JP6644980B2 - Neutron shielding structure and neutron shielding method - Google Patents

Description

  The present invention relates to a technique for shielding a neutron beam emitted from a source installed in an acceleration tube in a facility where an accelerator is used, for example, for radiotherapy, and more specifically, a cone shape or a truncated cone. TECHNICAL FIELD The present invention relates to a neutron shielding structure, a local shield, and a neutron shielding method using a local shield having a shape and a box shape.

  Radiation therapy is one of the cancer treatments along with surgery and chemotherapy, and is a treatment that shrinks the tumor by irradiating the affected part of the body with radiation. There are various types of radiation for irradiation, and they are appropriately used depending on the state of the tumor. For example, X-rays and gamma rays are used when the tumor is at a shallow position because the radiation dose decreases as it goes deeper into the body. If X-rays or gamma rays are used in cases where the tumor is deep inside the body, the radiation dose will be insufficient when the tumor reaches the essential tumor, and it will not only be effective, but it will also destroy the normal cells near the surface. On the other hand, proton beams and heavy ion beams emit a large amount of radiation at a distance corresponding to the energy to be irradiated, and can be effectively used for tumors deep inside the body.

  In heavy ion therapy and proton beam therapy, heavy particles and protons are accelerated by an accelerator until they have enough energy to reach deep inside the body. The sufficiently accelerated heavy particles and protons are sent to the patient in the treatment room as heavy particles and proton beams. Accelerators such as cyclotrons and synchrotrons are mainly used as accelerators for applying energy to heavy particles and protons.

  FIG. 8 is a plan view showing a schematic configuration of the synchrotron. As shown in this figure, the synchrotron includes an accelerating tube arranged in a substantially circular shape, a bending electromagnet, a high-frequency accelerating cavity, and the like. Usually, a proton beam or the like accelerated in advance by a linear accelerator or the like is incident on the synchrotron, and travels in a substantially vacuum accelerating tube. A bending electromagnet is provided for advancing a proton beam or the like in a circular trajectory, and a high-frequency accelerating cavity provided with a potential difference further promotes acceleration of the proton beam or the like. The proton beam and the like orbit around the accelerating tube and, when having sufficient energy, are emitted toward the treatment room.

  By the way, in order to measure the velocity of a proton beam or the like or to adjust the velocity of a proton beam or the like, a metal lump may be installed in the acceleration tube. When a proton beam or the like collides with this mass, a neutron beam is emitted. In addition, since it becomes a starting point from which a neutron beam is emitted, a lump installed for speed measurement or speed adjustment is called a “source”.

  Conventionally, the neutron shielding structure in the synchrotron has been constructed on a relatively large scale, for example, in a shape surrounding a radiation source. In Patent Document 1, a case where an electron beam traveling in an accelerator tube of a synchrotron deviates from a normal orbit due to a power failure or the like is assumed. The technology has been proposed to block (compartment) and inject water into the interior.

JP-A-06-215897

  The technique proposed in Patent Literature 1 utilizes the characteristic that neutron rays are hardly permeated through water, but specifies a place where neutron rays are likely to be emitted, and provides a shut-off valve for sealing the surrounding area with water. Large-scale facilities are required, such as constructing or installing a water tank that constantly stores water, and providing an open valve for discharging water in the water tank. In addition to detecting the danger of water leakage and the sign of a power outage or a sign of a power outage, a device that operates a closing valve and an opening valve in response to this is required, and the cost of installation and maintenance of facilities is large, which is considerable for such facilities. Space is also required.

  Heretofore, a concrete wall of considerable thickness has been constructed in front of the source where proton beams and the like travel. The purpose is to shield the neutron beam emitted from the radiation source with this concrete wall. Generally, however, since a very thick wall of about 2 to 3 m is provided, the construction cost is high and There was a problem that the use space of was restricted.

  An object of the present invention is to solve the problems of the prior art, that is, to shield a neutron beam from a source with a smaller structure than before, and furthermore, to reduce the thickness of a concrete wall for neutron beam shielding. An object of the present invention is to provide a neutron shielding structure, a local shielding body, and a neutron shielding method that can be reduced as compared with the related art.

  The present invention has been made by paying attention to the point that the local shield is installed near the place where the radiation source is installed, and furthermore, the local shield is made into a cone shape or a frustum shape, a box shape, This is based on an unprecedented idea.

  The neutron beam shielding structure of the present invention is a shielding structure for shielding neutron rays from a radiation source with a local shield. The local shield has a cone-shaped or frustum-shaped outer shape made of a material capable of absorbing neutrons, and is arranged in a posture in which the skirt spreads in the particle beam traveling direction. Note that a local shield is provided in front of (in the traveling direction of) the particle source as viewed in the traveling direction of the particle beam.

  The neutron shielding structure of the present invention may be a structure further provided with a shielding wall. This shielding wall is installed ahead of the local shielding body in the particle beam traveling direction. In this case, the neutron beam traveling along the side surface of the local shield collides outside the range of the wall surface of the shield wall, that is, the local shield has a position and a shape such that the shield wall cannot be seen through the local shield. Is installed.

  The neutron beam shielding structure of the present invention may be a structure having a box-shaped local shield. The local shield in this case is made of a material capable of absorbing neutrons, has a box-shaped outer shape in which one side surface of the hollow hexahedron is an opening surface, and the opening surface of the side surface is in front of the particle beam traveling direction. It is arranged in a posture. In this case as well, the local shield is provided in front of (in the traveling direction of) the particle source in the particle beam traveling direction.

  The neutron beam shielding structure of the present invention may have a structure in which a local shield is installed outside the acceleration tube. In this case, the local shield is installed on the linear tangent direction of the acceleration tube at the installation position of the radiation source.

  The neutron beam shielding structure of the present invention may be a structure having a local shield having a through hole. In this case, the local shield is attached to the accelerator by inserting the accelerator tube into the through hole of the local shield.

  The neutron beam shielding structure of the present invention may be a structure further provided with an additional shielding body. The additional shield is made of a material capable of absorbing neutrons and has a cross-sectional shape larger (or equivalent) than the cross section of the accelerator tube. The additional shield is installed in the linear tangent direction of the acceleration tube at the installation position of the radiation source and ahead of the radiation source in the particle beam traveling direction.

  The local shield of the present invention is an object that locally shields a neutron beam from a radiation source, and an iron-based metal including iron or iron oxide, or polyethylene or boron-containing polyethylene, or nickel, or a ferrite-based material, or It is made of epoxy resin or a mixture thereof, and has an outer shape of a cone or a truncated cone.

  The neutron shielding method of the present invention is a method in which neutrons emitted from a radiation source are absorbed and blocked by a local shield. The local shield is made of a material capable of absorbing neutrons, and has a cone shape or a truncated cone shape, and is arranged in a posture in which the skirt spreads in the particle beam traveling direction. In addition, the local shield is installed at a position in front of the radiation source installed in the acceleration tube when viewed in the particle beam traveling direction.

The neutron shielding structure, local shield, and neutron shielding method of the present invention have the following effects.
(1) The neutron beam can be sufficiently shielded simply by attaching the local shield to the accelerator tube or installing it outside the accelerator tube. Therefore, the structure is reduced in size compared to the conventional one, and the cost for equipment and installation is reduced. Can be.
(2) Since the thickness of the neutron shielding concrete wall can be significantly reduced as compared with the related art, the cost for constructing the concrete wall can be suppressed.
(3) Since the shielding structure is reduced in size and the thickness of the concrete wall can be reduced, the use space in the facility is expanded, and the degree of freedom in facility design is improved.

The top view which shows typically the neutron beam shielding structure of this invention installed in the synchrotron. (A) is a top view of the truncated cone-shaped local shield viewed from above, and (b) is a side view showing the truncated cone-shaped local shield attached to the acceleration tube. (A) is a plan view of the box-shaped local shield viewed from above, (b) is a rear view of the box, and (c) is a side view showing the box-shaped local shield attached to the acceleration tube. FIG. 3 is a plan view showing a local shield installed on a linear tangent of an acceleration tube at an installation position of a radiation source. The top view which shows the additional shield installed on the linear tangent of the acceleration tube in the installation position of a radiation source. FIG. 4 is a plan view showing a situation in which a frustum-shaped local shield diffuses neutron rays. (A) is a plan view showing a simulation result when a local shield is not installed, and (b) is a plan view showing a simulation result when a local shield is installed. FIG. 2 is a plan view showing a schematic configuration of a synchrotron.

  An example of an embodiment of a neutron shielding structure, a local shielding body, and a neutron shielding method of the present invention will be described with reference to the drawings.

1. FIG. 1 is a plan view schematically showing a neutron beam shielding structure of the present invention installed in a synchrotron ST. As described above, the synchrotron ST is a kind of a circular accelerator, and is configured by an acceleration tube 10 arranged in a substantially circular shape, a bending electromagnet, a high-frequency acceleration cavity, and the like. In addition, a radiation source 20 which is a lump of metal is installed in a part of the acceleration tube 10. In this figure, for convenience, the radiation source 20 is shown larger than the acceleration tube 10, but is actually installed in the acceleration tube 10. Further, the radiation source 20 is not limited to being installed at one place, but may be provided at a plurality of places.

  Heavy particles and protons accelerated by the linear accelerator are incident on the synchrotron ST. Since heavy particles such as argon and carbon or protons are accelerated in an electrified state, the heavy particles and protons are collectively referred to herein as "charged particles". The accelerated charged particle is incident on the synchrotron ST as a heavy particle beam when it is a heavy particle and as a proton beam when it is a proton.

  A heavy particle beam or a proton beam (hereinafter, collectively referred to as “particle beam”) orbits inside the accelerating tube 10 while maintaining a circular trajectory by the bending electromagnet and further accelerated by the high frequency accelerating cavity. It collides with a radiation source 20 installed in the acceleration tube 10 for measurement or speed adjustment. At this time, since the neutron beam is emitted, the local shield 30 is installed ahead of the radiation source 20 in the particle beam traveling direction. Here, as shown in FIG. 1, the destination of the particle beam is expressed as “forward”. In other words, when it is described as “forward in the particle beam traveling direction” or simply “forward”, the particle beam advances in the future. Means the direction to go.

  Hereinafter, the main elements constituting the neutron shielding structure, the local shielding body, and the neutron shielding method of the present invention will be described in detail.

2. 2. Shape of Local Shield FIG. 2 is a model diagram showing a truncated conical local shield 30 of the present invention, wherein FIG. 2 (a) is a plan view seen from above, and FIG. FIG. The local shield 30 shown in this figure has a so-called truncated cone shape obtained by cutting a part of the top of a cone. The local shield 30 of the present invention can be roughly divided into two types in terms of shape. One is a frustum-shaped or cone-shaped local shield 30 (hereinafter, referred to as a “cone-shaped local shield”) also shown in FIG. 31 "), and the other is a hollow box-shaped local shield 30 (hereinafter, referred to as a" box-shaped local shield 32 ") described later.

  The cone-shaped local shield 31 is not limited to the truncated cone shape shown in FIG. 2, and may have a shape (a so-called taper) in which a side surface is inclined so as to spread from the top to the bottom, such as a cone, a pyramid, or a truncated pyramid. Things included. However, when a through hole 31h for inserting into the accelerating tube 10 is provided as described later, a frustum-shaped local shield 31 having a truncated cone shape or a truncated pyramid shape may be used. The conical local shield 31 may have a hollow shape (that is, only the side wall), or a solid shape in which the bottom surface and the inside are made of the same material.

  FIGS. 3A and 3B are model diagrams showing the box-shaped local shield 32, wherein FIG. 3A is a plan view seen from above, FIG. 3B is a rear view, and FIG. 3C is a side view showing a state where the box is attached to the acceleration tube 10. It is. As shown in this figure, the box-shaped local shield 32 has a top surface, a bottom surface, and one side surface (front surface) of a hollow hexahedron (a rectangular parallelepiped or a cube) having four side surfaces, and a shape having the bottom surface as an opening surface. Is presented. In other words, the box-shaped local shield 32 has a hollow box shape formed by an upper surface and three side surfaces.

3. Local Shield Material The local shield 30 (cone-shaped local shield 31 and box-shaped local shield 32) is formed of a material that easily absorbs neutron rays in order to shield neutron rays emitted from the radiation source 20. You. Specifically, the local shield 30 may be formed using a so-called iron-based metal such as iron or iron oxide, polyethylene or boron-containing polyethylene, nickel, a ferrite-based material, an epoxy resin, or a mixture thereof.

4. Posture of Local Shield As shown in FIG. 2B, the conical local shield 31 is arranged in a posture in which the skirt spreads in the particle beam traveling direction. For example, in the case of a truncated cone as shown in FIG. 2, the bottom surface is located forward (in the figure, rightward) in the particle beam traveling direction rather than the top portion, that is, the farther the side surface is, the further away from the accelerating tube 10. Be placed. On the other hand, as shown in FIG. 3 (c), the box-shaped local shield 32 is disposed in such a posture that the opening surface of the side surface is located forward (to the right in the drawing) in the particle beam traveling direction. In other words, if the opening surface of the side surface is "front" and the surface facing this front is "back", the particles are arranged so that the front is the front in the particle beam traveling direction and the rear is the back. The front wall can be omitted because the back wall mainly blocks neutron rays. The bottom surface of the box-shaped local shield 32 in FIG. 3 is omitted because it is assumed that the box-shaped local shield 32 is placed directly on the floor surface. However, when the box-shaped local shield body 32 is installed at a high position away from the floor surface, the bottom surface may be provided.

5. Installation of Local Shield When the local shield 30 shields neutrons emitted from the radiation source 20, the local shield 30 is naturally installed ahead of the radiation source 20 in the particle beam traveling direction. The inventor has confirmed that when the distance L (shown in FIG. 2B) from the radiation source 20 to the local shield 30 is set to 70 to 180 cm, a more effective shielding effect is obtained. Further, the local shield 30 can be attached to and installed on the acceleration tube 10, or can be installed at a position off the acceleration tube 10.

  When attaching to the acceleration tube 10, it is good to provide a through-hole in the local shield 30. In the case of the conical local shield 31, as shown in FIG. 2A, a through-hole 31 h is provided at the top (to penetrate to the bottom in the case of a solid shape) as shown in FIG. For example, as shown in FIG. 3B, a through hole 32h is provided in the rear wall. Then, the local shielding body 30 is accelerated by inserting the acceleration tube 10 into the through-hole 31h (through-hole 32h) while maintaining the posture in which the through-hole 31h (through-hole 32h) is located rearward in the particle beam traveling direction. Attach to tube 10. In addition, since the weight of the local shield 30 is considerably large, avoid depositing a load only on the accelerating tube 10 and provide an auxiliary means for supporting the local shield 30 such as separately supporting from below or hanging from above. It is good to provide.

  When the local shield 30 is installed at a position off the acceleration tube 10, the local shield 30 is placed on the floor or the local shield 30 is suspended from above. Of course, also in this case, the local shield 30 is installed at a position ahead of the radiation source 20 in the particle beam traveling direction and at a height equivalent to the installation height of the acceleration tube 10. By the way, as shown in FIG. 1, there are some places where the linear shape of the accelerator tube 10 constituting the circular accelerator is curved. Therefore, the local shield 30 may be installed on the linear tangent (tangential direction) of the acceleration tube 10 at the installation position of the radiation source 20, as shown in FIG. It is considered that the neutron beam emitted from the radiation source 20 mainly advances in the tangential direction shown in FIG. is there. At this time, the local shield 30 may be installed at a position 70 to 180 cm in front of the radiation source 20.

6. As shown in FIGS. 2 and 3, if a through-hole is provided in the local shield 30, the through-hole portion cannot sufficiently shield the neutron beam. Therefore, an additional shield can be provided separately from the local shield 30 in order to supplement the through-hole range. Since the additional shield also shields the neutron beam, the same material as the local shield 30, that is, a so-called iron-based metal such as iron or iron oxide, polyethylene, boron-containing polyethylene, nickel, a ferrite-based material, an epoxy resin, Alternatively, an additional shield may be formed by using a mixture of these materials.

  Since the additional shield is used to supplement the through-hole range (that is, the cross section of the acceleration tube 10) of the local shield 30, it is preferable that the additional shield has the same shape and area as the cross section of the acceleration tube 10. Alternatively, an additional shield having a cross-sectional shape slightly larger than the cross section of the accelerating tube 10 may be used. If the cross section of the acceleration tube 10 is circular, the external shape of the additional shield can be, for example, a columnar shape. Like the cone-shaped local shield 31, the additional shield can be formed in a hollow shape (that is, only the outer wall), and the inside can be formed in a solid shape made of the same material.

  As shown in FIG. 2B, the additional shield 40 is installed at a position in front of the local shield 30 in the particle beam traveling direction and at the same height as the installation height of the acceleration tube 10. That is, when the additional shield 40 is attached to the acceleration tube 10, the additional shield 40 is inserted into the interior of the acceleration tube 10, but this impedes the progress of the particle beam and becomes a new neutron beam source. There is also a risk. Therefore, the additional shield 40 is installed outside the acceleration tube 10, and specifically, is mounted on the floor or suspended from above. In this case as well, similarly to the case where the local shield 30 is attached outside the acceleration tube 10, as shown in FIG. 5, the additional shield 40 is placed on the linear tangent (tangential direction) of the acceleration tube 10 at the installation position of the radiation source 20. Should be installed.

7. Diffusion by the cone-shaped local shield The cone-shaped local shield 31 is arranged in a flared attitude toward the particle beam traveling direction, so that it can absorb the neutron beam that has collided and diffuse a part thereof widely. . FIG. 6 is a plan view showing a situation where the cone-shaped local shield 31 diffuses a neutron beam. In this figure, the range in which the neutron beam spreads is indicated by a broken line, and it can be seen that the neutron beam spreads along the slope of the side surface of the conical local shield 31.

  Also, as shown in FIGS. 1 and 6, the neutron beam diffused by the cone-shaped local shield 31 is directed more outward than the wall surface of the concrete shielding wall 50 constructed in front (the range visible from the radiation source 20). ing. As a result, the neutron beam hitting the concrete shielding wall 50 is significantly reduced, that is, the wall thickness of the concrete shielding wall 50 can be reduced. In this manner, the neutron beam traveling along the side surface of the cone-shaped local shield 31 collides outside the wall area of the concrete shielding wall 50, that is, the concrete shielding wall 50 cannot be seen through the cone-shaped local shield 31. The conical local shield 31 may be provided with such a position or shape (particularly, inclination of the side surface).

8. Experimental Results Hereinafter, experimental results performed by the inventor of the present application to confirm the effects of the present invention will be described.

  FIGS. 7A and 7B are plan views showing simulation results performed to confirm the neutron shielding effect of the present invention. FIG. 7A shows the result when the local shield 30 is not installed, and FIG. The result when the body 30 is installed is shown. In FIG. 7A, the range over which the neutron beam exceeds the reference is 3 m thick, whereas in FIG. 7B, it is understood that the range is suppressed to 2.25 m. That is, when the local shield 30 of the present invention is not installed, a concrete shielding wall 50 having a wall thickness of 3 m must be constructed. However, when the local shield 30 of the present invention is installed, the concrete shielding wall 50 has a thickness of 2.25 m. It can be seen that the present invention greatly contributes to the reduction of the wall thickness of the concrete shielding wall 50.

  The neutron shielding structure, local shield, and neutron shielding method of the present invention can be effectively used in various facilities where neutron radiation is emitted, including medical institutions that perform heavy ion beam therapy or proton beam therapy. Can be. The present invention is intended to solve the problems presently present at medical sites equipped with synchrotrons, that is, not only can be used industrially but also contribute significantly to society, considering that it will contribute to the further spread of radiation therapy. It is an invention that can be expected.

DESCRIPTION OF SYMBOLS 10 Accelerator tube 20 Source 30 Local shield 31 Conical local shield 31h Through-hole (of cone local shield) 32 Box-shaped local shield 32h Through-hole (of box-shaped local shield) 40 Additional shield 50 Concrete Shielding wall ST synchrotron

Claims (8)

In a shielding structure in which a neutron beam emitted when a particle beam traveling in an acceleration tube of a circular accelerator collides with a radiation source is shielded by a local shield,
It is made of a material capable of absorbing neutrons, and comprises the frustum- shaped local shield,
The local shield is located in a position where the local shield is located forward of the radiation source as viewed in the particle beam traveling direction and in a position spaced apart from the radiation source in a posture in which the skirt spreads toward the particle beam traveling direction. Is placed,
By penetrating the accelerating tube into the through-hole provided in the local shield, the local shield was attached to the accelerating tube,
A neutron beam shielding structure, characterized in that: The separation between the local shield and the source is 70-180 cm,
2. The neutron shielding structure according to claim 1, wherein: Further comprising a shielding wall installed in front of the local shielding body in the particle beam traveling direction,
A straight line extending along the side surface of the local shield, the local shield was installed so as not to intersect with the shielding wall,
The neutron shielding structure according to claim 1 or 2, wherein: In a shielding structure for shielding a neutron beam emitted when a particle beam traveling in an acceleration tube of a circular accelerator collides with a radiation source arranged in the acceleration tube by a local shield,
A box-shaped local shield formed of a material capable of absorbing neutrons, wherein one side of the hollow hexahedron is an open surface and is formed by three sides ,
The local shield, before KiHiraki port surface is arranged in a posture which is a particle beam traveling forward,
Seen in the particle beam traveling direction, the local shield was installed ahead of the radiation source,
A neutron beam shielding structure, characterized in that: The local shield has a through hole,
The local shield is disposed at a position spaced from the source,
By inserting the acceleration tube into the through-hole, the local shield was attached to the acceleration tube,
The neutron shielding structure according to claim 4 , characterized in that: An additional shield made of a material capable of absorbing neutrons and having a cross-sectional shape larger than or equal to the cross-section of the accelerator tube is further provided.
The additional shield is installed in a linear tangent direction of the accelerator tube at the installation position of the radiation source, and is installed ahead of the radiation source in the particle beam traveling direction.
The neutron shielding structure according to any one of claims 1 to 5, wherein The additional shield is placed on the floor or suspended from above, at a position off the acceleration tube,
The neutron shielding structure according to claim 6, wherein: In a method of locally shielding a neutron beam emitted when a particle beam traveling in an acceleration tube of a circular accelerator collides with a source,
The local shield of Rikiri trapezoidal such a material capable of absorbing neutrons, placed in a posture to be flared toward the particle beam traveling direction,
The accelerating tube is inserted into a through-hole provided in the local shield at a position in front of the radiation source installed in the accelerating tube as viewed in the particle beam traveling direction and spaced apart from the radiation source. By attaching the local shield to the accelerator tube,
Neutron radiation emitted from the source is absorbed and shielded by the local shield,
A neutron shielding method, comprising:

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