non-rotating metal foil (fixed window). If a fixed window is used, it can be combined with both the vacuum window wheel 222A of FIG. 18 and the vacuum window wheel 222B of FIG. 19. For either vacuum window wheel 222A or 222B, the cooling gas supply system 26 shown in Figure 17B can be adopted to cool the vacuum windows. When this cooling gas supply system 26 is present, it is effective to adopt additional window components, such as fixed windows, downstream of the alpha-particle beam 2 to reduce the probability of astatine escaping through backflow via the cooling gas supply system 26.
Quantum beam generator to which the output window device 220 and the spoke-less rotating window are applicable include, in addition to the alpha-particle beam generator 200, devices for generating charged particle beams, i.e., devices for generating beams of any quantum (referred to herein as a "quantum beam"), including, for example, electron beams, proton beams, heavy particle beams, etc. For example, while a Li target used in a neutron source may be placed in a vacuum, adopting the output window device 220 or the spokeless rotating window of the present embodiment enables the possibility of extracting the quantum beam into a space filled with a gas atmosphere (including inert gases such as argon or helium). Compared to a vacuum, having the extraction of quantum beams filled with gas offers the advantage of utilizing that gas for cooling the target material irradiated by the quantum beams.

2-7-3. Differential Pumping

Another useful solution is the adoption of differential pumping. FIG 20 is a schematic diagram showing the structure of an At production system employing differential pumping. A system using differential pumping includes a differential pumping cylinder 72 in addition to the target chamber 7 capable of irradiating the target material with an alpha-particle beam. The differential pumping cylinder 72 is connected to the output opening of the alpha-particle beam generator 200 and possesses a passageway 74 through which the alpha-particle beam 2 emitted from that output opening passes. The passageway 74 is divided into multiple sub-chambers 721 to 726 arranged in the order of passage of the alpha-particle beam 2, and each sub-chamber is evacuated by its own individual pumping system 761 to 766. Each sub-chamber 721 to 726 is demarcated from the others by partitions having beam openings of a small diameter sufficient to allow the beam to pass through, but they are interconnected at those points. The alpha-particle beam 2 can pass through the beam openings without colliding with any solid objects at all, reaching the target material (not shown in Fig. 20) inside the target material container 3 from the output opening of the alpha-particle beam generator 200. Therefore, the potential problem of vacuum window durability associated with vacuum window (partition) usage does not arise. By appropriately operating each exhaust system 761 to 766, the pressure difference between the high vacuum inside the alpha-particle beam generator 200 and the atmospheric pressure (typically 1 atm) inside the target chamber 7 can be maintained. Furthermore, during device shutdown, the gate valve 78 can be closed to maintain the high vacuum within the alpha-particle beam generation device 200.

3. Experimental Verification

Experimental verification was conducted to confirm the feasibility and practicality of astatine production using the present embodiment. This verification first involved a preliminary experiment to confirm the effect of switching the thermal contact structure shown in FIGS. 14A-B. Next, a reference experiment (Experiment 0) using a bismuth plate as the target was conducted to establish a baseline for At yield. Subsequently, a series of online collection experiments were performed, involving the irradiation of an alpha-particle beam to produce ²¹¹At using ²⁰⁹Bi as the target material 4. This series of experiments includes unit experiments involving a total of eight alpha-particle beam irradiations: Experiments 1, 2, 3-1, 3-2, 3-3, 4-1, 4-2, and 5. Each experiment, excluding Experiment 0 (the reference experiment) but including the preliminary experiments, forms part of the examples for the embodiments of the present disclosure. The objectives of each experiment are as shown in Table 1.

[Table 1]

Table 1: Experiment List
Experiment Number	Purpose	Remarks
Preliminary Experiment	Confirmation of thermal contact switching effect (FIG. 14A, B)	No beam irradiation, no target material
Experiment 0 (Reference Experiment)	Reference experiment for determining At production quantity standard	Bismuth plate used instead of rotating target material container
Experiment 1	Determine At evaporation temperature	Changing evaporation temperature
Experiment 2	Determine At evaporation time	Confirmation as to how evaporation progresses
Experiments 3-1, 3-2, 3-3	Determine trap conditions	Confirmation of effect of oxygen mixing in ambient gas
Experiments 4-1, 4-2	High current test
Experiment 5	High current/long duration test

First, the conditions common to Experiments 1, 2, 3-1, 3-2, 3-3, and 4 to 5 are described. These were conducted to experimentally verify the conditions for At production using an alpha-particle beam. The target material container adopted a graphite (carbon) container with the shape of target material container 3 shown in FIGS. 14A and 14B. Target material container 3 was heated by induction heating coil 302. The power for induction heating was controlled while measuring the temperature of target material container 3 using a radiometer (not shown). The content, production, and collection of ²¹¹At were determined by measuring the intensity (radioactivity) of γ-rays originating from ²⁰⁷Bi, a decay product of ²¹¹At. Specifically, γ-ray measurements in the trap were performed using a CdTe detector during the experiment and a Ge semiconductor detector after the experiment to quantify the radioactivity. Note that the half-life of ²¹¹At is extremely short at 7.2 hours, causing ²¹¹At to decrease during the test. To compensate for this in the experiment, all measured radioactivity values were converted to values at a specific time (e.g., End Of Bombardment: EOB, i.e., the end of beam irradiation) to enable comparison.
Experimental verification using beam irradiation generally employed a setup combining either a charcoal trap or both a charcoal trap and a cold trap 696 in place of the cold trap 696 shown in FIG. 3A within the structures of FIGS. 1, 2, and 5A. In some experiments, a particulate filter 694 was also connected, as shown in FIG. 15A. The particulate filter 694 was combined with the heater 694c shown in FIG. 15B. Furthermore, the target material container 3, which employs a structure switching thermal contact via the gap shown in FIGS. 14A and B, was adopted. The At production method was performed as shown in FIG. 9. However, in Experiments 1, 2, 3-1, 3-2, 3-3, 4, and 5, after completing the ambient gas transport and trapping step S46, the target material 4 was cooled and solidified. The process then restarted from the heating and melting stage S34, and the loop indicated by the dotted line in FIG. 9 was not used. The following description of the process steps refers to the corresponding steps in FIG 9. During the experiments, the target material container 3 was rotated at 2000 rpm. This setting condition will be explained later in the preliminary experiment section. Note that rotating the target material container 3 at 2000 rpm caused the container 3 to thermally contact the container holding part 51, as shown in FIG. 14B. When stationary without rotation, the container 3 was thermally isolated from the container holding part 51 by a gap G, as shown in FIG. 14A. The vacuum window employed the spoke-less rotating window shown in FIG. 18. For the spoke-less rotating window in FIG. 18, a 15µm-thick Ti foil was adopted for the vacuum window 22SA, and the rotational speed of the vacuum window wheel 222A was set to 1000 rpm.The rotation of the vacuum window wheel 222A was maintained at least during the time the alpha-particle beam 2 was being output.

3-1. Preliminary Experiment

As a preliminary experiment, we conducted an experiment to confirm the relationship between thermal contact and the heating state in the target material container 3, which has a structure that switches thermal contact via the gap shown in FIGS. 14A and 14B. In this preliminary experiment, the target material 4 was not placed in the target material container 3, and no beam was irradiated. For the preliminary experiment, the target material container 3 and container holding part 51, with a thermal contact switching structure as shown in FIGS. 14A and B, were actually constructed. The relationship between the power input to the induction heating coil 302 and the target material container 3 temperature measured by a radiant thermometer was investigated for the target material container 3 in a stationary state and in a rotating state at 1000 and 2000 rpm. FIG. 21 shows a graph of this relationship. The horizontal axis represents the power input to the induction heating coil 302, shown as the actual power value and the ratio to the maximum output. The vertical axis shows the temperature measured by the radiant pyrometer on the target material container 3. FIG 21 shows that the temperature of the stationary target material container 3 increases more readily with the input power value, while the temperature of the target material container 3 rotating at 1000 rpm and 2000 rpm increases less readily with the input power value. Thus, it was confirmed that controlling gap G (Fig. 14A) to switch thermal contact between a stationary target material container 3 and a rotating target material container 3 directly affects the relationship between input power and temperature. In other words, the preliminary experiment results confirmed the usefulness of the target material container 3 structure that switches thermal contact. In subsequent experiments, the target material container 3 was rotated at 2000 rpm for the rotation start step S36 (Fig. 9).

3-2. Experiment 0 (Reference Experiment)

Next, a reference experiment (Experiment 0) was conducted to establish a baseline for At production in subsequent experiments. In Experiment 0, a plateshaped Bi target was selected and irradiated with an alpha-particle beam under the following conditions: beam current 0.1425 pµA (particle microampere), irradiation time 5 minutes. These conditions ensure that the temperature rise of the bismuth plate is negligible due to the extremely weak beam, meaning the generated At remains largely retained in the bismuth plate. Although the amount of produced ²¹¹At decreased during the test due to radioactive decay, the radiation dose at the end of the beam (EOB) could be calculated and compared. The measured value was 0.411 MBq, or 85.6% of the theoretical value of 0.480 MBq (10⁶ Bq). For the results of Experiments 1 and beyond, the calculated values at EOB are provided for reference. Furthermore, the yield or production amount for each experiment is expressed as a ratio relative to a proportionally calculated value based on the above measured value from Experiment 0, adjusted according to each experiment's conditions. This proportional calculation assumes that the ²¹¹At production amount for each experiment, under conditions where the beam intensity and beam irradiation time were identical to those of Experiment 0, is set as 100%. The yield is then calculated as a percentage relative to this baseline For Experiments 1 and beyond, conditions involve rotating the target 4 housed in target material container 3 using a strong beam to increase production. Consequently, the trap efficiency of ²¹¹At causes yields to be less than 100%. Note that the beam intensity pµA (particle microampere) is derived by dividing the beam current value µA (microampere), measured by a Faraday cup, i.e., eµA (electric microampere), by the valence of the particles forming the beam. For the alpha rays beam used in the experiments, the valence is 1, so the values are identical.

3-3. Experimental Verification (Experiments 1 - 5)

Table 2 summarizes the experimental conditions and results for each verification experiment (Experiments 1 - 5) described below.

[Table 2]

	Experimental conditions					Calculated Activity at EOB (MBq)	Yields of trap (%)
No.	Irradiation time (min)	Beam current (pµA)	O₂ (L/min)	He (L/min)	Evaporating temperature (°C)	Calculated Activity at EOB (MBq)	Cold trap	Charcoal	Total
1	5	0.245	-	1	750	0.83	-	72	72
2	5	0.24	-	1	750	0.81	-	74	74
3-1	5	0.2375	-	1	750	0.81	62	13	75
3-2	5	0.2725	0.25	1	750	0.93	72	7	79
3-3	5	0.2825	0.38	1.5	750	0.94	67	6	73
4-1	5	10	0.25	1	750	33.69	53	8	61
4-2	5	24.5	0.25	1	750	80.86	36	7	43
5	96	23	0.25	1	750	1445	53	3	56

Table 2 columns, from left to right, are: experiment number, conditions, radioactivity converted to EOB, and yield. The conditions are: alpha-particle beam irradiation time (min), beam current (pµA), oxygen flow rate (L/min), carrier gas (helium) flow rate (L/min), and At evaporation temperature, i.e., the temperature (°C) of target material container 3 at trapping step S46. The EOB-equivalent radioactivity is the measured value (MBq) decreasing according to a 7.2-hour half-life, converted to the value at the point traced back to EOB. The yield is the ratio of the radioactivity values in the cold trap (FIG. 3A, symbol 696) and charcoal trap (FIG. 15B, 6982), and their sum. Note that trap system 698, intended to prevent At leakage to the outside, is not measured. Details for each experiment are as follows.

3-3-1. Experiment 1: Determination of At Evaporation Temperature

Experiment 1 was conducted to determine the temperature to be set for the target material container 3 during the subsequent experiments' heating and melting stage S44 and ambient gas transport and trapping step S46, in order to establish the temperature conditions for these steps. FIG. 22 shows a graph of the time variation in the measured values from the radiation thermometer for the target material container 3 and the measured values from the rate meter indicating the radioactivity (atomic number) in Experiment 1 of the present embodiment. The figure also clearly shows the timing of irradiation step S40. Note that the temperature of the target material container 3 during the heating and melting step S44 and the ambient gas transport and trapping step S46 is limited to a detection lower limit of 135°C due to the constraints of the pyrometer. The trap in the trap device 69 was a charcoal trap only. In experiments (Experiment 1 and Experiment 2 described later) where At adsorption was performed using only a charcoal filter without employing cold trap 696, cold trap 696 was not installed. As shown in FIG. 22, the temperature of target material container 3 was sequentially increased from 450°C in 50°C steps after irradiation. Each temperature was maintained for 5 minutes. As the temperature of the target material container 3 was increased from 450°C, the rate meter detected by the charcoal trap, indicating the amount of ²¹¹At radioactivity, increased. At 700°C, a significant increase in slope was confirmed. Furthermore, this slope was maintained even at 750°C, after which the radioactivity value saturated. Based on the results of Experiment 1, the temperature of target material container 3 during the heating and melting step S44 and the ambient gas transport and trapping step S46 in subsequent experiments was set to 750°C. Furthermore, in Experiment 1, compared to experiments not described in this specification, the rate meter value corresponding to the radioactivity measured by the charcoal trap was large even at relatively low temperatures of target material container 3. This is considered to result from the target material container 3 being stationary during the heating and melting step S44 and the ambient gas transport and trapping step S46. As shown in FIG. 14A, a gap G is formed between the target material 4 and the container holding part 51. Consequently, local temperature variations in the target material 4 were small, leading to a more uniform temperature.

3-3-2. Experiment 2: Determination of At Evaporation Time

In Experiment 2, we confirmed how long At continued to volatilize in the target material container 3. Note that the cold trap (FIG. 3A, reference numeral 696) was not connected; instead, a charcoal trap was connected in its position. FIG. 23 shows a graph of the time variation in the measured values from the radiation thermometer for the target material container 3 and the measured values from the radioactivity rate meter indicating the At quantity in Experiment 2 of the present embodiment. As shown in FIG. 23, maintaining the target material container 3 at 750°C for approximately 20 minutes confirmed that the radioactivity value in the charcoal trap reached saturation. This indicates that At vaporization from the target material 4 completes in about 20 minutes at 750°C. Therefore, the At evaporation time was set to be 20 minutes or longer.

3-3-3. Experiments 3-1, 3-2, and 3-3: Determination of Collection Conditions

In Experiments 3-1, 3-2, and 3-3, we investigated the conditions for the gas used for collection - specifically, the ambient gas transport and trapping step S46 - that passes through the cold trap 696 shown in FIG 3A. This was done to ensure sufficient At vaporization within the target material container 3. The trap device 69 was configured as shown in FIG. 15B, connecting the cold trap 696 to the charcoal trap 6982, and as shown in FIG. 15A, also connecting the particulate filter 694. For the ambient gas transport and trapping step S46, the heating conditions for the target material container 3 were set to a temperature of 750°C and an evaporation time of 25 minutes. For Experiments 3-1, 3-2, and 3-3, respectively, the collection gases were, in the order of Experiments 3-1, 3-2, and 3-3: Ambient gas supplied to the carrier gas supply system 61 containing only helium (flow rate 1.0 L/min); a carrier gas supply system 61 supplied with helium (flow rate 1.0 L/min) to which oxygen (flow rate 0.25 L/min) was added immediately before the particulate filter 694, a carrier gas supply system 61 supplied with helium (flow rate 1.5 L/min) to which oxygen (flow rate 0.38 L/min) was added immediately before the particulate filter 694, were used.
FIG. 24 shows a graph depicting the time variation of the measured values obtained by the radiation thermometer for the target material container 3 in Experiments 3-1, 3-2, and 3-3 of the present embodiment, and the measured values obtained by the radioactivity rate meter indicating the At amount The value of the rate meter for each experiment is the value for charcoal trap 6982. In Experiment 3-1, At was adsorbed onto charcoal trap 6982. This indicates that a significant amount of At passed through cold trap 696 upstream of the charcoal trap. Therefore, Experiment 3-2 involved introducing oxygen into the collection gas at the position immediately before particulate filter 694. In Experiment 3-2, the radioactivity in the charcoal trap decreased significantly. This is thought to be because the oxygen made it easier for At to be trapped in the cold trap 696. Furthermore, in Experiment 3-3, the flow rate of the collection gas was increased by 1.5 times for both helium and oxygen compared to Experiment 3-2. As a result, the radioactivity measured in the charcoal trap system 698 increased in Experiment 3-3. This is thought to be because the gas collection flow rate was excessive, increasing the proportion of At contained in the collection gas immediately before entering the cold trap that passed straight through cold trap 696.

3-3-4. Experiments 4-1 and 4-2: High Current Test

Experiments 4-1 and 4-2 were conducted to investigate the amount of At produced under conditions where the beam current, an indicator of beam intensity, was increased to a high current level. The irradiation intensity of the alpha-particle beam was set to 10 pµA for Experiment 4-1 and 24.5 pµA for Experiment 4-2, with an irradiation time of 5 minutes as before. The cold trap 696 and charcoal trap 6982 were connected as shown in FIG. 15B, and the particulate filter 694 was also connected. The heating conditions for the target material container 3 during the ambient gas transport and trapping step S46, and the gas conditions for collection, were fixed to those of Experiment 3-2. Figure 25 shows a graph of the measured values from the radiation thermometer for the target material container 3 in Experiments 4-1 and 4-2 of the present embodiment.

3-3-5. Experiment 5: High Current, Long Duration Test

Experiment 5 was conducted to investigate behavior under irradiation with high current and for an extended duration. The alpha-particle beam irradiation intensity was set to 23 pµA, with continuous irradiation for 96 minutes. As shown in FIG. 15A, a particulate filter 694 was also connected, along with the charcoal trap and cold trap 696. The temperature conditions of the target material container 3 during the ambient gas transport and trapping step S46, and the gas conditions for collection, were fixed to those of Experiments 3-2, 4-1, and 4-2. However, during the ambient gas transport and trapping step S46, the heating time of the target material container 3 was originally planned for 25 minutes according to the evaporation time. Due to an increase in the target chamber 7 internal pressure, the pressure did not decrease even after reducing the helium flow rate, so the heating was terminated at 23.5 minutes.
FIG. 26 shows a graph of the measured values obtained by the radiation thermometer for the target material container 3 in Experiment 5 of the present embodiment. As shown in Table 2, the yield decreased.

3-4. Summary of Experiments

The above experiments confirmed that At can also be produced with high yield and production volume to perform. In this process, the spoke-less rotating window proved sufficiently practical. The target material container, which switches thermal contact via a gap, was also practical. Mixing oxygen into the collection gas was also useful.

4. Conclusion

The embodiments of the present disclosure have been described in detail above. The above embodiments, modifications, and experimental verifications are described to explain the invention of the present disclosure, and the scope of the invention of the present disclosure should be determined based on the claims. Furthermore, variants existing within the scope of the present disclosure, including other combinations of embodiments, are also included in the scope of the claims of the present disclosure. That is, a person skilled in the art can make various changes, combinations, subcombinations, and substitutions with respect to the components of the above embodiments within the scope of the technical scope of the present invention or its equivalents.
Japanese Patent Applications Nos. 2021-190640 and 2023-089284 , and PCT/JP2022/043283 ; the contents of all of which are incorporated herein by reference in their entirety.

Industrial Applicability

The approach to At production disclosed in the present disclosure can be applied to any apparatus that irradiates a target material containing Bi with an alpha-particle beam, or to any process utilizing such irradiation.

Reference Signs List

1, 1A, 1B target
1000 At production system
100 target holding device
11 axis of rotation
200 alpha-particle beam generator
2 alpha-particle beam
22 vacuum window
22A - 22D upstream vacuum window
22E - 22H downstream vacuum window
22SA, 22SB vacuum window
220 output window assembly
222, 222A, 222B vacuum window wheel
224 upstream disk
224f, 226f window retaining frame
224p O-ring
226 downstream disk
228 connecting shaft
2202 window outer frame (outer periphery retainer)
2204A, 2204B window spindle (inner periphery retainer)
2206 ball bearing
230 window rotation axis
242 window rotation mechanism
244 pulley
246 belt
248 motor
252 pipe axle
26 cooling gas supply system
282 magnetic fluid seal and bearing
284 spinning seal
3, 3A, 3D target material container
30 containing part
30B bottommost portion
31 opening
32 surrounding wall
33 inner surface
34 base
36 inner flange
302 induction heating coil (container heater)
304 matching device
306 high-frequency power supply
322 controller (container heater controller)
4 target material
42 supply tube (automatic supply mechanism)
44 suction nozzle (discharge mechanism)
5 rotational drive mechanism
51 container holding part
512 heat conduction member
52 arm
52N claw section
52W weight section
52P pivot pin
52S arm spring
53 arm mounting section
54G guide pin
54S spring
55 rotation feedthrough unit
56 rotary coupling
57 spindle
58 rotation transmission mechanism
59 motor
6 At collection system
61 carrier gas supply system
65 ambient gas exhaust system
66 exhaust pipe
66a exhaust pipe assembly
66f particulate filter
67 thermal conductive sheath
68 exhaust pipe heater
682 exhaust pipe heater controller
69 trap device
69a solvent elution system
691 hood
692a, b three-way valve
693a, b, c, d: fittings
694: particulate filter
694a: quartz tube
694b: quartz wool
694c: heater
695 liquid transfer pump
696 cold trap
697 vial
698 trap system
6982 charcoal trap
699 exhaust pump
7 target chamber
72 differential pumping cylinder
721-726 sub-chamber
74 passageway
761-766 exhaust system
78 gate valve
8A, 8B container cover
82 opening for irradiation
84 exhaust tube port
86 side cover section
s shield
R1 irradiation room
R2 laboratory room
P1, P2 position
C1, C2 circle
VC vacuum chuck through hole

Claims

A method for producing astatine-211 (²¹¹At) by alpha-particle beam irradiation, comprising:
an installation step wherein a target material containing bismuth-209 (²⁰⁹Bi) is placed and positioned in a target container, the target container capable of receiving irradiation with an alpha-particle beam from an alpha-particle beam generator inside of an interior of a target chamber, and wherein the target material container has a containing part and an opening for allowing the containing part to communicate with the exterior, and is rotatable about an axis of rotation passing through the containing part and the opening, and is placed within the interior of the target chamber;

a rotational drive irradiation step wherein an alpha-particle beam is irradiated onto at least a portion of the target material while rotating the target material container about the axis of rotation;

a transport step, after the rotational drive irradiation step, wherein a carrier gas is supplied into the target chamber and an ambient gas surrounding the target material is discharged outside the target chamber through an exhaust pipe; and

a trapping step, after the rotational drive irradiation step, wherein a trap device connected to the exhaust pipe is configured to trap ²¹¹At from the ambient gas, ²¹¹At being a radionuclide generated from ²⁰⁹Bi through irradiation with the alpha-particle beam.
The method according to claim 1, wherein the rotational drive irradiation step includes alpha-particle beam irradiation while rotating the target material container about the axis of rotation such that the irradiation axis passes through the opening and the containing part.
The method according to claim 1 or 2,
wherein, prior to the rotational drive irradiation step, the temperature of the target material container is controlled to be lower than the melting point of ²⁰⁹Bi, and

the rotational drive irradiation step is performed while maintaining the temperature of ²⁰⁹Bi in the target material at a temperature at which the target material would take a solid state under conditions where no alpha-particle beam is irradiated.
The method according to claim 3, further comprising
a target material thinning step, prior to the rotational drive irradiation step, wherein the target material container is rotated while controlling its temperature above the melting point of ²⁰⁹Bi, and then the temperature of the target material container is controlled below the melting point of ²⁰⁹Bi while maintaining rotation.
The method according to claim 1, wherein, in either or both of the transport step and the trapping step, the target material container is heated.
The method according to claim 5, wherein the temperature of the target material container is controlled such that the temperature of the target material container is higher than the melting point of ²⁰⁹Bi and lower than the boiling point of ²⁰⁹Bi.
The method according to claim 1, wherein the ambient gas is transported through the exhaust pipe while the exhaust pipe is heated during the transport step.
The method according to claim 1, wherein the transport step and the trapping step are performed at least temporarily in parallel.
The method according to claim 1, wherein, in the trapping step, the exhaust pipe is heated while the ambient gas is transported through the exhaust pipe.
The method according to claim 9, wherein
a particulate filter is connected in series in the exhaust pipe, midway to and upstream of the trap device, and

the particulate filter is maintained at least temporarily at a temperature at which ²¹¹At vaporizes and ²⁰⁹Bi does not melt, in either or both of the transport step and the trapping step.
The method according to claim 10, wherein the particulate filter includes quartz wool as a filter material.
The method according to claim 1, wherein
an inlet of the exhaust pipe is positioned facing the containing part of the target material container, and

the transport step is performed while the target material container's rotation is substantially stationary.
The method according to claim 12, wherein
the rotational drive step using the first target material container and either the transport step or the trapping step using the second target material container are performed in parallel at least temporarily, and

then, the rotational drive step using the second target material container and either the transport step or the trapping step using the first target material container are performed in parallel at least temporarily, and

wherein the alpha-particle beam is irradiated in the rotational drive irradiation step using the first target material container and then is irradiated in the rotational drive irradiation step using the second target material container.
The method according to claim 1,
wherein a particulate filter is connected in series in the exhaust pipe, midway to and upstream of the trap device,

the method further comprising a sublimation step wherein ²¹¹At within the particulate filter is sublimated by passing gas through the particulate filter while raising the temperature of the particulate filter,

wherein the gas passed through the particulate filter in the sublimation step is oxygen or oxygen-mixed gas.
A target holding device for alpha-particle beam irradiation, comprising:
a target material container made of a material capable of being induction heated, the target material container having a containing part for the target material to be irradiated with the alpha-particle beam, an opening for allowing the containing part to communicate with the exterior, a base, and a surrounding wall extending from the base, and the target material container being rotatable about an axis of rotation passing through the containing part, the opening, and the base, wherein the base and the surrounding wall demarcate at least a portion of the containing part from the exterior;

a rotational drive mechanism generating driving force for rotating the target material container;

a container heater including an induction heating coil capable of heating at least a portion of the surrounding wall of the target material container; and

a container heater controller for controlling heating operation by the container heater based on a temperature of the target material container or the target material, wherein

the axis of rotation is inclined relative to the horizontal direction, and

the base and the surrounding wall are configured such that an inner surface thereof that is in contact with a bottommost portion of the containing part of the inclined target material container is positioned close to a heated range of the induction heating coil.
A target holding device for alpha-particle beam irradiation, comprising:
a target material container made of a material capable of being induction heated, the target material container having a containing part for the target material to be irradiated with the alpha-particle beam, an opening for allowing the containing part to communicate with the exterior, a base, and a surrounding wall extending from the base, and the target material container being rotatable about an axis of rotation passing through the containing part, the opening, and the base, wherein the base and the surrounding wall demarcate at least a portion of the containing part from the exterior;

a rotational drive mechanism generating driving force for rotating the target material container;

a container heater including an induction heating coil capable of heating at least a portion of the surrounding wall of the target material container; and

a container heater controller for controlling heating operation by the container heater based on a temperature of the target material container or the target material,

wherein the target holding device comprises a first target material container and a second target material container,

wherein the target material in the second target material container can be heated by the container heater when the alpha-particle beam is irradiated onto the target material in the first target material container while the first target material container is rotated by the rotational drive mechanism, and

wherein the target material in the first target material container can be heated by the container heater when the alpha-particle beam is irradiated onto the target material in the second target material container while the second target material container is rotated by the rotational drive mechanism.
The target holding device according to claim 15 or 16, further comprising:
a container cover that covers at least a portion of the opening, permits rotation of the target material container about the axis of rotation, and is provided with an opening for irradiation by the alpha-particle beam; and

a movable cap for blocking the opening for irradiation.
The target holding device according to claim 15 or 16, further comprising:
a container cover that covers at least a portion of the opening, permits rotation of the target material container about the axis of rotation, and is provided with an opening for irradiation by the alpha-particle beam; and

an irradiation opening window comprising a metal foil that blocks the opening for irradiation and permits alpha-particle beam transmission over the thickness direction.
A target holding device for alpha-particle beam irradiation, comprising:
a target material container made of a material capable of being induction heated, the target material container having a containing part for the target material to be irradiated with the alpha-particle beam, an opening for allowing the containing part to communicate with the exterior, a base, and a surrounding wall extending from the base, and the target material container being rotatable about an axis of rotation passing through the containing part, the opening, and the base, wherein the base and the surrounding wall demarcate at least a portion of the containing part from the exterior;

a rotational drive mechanism generating driving force for rotating the target material container;

a container heater including an induction heating coil capable of heating at least a portion of the surrounding wall of the target material container; and

a container heater controller for controlling heating operation by the container heater based on a temperature of the target material container or the target material,

wherein the target material container is disposed inside a target chamber,

wherein the target material container further comprises a rotation feedthrough part that transmits the driving force of the rotation drive mechanism through the wall of the target chamber, with the space between it and the wall hermetically sealed, and a container holding part that holds the target material container via a gap and is attached to the rotation feedthrough part, and

wherein the gap closes during rotation of the target material container, bringing the container holding part and the target material container into thermal contact with each other, and the gap opens when the target material container is stationary, thermally isolating the container holding part and the target material container from each other.
The target holding device according to claim 19, further comprising:
a spring biased to open the gap by moving the container holding part away from the target material container; and

a plurality of arm parts provided around the container holding part,

wherein each arm part comprising: a weight section subjected to centrifugal force when the target material container rotates; a claw section that contacts the target material container to close the gap section when centrifugal force acts on the weight section; and a pivot axis fixed to the container holding part and positioned between the weight section and the claw section and

wherein, during rotation of the target material container, centrifugal force acting on the weight section causes each arm to pivot about the pivot axis, causing the claw section to close the gap against the biasing force of the spring, and wherein, when the target material container is stationary, the spring opens the gap.
A system for alpha-particle beam irradiation, comprising:
a differential pumping cylinder connected to an opening for irradiation of an alpha-particle beam generator, and

a target chamber capable of irradiating the alpha-particle beam onto a target material through the differential pumping cylinder,

wherein the differential pumping cylinder has a passageway through which the alpha-particle beam emitted from the output opening passes, and the passageway is divided into a plurality of sub-chambers arranged in the order of passage of the alpha-particle beam, each sub-chamber being vented by a separate pumping system.
An output window device connected to the output opening of an alpha-particle beam generator, comprising:
a vacuum window wheel rotatable about a window rotation axis substantially parallel to the irradiation axis of the alpha-particle beam; and

a window rotation mechanism for generating driving force for rotating the vacuum window wheel,

wherein the vacuum window wheel comprises:
an upstream disk having an upstream vacuum window array, wherein multiple upstream vacuum windows are arranged along a circumference of a first circle centered on the window rotation axis and lying in a first plane perpendicular to the window rotation axis; and

a downstream disk having a downstream vacuum window array, wherein multiple downstream vacuum windows are arranged along a circumference of a second circle centered on the window rotation axis and lying in a second plane perpendicular to the window rotation axis,

wherein the upstream vacuum windows and the downstream vacuum windows are equal in number, and each vacuum window has a window material comprising a metal foil that transmits the alpha-particle beam over the thickness direction

wherein the output window device further comprises a cooling gas supply system that supplies cooling gas to a gap provided between the upstream disk and the downstream disk.
The output window device according to claim 22, further comprising:
an upstream sealing mechanism that seals both spaces on either side of the upstream vacuum window while allowing rotation of the vacuum window wheel about the window rotation axis; and

a downstream sealing mechanism that seals both spaces on either side of the downstream vacuum window while allowing rotation of the vacuum window wheel about the window rotation axis.
An output window device connected to the output opening of a quantum beam generator, comprising:
a vacuum window wheel rotatable about a window rotation axis substantially parallel to the irradiation axis of the quantum beam; and

a window rotation mechanism for generating driving force for rotating the vacuum window wheel,

wherein the vacuum window wheel comprises a continuous doughnut-shaped vacuum window on a circumference centered on the window rotation axis, and

wherein the vacuum window has a window material comprising a metal foil that permits transmission of the quantum beam over the thickness direction.
The output window device according to claim 24,
wherein the quantum beam is an alpha-particle beam.
The output window device according to claim 24,
wherein the vacuum window wheel comprises an outer circumference retaining portion that defines the outer circumference of the vacuum window's donut-shaped section and holds the metal foil, and an inner circumference retaining portion that defines the inner circumference of the vacuum window's donut-shaped section and holds the metal foil,

wherein the outer circumference retaining portion and the inner circumference retaining portion both have a common rotating axis, and

wherein the window rotation mechanism drives the outer retaining portion.
The output window device according to claim 26,
wherein the metal foil has no opening provided inwardly from the outer periphery of the donut-shaped portion of the vacuum window, and

the inner circumference retaining portion is rotatable about the axis of rotation and supports the metal foil to suppress its displacement.
A system for alpha-particle beam irradiation, comprising:
a target chamber capable of irradiating an alpha-particle beam from an alpha-particle beam generator onto a target material,

wherein the output window device according to claim 22 is connected between the target chamber and the output opening of the alpha-particle beam generator.
The system according to claim 21 or 28,
wherein the target material contains bismuth-209 (²⁰⁹Bi), and astatine-211 (²¹¹At) is generated.
The system according to claim 21 or 28, further comprising:
a gas supply system for supplying carrier gas into the interior of the target chamber;

an exhaust pipe for exhausting the ambient gas surrounding the target material;

a trap device connected to said exhaust pipe for collecting ²¹¹At in the ambient gas;

an exhaust pipe heater for heating at least a portion of a path through which the ambient gas passes to the trap device of the exhaust pipe; and

an exhaust pipe heater controller for controlling the heating operation of the exhaust pipe heater.
A system for alpha-particle beam irradiation comprising:
a target holding device according to claim 15 or claim 16; and

an alpha-particle beam generating device oriented to have an irradiation axis directed toward the surrounding wall.
A system for alpha-particle beam irradiation, comprising:
the target holding device according to claim 15 or 16;

a target chamber for accommodating the target material container and capable of being irradiated with an alpha-particle beam from the alpha-particle beam generating device while the target material container is rotated;

a gas supply system for supplying carrier gas into the interior of said target chamber;

an exhaust pipe for exhausting an ambient gas within the containing part;

a trap device connected to said exhaust pipe for collecting radionuclides in the ambient gas;

an exhaust pipe heater for heating the exhaust pipe; and

an exhaust pipe heater controller for controlling the heating operation of the exhaust pipe heater,

wherein the radionuclide is astatine-211 (²¹¹At) generated when bismuth-209 (²⁰⁹Bi) contained in the target material is irradiated by the alpha-particle beam.
The system according to claim 32, wherein
a particulate filter is connected in series in the exhaust pipe, midway to and upstream of the trap device, and

the particulate filter is maintained at least temporarily in either or both of the transport step and the trapping step at a temperature that vaporizes ²¹¹At and does not melt ²⁰⁹Bi.
The system according to claim 33, wherein the particulate filter includes quartz wool as a filter material.