JP6732244B2 - Neutron moderation irradiation device - Google Patents

Description

The present invention relates to a neutron moderating irradiation device that decelerates and irradiates a neutron beam generated by a neutron source.

Boron neutron capture therapy (BNCT) is one type of radiation therapy for treating cancer. Boron neutron capture therapy destroys cancer cells by irradiating neutrons to boron compounds selectively accumulated in cancer cells, and α particles and lithium nuclei produced by the nuclear reaction of ¹⁰ B(n,α) ⁷ Li. It is a cure to do. Since the range of α particles and lithium nuclei is about the same as the size of cells, boron neutron capture therapy can selectively destroy only cancer cells without significantly damaging normal cells. ..

In boron neutron capture therapy, Borono-phenylalanine (BPA) and borocaptate (Sodium mercapto-undecahydro-dodecaborato; BSH) are administered to a patient, and neutron rays are given to cancer cells in which these compounds are accumulated. Irradiate. While the reaction cross section of neutrons increases as the energy decreases, so does the energy required to reach deep inside the tissue of the patient. However, if the energy is too large like fast neutrons, even normal cells will be greatly damaged. Therefore, the irradiated neutron beam is required to have a high intensity of epithermal neutrons and a low mixing rate of fast neutrons.

Traditionally, boron neutron capture therapy has often been performed using a research reactor as a neutron source. However, it takes time to start and stop the operation of the research reactor, and it is necessary to adjust the operation plan of the reactor and the treatment schedule when performing the treatment. In addition, existing research reactors will have a limit in the future in terms of maintenance cost and life, so that they can be used continuously. Therefore, in recent years, development of a device for generating a neutron beam using an accelerator has been underway.

A neutron generator that utilizes neutrons generated by an accelerator for boron neutron capture therapy is generally an accelerator that generates a charged particle beam, a target that is irradiated with a charged particle beam to generate a neutron beam, and a neutron beam that is generated by the target. And a neutron decelerating irradiation device for decelerating and irradiating the object to be irradiated. A target material that functions as a neutron source is held in the target, and a neutron generation reaction occurs when the target material is irradiated with a charged particle beam such as a proton beam. As the target material, lithium that causes a reaction of ⁷ Li(p,n) ⁷ Be, beryllium that causes a reaction of ⁹ Be(p,n) ⁹ B, ⁹ Be(p,xn), or a spallation reaction is generated. Heavy metals such as tantalum and tungsten are being studied.

When lithium or beryllium is used as the target material, less gamma rays are generated as compared with the spallation reaction by heavy metals, so the shielding is easy and the treatment safety is high. Further, comparing lithium and beryllium, lithium has a low neutron yield, is chemically unstable, and has a low melting point, but it is possible to generate neutrons with low incident proton energy. That is, in addition to the energy of the generated neutron beam, it has the advantage that the generation of secondary radiation can be suppressed to a low level. The threshold of incident proton energy is about 2.06 MeV in the reaction of ⁹ Be(p,n) ⁹ B, while it is about 1.88 MeV in the reaction of ⁷ Li(p,n) ⁷ Be, which is macroscopic. There is a difference in that the cross-sectional area of lithium is larger than that of lithium over the entire incident proton energy. Therefore, lithium is regarded as a promising target material suitable for reducing the size and weight of accelerators and neutron deceleration irradiation devices.

JP, 2014-032168, A

The International Atomic Energy Agency (IAEA) has set design target values for neutron beams used for boron neutron capture therapy. For example, the epithermal neutron intensity is recommended to be 1×10 ⁹ [n/cm ² /s] or more from the viewpoint of effectively performing treatment in a short time. Further, the fast neutron contamination rate is recommended to be 2×10 ⁻¹³ [Gy/cm ² ] or less from the viewpoint of avoiding damage to normal cells.

However, if the thickness of the moderator is increased in order to reduce the mixing rate of fast neutrons, the neutron moderating irradiation device becomes heavier and the intensity of the epithermal neutron beam emitted through the moderator also decreases. I will end up. In particular, when the target material is solid lithium, if the target material is excessively heated and melted, irradiation damage and uneven distribution of atomic number density may occur, or leakage may increase, and therefore, incident It is desired that the proton energy is small. Therefore, how to increase the intensity of epithermal neutrons while reducing the rate of mixing of fast neutrons is a major issue under the constraint of the incident proton energy.

Therefore, the present invention provides a neutron moderating irradiation device capable of decelerating a neutron beam generated by a neutron source and emitting a neutron beam having a high intensity of epithermal neutrons while the mixing rate of fast neutrons is reduced. The purpose is to

In order to solve the above problems, the neutron moderating irradiation apparatus according to the present invention is a moderator that decelerates the neutron beam emitted by a neutron source by being irradiated with a charged particle beam, and a reflector surrounding the moderator. The moderator comprises a collimator part for shaping the irradiation field of the neutron beam decelerated by the moderator, the moderator being a main body part arranged on the downstream side of the neutron source in the irradiation direction of the charged particle beam, and the irradiation. possess the upstream portion is disposed upstream of said main body portion in a direction surrounding the periphery of the neutron source, the collimator unit includes an opening whose diameter decreases toward the irradiation direction, the in the irradiation direction body portion An upstream member arranged on the downstream side, a downstream inner member forming the opening on the downstream side of the upstream member in the irradiation direction, and a downstream side of the upstream member in the irradiation direction, and A downstream side outer member arranged radially outside of the downstream side inner member, wherein the upstream side member and the downstream side inner member are formed of graphite, iron, lead or beryllium, and The downstream outer member is made of cadmium, indium, hafnium, gadolinium, bismuth, lead fluoride, boron compound, paraffin, water, lithium fluoride-polyethylene or boron-polyethylene .

The present invention provides a neutron moderating irradiation device capable of decelerating the neutron beam generated by the neutron source and emitting a neutron beam having a high intensity of epithermal neutrons while the mixing rate of fast neutrons is reduced. be able to. The epithermal neutron beam emitted from the neutron moderating irradiation device has a high intensity and a low mixing rate of fast neutrons, so that the moderator can be made thin. Therefore, a lightweight and small neutron moderation irradiation apparatus can be provided.

It is a figure which shows schematic structure of a neutron generator. It is a longitudinal cross-sectional view of the neutron moderation irradiation apparatus which concerns on 1st Embodiment of this invention. It is a longitudinal cross-sectional view of the neutron moderation irradiation apparatus which concerns on 2nd Embodiment of this invention. It is a longitudinal cross-sectional view of the neutron moderation irradiation apparatus which concerns on 3rd Embodiment of this invention. It is a figure which shows the inlet diameter of a collimator part, an outlet diameter, and the relationship between epithermal neutron intensity. It is a figure which shows the relationship between the entrance diameter and exit diameter of a collimator part, and a current/flux ratio. It is a figure which shows the relationship between the exit diameter of a collimator part, and epithermal neutron intensity. It is a figure which shows the relationship between the exit diameter of a collimator part, and a current/flux ratio. It is a figure which shows the structure of a collimator part, and the relationship between epithermal neutron intensity. It is a figure which shows the structure of a collimator part, and the relationship of a current/flux ratio. It is a figure which shows the shape of a moderator and the relationship between epithermal neutron beam intensity. It is a figure which shows the shape of a moderator, and the relationship between a fast neutron mixing rate. It is a figure which shows the relationship between the thickness and length of an upstream moderator, and epithermal neutron intensity. It is a figure which shows the relationship between the thickness and length of an upstream moderator, and a neutron air dose ratio. It is a figure which shows the relationship between the thickness of a downstream moderator and epithermal neutron intensity. It is a figure which shows the relationship between the thickness of a downstream moderator and the thickness of a moderator main body.

Hereinafter, a neutron moderation irradiation apparatus according to an embodiment of the present invention will be described in detail with reference to the drawings. In addition, the same reference numerals are given to the common configurations in the respective drawings, and the duplicated description will be omitted.

[Neutron generator]
First, a schematic configuration of a neutron generator provided with the neutron moderation irradiation apparatus according to the present embodiment will be described.

FIG. 1 is a diagram showing a schematic configuration of a neutron generator.
As shown in FIG. 1, the neutron generator 100 includes a charged particle beam generator 1, a neutron deceleration irradiation device 2, a conduit 4, and a target 5 as a neutron source. The target 5 as a neutron source holds solid lithium as a target material. The neutron generator 100 is preferably used as a neutron source in boron neutron capture therapy.

In the neutron generator 100, the charged particle beam generator 1 generates a proton beam or the like (charged particle beam 6) having a predetermined energy. The charged particle beam 6 reaches the target 5 through the conduit 4, and the target 5 is irradiated with the charged particle beam 6 to generate a neutron beam in a predetermined energy band. Then, the neutron deceleration irradiation device 2 decelerates the neutron beam emitted from the target 5 and emits the neutron beam 9 whose irradiation field is shaped. The neutron beam 9 emitted from the neutron deceleration irradiation device 2 is irradiated on the irradiation target body 3 to cause a neutron capture reaction. That is, when a cancer cell as the irradiated body 3 in which boron is accumulated is irradiated with the neutron beam 9, the cancer cell is destroyed by α rays and lithium particles generated by the nuclear reaction.

[Charged particle beam generator]
The charged particle beam generator 1 generates, for example, a proton beam as a charged particle beam. As shown in FIG. 1, the charged particle beam generator 1 for generating a proton beam includes an ion source 1a for generating a proton and an accelerator 1b for accelerating the proton.

As the ion source 1a, for example, an electron cyclotron resonance (ECR) ion source is used. The ECR ion source introduces hydrogen gas under a strong magnetic field and applies high frequency to generate electron cyclotron resonance, thereby generating hydrogen plasma at high density. Then, the generated hydrogen ions ( ¹ H ⁺ ) are collected by the magnetic mirror and extracted. The ECR ion source is capable of stable operation over a long period of time because of the electrodeless discharge.

As the accelerator 1b, for example, an electrostatic accelerator is used. The electrostatic accelerator applies a high DC voltage between the electrodes to accelerate charged particles under a constant electrostatic field. With the electrostatic accelerator, it is possible to generate a continuous charged particle beam 6. As the electrostatic accelerator, for example, a dynamitron accelerator (manufactured by IBA Co., Ltd.) can be used. Further, an electrostatic accelerator such as Cockcroft-Walton type or Van de Graaff type, or a high frequency type accelerator such as cyclotron or synchrotron can be used.

[conduit]
The conduit 4 connects the charged particle beam generator 1 and the target 5. The conduit 4 forms a path for guiding the charged particle beam 6 emitted by the charged particle beam generator 1 to the target 5. The conduit 4 is provided with a focusing lens 7 that suppresses divergence of the charged particle beam 6 in the width direction. As the focusing lens 7, for example, a plurality of quadrupole electromagnets are installed along the irradiation direction of the charged particle beam 6 and the polarities thereof are reversed. The conduit 4 is not limited to the linear shape as shown in FIG. 1, and may have a path of any shape having a curved portion. A bending electromagnet or the like for deflecting the charged particle beam 6 can be installed in the curved portion of the conduit 4.

[target]
The target 5 is installed at the tip of the conduit 4. The target 5 as a neutron source holds solid lithium as a target material. The target material made of solid lithium is held in, for example, a concave portion of a metal substrate formed by combining tantalum that shields a proton beam, copper having a high thermal conductivity, iron, and the like. Then, the solid lithium held in the recess is sealed by a metal foil made of titanium or the like, and leakage of lithium melted by the charged particle beam 6 is prevented. In addition, inside the metal substrate, a coolant flow path is formed for allowing cooling water to flow therethrough to cool the target material.

The target material lithium is irradiated with a proton beam to generate a neutron beam by a nuclear reaction of ⁷ Li(p,n) ⁷ Be. The threshold of the incident proton energy required for this nuclear reaction is about 1.88 MeV. Therefore, in the charged particle beam generator 1, the charged particle beam 6 having a low energy which is equal to or higher than this threshold value and which does not generate neutrons having excessive energy is generated. Specifically, the energy of the charged particle beam 6 generated by the charged particle beam generator 1 is in the range of 4.0 MeV or less, preferably 3.0 MeV or less, and more preferably 2.8 MeV or less. The current value is 10 mA or more and 100 mA or less, and more preferably 10 mA or more and 20 mA or less from the viewpoint of avoiding a heat load on the target material.

[First Embodiment]
Next, a specific configuration of the neutron moderation irradiation apparatus according to the first embodiment of the present invention will be described.

FIG. 2 is a vertical sectional view of the neutron moderating irradiation apparatus according to the first embodiment of the present invention.
As shown in FIG. 2, the neutron moderation irradiation apparatus 2 according to this embodiment includes a moderator (21A, 21B), a reflector 22, a shield (shield) 23, and a collimator (24A, 24B). Equipped with. The neutron deceleration irradiation device 2 surrounds the target 5 as a neutron source and is installed downstream of the target 5. The neutron deceleration irradiation device 2 decelerates the neutron beam generated by the target 5, decelerates mainly to the energy band of the epithermal neutron beam, and irradiates the irradiation target 3 with the neutron beam.

The neutron beams generated by the target 5 are roughly classified into thermal neutrons ( _Nther ), epithermal neutrons (N _epi ) and fast neutrons (N _fast ) according to their energies. In the present specification, neutrons having an energy of 0.5 eV or less are thermal neutrons ( _Nther ), neutrons exceeding 0.5 eV and 10 keV or less are epithermal neutrons (N _epi ), and neutrons exceeding 10 keV are _fast neutrons (N _fast) ) Is defined. In the following description, the terms “downstream” and “upstream” mean downstream and upstream in the irradiation direction (traveling direction) of the charged particle beam 6, respectively.

(Moderator)
The moderator (21A, 21B) mainly decelerates the neutron beam generated by the target 5 as the neutron source to the energy band of the epithermal neutron beam. As shown in FIG. 2, the moderator (21A, 21B) according to the present embodiment includes a moderator main body (body) 21A and an upstream moderator (upstream) 21B.

Both the moderator main body 21A and the upstream moderator 21B are preferably made of magnesium fluoride (MgF ₂ ). Magnesium fluoride has a texture of either a single crystal body or a sintered body in which single crystals are sintered together. Further, magnesium fluoride has a bulk density (relative density) of 95% or more, preferably 98% or more, more preferably 99% or more with respect to the true density. Magnesium fluoride has the ability to effectively slow down the generated neutrons when the incident proton energy to the target 5 is 10 MeV or less. That is, since the moderator (21A, 21B) is made of magnesium fluoride, the neutron beam emitted from the target 5 is mainly decelerated to the energy band of the epithermal neutron beam, and most of the fast neutrons are absorbed. Therefore, the mixing ratio of fast neutrons is reduced, and it becomes possible to emit epithermal neutron rays with high intensity.

The moderator main body (main body portion) 21A is arranged on the downstream side of the target 5 in a columnar shape or a polygonal columnar shape. Specifically, the moderator body 21A is arranged so as to be concentric with the irradiation axis of the charged particle beam 6 and also concentrically with the target 5. The upstream end surface (the left end surface in FIG. 1B) of the target 5 is arranged, for example, 2 cm to 6 cm apart from the upstream end surface of the moderator main body 21A. The moderator material main body 21A may be configured by a single moderator material having a columnar shape or a polygonal column shape, or may be configured by combining a plurality of moderator materials so as to exhibit a columnar shape or a polygonal columnar shape. Good. Further, the moderator body 21A may have a truncated cone shape whose diameter decreases toward the downstream side in the irradiation direction of the charged particle beam 6. In the truncated cone moderator main body 21A, the ratio of the diameter of the upstream end face to the diameter of the downstream end face is preferably 1 or more and 3/2 or less. However, from the viewpoint of facilitating the molding of the moderator body 21A, a columnar shape or a polygonal columnar shape is preferable.

Moderator body 21A, the thickness (the length of the irradiation direction of the charged particle beam 6) _{(L B)} is preferably at less than 20.0 cm 35.0Cm, in this embodiment, more than 25.7cm It is more preferably 32.7 cm or less, and further preferably 25.7 cm or more and 29.3 cm or less. If the thickness _{(L B)} is in this range, while the mixing ratio of the fast neutrons to ^{2 × 10 -13 [Gy / cm} 2] below neutrons emitted from the neutron moderating irradiation apparatus 2, epithermal The intensity of neutron rays can be maximized.

It moderator body 21A, the diameter (the length of the normal direction with respect to the irradiation axis of the charged particle beam 6) _{(R B)} is a 10 / 7-2 times the thickness of the moderator body 21A _{(L B)} Is preferred. If the diameter (R _B) is within this range, the neutron beam by the target 5 is emitted is effectively decelerated to reach the opening 124a of the collimator unit. Therefore, it is possible to sufficiently reduce the mixing ratio of fast neutrons in the neutron beam emitted from the neutron deceleration irradiation device 2. More specifically, the diameter (R _B ) is preferably 40 cm or more and 60 cm or less, and more preferably 48 cm or more and 52 cm or less.

The upstream moderator (upstream portion) 21B is arranged in a concentric cylindrical shape or a polygonal tube shape from the upstream end surface of the moderator main body 21A toward the upstream side. The downstream end surface of the upstream moderator 21B is in close contact with the upstream end surface of the moderator main body 21A, and the upstream moderator 21B encloses the target 5 inside a cylindrical shape or a polygonal tube shape. It surrounds you. In detail, the upstream moderator 21B is arranged so as to be concentric with the moderator main body 21A, and so as to be concentric with the irradiation axis of the charged particle beam 6. The upstream moderator 21B may be configured as a single-body moderator having a cylindrical shape or a polygonal tube shape, or may be configured by combining a plurality of moderator materials so as to exhibit a cylindrical shape or a polygonal cylinder shape. Alternatively, it may be formed integrally with the moderator main body 21A.

The upstream moderator 21B preferably has a peripheral wall thickness (difference between outer shape and inner diameter) (R _U ) that is 1/6 to 2/5 times the diameter (R _B ) of the moderator main body 21A. When the thickness (R _U ) of the peripheral wall is in such a range, the target 5 effectively decelerates the neutron beam emitted toward the normal direction to the irradiation axis of the charged particle beam 6 and the upstream side, and The size of the upstream moderator 21B can be kept small. More specifically, the thickness (R _U ) of the peripheral wall is preferably 8 cm or more and 20 cm or less, and more preferably 9 cm or more and 20 cm or less.

The upstream moderator 21B preferably has a length (L _U ) in the irradiation direction of the charged particle beam 6 that is ¼ to 4/5 times the thickness (L _B ) of the moderator body 21A. When the length (L _U ) is within such a range, the neutron beam emitted from the target 5 toward the normal direction to the irradiation axis of the charged particle beam 6 and the upstream side is effectively decelerated, and The size of the side moderator 21B can be kept small. More specifically, the length (L _U ) is preferably 8 cm or more and 20 cm or less, and more preferably 9 cm or more and 20 cm or less.

(Reflective material)
The reflector 22 surrounds the moderators (21A, 21B) and reflects the neutron beam emitted from the target 5 as a neutron source. Specifically, the reflecting material 22 is arranged radially outside the moderator material main body 21A and the upstream moderator material 21B, and covers the moderator material main body 21A and the upstream moderator material 21B from the outside. Further, the reflecting material 22 also covers the upstream end surface of the upstream moderator 21B from the upstream side. An opening having a circular cross section is formed on the upstream end surface of the reflector 22 so as to be concentric with the irradiation axis of the charged particle beam 6. The inner surface of the opening and the inner peripheral surface of the upstream moderator 21B are provided substantially flush with each other, and a path of the charged particle beam 6 reaching the target 5 included in the reflector and the moderator is formed.

The reflector 22 is made of a material having a large neutron scattering cross section. Examples of the material of the reflective material 22 include graphite, iron, lead, beryllium, and the like. The reflection material 22 preferably has a large neutron scattering cross-section and a small neutron absorption cross-section, and is particularly preferably made of lead or a lead alloy such as a lead-bismuth alloy. By forming the reflective material 22 with such a material, the neutron beam emitted from the target 5 toward the normal direction to the irradiation axis of the charged particle beam 6 or toward the upstream side is reflected with a high reflectance, and the collimator unit has a high reflectivity. The opening 124a can be reached. Therefore, the intensity of the epithermal neutron beam emitted from the neutron moderation irradiation device 2 can be increased.

(Shielding material)
The shielding material (shielding portion) 23 mainly shields fast neutron rays and gamma rays emitted from the moderator (21A, 21B). The shielding material 23 is arranged on the downstream side of the moderator material main body 21A and the reflecting material 22. In detail, the shielding material 23 is in close contact with the downstream end surface of the moderator material main body 21A and the downstream end surface of the reflecting material 22, which are provided substantially flush with each other.

The shielding material 23 is formed of a material having a large neutron capture reaction cross-section and a high neutron absorption ability, or a material having a high gamma ray shielding ability. Examples of the material of the shielding material 23 include cadmium, indium, hafnium, gadolinium, bismuth, lead, iron, lead fluoride, boron compounds such as boron carbide, paraffin, water, lithium fluoride-polyethylene, and boron-polyethylene. Etc. The shielding material 23 may be a shielding material 23 having a multilayer structure by laminating one or more of these materials, but it is particularly preferable to include a shielding material made of cadmium or cadmium alloy. By forming the shielding material 23 with such a material, neutrons of low energy emitted from the moderator (21A, 21B) can be effectively absorbed, while high intensity of epithermal neutron rays can be maintained. It will be possible. Further, it may be constituted only by a shielding material made of cadmium or a cadmium alloy, or a shielding material made of bismuth may be used together. When such a material is used in combination, gamma rays can be effectively shielded.

The shielding material 23 has a thickness (length in the irradiation direction of the charged particle beam 6) of preferably 0.005 cm or more and 0.2 cm or less, more preferably 0.005 cm or more and 0.15 cm or less, It is more preferably 0.008 cm or more and 0.012 cm or less. The shielding material made of cadmium or cadmium alloy is more preferably 0.005 cm or more and 0.05 cm or less, and the shielding material made of bismuth is more preferably 0.005 cm or more and 0.15 cm or less. When the thickness is in such a range, while maintaining the intensity of the epithermal neutron beam emitted from the neutron deceleration irradiation device 2, it effectively absorbs neutrons of low energy and emits from the neutron deceleration irradiation device 2. It is possible to sufficiently reduce the generated thermal neutron rays. Further, since the shielding material is thinned and the number of secondary gamma ray sources is reduced, it is possible to reduce the mixing rate of gamma rays in the neutron rays emitted from the neutron deceleration irradiation device 2.

(Collimator part)
The collimator section (24A, 24B) is decelerated by the moderator (21A, 21B) and shapes the irradiation field of the neutron beam that has passed through the shielding material 23. The collimator section (24A, 24B) is composed of an upstream member 24A and a downstream member 24B. The collimator section (24A, 24B) has an opening 124a that is tapered toward the downstream side. The opening 124a is provided through the collimator portions (24A, 24B) so as to be concentric with the irradiation axis of the charged particle beam 6. Therefore, the irradiation field of the neutron beam that has been decelerated by the moderators (21A, 21B) and transmitted through the shielding material 23 is narrowed down along the opening 124a.

The upstream member 24A is arranged in a cylindrical shape or a polygonal cylindrical shape downstream of the reflector 22 and the moderator body 21A. The upstream member 24A is arranged such that the opening 124a of the collimator portion is concentric with the moderator main body 21A. The upstream member 24A may be a single body member having a cylindrical shape or a polygonal tube shape, or may be a combination of a plurality of members having a cylindrical shape or a polygonal tube shape.

The upstream member 24A is made of a material having a large neutron scattering cross-section or a material having a high neutron absorption ability. Examples of the material of the upstream member 24A include graphite, iron, lead, beryllium, cadmium, indium, hafnium, gadolinium, bismuth, lead fluoride, boron compounds including boron carbide, paraffin, water, lithium fluoride- Examples thereof include polyethylene and boron-polyethylene. By forming the downstream member 24B with such a material, it is possible to effectively shield neutron rays and the like that pass through the collimator portions (24A, 24B).

Upstream member 24A is inner diameter _{(D 1)} at the upstream end surface is preferably 4 / 5-1 times the diameter of the moderator body 21A _{(R B).} When the inner diameter (D ₁ ) is in such a range, the neutron flux incident on the opening 124a of the collimator section becomes high, so that the intensity of the epithermal neutron beam emitted from the neutron deceleration irradiation device 2 can be increased. More specifically, the inner diameter (D ₁ ) is more preferably 40 cm or more and 60 cm or less, more preferably 48 cm or more and 52 cm or less, and is the same as the diameter (R _B ) of the moderator main body 21A. Is particularly preferable. In detail, the intensity of the epithermal neutron beam and the fast neutron beam decreases as the distance from the irradiation axis of the charged particle beam 6 decreases, but only the fast neutron beam has a maximum value in the range where the inner diameter (D ₁ ) exceeds 50 cm. Therefore, when the inner diameter (D ₁ ) is larger than 50 cm, the intensity of fast neutron beam is slightly increased. Therefore, if the inner diameter (D ₁ ) is in such a range, the intensity of epithermal neutron rays can be increased while reducing the mixing rate of fast neutrons with respect to the neutron rays emitted from the neutron moderating irradiation device 2. ..

The downstream member 24B is arranged in the shape of a cylinder or a polygonal cylinder on the downstream side of the upstream member 24A, in close contact with the downstream end surface of the upstream member 24A. Specifically, the downstream member 24B is arranged such that the opening 124a of the collimator portion is concentric with the moderator main body 21A. The downstream member 24B may be a single member having a cylindrical shape or a polygonal tube shape, or may be a combination of a plurality of members having a cylindrical shape or a polygonal tube shape.

The downstream member 24B is formed of a material having a large neutron scattering cross-section or a material having a high neutron absorption capacity. Examples of the material of the downstream member 24B include graphite, iron, lead, beryllium, cadmium, indium, hafnium, gadolinium, bismuth, lead fluoride, boron compounds including boron carbide, paraffin, water, lithium fluoride- Examples thereof include polyethylene and boron-polyethylene. By forming the downstream member 24B with such a material, the neutron beam or the like passing through the collimator part (24A, 24B) is effectively shielded, and the neutron dose to normal tissue that does not need to be irradiated with neutrons is reduced. can do.

Downstream member 24B is an inner diameter _{(D 2)} at the downstream side end face, is preferably 1 / 5-1 / 3 times the inner diameter of the upstream end surface of the upstream-side member 24A _{(D 1).} When the inner diameter (D ₂ ) is in such a range, the neutron beam is largely shaped by the opening 124a of the collimator section, so that the straightness of the neutron beam emitted from the neutron deceleration irradiation device 2 can be enhanced. More specifically, the inner diameter (D ₂ ) is preferably 8 cm or more and 20 cm or less, and more preferably 9 cm or more and 15 cm or less.

A preferable form of the collimator portion (24A, 24B) is an upstream side member 24A (reflecting material) made of lead or a lead alloy such as a lead-bismuth alloy, and a downstream side member 24B (shielding material) made of lithium fluoride-polyethylene. Or a combination of an upstream side member 24A (shielding material) made of lithium fluoride-polyethylene and a downstream side member 24B (reflecting material) made of lead or a lead alloy such as a lead-bismuth alloy. It is a thing. With such a configuration, it is possible to increase the intensity of the epithermal neutron beam emitted from the neutron deceleration irradiation device 2 while reducing the spatial dose of neutron beam or gamma ray around the exit of the opening 124a of the collimator portion. Further, the straightness of the neutron beam emitted from the neutron deceleration irradiation device 2 can be enhanced.

More preferable forms of the collimator parts (24A, 24B) are an upstream side member 24A (reflecting material) made of lead or a lead alloy such as a lead-bismuth alloy, and a downstream side member 24B (shielding material) made of lithium fluoride-polyethylene. ) Is formed by the combination with. With such a configuration, epithermal neutrons can pass through the upstream member 24A and be guided to the emission port, and the rate of incorporation of thermal neutrons can be further reduced, as compared with the configuration in which the configuration is reversed. You can Therefore, the moderator can be made thinner while reducing the mixing rate of fast neutrons. In addition, in this embodiment, the upstream side member 24A can be provided with a thick wall on the inner side that forms the peripheral wall of the opening 124a of the collimator section, and can be provided with a thinner wall on the outer side away from the opening 124a of the collimator section than the inner side. is there. Even if the upstream member 24A is thin in this way, the straightness of the neutron beam emitted from the neutron deceleration irradiation device 2 can be improved. Further, since it becomes possible to thicken the outside of the downstream member 24B made of lithium fluoride-polyethylene by that much, it is possible to improve the shielding property of the outside.

Upstream member 24A, the irradiation direction of the length of the charged particle beam 6 _{(L 1)} is preferably 1 / 7-2 / 5 times the thickness of the moderator body 21A _{(L B).} More specifically, the length (L ₁ ) is preferably 3.5 cm or more and 14 cm or less, and more preferably 4 cm or more and 6 cm or less. The length (L ₂ ) of the downstream member 24B in the irradiation direction of the charged particle beam 6 is preferably 1/7 to 2/5 times the thickness (L _B ) of the moderator body 21A. More specifically, the length (L ₂ ) is preferably 3.5 cm or more and 14 cm or less, and more preferably 4 cm or more and 12 cm or less.

According to the neutron moderation irradiation apparatus 2 according to the present embodiment described above, since the upstream moderator 21B (upstream portion) that surrounds the target 5 is provided, the target 5 is also covered with the moderator from the upstream side. Therefore, a part of the neutron beam emitted from the target 5 in the direction normal to the irradiation axis of the charged particle beam 6 and toward the upstream side follows a long track and reaches the opening 124a of the collimator unit, but the upstream moderator The neutron beam is effectively decelerated by installing 21B. Therefore, in the neutron beam emitted from the neutron deceleration irradiation device 2, it is possible to sufficiently reduce the mixing rate of the fast neutrons emitted particularly in the normal direction or the upstream side with respect to the irradiation axis. As a result, the fast neutron component flowing into the opening 124a of the collimator portion is sufficiently reduced, and the moderator can be made thin. Therefore, the moderator is made thin and emitted from the neutron moderator/irradiator 2. The intensity of epithermal neutron rays can also be increased. Further, the neutron moderation irradiation device 2 can be made small and lightweight. As an example, the thickness of the moderator body 21A of the _{(L B),} as compared with the case without the upstream moderator 21B, it is possible to reduce the order of 3.4cm.

[Second Embodiment]
Next, a specific configuration of the neutron moderation irradiation apparatus according to the second embodiment of the present invention will be described.

FIG. 3 is a vertical cross-sectional view of the neutron moderating irradiation device according to the second embodiment of the present invention.
As shown in FIG. 3, the neutron moderation irradiation apparatus 2A according to the present embodiment includes a moderator (21A, 21B), a reflector 22, a shield (shield) 23, and a collimator (24A, 24C, 24D). ) And. The neutron deceleration irradiation device 2A is different from the neutron deceleration irradiation device 2 according to the above-described embodiment in that a downstream inner member 24C and a downstream outer member 24D are provided on the downstream side of the upstream member 24A of the collimator section. That is the point. The rest of the configuration is the same as that of the above embodiment.

The downstream inner member 24C and the downstream outer member 24D are arranged downstream of the upstream member 24A in close contact with the downstream end face of the upstream member 24A in a cylindrical shape or a polygonal cylindrical shape. The downstream inner member 24C has a cylindrical or polygonal cylindrical inner surface that forms an opening 124a, and the downstream outer member 24D is arranged in close contact with the outer side of the downstream inner member 24C in the radial direction. Specifically, the downstream inner member 24C and the downstream outer member 24D are arranged such that the opening 124a is concentric with the moderator main body 21A. Note that each of the downstream inner member 24C and the downstream outer member 24D may be configured by a single member having a cylindrical shape or a polygonal tubular shape, or a plurality of members having a cylindrical shape or a polygonal tubular shape. You may comprise combining.

The upstream member 24A and the downstream inner member 24C are made of a material having a large neutron scattering cross section. Examples of the material of the upstream side member 24A and the downstream side member 24C include graphite, iron, lead, beryllium, and the like. The upstream-side member 24A and the downstream-side inner member 24C have a large neutron scattering cross-sectional area, and preferably have a small neutron absorption cross-sectional area, and are particularly preferably made of lead or a lead alloy such as a lead-bismuth alloy. .. By forming the upstream member 24A and the downstream inner member 24C with such a material, it is possible to increase the intensity of the epithermal neutron beam emitted from the neutron deceleration irradiation device 2 through the opening 124a of the collimator portion. Further, it is possible to reduce the mixing rate of gamma rays with respect to the neutron rays emitted from the neutron moderating and irradiating device 2. The upstream member 24A and the downstream inner member 24C may be integrated.

The downstream outer member 24D is made of a material having a large neutron capture reaction cross-section and a high neutron absorption ability. Examples of the material of the downstream outer member 24D include cadmium, indium, hafnium, gadolinium, bismuth, lead fluoride, boron compounds such as boron carbide, paraffin, water, lithium fluoride-polyethylene, and boron-polyethylene. Can be mentioned. It is particularly preferable that the downstream outer member 24D is made of lithium fluoride-polyethylene. By forming the downstream outer member 24D with such a material, it is possible to reliably reduce the spatial dose of neutron rays around the exit of the opening 124a of the collimator portion.

Upstream member 24A, when the downstream-side inner member 24C is provided, the irradiation direction of the length of the charged particle beam 6 _{(L 1)} is, the thickness of the moderator body 21A _{(L B)} of 1 / 7-2 / It is preferably 5 times. Further, in the downstream side inner member 24C and the downstream side outer member 24D, the length (L ₂ ) in the irradiation direction of the charged particle beam 6 is 1/25 to 5/7 of the thickness (L _B ) of the moderator main body 21A. It is preferably double. The intensity of the epithermal neutron beam emitted from the neutron deceleration irradiation device 2 increases as the length (L ₁ ) of the upstream member 24A in the irradiation direction increases. Further, the mixing ratio of thermal neutrons with respect to the neutron beam emitted from the neutron moderating irradiation device 2 is reduced. On the other hand, the shorter the length (L ₁ ) in the irradiation direction of the upstream member 24A, the higher the straightness of the neutron beam emitted from the neutron deceleration irradiation device 2. Further, the downstream side inner member 24C greatly reduces the spatial dose of neutron rays and gamma rays around the outlet of the opening 124a of the collimator portion. Therefore, when the length of the upstream member 24A (L ₁ ) and the length of the downstream inner member 24C and the downstream outer member 24D (L ₂ ) are in such ranges, the intensity of epithermal neutron rays is increased. It is possible to reduce the space dose around the exit of the opening 124a while reducing the thickness of the moderator. More specifically, the length (L ₁ ) of the upstream member 24A is preferably 3.5 cm or more and 14 cm or less, and more preferably 8 cm or more and 12 cm or less. Further, more specifically, the length (L ₂ ) of the downstream inner member 24C and the downstream outer member 24D is preferably 1 cm or more and 10 cm or less, and more preferably 4 cm or more and 6 cm or less.

The downstream inner member 24C preferably has a peripheral wall thickness (difference between outer shape and inner diameter) (R ₂ ) that is ¼ to 1 times the inner diameter (D ₂ ) of the downstream end surface. The thicker the peripheral wall (R ₂ ) is, the higher the intensity of the epithermal neutron beam emitted from the neutron moderating irradiation device 2 is. Further, the space dose of neutron rays and gamma rays is greatly reduced around the exit of the opening 124a of the collimator section. On the other hand, the thinner the peripheral wall (R ₂ ) is, the lower the mixing rate of fast neutrons in the neutron beam emitted from the neutron moderating irradiation device 2 is. Further, the straightness of the neutron beam emitted from the neutron deceleration irradiation device 2 becomes high. Therefore, when the thickness (R ₂ ) of the peripheral wall is in such a range, it is possible to increase the intensity of epithermal neutron rays and reduce the thickness of the moderator while reducing the air dose around the exit of the opening 124a. Becomes More specifically, the thickness (R ₂ ) of the peripheral wall is preferably 2 cm or more and 16 cm or less, and more preferably 4 cm or more and 12 cm or less.

According to the neutron deceleration irradiation apparatus 2A according to the present embodiment described above, the downstream inner member 24C formed of a material having a large neutron scattering cross-sectional area is provided on the downstream side of the upstream member 24A of the collimator section. , The energy-preserved epithermal neutron beam is emitted from the opening 124a with high intensity. Further, since the downstream side outer member 24D formed of a material having a high neutron absorption ability is provided on the downstream side of the upstream side member 24A of the collimator section, the epithermal neutron beam emitted from the neutron moderating irradiation device 2 is provided. It is possible to reduce the air dose around the exit of the opening 124a of the collimator while increasing the strength of the. In particular, if the upstream side member 24A and the downstream side inner member 24C are made of lead or lead alloy, the entire surface of the opening 124a of the collimator portion is made of lead or lead alloy, so that the neutron deceleration irradiation device 2 emits light. It is possible to reduce the air dose around the outlet while greatly increasing the intensity of the epithermal neutron beam generated.

[Third Embodiment]
Next, a specific configuration of the neutron moderating irradiation apparatus according to the third embodiment of the present invention will be described.

FIG. 4 is a vertical cross-sectional view of the neutron moderating irradiation device according to the third embodiment of the present invention.
As shown in FIG. 4, the neutron moderation irradiation apparatus 2B according to the present embodiment includes a moderator (21A, 21B, 21C), a reflector 22, a shield (shield) 23A, and a collimator (24A, 24C). , 24D). The neutron deceleration irradiation device 2B differs from the neutron deceleration irradiation device 2A according to the above-described embodiment in that a downstream moderator 21C and a shielding member 23A are provided on the downstream side of the moderator body 21A. .. In addition, in FIG. 4, a mode in which the thickness (R _U ) of the peripheral wall of the upstream moderator 21B is different is illustrated, but other configurations are the same as those in the above-described embodiment.

The downstream moderator (downstream part) 21C is arranged in a truncated cone shape from the downstream end surface of the moderator main body 21A toward the downstream side, and is surrounded by the opening 124a of the collimator part. The upstream end surface of the downstream moderator 21C is in close contact with the downstream end surface of the moderator main body 21A, and the frustoconical peripheral surface of the downstream moderator 21C is in close contact with the opening 124a of the collimator portion. In detail, the downstream moderator 21C is arranged so as to be concentric with the moderator main body 21A, and is also arranged so as to be concentric with the irradiation axis of the charged particle beam 6. The downstream moderator 21C may be a single-body moderator having a truncated cone shape, may be a combination of a plurality of moderators having a truncated cone shape, or may be a moderator. It may be integrally formed with the main body 21A. The downstream moderator 21C is preferably made of the same magnesium fluoride as the moderator main body 21A.

Downstream moderator 21C, the irradiation direction of the length of the charged particle beam 6 _{(L D)} is preferably 1 / 30-2 / 5 times the thickness of the moderator body 21A _{(L B).} When the length (L _D ) is in such a range, the neutron beam emitted from the target 5 is effectively decelerated while shaping the irradiation field. Therefore, it is possible to further reduce the thickness of the moderator while reducing the mixing rate of fast neutrons, and it is also possible to increase the intensity of the epithermal neutron beam emitted from the neutron moderating irradiation device 2. Further, the neutron deceleration irradiation device 2 can be made smaller and lighter. More specifically, the length (L _D ) is preferably 1 cm or more and 10 cm or less, more preferably 1 cm or more and 4.5 cm or less, and further preferably 3.5 cm or more and 4.5 cm or less. preferable.

The shielding material (shielding portion) 23A mainly shields fast neutron rays and gamma rays emitted from the moderators (21A, 21B, 21C). The shielding material 23A is arranged on the downstream side of the downstream moderator 21C. Specifically, the shielding material 23A is in close contact with the downstream end surface of the downstream moderator 21C. The material and thickness of the shielding material 23A are the same as those of the shielding material 23 in the above-described embodiment. It is particularly preferable that the shielding material 23A includes a shielding material made of cadmium or a cadmium alloy. Further, it may be constituted only by a shielding material made of cadmium or a cadmium alloy, or a shielding material made of bismuth may be used together.

According to the neutron moderator/irradiator 2B according to the present embodiment described above, the downstream moderator 21C is provided on the downstream side of the moderator main body 21A, so that the neutrons incident on the opening 124a of the collimator section are shaped and are effective. Will be slowed down. Therefore, it is possible to emit a high intensity epithermal neutron beam while surely reducing the mixing rate of fast neutrons. That is, the moderator can be made thin while reducing the mixing ratio of fast neutrons, and the neutron moderating and irradiating device 2 can be made small and lightweight. As an example, the thickness of the moderator body 21A of the _{(L B),} as compared with the embodiment shown in FIG. 3, it is possible to reduce the order of 5.5cm from 3.6 cm.

The neutron moderating irradiation apparatus according to the above-described embodiment can be variously modified, replaced with a configuration, omitted with a configuration, and the like without departing from the scope of the present invention.

For example, the configurations of the neutron moderating irradiation device 2, the neutron moderating irradiation device 2A, and the neutron moderating irradiation device 2B can be mutually replaced. For example, the downstream moderator 21C may be added to the configuration shown in FIG. By adding the downstream moderator 21C, it is possible to reduce the speed while shaping the neutrons that have entered the opening 124a of the collimator section. Further, at this time, in the upstream side member 24A, the inner side forming the peripheral wall of the opening 124a of the collimator section is provided with a thick wall, and the outer side away from the opening 124a of the collimator section is provided with a smaller wall thickness than the inner side, and the upstream side member 24A is provided with a thin wall. A shielding material or a reflecting material may be provided on the thinned portion.

In addition, the moderators (21A, 21B) in the above-described embodiment are arranged in direct contact with the reflector 22. However, the moderator (21A, 21B) may be provided with a reinforcing material along the contact surface between the moderator (21A and 21B). The reinforcing material can be formed of, for example, a light metal such as aluminum or an aluminum alloy, another metal material, or a carbon material. The thickness of the reinforcing material can be, for example, in the range of 0.5 cm or more and 2 cm or less. By covering the moderator (21A, 21B) with the reinforcing material, it is possible to stabilize the structure of the neutron moderating irradiation device.

Further, the moderators (21A, 21B) in the above embodiment are arranged in a cylindrical shape or a polygonal cylinder shape, and the moderators (21C) are arranged in a truncated cone shape. However, the moderators (21A, 21B, 21C) may be discontinuously arranged on the longitudinal section or the transverse section as long as the general arrangement is maintained. For example, a plurality of partially circular or rod-shaped moderators (21A, 21B) may be arranged at intervals in the circumferential direction to form a substantially circular shape. A reflective material or the like can be placed in the space provided.

Hereinafter, the present invention will be specifically described with reference to examples, but the technical scope of the present invention is not limited thereto.

The results of the simulation analysis of the geometric structure of the neutron moderating irradiation device are shown. In the following analysis, calculation simulation is performed by the Monte Carlo method.

The calculation code is PHITS (ver. 2.660), and the nuclear data is ENDF/B-VII. 1 was used. As the neutron source, a flat lithium target having a diameter of 15 cm was set near the upstream end face of the moderator, and the neutron energy and emission angle were calculated using LIYILED. In addition, as the target, titanium 0.01 mm, lithium 0.14 mm, tantalum 0.36 mm, copper 1.5 mm, water 5.0 mm, and copper 3.0 mm are laminated in this order from the upstream side to the downstream side. I assumed. Further, neutrons are assumed to be generated with a Gaussian distribution having a half width of 3.75 cm at a depth of 0.0011 cm from the upstream end surface of the target flat plate.

The performance of the moderator was evaluated for the neutron beam emitted from the neutron moderating and irradiating device and measured in "Tally" using the design target value set by the IAEA as an index. Specifically, the following epithermal neutron intensity, fast neutron mixing rate, current/flux ratio, and neutron air dose rate were evaluated. The number of calculated particles was set to 10 ⁸ at which the relative uncertainty of the evaluation item was less than 3%. When the statistics were insufficient, the number of calculated particles was increased and analysis was performed. Note that “Tally” is a virtual detection site on the simulation. “Tally” is a circular flat surface with a diameter of 10 cm, and is arranged concentrically at a position separated by 2.5 cm from the exit of the collimator section to detect emitted neutron rays and the like.

Epithermal neutron intensity (N _epi ) is defined as the integral of the neutron flux above 0.5 eV and with energies below 10 keV. For the evaluation of epithermal neutron intensity (N _epi ), the target value was 1×10 ⁹ [n/cm ² /s] or more.

The fast neutron mixing ratio (D _f ) is a ratio of absorbed dose and epithermal neutron flux given by fast neutrons, and is defined by the following mathematical formula 1. N _fast is the fast neutron intensity, and K _n is the neutron kerma _coefficient . For the evaluation of the fast neutron contamination rate (D _f ), the target value was 2×10 ⁻¹³ [Gy/cm ² ] or less.

The gamma ray contamination rate (D _g ) is the ratio of absorbed dose given by gamma rays and epithermal neutron flux, and is defined by the following mathematical formula 2. G is the gamma ray dose in total energy, and _Kg is the gamma ray kerma coefficient. For the evaluation of the gamma ray contamination rate (D _g ), the target value was 2×10 ⁻¹³ [Gy/cm ² ] or less.

The thermal neutron ratio (N _t/e ) is a ratio of thermal neutron flux and epithermal neutron flux, and is defined by the following mathematical formula 3. The thermal neutron ratio (N _t/e ) was evaluated with a target value of 0.05 or less.

The current/flux ratio (C/F ratio) is an index showing the straightness of the neutron beam and is defined by the following mathematical formula 4. θ corresponds to the incident angle to “Tally”, and represents the angle formed by the Tally normal line and the incident direction of the neutron beam. The current/flux ratio (C/F ratio) was evaluated with a target value of 0.7 or more.

The neutron air dose rate (S _n ) is an index showing the air dose rate distribution around the exit of the collimator unit, and is defined by the following mathematical formula 5. r is a radial distance with the origin of the center of "Tally". W _R is a radiation weighting coefficient defined by the following Equation 6. In the evaluation of the neutron air dose rate (S _n ), 0.01 or less was set as a target value at r=25 cm.

First, the shape of the collimator section was evaluated. Specifically, the inlet diameter of the opening 124a of the collimator portion (the inner diameter of the upstream end surface of the upstream member 24A (D ₁ )) and the outlet diameter (the inner diameter of the downstream end surface of the downstream member 24B (D ₂ )) are variable. The analysis was performed by changing the thickness of the moderator so that the fast neutron mixing rate (D _f ) satisfies the target value of 2×10 ⁻¹³ [Gy/cm ² ] or less.

The neutron moderator/irradiator is provided with only the moderator material body 21A having a diameter of 50 cm as the moderator material and not the upstream moderator material 21B in the mode shown in FIG. As the shielding material 23, a 0.1 cm cadmium plate and a 1 cm bismuth plate are laminated in this order from the upstream side, and the collimator section has a 5 cm lithium fluoride-polyethylene upstream member 24A and a 5 cm lead downstream. It is configured by the side member 24B. The total diameter of the neutron moderating irradiation device was 100 cm, and the opening of the reflecting material 22 including the target 5 was 20 cm in diameter and 37 cm in length.

FIG. 5 is a diagram showing a relationship between the entrance diameter and the exit diameter of the collimator section and the epithermal neutron intensity. FIG. 6 is a diagram showing the relationship between the entrance diameter and the exit diameter of the collimator section and the current/flux ratio.
5 and 6, the horizontal axis represents the inlet diameter of the collimator portion (the inner diameter of the upstream end surface of the upstream member 24A (D ₁ )) [cm]-the outlet diameter (the inner diameter of the downstream end surface of the downstream member 24B ( D ₂ )) [cm] is shown. 5, the vertical axis represents epithermal neutron intensity (N _epi ) [×10 ⁹ n/cm ² /s], and the vertical axis in FIG. 6 represents current/flux ratio (C/F ratio). ..

As shown in FIG. 5 and FIG. 6, the epithermal neutron intensity (N _epi ) is maximum at the inlet diameter 50 [cm]−the outlet diameter 10 [cm], and the current/flux ratio (C/F ratio) is indicated by a broken line. Exceeded the target value shown. Further, in any combination, the gamma ray contamination rate (D _g ) and the thermal neutron ratio (N _t/e ) satisfied the target values. However, the neutron air dose rate (S _n ) was about 0.1, which did not satisfy the target value.

Subsequently, the inlet diameter of the opening 124a of the collimator section (the inner diameter (D ₁ ) of the upstream end surface of the upstream member 24A) is fixed to 50 cm, and the outlet diameter (the inner diameter (D ₂ ) of the downstream end surface of the downstream member 24B). ) Was changed and analyzed.

FIG. 7: is a figure which shows the relationship between the exit diameter of a collimator part, and an epithermal neutron beam intensity. FIG. 8 is a diagram showing the relationship between the exit diameter of the collimator section and the current/flux ratio.
7 and 8, the horizontal axis represents the exit diameter of the collimator portion (the inner diameter (D ₂ ) of the downstream end surface of the downstream member 24B) [cm]. 7, the vertical axis represents epithermal neutron intensity (N _epi ) [×10 ⁹ n/cm ² /s], and the vertical axis in FIG. 8 represents current/flux ratio (C/F ratio). ..

As shown in FIG. 7, the epithermal neutron intensity (N _epi ) is high when the outlet diameter is 9 to 15 cm. On the other hand, as shown in FIG. 8, the current/flux ratio (C/F ratio) exceeded the target value indicated by the broken line when the outlet diameter was about 11 cm or less. As described above, the combination of the inlet diameter of 50 cm and the outlet diameter of 10 cm was excellent.

Next, the structure of the collimator section was evaluated. Specifically, the thickness of the downstream side inner member 24C (the charged particle beam 6 of the charged particle beam 6 and the case where the downstream side inner member 24C made of lithium fluoride-polyethylene is provided and the case where the downstream side inner member 24C made of lead is provided. Analysis was performed by changing the length in the irradiation direction (L ₂ ).

The neutron moderator/irradiator is provided with only the moderator material main body 21A having a diameter of 50 cm and not the upstream moderator 21B in the configuration shown in FIG. The shielding material 23 is formed of a 0.01 cm cadmium plate, and the collimator section has a lead upstream member 24A having a length (L ₁ ) in the irradiation direction of 10 cm and a peripheral wall thickness (R ₂ ) of 5 cm. The downstream inner member 24C and the downstream outer member 24D made of lithium fluoride-polyethylene. The total diameter of the neutron moderating irradiation device was 100 cm, and the opening of the reflecting material 22 including the target 5 was 20 cm in diameter and 37 cm in length.

FIG. 9 is a diagram showing the relationship between the structure of the collimator section and the epithermal neutron intensity. FIG. 10 is a diagram showing the relationship between the structure of the collimator section and the current/flux ratio.
9 and 10, the horizontal axis represents the thickness of the downstream inner member 24C of the collimator section (the length (L ₂ ) in the irradiation direction of the charged particle beam 6) [cm], and the plot of ○ is lithium fluoride. -When the downstream inner member 24C made of polyethylene is provided, the plot of ● indicates the case where the downstream inner member 24C made of lead is provided. 9, the vertical axis represents epithermal neutron intensity (N _epi ) [×10 ⁹ n/cm ² /s], and the vertical axis in FIG. 10 represents current/flux ratio (C/F ratio). ..

As shown in FIG. 9, in the range of the analyzed thickness (length (L ₂ ) in the irradiation direction of the charged particle beam 6), the downstream inner member 24C is made of lithium fluoride-polyethylene, and The epithermal neutron intensity (N _epi ) exceeded the target value in all cases made of lead. However, when the downstream inner member 24C made of lead was provided, the epithermal neutron intensity (N _epi ) was further increased. On the other hand, as shown in FIG. 10, the current/flux ratio (C/F ratio) was lower when the downstream inner member 24C made of lead was provided than when it was made of lithium fluoride-polyethylene. However, by increasing the thickness, the current/flux ratio (C/F ratio) exceeded the target value shown by the broken line. The target value was satisfied in all cases for the gamma ray contamination rate (D _g ). On the other hand, the thermal neutron ratio (N _t/e ) satisfied the target value only when the downstream inner member 24C made of lead was provided.

Next, the shape of the moderator was evaluated. A neutron moderator/irradiator including a moderator main body 21A and an upstream moderator 21B as a moderator was compared with that without the upstream moderator 21B and evaluated for each direction component emitted by the neutron source. Specifically, when the neutron source emits in all directions from the generation point, when it emits only to the downstream side (0 to 90 sr) from the generation point, and when it emits only to the upstream side (90 to 180 sr) from the generation point Each was analyzed.

In addition, the neutron moderating irradiation apparatus is provided with a moderator body 21A having a diameter of 50 cm and a thickness of 31.5 cm in the form shown in FIG. The upstream moderator 21B had a length (L _U ) in the irradiation direction of the charged particle beam 6 of 20 cm and a peripheral wall thickness (R _U ) of 15 cm. The shielding material 23 is formed of a 0.01 cm cadmium plate, and the collimator portion has a lead upstream member 24A having a length (L ₁ ) in the irradiation direction of 10 cm and a length (L ₂ ) in the irradiation direction. is 5 cm, the thickness of the peripheral wall (R ₂₎ and a lead made of the downstream side inner member 24C of 5 cm, lithium fluoride - was constituted by the downstream-side outer member 24D made of polyethylene. The total diameter of the neutron moderating irradiation device was 100 cm, and the opening of the reflecting material 22 including the target 5 was 20 cm in diameter and 37 cm in length.

FIG. 11 is a diagram showing the relationship between the moderator shape and the epithermal neutron beam intensity. FIG. 12 is a diagram showing the relationship between the moderator shape and the fast neutron mixing rate.
In FIG. 11 and FIG. 12, “absent” indicates a mode without the upstream moderator 21B, and “present” indicates a mode including the upstream moderator 21B. In the case where the upstream moderator 21B is not provided, the relevant portion is replaced with the reflector 22. "All" means that the neutron source emits in all directions, "Downstream" means that the neutron source emits only to the downstream side, and "Upstream" means that the neutron source emits only to the upstream side. That is the case. The vertical axis in FIG. 11 represents the epithermal neutron intensity (N _epi ) [×10 ⁹ n/cm ² /s], and the vertical axis in FIG. 12 represents the fast neutron mixing rate [×10 ⁻¹³ Gy·cm]. ² ] is shown.

As shown in FIG. 11, by providing the upstream moderator 21B, the epithermal neutron intensity (N _epi ) was increased particularly for the upstream component. Further, as shown in FIG. 12, the provision of the upstream moderator 21B reduced the fast neutron mixing ratio (D _f ). In particular, the fast neutron mixing ratio (D _f ) of the upstream component is remarkably reduced, and the neutrons emitted to the upstream side are scattered by the upstream moderator 21B and emitted along a long track. Was confirmed.

Then, the length (L _U ) of the upstream moderator 21B in the irradiation direction of the charged particle beam 6 and the thickness (R _U ) of the peripheral wall are used as variables, and the fast neutron mixing rate (D _f ) is 2×10 ^−. Analysis was performed by changing the thickness of the moderator so that the target value of ¹³ [Gy/cm ² ] or less was satisfied.

FIG. 13 is a diagram showing the relationship between the thickness and length of the upstream moderator and the epithermal neutron intensity. FIG. 14 is a diagram showing the relationship between the thickness and length of the upstream moderator and the neutron air dose ratio.
13 and 14, the horizontal axis represents the thickness (R _U ) [cm] of the peripheral wall of the upstream moderator 21B−the length (L _U ) [cm] of the upstream moderator 21B. The vertical axis in FIG. 13 shows the epithermal neutron intensity (N _epi ) [×10 ⁹ n/cm ² /s], and the vertical axis in FIG. 14 shows the neutron air dose rate (S _n ).

As shown in FIGS. 13 and 14, by providing the upstream moderator 21B, the epithermal neutron intensity (N _epi ) is higher than that in the case where the upstream moderator 21B is not provided (“0-0” in the figure). ) Became higher, and the neutron air dose rate (S _n ) became lower. Further, by providing the upstream moderator 21B, the fast neutron mixing rate (D _f ), the gamma ray mixing rate (D _g ), the thermal neutron ratio (N _t/e ) and the current/flux ratio (C/F ratio) are , All met the target value. And the thickness of the moderator main body 21A satisfies the target value of the fast neutron mixing rate (D _f ) at 32.7 cm when the upstream moderator 21B is not provided (“0-0” in the figure). When the upstream moderator 21B was provided, the maximum (“10-20” in the figure) was shortened to 29.3 cm.

Next, the thickness of the downstream moderator was evaluated. Specifically, the thickness of the moderator body 21A (the length in the irradiation direction of the charged particle beam 6) for the neutron moderating and irradiating device including the moderator body 21A, the upstream moderator 21B, and the downstream moderator 21C as the moderator. is _(L B)), and the thickness of the downstream moderator 21C (irradiation length of the charged particle beam 6 _(L D) a) a variable, high-speed neutrons mixing ratio _{(D f)} is, 2 × ^{10 -13} Analysis was performed by changing the thickness of the moderator so as to satisfy the target value of [Gy/cm ² ] or less.

In addition, the neutron moderation irradiation apparatus was equipped with the moderator main body 21A with a diameter of 50 cm in the form shown in FIG. The upstream moderator 21B had a length (L _U ) in the irradiation direction of the charged particle beam 6 of 20 cm and a peripheral wall thickness (R _U ) of 10 cm. The shielding material 23A is formed of a 0.01-cm cadmium plate, and the collimator portion has a lead upstream member 24A having a length (L ₁ ) in the irradiation direction of 10 cm and a length (L ₂ ) in the irradiation direction. is 5 cm, the thickness of the peripheral wall (R ₂₎ and a lead made of the downstream side inner member 24C of 5 cm, lithium fluoride - was constituted by the downstream-side outer member 24D made of polyethylene. The total diameter of the neutron moderating irradiation device was 100 cm, and the opening of the reflecting material 22 including the target 5 was 20 cm in diameter and 37 cm in length.

FIG. 15 is a diagram showing the relationship between the thickness of the downstream moderator and the epithermal neutron intensity. 16 is a diagram showing the relationship between the thickness of the downstream moderator and the thickness of the moderator body.
15 and 16, the horizontal axis represents the thickness of the downstream moderator 21C (irradiation length of the charged particle beam 6 _(L D)) showing a [cm]. 15, the vertical axis represents the epithermal neutron intensity (N _epi ) [×10 ⁹ n/cm ² /s], and the vertical axis in FIG. 16 represents the thickness of the moderator body 21A (charged particle beam 6). Shows the length of the irradiation direction (L _B ).

As shown in FIG. 15, the epithermal neutron intensity (N _epi ) was increased in the range where the thickness of the downstream moderator 21C was approximately 0.5 cm or more and 4.5 cm or less. Then, as shown in FIG. 16, the thickness of the moderator material main body 21A could be reduced as the thickness of the downstream moderator material 21C increased. Specifically, the thickness of the moderator body 21A _{(L B)} were reduced by about 3.6 cm. Further, by providing the downstream moderator 21C, the fast neutron mixing rate (D _f ), gamma ray mixing rate (D _g ), thermal neutron ratio (N _t/e ) and current/flux ratio (C/F ratio) are , All met the target value.

100 Neutron generator 1 Charged particle beam generator 1a Ion source 1b Accelerator 2 Neutron deceleration irradiation device 4 Conduit 5 Target (neutron source)
6 Charged particle beam (proton beam)
7 Focusing lens 9 Neutron beam 21A Moderator main body (main body)
21B upstream moderator (upstream part)
22 reflective material 23 shielding material (shielding portion)
24A upstream member (collimator part)
24B Downstream member (collimator part)
124a opening

Claims (3)

A moderator that decelerates the neutron beam generated by the neutron source by being irradiated with a charged particle beam,
A reflector surrounding the moderator,
A collimator section for shaping the irradiation field of the neutron beam decelerated by the moderator,
The moderator is a main body part arranged on the downstream side of the neutron source in the irradiation direction of the charged particle beam, and an upstream part that is arranged on the upstream side of the main body part in the irradiation direction and surrounds the periphery of the neutron source. It has a door,
The collimator portion has an opening that reduces in diameter in the irradiation direction, an upstream member arranged on the downstream side of the main body portion in the irradiation direction, and the opening on the downstream side of the upstream member in the irradiation direction. A downstream side inner member that forms, and a downstream side of the upstream side member in the irradiation direction, and a downstream side outer member arranged radially outside the downstream side inner member,
The upstream member and the downstream inner member are formed of graphite, iron, lead or beryllium,
The downstream side outer member is a neutron moderating irradiation device formed of cadmium, indium, hafnium, gadolinium, bismuth, lead fluoride, boron compound, paraffin, water, lithium fluoride-polyethylene or boron-polyethylene . Arranged on the downstream side of the moderator in the irradiation direction of the charged particle beam, and comprising a shield part for shielding the fast neutron beam,
The neutron moderation irradiation apparatus according to claim 1, wherein the shielding unit includes a shielding material made of cadmium or a cadmium alloy. The moderator further has a downstream portion arranged in a truncated cone shape from the downstream end of the main body portion toward the downstream side in the irradiation direction,
The neutron deceleration irradiation apparatus according to claim 1 or 2 , wherein the downstream portion is surrounded by the opening included in the collimator portion.

Images