RU2282908C2 - Neutron-producing target assembly - Google Patents

Abstract

FIELD: neutron-entrapping therapy.

SUBSTANCE: proposed neutron-producing target assembly has target with active material which is essentially thin-walled shell of revolution made in the form of sphere or cylinder and disposed within casing filled with circulating cooling medium. Charged particles arrive at target from accelerator outlet through inlet window whose dimensions are much lower than diameter of thin-walled shell. Neutrons being produced escape cavity filled with cooling medium through collimating outlet window. Assembly is provided with means for setting target in rotary motion and means preventing cooling medium ingress in inlet window space. Target in one of assembly alternatives is set in motion by means of drive. In other alternative target made in the form of sphere resides in cooling liquid within casing in suspended condition and is set in rotary motion by means of cooling medium flow.

EFFECT: simplified design and reduced size of apparatuses for conducting neutron therapy directly in cancer clinics and centers.

13 cl, 4 dwg

Description

Technical field

The invention relates to nuclear physics and medicine and can be used in sources of epithermal neutrons based on charged particle accelerators. Such sources are mainly intended for use in medical equipment used in neutron therapy, mainly in neutron capture therapy (NRT).

State of the art

The concept of neutron capture therapy in oncology was proposed in 1936, 4 years after the discovery of the neutron. Its physical principle is as follows. A solution containing the stable boron-10 isotope is introduced into human blood and after some time, boron is sorbed in the cells. Then the tumor is irradiated with a stream of epithermal neutrons. As a result of neutron absorption by the stable ¹⁰ V isotope, a nuclear reaction occurs, and the resulting α energy particle and ⁷ Li ion are rapidly inhibited along the length of the tumor cell, releasing an energy of ~ 2.3 MeV within the very cell that contained the boron nucleus, which leads to her defeat. Thus, if a higher concentration of ¹⁰ V in the cancer cell is achieved compared to a healthy one, then boron neutron capture therapy (BRNT) will allow for selective damage to malignant tumor cells.

Over the period since then, the concept of NRT and, in particular, BNCT, has experienced falls and rises. However, in the late 60s, interest in NRT increased significantly. In this, the research of the Japanese scientist Hatanaki and the progress in the synthesis of ¹⁰ V isotope-containing pharmaceutical preparations played a large role. Preparations were obtained that create a concentration of ¹⁰ V isotope in tumor tissue up to 40 μg / g, which is 3.5 times more than in healthy tissue. This concentration makes it possible to selectively damage a cancerous tumor, which was not possible in early studies due to the lack of such pharmaceuticals. To obtain the neutron beam, special medical reactors built for this purpose in Brookhaven and Massachusetts (USA) were used.

Today, interest in BNCT is shown in many countries of the world. At the same time, work is underway aimed at obtaining neutrons of the energies necessary for BNCT without the use of reactor facilities.

Various principles are proposed for constructing BNCT systems based on an accelerator with a particle beam interacting with a neutron generation target.

A promising achievement is the concept of a proton beam - a lithium target, according to which a low-energy neutron beam (with an energy of the order of 2 MeV) hits a lithium target, generating neutrons by (p, n) reaction. Attractive features of the concept are:

- a relatively high neutron yield (about 10 ^-4 ),

- the ability to provide low maximum energy of the generated neutrons (not more than 30 keV),

- a relatively simple low-energy proton accelerator,

- the ability to work in an "open" geometry, when the beam of formed neutrons is directed forward at an angle of 20-30 °, and the distance from the target to the patient is minimal (no more than 20 cm),

- minimum - required shielding and low residual radioactivity.

In Russia, the proton beam - lithium target concept has been developed, including the use of a tandem proton accelerator to act on a lithium target with an energy exceeding the production threshold of 1.88 MeV in the reaction ⁷ Li (p, n) ⁷ Be, similar to that described in US patent No. 59976066, publ. 11/02/99, Yanch J.C. et al. Patentee Massachusetts Institut of Technology, Newton Scientific, Inc. Preliminary studies have shown that this proposal is acceptable, while the required proton beam current should be 10 mA or more to provide the necessary dose in the treatment area.

Nevertheless, the accelerator-based lithium target concept still has unresolved critical problems, including the requirements of high proton currents, excess energy neutrons, and the cooling of a lithium target with a low melting point. As an alternative, beryllium, which has a higher melting point than lithium, is more easily cooled and successfully used in clinical equipment for fast neutron therapy, is considered as the target material. For example, for the ⁹ Be (d, n) ¹⁰ B reaction at E = 2.5 MeV and I = 10 mA, the neutron yield is S ~ 4 × 10 ¹³ n / s, which is 4 times more than for the other ⁷ Li reaction ( p, n) ⁷ V for the same conditions. However, at the same time, high-energy neutrons with an energy of 100-800 keV are observed, and to convert them into epithermal neutrons with an energy of 0-30 keV, which are needed for NRTs, they use moderators based on beryllium oxide, alumina, and heavy water. This requires a distance from the target to the patient of at least 50 cm, which reduces the order of magnitude of the flux density of epithermal neutrons in the tumor.

Using the reaction ⁷ Li (p, n) ⁷ B in the near supercritical region of proton energy, when the difference between the reaction threshold Ecr = 1.88 MeV and the energy incident on the target is 0-30 keV, it allows:

- bring the patient closer to the target at a distance of 10-20 cm, i.e. work in "open geometry"

- use the so-called "kinematic collimation" when epithermal neutrons propagate forward within a spatial angle of 20-30 °.

In experiments, the last reaction was studied at currents of 1-2 mA. This corresponds to a beam power of 2-4 kW. The current level of development of NRT, taking into account the achievements of medicine, technology and pharmaceuticals, requires an increase in capacity by about an order of magnitude.

Known neutron-producing target sites can be divided into two main types with static and dynamic targets.

In static targets, the flow of charged particles directed at the target and the target working medium containing the active neutron-forming substance do not change their position relative to each other. In such targets, significant wear of the active (neutron-producing) layer and accumulation of induced activity are observed. In addition, modern static targets require cooling systems with large specific heat fluxes (> 1 kW / cm ² ), but even intensive liquid cooling is not able to remove such high thermal loads. As an example of a static target for a relatively small specific heat flux, neutron-producing targets described in US Pat. No. 4,666,651, publ. 05/19/1987, Barjon Robert, patent holder: Commissariat a l'Energie Atomique (Paris, Fr), Center Antoine-Lacassagne (Nice, Fr.) and No. 5920601, publ. 07/06/1999, Nigg David W. et al., Patent holder: Lockheed Martin Idaho Technologies Company (Idaho Falls, ID).

In dynamic targets, the particle flux and the target working medium containing a neutron-producing substance move relative to each other. Dynamic targets, unlike static ones, do not impose strict requirements on the cooling system, however, they require special devices that realize the movement of the target’s working medium or particle flow.

As a prototype, a neutron-producing target site with a target moving relative to the flow of charged particles - protons generated by the accelerator (see US patent No. 4582667, publ. 04/15/1986, Gunter Bauer, patent holder: Kemforschungsanlage Juljich Gesellschaft, Fed. Rep. Of Germany, was selected ) The known neutron-producing target node based on the accelerator contains a movable target with active material on which a nuclear reaction is made in the form of a rotating disk, a housing for accommodating the target with an input window designed to enter charged particles from the accelerator, and with an output collimating window designed for neutron output, as well as drive the target into motion and means of its cooling. For cooling directly in the target, an extensive network of cooling channels is carried out.

The disadvantage of this device is that cooling is carried out through the target body, i.e. not effective enough. In addition, at high capacities it is technologically difficult to organize cooling of a large area of the active material. At the same time, the larger the area of the active material, the larger the size of the disk and, as a result, the dimensions of the entire device.

Disclosure of invention

The objective of the invention is to create a more compact and simple, including in operation, neutron-producing target site.

To solve the problem in a neutron-producing target site for an accelerator-based neutron source containing a target of an active material that produces a nuclear reaction, a housing for placing the target with an input window designed to enter charged particles from the accelerator, and with an output collimating window designed for neutron output, as well as means for bringing the target into rotational motion and means for cooling it, according to the invention, the target is made in the form of a thin-walled a rotation shell, the diameter of which is significantly larger than the size of the input window, while the cavity of the housing in which the target is placed is filled with a cooling medium and equipped with a means to prevent the cooling medium from entering the volume of the input window (and, as a consequence, into the volume of the accelerator).

Thin-walled shell can be made either in the form of a sphere, or in the form of a cylinder.

In this embodiment of the target assembly, heat is removed from the target directly from its active surface by immersing the heated layer in a cooling medium. This provides a more efficient cooling of the active material. In addition, while maintaining the same working area of the active material as on the disk, the dimensions of the assembly are reduced during the transition to the shell of rotation.

As a cooling medium filling the cavity of the casing, liquid lithium can be used, which simultaneously performs the function of the active material.

In addition, the target itself can be made of an active material, for example, of beryllium or an alloy of lithium with other metals. At the same time, for cooling in certain modes, gas media, for example, nitrogen, inert gases, etc., can be used.

In another embodiment, the target consists of a carrier layer made of a material with high thermal conductivity and a solid active layer deposited on its surface, the thickness of which slightly exceeds the supercritical range of charged particles.

The active layer can be coated with a protective layer with low energy loss of charged particles.

To bring the target into rotational motion, a drive can be used whose shaft is rigidly connected to the axis of rotation of the target, with the axis of the target coinciding with its axis of symmetry and perpendicular to the axis of the input window.

The working surface in this case will be limited to the portion of the lateral surface of the sphere or cylinder.

When using a thin-walled shell in the form of a sphere, the latter can be placed in the coolant in the counter-spherical cavity of the housing in suspension. In this case, the flow of the cooling medium can serve as a means of rotation of the sphere.

In this case, the entire surface of the sphere can serve as the working surface of the active material, since during arbitrary rotation, it will turn to the input window in different parts of its side surface.

The means for cooling comprises an external reservoir and a heat exchanger with a cooling medium connected by input / output channels to the cavity of the housing in which the target is placed.

The tool that prevents the cooling medium from entering the body cavity into the volume of the inlet window may be formed by an annular protrusion made in the body in the region of the inlet window towards the target with a minimum clearance required for free rotation of the specified target.

As will be shown below, with the right choice of the gap between the sphere and the annular protrusion with a gas cooling medium flowing into the accelerator volume through the inlet window, the working pumping system can cope.

In addition, an annular magneto-liquid seal may be provided in the annular protrusion. At the same time, a hermetic separation of the body cavity from the inlet channel is guaranteed.

In addition, the window can be hermetically sealed by a cup-shaped gasket made of a foil of light metals or their alloys, which have a small effect on the energy of charged particles.

Brief Description of the Drawings

Figure 1 shows a General view of the inventive neutron-producing site with a drive rotation of the target. Figure 2 and figure 3 shows fragments of the device in the area of the input window with various means of separating the cavity of the housing from the input window. Figure 4 shows a variant of a neutron-producing assembly with a target in the form of a sphere located in the cooling fluid of the housing cavity in suspension.

Embodiments of the invention

The neutron-producing unit contains a target made in the form of a thin-walled shell 1 having the shape of a sphere or cylinder and placed in the cavity of the housing 2. Providing an active layer on the target surface can be solved in various ways. In particular, the entire thin-walled target shell can be made of an active homogeneous material, for example, beryllium or a lithium alloy. In another embodiment, the thin-walled shell comprises a substrate of a passive material with good heat-conducting properties, for example, copper, on which the active material is deposited, for example, lithium or its compounds. A protective layer may be applied over the active material, for example, aluminum having a small thickness of not more than 1 μm. In the embodiment shown in FIG. 1, the target is placed on the axis 3 and rotationally driven by the drive 4. The body cavity is filled with cooling medium 5, driven by pumps (not shown) through the heat exchanger 6. If lithium is used as the cooling medium, then it can itself form an active layer on the surface of the shell, while the latter can be made of a passive heat-conducting material such as copper. A beam of charged particles in the form of proton “P” protons from the accelerator enters the body cavity through the inlet window 7. To ensure the separation of the body cavity 2 from the inlet window 7, an annular protrusion 8 is made in the body toward the target 1 with a minimum clearance required for free rotation the target. For example, in the developed model with a thin-walled (wall thickness ~ 0.5 mm) sphere of beryllium or copper with a diameter of 150 mm, with an entrance window with a diameter of 50 mm and an annular protrusion of 10 mm wide, a gap of ~ 1 μm is made. This ensures satisfactory cooling of the heated region of the target, and the leakage of the cooling medium (inert gas) through the inlet window 7 into the accelerator volume does not exceed 1 l / s and is effectively eliminated by the working pumping system. The neutron flux n, formed as a result of the reaction on the active material of the target, leaves the cavity of the casing 2 through the output collimating window 9.

Figure 2 shows an embodiment of a housing with a magneto-liquid seal 10. Such a seal hermetically separates the cooling volume from the area of influence of a beam of charged particles on the target 1. Obviously, the target must have a solid active layer of active material.

Figure 3 shows a variant using a special thin-walled cup-shaped strip 11 of a foil of light metals or alloys. Due to the small thickness of the material, more than 10% of the energy of the proton flux should not be lost in such a gasket. This ensures a sufficiently high degree of sealing of the separation of cooling volumes and the inlet window.

Figure 4 shows a variant in which the target 12, made in the form of a sphere, is in the cooling fluid in the cavity of the housing 2 in a suspended state. An arbitrary (in the general case) rotation of a sphere 12 around its own center of symmetry occurs under the action of an eccentric jet of a cooling stream flowing out of channel 13 and creating a moment M = N × d, where N is the hydrodynamic force and d is its shoulder.

Neutron-producing target site operates as follows.

The proton beam "P" from the output of the accelerator through the input window 7 enters the cavity of the housing 2 and acts on the surface area of the target 1, which is currently opposite the input window. For use in BNCTs, the proton beam parameters are: energy 1.91 MeV, current 10 mA, which corresponds to a power of ~ 20 kW. In the embodiment with a drive (Fig. 1), target 1 is set in motion relative to an axis passing through the center of symmetry of the target, which can be made either in the form of a cylinder or in the form of a sphere. During the rotational movement of the target 1 relative to the window, a part of the side surface limited by the size of the window 7 moves. In the cavity of the casing with the target, this or that cooling medium 5 constantly passes through the heat exchanger 6. In one embodiment, when liquid lithium is used as the cooling medium 5 , it also acts as an active material on the target surface, on which it is retained due to wettability forces. In any case, regardless of whether the active material is an integral part of the target and is in the solid state, or whether it is liquid and flows around the outer surface of the thin-walled shell - the target, when it interacts with the proton beam, a nuclear reaction occurs, leading to the formation of neutrons. Neutrons at each moment of time are formed on a small portion of the target facing the window. Passing through the inner volume of the thin-walled shell and the opposite thin wall, the neutrons exit through the exit collimating window 9. Naturally, the surface area on which the reaction has just occurred experiences heating. However, at the next point in time, this section, immersed in the cooling medium 5, gives it the accumulated heat and cools during the complete rotation of the target to almost the temperature of the cooling medium. In this case, the surface of the active material on which neutron production is carried out exceeds a similar area in the prototype device with a target in the form of a disk while maintaining the same dimensions of the assembly.

In the solid target variant, particular attention is paid to the choice of the thickness of the active material, which can be used as one of the Li compounds, for example, LiH hydride, Li ₂ O oxide. LiF fluoride or an alloy of Li with metals so that the range of charged particles is somewhat smaller thickness of such a layer.

Since almost all the energy of a beam of charged particles (protons) is transformed into thermal energy, heating the mean free path at the target surface, special attention should be paid to the temperature regimes. In this regard, it is necessary to show the following.

The range of protons with an energy of 1.91 MeV in lithium-containing neutron-producing materials is shown in the table:

Material Li Lih Li ₂ O LiF Mileage R, microns 140 77 40 33 Supercritical 2.2 1,2 0.65 0.5 range ΔR, microns

Neutrons are formed only in the ΔR layer, at an energy difference of 1.91-1.88 MeV.

By approximate calculations it is easy to show that at a rotation speed of 200-300 rpm a spot with a diameter of d = 50 mm on a sphere d = 150 mm will be heated under the beam to an additional temperature of 150-200 ° C.

Further, by directly contacting the surface with the cooling medium on a path that is an order of magnitude larger than the size of the spot (or over a period of time greater by an order of magnitude), this surface energy is transferred to the environment.

Nevertheless, given the high surface energy density on the target (> 2 kW / cm ² ), the use of gas cooling is more limited, which will require, for example, compression of a heavy inert gas (for example, xenon) and its use in a closed cycle with cooling to negative temperatures.

It is easier to implement the option with liquid cooling, which looks most functionally when cooled by liquid lithium. For this, it is necessary to warm the entire target in working condition to a temperature above the melting temperature of lithium (185 ... 200 ° C) and use a high-temperature liquid in the second circuit of the cooler, for example, glycerin in the operating temperature range of 200-220 ° C.

Nevertheless, liquid lithium, wetting a spherical or cylindrical surface and falling under a proton beam, will produce the necessary epithermal neutrons and then mix with a much larger volume of lithium in the cooling cavity, only slightly increasing its temperature.

When using other liquids as a cooler, it is desirable to use the sealing of the volume of the inlet window from the cooling space using, for example, a magneto-liquid seal encircling the inlet window. Such seals are already used in working target nodes.

Another option is a separation cup made of thin aluminum foil, which is hermetically connected to the walls of the entrance window and rests on a spherical or cylindrical surface of the target. In this case, it will be necessary to set a large energy of protons, because, for example, ~ 0.4 MeV will be lost in a foil 10 μm thick. The foil can be cooled by the same system and the same liquid, which cools the target.

In another embodiment, with the sphere 12 in suspension, the proton beam P interacts at different points in time with almost the entire surface of the sphere, because its rotation occurs relative to two axes with arbitrary angular velocities ω ₁ and ω ₂ . In this case, the rotation itself is created by the flow of the cooling medium, for example, due to the implementation of the supply channel 13 with a certain shoulder d relative to the center of the target. And in this embodiment, the active areas of the target after exposure to the proton beam are immediately immersed in the cooling medium and lose to some extent the heat accumulated as a result of the action of the flow of charged particles. An example is a spherical shell with a diameter of 150 mm, with a wall thickness of 2.5 mm, placed in a silicone fluid. In this case, the weight of the sphere ~ 1.6 kg will be balanced by the buoyancy of the liquid ~ 1.6 kg. In this sphere, the entire surface will be working, i.e. the target track with the specified dimensions will increase in area several times, which will reduce the specific heat load or allow you to work under high conditions.

An additional quality of the latter option will be functional simplification and, as a result, increased reliability of the device.

Industrial applicability

This embodiment will allow you to create a compact and simple, including in operation, neutron-producing target site for accelerators of charged particle flows. This will allow obtaining the required epithermal neutron fluxes for use in neutron (mainly neutron capture) therapy.

Using the invention, it is possible to create simple and compact accelerator facilities for neutron therapy directly in cancer clinics and centers.

Claims (13)

1. Neutron-producing target site for an accelerator-based neutron source, containing a target of the active material on which the nuclear reaction is carried out, a housing for placing the target with an input window for inputting charged particles from the accelerator, and with an output collimating window for outputting neutrons and also means for bringing the target into rotational motion and means for cooling it, characterized in that the target is made in the form of a thin-walled shell of revolution, the diameter of which is substantially larger than the entrance window, while the cavity of the housing in which the target is placed is filled with a cooling medium and is equipped with a means to prevent the cooling medium from entering the cavity of the body into the volume of the entrance window. 2. The neutron-producing target site according to claim 1, characterized in that the thin-walled shell is made in the form of a sphere. 3. The neutron-producing target site according to claim 1, characterized in that the thin-walled shell is made in the form of a cylinder. 4. The neutron-producing target site according to any one of claims 1 to 3, characterized in that liquid lithium is used as the cooling medium filling the body cavity, which simultaneously performs the function of the active material. 5. The neutron-producing target site according to any one of claims 1 to 3, characterized in that the thin-walled shell is made of an active material, for example, beryllium. 6. The neutron-producing target site according to any one of claims 1 to 3, characterized in that the thin-walled shell consists of a carrier layer made of a material with high thermal conductivity and a solid active layer deposited on its surface, the thickness of which slightly exceeds the supercritical range of charged particles. 7. The neutron-producing target site according to claim 6, characterized in that the active layer is coated with a protective layer with low energy loss of charged particles. 8. The neutron-producing target assembly according to any one of claims 1 to 7, characterized in that the means for bringing the target into rotational motion comprises a drive whose shaft is rigidly connected to the axis of rotation of the thin-walled shell, coinciding with its axis of symmetry and perpendicular to the axis of the input window. 9. The neutron-producing target site according to any one of claims 2 or 4-7, characterized in that the sphere is placed in the cooling fluid of the body cavity in suspension, and the flow of the cooling medium serves as a means of bringing it into rotation. 10. The neutron-producing target assembly according to any one of claims 1 to 9, characterized in that the means for cooling the target comprises an external reservoir with a heat exchanger connected by input / output channels to the body cavity. 11. Neutron-producing target assembly according to any one of claims 1 to 10, characterized in that the means preventing the cooling medium from entering the body cavity into the volume of the inlet window is formed by an annular protrusion made in the body in the region of the inlet window towards the thin-walled shell with the minimum clearance required for free rotation of the specified thin-walled shell. 12. The neutron-producing target site according to claim 11, characterized in that an annular magneto-liquid seal is made in the annular protrusion. 13. The neutron-producing target site according to claim 11, characterized in that the input window is overlapped by a cup-shaped spacer made of a foil of light metals or their alloys, which has little effect on the energy of charged particles.

Images