Ion source and neutron capture therapy device

Abstract

To provide an ion source capable of reducing influence of discharge between electrodes, and a neutron capture therapy device.SOLUTION: A plasma electrode 90 includes at least a first electrode member 91 and a second electrode member 92, the first electrode member 91 being a magnetic material. In a radial direction orthogonal to an axial direction, a distance R3' of an end 91b on the outer peripheral side of the first electrode member 91 from an extraction opening 66 is equal to or larger than a distance R1'of an end 44b on the outer peripheral side of a first extraction electrode 44 from the extraction opening 66. Generation of a leakage magnetic field from the outer peripheral side of the first electrode member 91 can be prevented. The prevention of the generation of the leakage magnetic field can prevent generation of discharge between the plasma electrode 90 and the first extraction electrode 44. Further, the second electrode member 92 is a member higher in at least one of melting point and thermal conductivity than the magnetic material. Even when discharge occurs, the second electrode member 92 can protect he first electrode member 91 being the magnetic material.SELECTED DRAWING: Figure 4

Description

Technical Field

[0001] The present invention relates to an ion source and a neutron capture therapy device.

Background Art

[0002] There is known an ion source configured to generate plasma in a plasma chamber and extract ions from the plasma chamber using extraction electrodes (see, for example, Patent Document 1). Ions generated in the plasma chamber are extracted to the outside of the plasma chamber by the potential difference between the plasma electrode and the extraction electrode of the plasma chamber.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] For example, when generating plasma by inputting microwaves, if a magnetic field coaxial with the direction of extracting ions is formed by a coil or the like, the microwave absorption efficiency is improved, and the efficiency of generating ions is improved. However, a strong electric field is applied between the extraction electrode and the plasma electrode, and if a magnetic field leaks to this portion, the following problems occur. That is, electrons move so as to wrap around the magnetic field. Due to such electrons, discharge occurs between the electrodes. When discharge occurs, there may be a problem that the plasma electrode is damaged.

[0005] Therefore, an object of the present invention is to provide an ion source and a neutron capture therapy device capable of reducing the influence of discharge between electrodes.

Means for Solving the Problems

[0006] The ion source according to the present invention includes a plasma electrode provided in a plasma chamber that generates ions by plasma, and an extraction electrode that faces the plasma electrode and extracts ions from the plasma chamber. The plasma electrode has at least a first electrode member and a second electrode member, and an extraction opening for extracting ions is formed. The first electrode member is a magnetic material, and the second electrode member is a member having at least one of a melting point and a thermal conductivity higher than that of the magnetic material. In a second direction orthogonal to a first direction in which the plasma electrode and the extraction electrode face each other, a distance from the extraction opening to an end portion on the outer peripheral side of the first electrode member is equal to or greater than a distance from the extraction opening to an end portion on the outer peripheral side of the extraction electrode.

[0007] In the ion source according to the present invention, the plasma electrode has at least a first electrode member and a second electrode member, and the first electrode member is a magnetic material. Thereby, by allowing the magnetic field to escape to the outer peripheral side with the first electrode member being a magnetic material, it is possible to suppress a leakage magnetic field between the plasma electrode and the extraction electrode. In particular, in a second direction orthogonal to a first direction in which the plasma electrode and the extraction electrode face each other, a distance from the extraction opening to an end portion on the outer peripheral side of the first electrode member is equal to or greater than a distance from the extraction opening to an end portion on the outer peripheral side of the extraction electrode. Thereby, it is possible to suppress the occurrence of a leakage magnetic field from the outer peripheral side of the first electrode member. Thus, by suppressing the occurrence of a leakage magnetic field, it is possible to suppress the occurrence of discharge between the plasma electrode and the extraction electrode. Further, the second electrode member is a member having at least one of a melting point and a thermal conductivity higher than that of the magnetic material. Therefore, even if discharge occurs, the second electrode member can protect the first electrode member, which is a magnetic material. From the above, it is possible to reduce the influence of discharge between electrodes. Further, by suppressing damage to the first electrode member, the frequency of failures and the frequency of maintenance can be reduced.

[0008] The material of the second electrode member may be selected from tungsten, tantalum, and molybdenum. In this case, the second electrode member can have a high melting point.

[0009] The material of the second electrode member may be selected from copper, silver, and aluminum. In this case, the second electrode member can be made to have high thermal conductivity.

[0010] The ion source is provided on the outer peripheral side of the plasma chamber, and further includes a magnetic field generating unit that generates a magnetic field in a first direction in the plasma chamber. The end portion on the outer peripheral side of the first electrode member may be disposed at a position spaced from the magnetic field generating unit toward the inner peripheral side. Even in this case, it is possible to suppress the generation of a leakage magnetic field from the end portion on the outer peripheral side of the first electrode member.

[0011] The ion source is provided on the outer peripheral side of the plasma chamber, and further includes a magnetic field generating unit that generates a magnetic field in a first direction in the plasma chamber. The first electrode member may reach the magnetic field generating unit. In this case, it is possible to further suppress the generation of a leakage magnetic field from the end portion on the outer peripheral side of the first electrode member.

[0012] The magnetic field generating unit has a coil and a yoke, and the first electrode member may have a connecting portion connected to the yoke of the magnetic field generating unit. In this case, the connecting portion can connect the first electrode member to the magnetic circuit with the coil, and further suppress the generation of a leakage magnetic field.

[0013] The neutron capture therapy apparatus according to the present invention is a neutron capture therapy apparatus having an ion source that generates ions, an accelerator that accelerates the ions to emit a particle beam, and an irradiation unit that generates a neutron beam by the particle beam and irradiates an object. The ion source includes a plasma electrode provided in a plasma chamber that generates ions by plasma, and a draw-out electrode that faces the plasma electrode and draws out ions from the plasma chamber. The plasma electrode has at least a first electrode member and a second electrode member, and a draw-out opening for drawing out ions is formed. The first electrode member is a magnetic body, and the second electrode member is a member having at least one of a melting point and thermal conductivity higher than that of the magnetic body. In a second direction orthogonal to a first direction in which the plasma electrode and the draw-out electrode face each other, the distance from the end portion on the outer peripheral side of the first electrode member to the draw-out opening is greater than or equal to the distance from the end portion on the outer peripheral side of the draw-out electrode to the draw-out opening.

[0014] In the neutron capture therapy device according to the present invention, the plasma electrode has at least a first electrode member and a second electrode member, and the first electrode member is a magnetic material. Thereby, by allowing the magnetic field to escape through the first electrode member which is a magnetic material, the leakage magnetic field between the plasma electrode and the extraction electrode can be suppressed. In particular, in a second direction orthogonal to a first direction in which the plasma electrode and the extraction electrode face each other, the distance from the extraction opening at the outer peripheral side end of the first electrode member is equal to or greater than the distance from the extraction opening at the outer peripheral side end of the extraction electrode. Thereby, generation of a leakage magnetic field from the outer peripheral side of the first electrode member can be suppressed. By suppressing the generation of such a leakage magnetic field, generation of discharge between the plasma electrode and the extraction electrode can be suppressed. Further, the second electrode member is a member having at least one of a melting point and a thermal conductivity higher than that of the magnetic material. Therefore, even if discharge occurs, the second electrode member can protect the first electrode member which is a magnetic material. As described above, the influence of discharge between electrodes can be reduced. Further, by suppressing damage to the first electrode member, the frequency of failures that cause the neutron beam not to be irradiated can be reduced, and the maintenance frequency can be reduced.

Effect of the Invention

[0015] According to the present invention, an ion source and a neutron capture therapy device capable of reducing the influence of discharge between electrodes can be provided.

Brief Description of the Drawings

[0016]

Figure 1

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Figure 12

MODE FOR CARRYING OUT THE INVENTION

[0017] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings.

[0018] FIG. 1 is a schematic view showing a neutron capture therapy apparatus 100 including an ion source 10 according to an embodiment of the present invention. The neutron capture therapy apparatus 100 is an apparatus for performing cancer treatment using boron neutron capture therapy (BNCT). The neutron capture therapy apparatus 100 includes a treatment unit 102, a treatment couch 160, and a moving mechanism 110.

[0019] The treatment unit 102 (irradiation unit) has an irradiation port 106 that irradiates the patient 150 with neutron rays N. The treatment unit 102 is composed of a structure that arranges the irradiation port 106 and the moving mechanism 110, etc. The treatment unit 102 is provided in the treatment room 101. The irradiation port 106 is provided in the vertical wall portion of the treatment room 101. Neutron rays N are emitted horizontally from the irradiation port 106. The irradiation port 106 includes a collimator 120 and an irradiation unit peripheral wall 115, which will be described later. The moving mechanism 110 is a mechanism that can move the treatment bed 160 on which the patient 150 is placed in the treatment unit 102. The moving mechanism 110 is provided at a position in front of the irradiation port 106 in the treatment room 101.

[0020] In the treatment unit 102, for example, boron ( 10 B) is administered to irradiate the tumor of the patient 150 with neutron rays N. The irradiation port 106 irradiates the patient 150 placed on the treatment bed 160 with neutron rays N (particle rays).

[0021] The neutron capture therapy device 100 includes an accelerator 112. The accelerator 112 has an ion source 10, which will be described later. The accelerator 112 accelerates the ions generated by the ion source and emits a particle beam R. For example, a cyclotron, a linear accelerator, etc. may be adopted as the accelerator 112.

[0022] The particle beam R emitted from the accelerator 112 passes through a transport path 109 called a beam duct whose inside is kept under vacuum and through which the beam can pass, and is transported to the target placement unit 130. The target placement unit 130 is a part where the target 111 is placed and has a mechanism for holding the target 111 in the posture during irradiation. The target placement unit 130 places the target 111 at a position facing the end (exit) of the transport path 109. The particle beam R emitted from the accelerator 112 travels through the transport path 109 and heads toward the target 111 placed at the end of the transport path 109. A plurality of electromagnets 104 (such as quadrupole electromagnets) and a scanning electromagnet 116 are provided along this transport path 109. The plurality of electromagnets 104 perform, for example, beam axis adjustment of the particle beam R using electromagnets.

[0023] The scanning electromagnet 116 scans the particle beam R and controls the irradiation of the particle beam R onto the target 111. This scanning electromagnet 116 controls the irradiation position of the particle beam R onto the target 111.

[0024] The neutron capture therapy apparatus 100 generates a neutron beam N by irradiating the particle beam R onto the target 111 and emits the neutron beam N toward the patient 150. The neutron capture therapy apparatus 100 includes a target 111, a shielding body 108, a moderator 139, and a collimator 120.

[0025] The target 111 generates a neutron beam N by receiving the irradiation of the particle beam R. The target 111 is a solid member formed of a material that generates a neutron beam N when irradiated with the particle beam R. Specifically, the target 111 is formed of, for example, beryllium (Be), lithium (Li), tantalum (Ta), or tungsten (W), and has, for example, a disk shape with a diameter of 160 mm. Note that the target 111 is not limited to a disk shape and may have other shapes.

[0026] The moderator 139 decelerates the neutron beam N generated by the target 111 (reduces the energy of the neutron beam N). The moderator 139 may have a laminated structure including a layer 139A that mainly decelerates fast neutrons contained in the neutron beam N and a layer 139B that mainly decelerates epithermal neutrons contained in the neutron beam N.

[0027] The shielding body 108 shields the generated neutron beam N and gamma rays and the like generated along with the generation of the neutron beam N from being emitted to the outside. The shielding body 108 is provided so as to surround the moderator 139. The upper and lower portions of the shielding body 108 extend upstream of the particle beam R from the moderator 139.

[0028] The collimator 120 shapes the irradiation field of the neutron beam N and has an irradiation port 120a through which the neutron beam N passes. The collimator 120 is, for example, a block-shaped member having the irradiation port 120a at the center. The collimator 120 is attached to the irradiation part peripheral wall 115 which is a wall part at the location where the neutron beam N is irradiated into the treatment room 101.

[0029] The moving mechanism 110 is a so-called six-axis mechanism that moves the treatment table 160 on which the patient 150 is placed in six axial directions. The moving mechanism 110 enables horizontal movement and rotational movement of the treatment table 160. In the present embodiment, the moving mechanism 110 supports the patient 150 placed on the treatment table 160 and moves the treatment table 160 together with the patient 150.

[0030] Next, with reference to FIG. 2, the detailed configuration of the ion source 10 will be described. FIG. 2 is a schematic configuration diagram of the ion source 10. As shown in FIG. 2, the ion source 10 is an ion source that generates high-density plasma by inputting microwave power in the direction of magnetic field lines into a plasma chamber 12 to which a magnetic field satisfying the electron cyclotron resonance (ECR) condition or a magnetic field higher than that is applied, and extracts ions. The ion source 10 is configured to generate plasma of a raw material gas by the interaction between a magnetic field and microwaves, and extract ions from the plasma to the outside of the plasma chamber 12.

[0031] The strength of the magnetic field satisfying the ECR condition is uniquely determined with respect to the frequency of the microwave used. When the microwave frequency is, for example, 2.45 GHz, a magnetic field of 87.5 mT (875 gauss) is required. Hereinafter, for convenience of explanation, the magnetic field satisfying the ECR condition may be referred to as a resonance magnetic field.

[0032] The ion source 10 includes an ion source main body 14. The ion source main body 14 includes a plasma chamber 12, a magnetic field generator 16 (magnetic field generation part), and a vacuum vessel 18.

[0033] The plasma chamber 12 has a cylindrical shape with both ends. The direction from one end to the other end of the plasma chamber 12 may be referred to as the axial direction for convenience hereinafter. Also, the direction orthogonal to the axial direction may be referred to as the radial direction, and the direction surrounding the axial direction may be referred to as the circumferential direction. However, these do not necessarily mean that the plasma chamber 12 has a rotationally symmetric shape. The axial direction corresponds to the "first direction" in the claims. The radial direction corresponds to the "second direction" in the claims. In the illustrated example, the plasma chamber 12 has a cylindrical shape, but the plasma chamber 12 may have any shape as long as it can appropriately contain the plasma. Also, the axial length of the plasma chamber 12 may be longer or shorter than the radial length of the end portion of the plasma chamber 12.

[0034] The magnetic field generator 16 is provided to apply a magnetic field to the plasma chamber 12. The magnetic field generator 16 is disposed around the plasma chamber 12. The magnetic field generator 16 is configured to generate a magnetic field along the central axis of the plasma chamber 12. The direction of its magnetic field lines is indicated by an arrow M in FIG. 2. The magnetic field generator 16 is configured to generate a resonance magnetic field or a magnetic field of higher intensity at at least a part on the axis of the plasma chamber 12. The magnetic field generator 16 can also generate a magnetic field lower than the resonance magnetic field at at least a part on the axis of the plasma chamber 12.

[0035] The vacuum vessel 18 is a housing for accommodating the plasma chamber 12 in a vacuum environment. The vacuum vessel 18 is also a structure for holding the magnetic field generator 16. The plasma chamber 12 has a vacuum window 24 for receiving microwaves inside. The plasma chamber 12, the magnetic field generator 16, and the vacuum vessel 18 will be described in more detail later.

[0036] The ion source 10 includes a microwave supply system 26. The microwave supply system 26 is configured to input microwave power into the plasma chamber 12 through the vacuum window 24. The microwave supply system 26 includes a microwave source 28, a waveguide 30, and a matching section 32. The microwave source 28 is, for example, a magnetron. The microwave source 28 outputs microwaves having a frequency of, for example, 2.45 GHz. The waveguide 30 is a three-dimensional circuit for transmitting the microwaves output from the microwave source 28 to the plasma chamber 12. One end of the waveguide 30 is connected to the microwave source 28, and the other end is connected to the vacuum window 24 via the matching section 32. The matching section 32 is provided for microwave matching.

[0037] In this way, microwaves are introduced from the microwave supply system 26 into the plasma chamber 12 through the vacuum window 24. The introduced microwaves propagate inside the plasma chamber 12 toward the end of the plasma chamber 12 facing the vacuum window 24. The propagation direction of the microwaves is indicated by an arrow P in FIG. 2. The propagation direction P of the microwaves is the same as the magnetic field line direction M by the magnetic field generator 16. Therefore, the propagation direction P of the microwaves coincides with the axial direction of the plasma chamber 12.

[0038] Also, the microwave supply system 26 includes a microwave detector 33 provided in the waveguide 30. The microwave detector 33 includes, for example, a directional coupler for monitoring the incident power to the plasma chamber 12 and the reflected power from the plasma chamber 12. The microwave detector 33 is configured to output the measurement results to the control device C.

[0039] The ion source 10 includes a gas supply system 34. The gas supply system 34 is configured to supply the raw material gas for the plasma to the plasma chamber 12. The gas supply system 34 includes a gas cylinder 36 as a gas source and a gas flow controller 38. The tip of the gas pipe 40 of the gas supply system 34 is connected to the plasma chamber 12 through the vacuum vessel 18. The gas pipe 40 is connected to, for example, the side wall 64 of the plasma chamber 12. The gas flow controller 38 includes an on-off valve for connecting or disconnecting the gas cylinder 36 to the plasma chamber 12, or a flow control valve for adjusting the gas flow rate from the gas cylinder 36 to the plasma chamber 12. Thus, the raw material gas is supplied from the gas cylinder 36 to the plasma chamber 12 at a controlled flow rate.

[0040] The ion source body 14 includes an extraction electrode system 42. The extraction electrode system 42 is configured to extract ions from the plasma through the extraction opening 66 of the plasma chamber 12. The extraction electrode system 42 includes a first extraction electrode 44 and a second extraction electrode 46. The first extraction electrode 44 is provided between the plasma chamber 12 and the second extraction electrode 46. The terminal portion 62 having the extraction opening 66 and the first extraction electrode 44 are arranged with a gap therebetween, and the first extraction electrode 44 and the second extraction electrode 46 are arranged with a gap therebetween. The first extraction electrode 44 and the second extraction electrode 46 are each formed, for example, in an annular shape and have an opening portion for passing the ions extracted from the plasma chamber 12 at the center portion.

[0041] The first extraction electrode 44 is provided to extract positive ions from the plasma and to prevent electrons from returning from the beam line 52 to the plasma chamber 12. For this purpose, a negative high voltage is applied to the first extraction electrode 44. A first extraction power supply 48 is provided to apply a negative high voltage to the first extraction electrode 44. The second extraction electrode 46 is grounded. Also, a positive high voltage is applied to the vacuum vessel 18. A second extraction power supply 50 is provided to apply a positive high voltage to the vacuum vessel 18. The absolute value of the positive high voltage applied to the vacuum vessel 18 is larger than the absolute value of the negative high voltage applied to the first extraction electrode 44. In this way, an ion beam 20 of positive ions is extracted from the plasma chamber 12. The extraction direction of the ion beam 20 from the plasma chamber 12 is the same as the propagation direction P of the microwave.

[0042] The ion source 10 is provided with a beam line 52 for transporting the ion beam 20 extracted by the extraction electrode system 42. The beam line 52 is connected to the ion source body 14 on the side opposite to the microwave supply system 26. The beam line 52 is a vacuum vessel that communicates with the vacuum vessel 18. The beam line 52 is insulated from the vacuum vessel 18 of the ion source body 14 and is attached to the vacuum vessel 18. For this purpose, a bushing 54 is provided between the beam line 52 and the vacuum vessel 18.

[0043] The bushing 54 maintains the vacuum inside the beam line 52 and the vacuum vessel 18 while maintaining the withstand voltage between the vacuum vessel 18 and the ground side. The bushing 54 is formed of an insulating material. The bushing 54 has an annular shape and surrounds the extraction electrode system 42. The bushing 54 is sandwiched and attached between the mounting flanges of the vacuum vessels of the beam line 52 and the ion source body 14 respectively.

[0044] A vacuum evacuation system 56 is provided for providing a vacuum environment to the vacuum vessel 18 and the plasma chamber 12. In the illustrated example, the vacuum evacuation system 56 is provided in the beam line 52. Since the beam line 52 communicates with the vacuum vessel 18 and the plasma chamber 12, the vacuum evacuation system 56 can evacuate the vacuum vessel 18 and the plasma chamber 12. The vacuum evacuation system 56 includes a high-vacuum pump such as a cryopump or a turbomolecular pump, for example.

[0045] The ion source 10 may include a control device C for controlling the output of the ion beam 20. The control device C controls each component of the ion source 10, controls the plasma generated in the plasma chamber 12, and thereby controls the output of the ion beam 20. The control device C is configured to control the operation of, for example, the microwave supply system 26, the gas supply system 34, and the coil power supply 76. The control device C may control the output of the ion beam 20 by adjusting at least one of, for example, the flow rate of the source gas, the microwave power, and the magnetic field strength.

[0046] The plasma chamber 12 is configured to generate and maintain plasma in its internal space. The internal space of the plasma chamber 12 may hereinafter be referred to as the plasma generation space 58.

[0047] The plasma chamber 12 includes a starting end portion 60, an ending end portion 62, and a side wall 64. The starting end portion 60 and the ending end portion 62 face each other with the plasma generation space 58 therebetween. The side wall 64 surrounds the plasma generation space 58 and connects the starting end portion 60 and the ending end portion 62. In this way, the plasma generation space 58 is defined inside the vacuum vessel 18 by the starting end portion 60, the ending end portion 62, and the side wall 64. When the plasma chamber 12 has a cylindrical shape, the starting end portion 60 and the ending end portion 62 are disc-shaped, the side wall 64 is cylindrical, and the ends of the side wall 64 are fixed to the outer peripheral portions of the starting end portion 60 and the ending end portion 62.

[0048] The starting end portion 60 has a vacuum window 24. The vacuum window 24 may occupy the entire starting end portion 60, or may be formed in a part (for example, the central part) of the starting end portion 60. One side of the vacuum window 24 faces the plasma generation space 58, and the other side of the vacuum window 24 is directed toward the microwave supply system 26. The vacuum window 24 seals the inside of the plasma chamber 12 in a vacuum. The propagation direction P of the microwave is perpendicular to the vacuum window 24. The vacuum window 24 is formed of a dielectric with low dielectric loss (for example, alumina or boron nitride, etc.). Note that the portions of the plasma chamber 12 other than the vacuum window 24 are formed of a non-magnetic metal material such as stainless steel or aluminum.

[0049] At least one extraction opening 66 is formed in the ending end portion 62. The extraction opening 66 is formed at a position facing the vacuum window 24 with the plasma generation space 58 interposed therebetween. That is, the vacuum window 24, the plasma generation space 58, and the extraction opening 66 are arranged along the axial direction of the plasma chamber 12.

[0050] The vacuum vessel 18 has a double cylinder structure in which the plasma chamber 12 is integrally formed. That is, the plasma chamber 12 is the inner cylinder of the vacuum vessel 18, and an outer cylinder 68 that houses the plasma chamber 12 is provided outside thereof. The outer cylinder 68 may have a cylindrical shape coaxial with the plasma chamber 12. There is a gap between the outer cylinder 68 and the side wall 64 of the plasma chamber 12, and the tip of the gas pipe 40 of the gas supply system 34 described above enters this gap and is attached to the side wall 64. The vacuum vessel 18 is formed of a non-magnetic metal material, for example.

[0051] The vacuum vessel 18 may not be integrally formed with the plasma chamber 12. The vacuum vessel 18 and the plasma chamber 12 may be separate bodies and separable. Also, the vacuum vessel 18 itself may form the plasma chamber 12. When the vacuum vessel 18 also serves as the plasma chamber 12 in this way, an end plate having the extraction opening 66 may be attached to the beam line 52 side of the outer cylinder 68.

[0052] One end of the vacuum vessel 18 is closed by an end plate 70, and the other end is open toward the beam line 52. The starting end portion 60 of the plasma chamber 12 is formed at the center of the end plate 70. The outer peripheral portion of the end plate 70 extends radially outward to the outside of the outer cylinder 68. An attachment flange 72 for the bushing 54 is provided at the end of the vacuum vessel 18 on the beam line 52 side. The attachment flange 72 extends radially outward from the outer cylinder 68. The vacuum vessel 18 and the plasma chamber 12 have the same axial length, and the attachment flange 72 and the terminal end portion 62 of the plasma chamber 12 are in the same axial position. The vacuum vessel 18 and the plasma chamber 12 may have different axial lengths.

[0053] A magnet holding portion 74 for holding the magnetic field generator 16 is formed in the vacuum vessel 18. The magnet holding portion 74 is formed, for example, on the outer surface of the outer cylinder 68 of the vacuum vessel 18. In this embodiment, the magnetic field generator 16 is provided outside the vacuum vessel 18 (i.e., in the atmosphere). The magnetic field generator 16 is arranged so as to surround the vacuum vessel 18. However, in another example, the vacuum vessel 18 may include a magnet holding portion 74 for holding the magnetic field generator 16 inside the vacuum vessel 18 (i.e., in a vacuum). In this case, the same effects as in this example can be obtained. In this way, the magnetic field generator 16 is arranged so as to surround the plasma generation space 58.

[0054] The magnetic field generator 16 includes a coil configured to generate a magnetic field directed along the axial direction of the plasma chamber 12. In this example, the plasma chamber 12 and the vacuum vessel 18 are cylindrical, the coil is formed in an annular shape, and a conductor is wound in the circumferential direction of the plasma chamber 12. The magnetic field generator 16 includes a coil power supply 76 for passing an electric current through the coil. The number of coils included in the magnetic field generator 16 is not particularly limited, and it may include one coil or a plurality of coils arranged along the axial direction of the plasma chamber 12.

[0055] FIG. 3 is a schematic configuration diagram showing an enlarged view of the vicinity of the plasma chamber of the ion source 10. As shown in FIG. 3, the ion source 10 includes the above-described plasma chamber 12 that generates ions by plasma, and extraction electrodes 44 and 46 for extracting ions to the outside from the extraction opening 66 of the plasma chamber 12. The plasma chamber 12 includes a terminal portion 62 that defines the end of the plasma generation space 58 in the axial direction. The terminal portion 62 faces the first extraction electrode 44 with an extraction gap 78 in the axial direction therebetween. In FIG. 3, the central axis CL1 of the plasma chamber 12 is shown. The direction in which the central axis CL1 extends is the axial direction. Also, among the axial directions, the side from which ions are extracted may be referred to as the “downstream” side, and the opposite side may be referred to as the “upstream” side. Further, the side away from the central axis CL1 may be referred to as the “outer circumference” side, and the side approaching may be referred to as the “inner circumference” side.

[0056] On the other hand, a vacuum window 24 is provided at the start end portion 60 of the plasma chamber 12. The vacuum window 24 has a two-layer structure including a window main body 80 and a window protective material 82. The window protective material 82 is the inner layer of the vacuum window 24 facing the plasma generation space 58, and the window main body 80 is the outer layer of the vacuum window 24 adjacent to the waveguide 30 side of the window protective material 82. The window protective material 82 covers the window main body 80 in order to protect the window main body 80 from electrons flowing back into the plasma chamber 12 from the outside of the plasma chamber 12 through the extraction opening 66. The window main body 80 is, for example, an alumina plate, and the window protective material 82 is, for example, a boron nitride plate. In addition, in order to protect the side wall 64 of the plasma chamber 12 from plasma, a liner (for example, made of boron nitride) covering the inner surface of the side wall 64 may be provided.

[0057] A magnetic field generator 16 that generates a magnetic field in the axial direction is provided around the side wall 64 of the plasma chamber 12. The magnetic field generator 16 includes a doughnut-shaped coil 84 that surrounds the plasma chamber 12 with the central axis CL1 as the center, and a yoke 86 attached to the coil 84. The yoke 86 is provided adjacent to the outer circumference and both axial ends of the coil 84. As the material of the yoke 86, a magnetic material such as iron may be adopted.

[0058] The terminal portion 62 includes an end wall 69 and a plasma electrode 90. The end wall 69 is a wall that projects inward from the downstream end of the side wall 64. The plasma electrode 90 is provided in the plasma chamber 12 and is an electrode having an extraction opening 66 that opens axially from the plasma chamber 12. The plasma electrode 90 extends perpendicular to the axial direction toward the outer peripheral side around the central axis CL1. The plasma electrode 90 has an axially symmetric shape with respect to the central axis CL1. The above-described extraction opening 66 that penetrates axially is formed at the center of the plasma electrode 90. A high voltage is applied to the plasma electrode 90 (the second electrode member 92) by the above-described second extraction power supply 50 (see FIG. 2).

[0059] The plasma electrode 90 includes a first electrode member 91, a second electrode member 92, and a third electrode member 93. The second electrode member 92 is provided on the downstream side in the axial direction of the first electrode member 91. The third electrode member 93 is provided on the upstream side in the axial direction of the first electrode member 91.

[0060] The first electrode member 91 is a member for suppressing the leakage magnetic field from the plasma chamber 12. The first electrode member 91 is a magnetic material. Specifically, as the magnetic material, a soft magnetic material such as iron may be adopted. In addition, as the magnetic material, cobalt, nickel, etc. may be adopted. The second electrode member 92 is a member for protecting the first electrode member 91 from the discharge when discharge occurs between the first extraction electrode 44 and the plasma electrode 90. The second electrode member 92 is a member having at least one of a melting point and a thermal conductivity higher than that of the magnetic material of the first electrode member 91. A non-magnetic material may be adopted as the second electrode member 92. As the material of the second electrode member 92, a material selected from high melting point materials such as tungsten, tantalum, and molybdenum may be adopted. As the material of the second electrode member 92, a material with high thermal conductivity may be a material selected from copper, silver, and aluminum. The third electrode member 93 is a member for protecting the first electrode member 91 from the plasma. As the third electrode member 93, for example, an insulator having plasma resistance such as boron nitride or alumina may be adopted.

[0061] The first extraction electrode 44 is axially opposed to the plasma electrode 90 at a position spaced downstream from the plasma electrode 90. The second extraction electrode 46 is axially opposed to the first extraction electrode 44 at a position spaced downstream from the first extraction electrode 44. The extraction electrodes 44 and 46 have openings 44a and 46a through which the extracted ions pass. In the present embodiment, the extraction electrodes 44 and 46 have a conical shape such that they face upstream in the axial direction as they extend from the outer peripheral side toward the inner peripheral side. Therefore, the first extraction electrode 44 is closest to the plasma electrode 90 at a position near the opening 44a.

[0062] Next, FIG. 4 is a cross-sectional view showing the configuration of the plasma electrode 90 shown in FIG. 3 in more detail. With reference to FIG. 4, the positional relationship of the plasma electrode 90 will be described. A reference line SL1 extending in the axial direction (the first direction) is set with respect to the outer peripheral side end portion 44b of the first extraction electrode 44. The radius (outer diameter) of the outer peripheral side end portion 44b of the first extraction electrode 44, that is, the distance from the central axis CL1 of the end portion 44b in the radial direction (the second direction) is defined as the distance R1. A reference line SL2 extending in the axial direction is set with respect to the inner peripheral side end portion 84a of the coil 84. The radius (inner diameter) of the inner peripheral side end portion 84a of the coil 84, that is, the distance from the central axis CL1 of the end portion 84a in the radial direction is defined as the distance R2. The radial distances of the reference lines SL1 and SL2 from the central axis CL1 are constant at each position in the axial direction.

[0063] The extraction opening 66 is formed by a through-hole that axially penetrates the end wall 69, the first electrode member 91, the second electrode member 92, and the third electrode member 93. In the present embodiment, the extraction opening 66 is configured by a circular through-hole centered on the central axis CL1. However, the extraction opening 66 may have any shape as long as it can extract ions from the plasma chamber 12, and the shape, size, etc. are not particularly limited. As shown in FIG. 4, the inner diameters of the extraction openings 66 formed in the end wall 69, the first electrode member 91, the second electrode member 92, and the third electrode member 93 do not have to be the same as each other and may have different inner diameters. The extraction opening 66 has an inner peripheral edge 66a. The inner peripheral edge 66a is the location where the inner diameter of the extraction opening 66 is the smallest. In the present embodiment, the tip of the inner peripheral edge of the second electrode member 92 becomes the inner peripheral edge 66a. However, the inner peripheral edge of another member may become the inner peripheral edge 66a. A reference line SL3 extending in the axial direction (the first direction) is set with respect to the inner peripheral edge 66a of the extraction opening 66. The radius (outer diameter) of the outer peripheral side end 44b of the first extraction electrode 44, that is, the distance from the extraction opening 66 (here, the reference line SL3) of the end 44b in the radial direction (the second direction) is defined as the distance R1'. The radius (inner diameter) of the inner peripheral side end 84a of the coil 84, that is, the distance from the extraction opening 66 (here, the reference line SL3) of the end 84a in the radial direction is defined as the distance R2'. The radial distances of the reference lines SL1 and SL2 from the reference line SL3 are constant at each position in the axial direction.

[0064] The first electrode member 91 of the plasma electrode 90 has a disc-shaped configuration that expands radially about the central axis CL1. The first electrode member 91 has an inner peripheral side end 91a and an outer peripheral side end 91b. The positional relationship of the inner peripheral side end 91a of the first electrode member 91 can be appropriately changed within a range that does not affect the extraction of ions. In the example shown in FIG. 4, it is disposed on the inner peripheral side of the inner peripheral side end 69a of the end wall 69 and on the outer peripheral side of the inner peripheral side openings 44a and 46a of the extraction electrodes 44 and 46.

[0065] In the radial direction, the distance R3 from the central axis CL1 to the outer peripheral side end portion 91b of the first electrode member 91 is equal to or greater than the distance R1 from the central axis CL1 to the outer peripheral side end portion 44b of the first lead electrode 44. In the radial direction, the distance R3' from the extraction opening 66 (here, the reference line SL3) to the outer peripheral side end portion 91b of the first electrode member 91 is equal to or greater than the distance R1' from the extraction opening 66 (here, the reference line SL3) to the outer peripheral side end portion 44b of the first lead electrode 44. That is, the outer peripheral side end portion 91b of the first electrode member 91 is disposed at the same position as the outer peripheral side end portion 44b of the first lead electrode 44 in the radial direction, or on the outer peripheral side of the end portion 44b. In the example shown in FIG. 4, the outer peripheral side end portion 91b of the first electrode member 91 is disposed on the outer peripheral side of the outer peripheral side end portion 69b of the end wall 69.

[0066] In the radial direction, the distance R3 from the central axis CL1 to the outer peripheral side end portion 91b of the first electrode member 91 is equal to or less than the distance R2 from the central axis CL1 to the inner peripheral side end portion 84a of the coil 84. In the radial direction, the distance R3' from the extraction opening 66 (here, the reference line SL3) to the outer peripheral side end portion 91b of the first electrode member 91 is equal to or less than the distance R2' from the extraction opening 66 (here, the reference line SL3) to the inner peripheral side end portion 84a of the coil 84. That is, the outer peripheral side end portion 91b of the first electrode member 91 is disposed at the same position as the inner peripheral side end portion 84a of the coil 84 in the radial direction, or on the inner peripheral side of the end portion 84a.

[0067] Therefore, the outer peripheral side end portion 91b of the first electrode member 91 may be disposed on the reference line SL1, on the reference line SL2, or in the region between the reference line SL1 and the reference line SL2 in the radial direction. FIG. 5 shows a state in which the outer peripheral side end portion 91b of the first electrode member 91 is disposed at the same position (on the reference line SL1) as the outer peripheral side end portion 44b of the first lead electrode 44 in the radial direction. FIG. 6 shows a state in which the outer peripheral side end portion 91b of the first electrode member 91 is disposed at the same position (on the reference line SL2) as the inner peripheral side end portion 84a of the coil 84 in the radial direction. In this state, the first electrode member 91 has reached the magnetic field generator 16.

[0068] The second electrode member 92 of the plasma electrode 90 has a substantially disk-shaped form that extends radially about the central axis CL1. The second electrode member 92 has a portion that protrudes toward the downstream side in the axial direction near the outer peripheral end portion 92b, but the shape is not particularly limited. The second electrode member 92 has an inner peripheral end portion 92a and an outer peripheral end portion 92b. The positional relationship of the inner peripheral end portion 92a of the second electrode member 92 can be appropriately changed within a range that does not affect the extraction of ions, but it is arranged so that a radius necessary for the extraction opening 66 can be secured. Further, the inner peripheral end portion 92a of the second electrode member 92 may be arranged on the inner peripheral side of the inner peripheral end portion 91a of the first electrode member 91 so as to protect against discharge from the first extraction electrode 44. The outer peripheral end portion 92b of the second electrode member 92 is arranged so as to cover the first electrode member 91 within a range where discharge from the first extraction electrode 44 can occur. In the example shown in FIG. 4, the outer peripheral end portion 92b of the second electrode member 92 is arranged on the inner peripheral side of the outer peripheral end portion of the first extraction electrode 44.

[0069] The third electrode member 93 of the plasma electrode 90 has a disk-shaped form that extends radially about the central axis CL1. The third electrode member 93 has an inner peripheral end portion 93a and an outer peripheral end portion 93b. The positional relationship of the inner peripheral end portion 93a of the third electrode member 93 can be appropriately changed within a range that does not affect the extraction of ions, but it is arranged at a position where the first electrode member 91 is not exposed so as to protect the first electrode member 91 from the plasma. Further, the outer peripheral end portion 93b of the third electrode member 93 is arranged on the outer peripheral side of the inner peripheral end portion 69a of the end wall 69 so as to protect the first electrode member 91 from the plasma.

[0070] The operation and effects of the ion source 10 and the neutron capture therapy apparatus 100 according to the present embodiment will be described.

[0071] In the ion source 10 according to this embodiment, the plasma electrode 90 has at least a first electrode member 91 and a second electrode member 92, and the first electrode member 91 is a magnetic material. Thereby, by allowing the magnetic field to escape to the outer peripheral side with the first electrode member 91 which is a magnetic material, it is possible to suppress the leakage magnetic field between the plasma electrode 90 and the first extraction electrode 44. In particular, in the radial direction (second direction) orthogonal to the axial direction (first direction) in which the plasma electrode 90 and the first extraction electrode 44 face each other, the distance R3' from the extraction opening 66 (here, the reference line SL3) of the outer peripheral side end 91b of the first electrode member 91 is equal to or greater than the distance R1' from the extraction opening 66 (here, the reference line SL3) of the outer peripheral side end 44b of the first extraction electrode 44. Thereby, it is possible to suppress the generation of a leakage magnetic field from the outer peripheral side of the first electrode member 91. For example, when the outer diameter of the magnetic material of the first electrode member 91 is smaller than the outer diameter of the first extraction electrode 44, there is a possibility that a leakage magnetic field is generated from the outer peripheral side end 91b of the first electrode member 91 into the space between the electrodes. In this embodiment, by suppressing the generation of the leakage magnetic field, it is possible to suppress the occurrence of discharge between the plasma electrode 90 and the first extraction electrode 44. Further, the second electrode member 92 is a member having at least one of a melting point and a thermal conductivity higher than that of a magnetic material. During discharge, electrons run from the extraction electrode 44 toward the plasma electrode 90, and the plasma electrode 90 is locally heated at the discharge location. If the heated location is a magnetic material, damage such as melting and cracking of the material occurs due to an instantaneous temperature rise. On the other hand, in this embodiment, even when discharge occurs, the second electrode member 92 can protect the first electrode member 91 which is a magnetic material. Even if the temperature locally rises due to discharge, a material with a high melting point can withstand the temperature rise. Also, if it is a material with high thermal conductivity, heat can be quickly diffused even if the temperature rises. As described above, the influence of discharge between the electrodes can be reduced. Further, by suppressing damage to the first electrode member 91, the frequency of failures and the frequency of maintenance can be reduced.

[0072] Here, with reference to FIGS. 7 to 9, the relationship between the outer diameter of the first electrode member 91 and the leakage magnetic field suppression effect will be described. FIG. 7 is a diagram showing the result of creating a model of the ion source 10 and simulating a two-dimensional magnetic field calculation system. In FIG. 7, the darker the color, the stronger the magnetic field. In this simulation, "FEMM" was used as the calculation software. As shown in FIG. 8, models of Examples 1 to 5 were prepared. Example 1 is a model in the case where the outer diameter of the first electrode member 91 is approximately the same as the outer diameter of the first extraction electrode 44. Examples 2 to 4 are models in which the outer diameter of the first electrode member 91 is gradually increased. Example 5 is a model in the case where the outer diameter of the first electrode member 91 is the same as the inner diameter of the coil 84. FIG. 9 is a graph showing the magnetic field strength on the central axis CL in Examples 1 to 5. The region "E1" in the graph indicates the beam extraction region indicated by "E1" in FIG. 7. "E2" in the graph indicates the region of the plasma chamber 12 indicated by "E2" in FIG. 7. As shown in FIG. 9, as the outer diameter of the magnetic body is increased, the generation of the leakage magnetic field in the beam extraction region can be reduced.

[0073] The material of the second electrode member 92 may be selected from tungsten, tantalum, and molybdenum. In this case, the second electrode member 92 can be made to have a high melting point.

[0074] The material of the second electrode member 92 may be selected from copper, silver, and aluminum. In this case, the second electrode member 92 can be made to have high thermal conductivity.

[0075] The ion source 10 is provided on the outer peripheral side of the plasma chamber 12 and further includes a magnetic field generator 16 that generates an axial magnetic field in the plasma chamber 12, and the outer peripheral side end portion 91b of the first electrode member 91 may be disposed at a position spaced apart from the magnetic field generator 16 toward the inner peripheral side. Even in this case, it is possible to suppress the generation of a leakage magnetic field from the outer peripheral side end portion 91b of the first electrode member 91.

[0076] The ion source 10 is provided on the outer peripheral side of the plasma chamber 12 and further includes a magnetic field generator 16 that generates an axial magnetic field in the plasma chamber 12. The first electrode member 91 may reach the magnetic field generator 16. In this case, it is possible to further suppress the generation of a leakage magnetic field from the end portion 91b on the outer peripheral side of the first electrode member 91.

[0077] The neutron capture therapy apparatus 100 according to the present embodiment includes an ion source 10 that generates ions, an accelerator 112 that accelerates the ions and emits a particle beam, and an irradiation unit that generates a neutron beam by the particle beam and irradiates a subject. The ion source 10 includes a plasma electrode 90 provided in a plasma chamber 12 that generates ions by plasma, and a first extraction electrode 44 that faces the plasma electrode 90 and extracts ions from the plasma chamber 12. The plasma electrode 90 has at least a first electrode member 91 and a second electrode member 92, and an extraction opening 66 for extracting ions is formed. The first electrode member 91 is a magnetic body, and the second electrode member 92 is a member having at least one of a melting point and a thermal conductivity higher than that of the magnetic body. In the radial direction (second direction) orthogonal to the axial direction (first direction) in which the plasma electrode 90 and the first extraction electrode 44 face each other, the distance R3' from the extraction opening 66 (here, the reference line SL3) of the end portion 91b on the outer peripheral side of the first electrode member 91 is equal to or greater than the distance R1' from the extraction opening 66 (here, the reference line SL3) of the end portion 44b on the outer peripheral side of the first extraction electrode 44.

[0078] In the neutron capture therapy apparatus 100 according to the present embodiment, the plasma electrode 90 has at least a first electrode member 91 and a second electrode member 92, and the first electrode member 91 is a magnetic material. Thereby, by allowing the magnetic field to escape to the outer peripheral side by the first electrode member 91 which is a magnetic material, the leakage magnetic field between the plasma electrode 90 and the first extraction electrode 44 can be suppressed. In particular, in the radial direction (second direction) orthogonal to the axial direction (first direction) in which the plasma electrode 90 and the first extraction electrode 44 face each other, the distance R3' from the extraction opening 66 (here, the reference line SL3) of the outer peripheral side end portion 91b of the first electrode member 91 is not less than the distance R1' from the extraction opening 66 (here, the reference line SL3) of the outer peripheral side end portion 44b of the first extraction electrode 44. Thereby, generation of a leakage magnetic field from the outer peripheral side of the first electrode member 91 can be suppressed. Thus, by suppressing the generation of the leakage magnetic field, generation of discharge between the plasma electrode 90 and the first extraction electrode 44 can be suppressed. Further, the second electrode member 92 is a member having at least one of a melting point and a thermal conductivity higher than those of the magnetic material. Therefore, even if discharge occurs, the first electrode member 91 which is a magnetic material can be protected by the second electrode member 92. As described above, the influence of the discharge between the electrodes can be reduced. Further, by suppressing damage to the first electrode member 91, the frequency of failures that cause the neutron beam N not to be irradiatable can be reduced, and the frequency of maintenance can be reduced.

[0079] The present invention is not limited to the above-described embodiment.

[0080] For example, as shown in Example 6 of FIG. 10(b), the first electrode member 91 may have a connection portion 95 connected to the yoke 86 of the magnetic field generator 16. In Example 6, the first electrode member 91 reaches the coil 84, and the end portion 91b and the yoke 86 are connected by the connection portion 95. In this case, the connection portion 95 can connect the first electrode member 91 to the magnetic circuit with the coil 84, and further suppress the generation of leakage magnetic fields. FIG. 10(a) shows a form in which the first electrode member 91 reaches the coil 84 but does not have the connection portion 95, which corresponds to Example 5 in FIG. 8. As shown in FIG. 11, compared with Example 5 without the connection portion 95, Example 6 with the connection portion 95 can significantly reduce the leakage magnetic field in the beam extraction region. Thus, even if the outer diameter of the magnetic body is the same, the degree of reduction of the leakage magnetic field can be further improved depending on the shape. Thus, the shape of the magnetic body is not limited to a disk, and an overall axisymmetric shape provided with a portion protruding in the axial direction may be adopted.

[0081] In the above-described embodiment, the extraction electrodes 44 and 46 were inclined, but they may be in a disk shape extending straight in the radial direction. When the extraction electrodes 44 and 46 are in a disk shape, discharge is likely to occur also on the outer peripheral side. Therefore, it is preferable that the second electrode member 92 covers the first electrode member 91 until it reaches the position on the outer peripheral side of the extraction electrode 44.

[0082] The second electrode member 92 is not limited to the above-described embodiment. For example, as shown in FIG. 12, the second electrode member 92 may have a multilayer structure. Here, the second electrode member 92 has a first layer 92A with high thermal conductivity on the upstream side and a second layer 92B with a high melting point on the downstream side. The temperature of the second layer 92B on the downstream side rises instantaneously during discharge. Therefore, by setting the second layer 92B to have a high melting point, the second electrode member 92 with excellent durability can be obtained.

[0083] In the above-described embodiment, the ion source 10 was applied to the accelerator 112 of the neutron capture therapy apparatus 1. However, the use of the ion source 10 is not limited, and for example, it is used as an ion source for an ion implantation apparatus or another particle beam therapy apparatus. The ion source 10 is used, for example, as a monovalent ion source. Further, the ion source 10 can also be used as an ion source for a proton accelerator or an X-ray source.

[0084] In the above-described ion source 10, positive ions were extracted, but whether the generated ions are positive ions or negative ions is not particularly limited.

[0085] Further, the ion source 10 is not limited to a microwave ion source, and can be applied to ion sources using magnetic fields in general (e.g., ECR ion sources).

Explanation of Reference Numerals

[0086] 10... ion source, 16... magnetic field generator (magnetic field generation unit), 44... first extraction electrode, 84... coil, 86... yoke, 90... plasma electrode, 91... first electrode member, 92... second electrode member, 100... neutron capture therapy apparatus, 102... treatment unit (irradiation unit), 112... accelerator.

Claims

1. A plasma electrode provided in a plasma chamber that generates ions by plasma, and a extraction electrode that faces the plasma electrode and extracts the ions from the plasma chamber, comprising: the plasma electrode has at least a first electrode member and a second electrode member, and a extraction opening for extracting the ions is formed, the first electrode member is a magnetic material, the second electrode member is a member having at least one of a melting point and a thermal conductivity higher than that of the magnetic material, in a second direction orthogonal to a first direction in which the plasma electrode and the extraction electrode face each other, a distance from the extraction opening of an outer peripheral side end portion of the first electrode member is equal to or greater than a distance from the extraction opening of an outer peripheral side end portion of the extraction electrode, an ion source.

2. The ion source according to claim 1, wherein the material of the second electrode member is selected from tungsten, tantalum, and molybdenum.

3. The ion source according to claim 1, wherein the material of the second electrode member is selected from copper, silver, and aluminum.

4. further comprising a magnetic field generation unit provided on an outer peripheral side of the plasma chamber and generating a magnetic field in the first direction in the plasma chamber, the outer peripheral side end portion of the first electrode member is disposed at a position spaced inward from the magnetic field generation unit, the ion source according to claim 1.

5. further comprising a magnetic field generation unit provided on an outer peripheral side of the plasma chamber and generating a magnetic field in the first direction in the plasma chamber, the first electrode member reaches the magnetic field generation unit, the ion source according to claim 1.

6. the magnetic field generation unit has a coil and a yoke, the first electrode member has a connection portion connected to the yoke of the magnetic field generation unit, the ion source according to claim 5.

7. an accelerator having an ion source that generates ions and accelerating the ions to emit a particle beam, and a neutron capture therapy apparatus comprising an irradiation unit that generates a neutron beam by the particle beam and irradiates an object, wherein the ion source is a plasma electrode provided in a plasma chamber that generates ions by plasma, and a extraction electrode that faces the plasma electrode and extracts the ions from the plasma chamber, comprising: the plasma electrode has at least a first electrode member and a second electrode member, and a extraction opening for extracting the ions is formed, the first electrode member is a magnetic material, The second electrode member is a member having at least one of a melting point and a thermal conductivity higher than that of the magnetic material. A neutron capture therapy device in which, in a second direction orthogonal to a first direction in which the plasma electrode and the extraction electrode face each other, a distance from the extraction opening of an outer peripheral side end of the first electrode member is equal to or greater than a distance from the extraction opening of an outer peripheral side end of the extraction electrode.

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