JP2009047432A - Target device for generating neutron and neutron generator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a target device for generating neutrons and a neutron generator capable of reducing the size of the structure for shielding and capable of efficiently generating cold neutrons. <P>SOLUTION: The target device 20 for the neutron generator is configured with a lithium film 28 generating neutrons by the Li(p, n) reaction, a beryllium film 30 covering the incident face of the lithium film 28 for receiving a proton beam, and a packing foil 32 to give the strength to the lithium film 28 fixed on the opposite face of the incident face of the lithium film 28 for receiving the proton beam and the beryllium film 30. The thickness of the lithium film 28 is set at such a thickness that the incident proton beam loses its energy to a threshold value of the Li(p, n) reaction or lower before being output from the lithium film 28. By so doing, the structure for shielding can be made smaller because cold neutrons having low energy can be efficiently generated and the generation of unnecessary radiation can be suppressed. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a technology for generating neutrons, and more particularly to a neutron generating target device and a neutron generating device for generating cold neutrons used for analysis of light elements and soft matter.

  In recent years, cold neutrons are used as probes for non-invasive observation of light element analysis and the dynamic state of soft matter on the nanoscale (see Patent Document 1). In particular, since it has higher sensitivity to hydrogen, oxygen, and the like than X-rays, it has been attracting attention as being able to provide information different from material analysis using X-rays. However, the intensity of cold neutron beams is orders of magnitude lower than that of synchrotron radiation such as X-rays, and at present, the only facility that can use neutrons in Japan is the research reactor in the Japan Atomic Energy Research Institute Tokai Research Institute. Therefore, the analysis method using cold neutrons is not as popular as the analysis using photons such as synchrotron radiation.

The former problem has been improved by improving the use efficiency of the neutron beam by the recent development of neutron optics. On the other hand, the J-PARC (Japan Proton Accelerator Research Complex) is being constructed for the latter issue, but there is only a large-scale facility and there is an improvement in strength, but it lacks a small turn, human resource development, etc. The problem remains. For this reason, it is desirable to construct a new facility that can be used easily, but it is difficult to reduce the size of the facility due to the problem of shielding the neutron source, and large equipment such as a nuclear reactor requires huge construction costs. Besides, location restrictions are large.
JP 2007-71799 A

  The problem to be solved by the present invention is to provide a neutron generating target device and a neutron generating device capable of reducing the structure for shielding and generating cold neutrons efficiently.

The target device for neutron generation of the present invention made to solve the above problems is as follows.
a) a lithium thin film having a predetermined thickness that generates neutrons by Li (p, n) reaction caused by the incidence of a proton beam;
b) a beryllium thin film covering the proton beam incident surface of the lithium thin film;
c) a reinforcing sheet that is fixed to a surface opposite to the proton beam incident surface of the lithium thin film and imparts mechanical strength to the lithium thin film and the beryllium thin film;
With
The lithium thin film is characterized in that it is set to a thickness that reduces its energy below the threshold value of the Li (p, n) reaction until the incident proton beam is emitted from the lithium thin film.

A target device for neutron generation according to another aspect of the present invention is provided.
a) a heat dissipation member having a conical recess,
b) a lithium thin film having a predetermined thickness that is provided along the inner peripheral surface of the concave portion and generates neutrons by a Li (p, n) reaction caused by incidence of a proton beam;
c) a beryllium thin film closing the recess,
With
The lithium thin film is characterized in that it is set to a thickness that reduces its energy below the threshold value of the Li (p, n) reaction until the incident proton beam is emitted from the lithium thin film.

  Moreover, the neutron generator of this invention is comprised using the above-mentioned neutron generation target apparatus.

Conventionally, cold neutrons are obtained by irradiating a proton beam to lithium as a target material to generate neutrons and decelerating the neutrons with a moderator. On the other hand, in the neutron generating target device of the present invention and the neutron generating device using the neutron generating device, a lithium thin film having a predetermined thickness is provided as a target material, and the lithium thin film is generated by irradiating a low energy proton beam. Reduce the initial energy of neutrons. Specifically, the surplus energy of generated neutrons is reduced as much as possible by making the incident proton energy slightly above the threshold of the Li (p, n) reaction. Thereby, the amount of moderator can be reduced. In addition, by suppressing energy, generation of X-rays and gamma rays that cause noise can be suppressed, and shielding for that purpose can also be reduced. With these two effects, the flux of cold neutrons can be earned.
Therefore, the moderator and cooling water for obtaining cold neutrons can be eliminated or reduced, and the size can be reduced accordingly. Further, since the generation of unnecessary radiation can be suppressed by lowering the energy of the proton beam, the shielding structure can be miniaturized.

  The proton beam loses energy in the lithium film, but is no longer just a heat source below the reaction threshold. By reducing the thickness of the lithium film, the proton beam that has fallen below the threshold value escapes from the lithium, so that it does not become a thermal load of lithium having a low melting point, and a long life can be expected. Moreover, since the proton beam incident surface of the lithium thin film is covered with the beryllium thin film, even when the lithium thin film is overheated and melted, transpiration and scattering of lithium can be prevented. Therefore, even if the target is irradiated with a proton beam for a long time, the Li (p, n) reaction can be stably caused to extract neutrons.

In the neutron generating target device according to another aspect of the present invention and the neutron generating device using the neutron generating device, the lithium thin film can be efficiently cooled because the lithium thin film is provided on the heat dissipation member. Further, a lithium thin film was provided in the conical recess of the heat dissipation member, and the proton beam incident surface of the lithium thin film was conical. For this reason, a proton beam can be disperse | distributed and a lithium thin film can be irradiated, and the temperature rise of a lithium thin film can be suppressed.
In this case, if the proton beam current density is higher in the region, the inclination of the proton beam entrance surface of the lithium thin film with respect to the direction orthogonal to the traveling direction of the proton beam becomes larger. Is equalized. Therefore, the lithium thin film can be prevented from being locally heated.

  The present invention relates to a neutron generating target device used for extracting cold neutrons having low energy used for light element analysis and analysis of the state of substances, and a neutron generating device using the same. The main feature of the present invention is that a low-energy proton beam is used so that the energy after passing through the lithium thin film that is the target material is equal to or lower than the threshold value of the Li (p, n) reaction. It is in the point which suppressed the lifetime reduction. Hereinafter, specific embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a schematic configuration diagram of a neutron generator 10 according to a first embodiment of the present invention. The neutron generator 10 includes an accelerator 12 that accelerates a proton beam, a beam transport system (not shown) that transports a fast proton beam accelerated by the accelerator 12, and a target device 20 that is irradiated with the fast proton beam. It is configured. The accelerator 12, the beam transport system, and the target device 20 are all placed under vacuum.
The accelerator 12 is a linear accelerator composed of an ion source 14, a high-frequency quadrupole accelerator 16 (RFQ), and a post-accelerator 18 (PAcc: Post Accelerator). The target device 20 is irradiated with a proton beam having energy slightly higher than the threshold value of Li (p, n) reaction (about 2.0 MeV).

  As shown in FIG. 2, the target device 20 is disposed in a shielding container 26 having a proton beam entrance 22 and a neutron take-out port 24, and covers the lithium thin film 28 and the proton beam entrance surface of the lithium thin film 28. It consists of a beryllium thin film 30 and a backing foil 32 fixed to the surface of the lithium thin film 28 opposite to the proton beam incident surface. The backing foil 32 is made of a low atomic number element such as beryllium. Thus, the target device 20 of this embodiment has a three-layer structure in which the lithium thin film 28 as the target material is sandwiched between the beryllium thin film 30 and the backing foil 32. The backing foil 32 is provided to give mechanical strength to the lithium thin film 28 and the beryllium thin film 30.

Here, the target device 20 will be described.
When the target device 20 is irradiated with a proton beam, a Li (p, n) reaction occurs and neutrons are generated. At this time, if the energy of the proton beam is kept as low as possible, cold neutrons with low energy are generated. Further, generation of unnecessary radiation such as α rays and γ rays can be suppressed. For this reason, the neutron generator 10 as a whole including the shielding container 26 can be downsized. However, neutrons are not generated when the incident energy of the proton beam on the lithium thin film 28 is lower than the threshold (about 2.0 MeV).

On the other hand, the energy of the proton beam decreases when it passes through the lithium thin film 28. FIG. 3 shows the relationship between proton energy and energy loss in lithium. As shown in FIG. 3, the lower the proton beam energy, the larger the energy loss. For example, energy slightly larger than the Li (p, n) reaction threshold (about 2.0 MeV) (the region enclosed by the rectangular frame in FIG. 3). In this case, the proton beam only passes about 70 μm through lithium and falls below the threshold of the Li (p, n) reaction, and does not contribute to neutron generation thereafter.
Therefore, in order to efficiently generate cold neutrons by the proton beam incident on the lithium thin film 28, it is necessary to set the incident energy of the proton beam and the thickness dimension of the lithium thin film 28 in consideration of energy loss in lithium. .

Further, the lithium thin film 28 on which the proton beam is incident generates heat due to the energy. Lithium has a high chemical activity and a melting point as low as about 454 K (about 181 ° C.). Therefore, when heat is generated by the energy of the proton beam, it may be melted and scattered. If lithium is scattered and the lithium thin film 28 becomes very thin, the Li (p, n) reaction cannot be caused stably, so the operation must be stopped and the target device 20 must be replaced by breaking the vacuum. .
Therefore, in this embodiment, the proton beam incident surface of the lithium thin film 28 is covered with the beryllium thin film 30 to prevent the molten lithium from scattering. However, considering the energy loss in beryllium, it is preferable to make the beryllium thin film 30 as thin as possible.

Incidentally, Li (p, n) in the reaction, proton lithium ⁷ (7 Li) (p) of lithium 7 by the collision ⁽⁷ Li) neutrons in the process of is converted to beryllium ⁷ (7 Be) (n) Occurs ( ⁷ Li + p → ⁷ Be + n). The half-life of beryllium 7 is about 2 months and returns to lithium 7 by β decay. That is, the beryllium 7 formed by the Li (p, n) reaction returns to the lithium 7 and is used again for the Li (p, n) reaction.
Thus, since the beryllium thin film 30 prevents not only lithium but also the beryllium 7 from being scattered, the life as a target can be increased and the surrounding contamination can be prevented.

  Further, the target device 20 is arranged in an inclined state with respect to a direction orthogonal to the traveling direction of the proton beam (the direction indicated by the arrow B in FIG. 1). With such a configuration, the irradiation area of the proton beam on the lithium thin film 28 increases. For this reason, the current density of the proton beam in the lithium thin film 28 can be reduced, and the temperature rise of the lithium thin film 28 can be suppressed. Furthermore, since the distance that the proton beam travels in the lithium thin film 28 can be increased by tilting the target device 20, the thickness of the lithium thin film 28 can be further reduced.

  According to a study by the inventors, when the thickness of the lithium thin film 28 is set to 70 μm, the thickness of the beryllium thin film 30 is set to 25 μm, and the target device 20 is irradiated with a proton beam having an energy of about 3 MeV. As a result, the incident energy of the proton beam after passing through the beryllium thin film 30, that is, the proton beam with respect to the lithium thin film 28 becomes 2.5 MeV, and the energy of the proton beam after passing through the target device 20 decreases below the threshold (˜1.5 MeV). Therefore, cold neutrons can be generated efficiently and unnecessary radiation can be eliminated.

  FIG. 4 shows a target device 40 of a neutron generator according to the second embodiment of the present invention. This target device 40 is arranged in the same shielding container as that of the first embodiment, and is deposited on the inner surface of the cylindrical heat radiation container 44 having a circular beam incident port 42 in the lower part thereof. A lithium thin film 46 and a lid 48 for closing the beam entrance 42 are provided.

  The heat radiating container 44 includes a cylindrical outer container 44a and a conical inner container 44b fitted to the outer container 44a. The outer container 44a is made of a moderator such as polyethylene. The inner container 44b is made of a metal excellent in heat dissipation, such as copper. A spiral cooling water channel 50 is formed on the outer peripheral surface of the inner container 44b. The outer container 44a is formed with an introduction path 52 for introducing cooling water into the cooling water path 50 and a lead-out path 54 for leading it out. When both containers 44 a and 44 b are combined, the introduction path 52 and the outlet path 54 communicate with both ends of the cooling water path 50. In this manner, a part of the heat radiation container 44 is made of a moderator, so that the size can be reduced.

  The inner peripheral surface of the inner container 44 b is composed of a conical portion 56 and a cylindrical portion 58 below the conical portion 56, and a flange 60 is formed on the outer peripheral surface of the cylindrical portion 58. A lithium thin film 46 is deposited on the conical portion 56 of the inner container 44b. Thus, the lithium thin film 46 can be efficiently cooled by providing the lithium thin film 46 on the inner surface of the inner container 44b.

  Further, with the above configuration, the proton beam incident surface of the lithium thin film 46 is inclined with respect to the direction orthogonal to the traveling direction of the proton beam (the direction indicated by the arrow B in the drawing). The inclination of the proton beam incident surface is set so that the current density of the proton beam is substantially uniform. The shape of the incident surface of the proton beam can be obtained by assuming that the current density distribution of the proton beam is a normal distribution, a bell-shaped distribution, or the like.

（ただし、Ｉ_０は全電流、σ^２は分散、ｒは陽子ビームの中心軸からビーム入射面までの距離を示す。） For example, assuming that the current density distribution of the proton beam is a normal distribution, the current density distribution i is expressed by the following equation (1).

(However, I ₀ is the total current, σ ² is the dispersion, and r is the distance from the central axis of the proton beam to the beam incident surface.)

ただし、ｒ_１は円錐状部５６の最外半径を示す。 At this time, in order to make the current density at the proton beam incident surface of the lithium thin film 46 constant, the traveling direction of the proton beam and the normal direction of the proton beam incident surface are as shown in the following equation (2). The cosine of the angle θ formed may be the reciprocal of the current density.

Here, r ₁ represents the outermost radius of the conical portion 56.

In general, if r _{1 is set} to about 2σ, the proton beam outside the conical portion 56 is 14% or less of the total amount, and the current density is 14% or less of the center value, so that it is sufficiently small. When handling a very strong proton beam, r ₁ can be selected to be about 3σ.
Since the shape of the conical portion 56 is obtained by integrating the tangent of the angle θ, if the coordinate in the traveling direction of the proton beam is Z, the coordinate Z is the angle θ, the maximum depth Z ₀ of the conical portion 56, It can be expressed by the following equation (3) using the distance r from the central axis of the proton beam to the beam incident surface.

Further, FIG. 7 shows the current density at this time and the relationship between Z / Z ₀ and r / σ. The shape represented by the solid line in FIG. 7 is the shape of the proton beam incident surface of the conical portion 56.
With the above configuration, the intensity of the proton beam applied to the lithium thin film 46 can be equalized, and the lithium thin film 46 can be prevented from being locally heated.

  The lid 48 is fixed to the heat dissipation container 44 via a plurality of bolts 64. The lid 48 includes a stainless outer frame 48b and an inner frame 48c having a circular opening 48a, and a beryllium thin film 66 stretched over the opening 48a and sandwiched between the outer frame 48b and the inner frame 48c. Has been. The inner frame 48c is screwed to the outer frame 48b with the beryllium thin film 66 sandwiched between it and the outer frame 48b.

A housing portion 62 that houses the lithium block 61 is disposed below the cylindrical portion 58. As shown in FIGS. 4 and 5, the accommodating portion 62 is disposed along the inner peripheral surface of the cylindrical portion 58, for example, and has an annular bottom surface portion 62 a and an inner peripheral wall erected on the inner and outer peripheral edges thereof. 62b and an outer peripheral wall 62c.
The accommodating part 62 is made of a material having good thermal conductivity and not reacting with lithium (not alloyed) at a high temperature, for example, tantalum (Ta). A heater element 63 such as a sheathed heater is embedded in the bottom surface portion 62a. A current is supplied to the heater element 63 via a vacuum hermetic current introduction terminal 65. When a current is supplied to the heater element 63, the entire accommodating portion 62 is heated almost uniformly to a predetermined temperature. As a result, the lithium block 61 is vaporized or sublimated and adheres to the surface of the lithium thin film 46 to form a thin film.

  5 shows a state in which the annular lithium block 61 is accommodated in the accommodating portion 62, a large number of lithium blocks may be arranged at almost equal intervals. Moreover, as shown in FIG. 6, the inside of the accommodating part 62 may be divided into a plurality of small chambers, and the lithium block 61 may be accommodated in each small chamber. Furthermore, instead of solid lithium, liquid lithium may be accommodated in the accommodating portion 62 so that liquid lithium can be appropriately supplied from the outside.

  In the target device 40, the proton beam that has entered the radiation container 44 from the beam entrance 42 passes through the beryllium thin film 66 and then enters the lithium thin film 46 to generate neutrons. Therefore, by setting the thickness of the lithium thin film 46 to 70 μm, the thickness of the beryllium thin film 66 to 25 μm, and the incident energy of the proton beam to 3 MeV, the same operation and effect as the target device 20 of the first embodiment can be obtained. It is done.

  In the target device 40 of this embodiment, since the beryllium thin film 66 and the lithium thin film 46 are separated from each other, the lithium scattered from the lithium thin film 46 floats in the space in the heat dissipation container 44 but leaks from the heat dissipation container 44. There is no. By confining the heat-dissipating container 44 with the scattered lithium, the lithium can be attached to the lithium thin film 46 again.

  Since the lithium thin film 46 has a complicated shape, lithium is not always reproduced uniformly. In the portion where the thickness of the lithium thin film 46 is larger than the predetermined thickness, the useless beam decelerated to 2 MeV or less generates heat in lithium having poor thermal conductivity and the temperature rises. Therefore, the thickness of the lithium thin film 46 is increased by sublimation or the like. Is expected to decline early. Such a phenomenon occurs until the thickness of the lithium thin film 46 reaches a predetermined thickness. Thereafter, the temperature rise is reduced, so that rapid sublimation is reduced. Accordingly, operation is possible until the thickness of the lithium thin film 46 becomes, for example, half of the predetermined value. Then, the regeneration operation of the lithium thin film 46 is repeated at the stage where the amount of neutron generation has decreased.

In the second embodiment, the heat radiating container is composed of the outer container and the inner container, but the heat radiating container may be composed of one container.
If the lithium thin film can be sufficiently cooled without circulating the cooling water, the cooling water channel can be omitted. Both the inner container and the outer container may be made of metal having excellent heat dissipation, and a moderator may be provided separately.
Furthermore, the above-described embodiment is merely an example of the present invention, and changes and modifications can be appropriately made within the scope of the gist of the present invention.

The figure which shows schematic structure of the neutron generator which concerns on 1st Example of this invention Vertical side view showing the configuration of the target device Diagram showing the relationship between proton energy and energy loss in lithium Longitudinal front view showing a configuration of a target device according to a second embodiment of the present invention The top view which shows an example of the accommodating part in which the lithium block was accommodated The top view which shows the other example of the accommodating part in which the lithium block was accommodated Diagram showing current density and the relationship between Z / Z ₀ and r / σ

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... Neutron generator 12 ... Accelerator 20, 40 ... Target device 26 ... Shielding container 28, 46 ... Lithium thin film 30, 66 ... Beryllium thin film 32 ... Backing foil 42 ... Beam entrance 44 ... Radiation container 44a ... External container 44b ... Inside Container 48 ... Lid 61 ... Lithium block 62 ... Storage section 62a ... Bottom surface 62b ... Inner peripheral wall 62c ... Outer peripheral wall 63 ... Heater element

Claims (8)

a) a lithium thin film having a predetermined thickness that generates neutrons by Li (p, n) reaction caused by the incidence of a proton beam;
b) a beryllium thin film covering the proton beam incident surface of the lithium thin film;
c) a reinforcing sheet that is fixed to a surface opposite to the proton beam incident surface of the lithium thin film and imparts mechanical strength to the lithium thin film and the beryllium thin film;
In a neutron generation target device comprising:
The lithium thin film is set to a thickness that reduces its energy below the threshold of the Li (p, n) reaction until the incident proton beam is emitted from the lithium thin film. Target device. a) a heat dissipation member having a conical recess,
b) a lithium thin film having a predetermined thickness that is provided along the inner peripheral surface of the concave portion and generates neutrons by a Li (p, n) reaction caused by incidence of a proton beam;
c) a beryllium thin film closing the recess,
In a neutron generation target device comprising:
The lithium thin film is set to a thickness that reduces its energy below the threshold of the Li (p, n) reaction until the incident proton beam is emitted from the lithium thin film. Target device.   The target device for neutron generation according to claim 2, wherein solid or liquid lithium is annularly arranged in the recess.   4. The neutron generating target device according to claim 2, wherein the heat dissipating member is provided with a cooling passage through which a cooling medium flows.   3. The structure according to claim 2, wherein an angle formed between a normal direction of a proton beam incident surface of the lithium thin film and an incident direction of the proton beam is larger in a region where the current density of the proton beam is larger. 5. The neutron generating target device according to any one of 4 above. Assuming that the current density distribution of the proton beam is a normal distribution, the angle θ between the normal direction of the proton beam incident surface of the lithium thin film and the incident direction of the proton beam is given by

(Where σ ² is dispersion, r ₁ is the outermost radius of the recess, and r is the distance from the center of the proton beam.)
The target device for neutron generation according to claim 5, wherein   2. The energy of the proton beam and the thickness of the beryllium thin film are set so that the energy of the proton beam when incident on a lithium thin film having a thickness of 70 μm is 2.5 ± 0.5 MeV. 6. The target device for generating neutrons according to any one of 6 above. An accelerator,
A target device for generating neutrons according to any one of claims 1 to 7, wherein a proton beam accelerated by the accelerator is irradiated.
A neutron generator characterized by comprising:

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