Beam monitor device, accelerator, radiation therapy device, and beam measurement method

Abstract

To provide a beam monitor device which enables a three-dimensional structure of a beam to be grasped.SOLUTION: An imaging unit generates a plurality of photographed images obtained by imaging fluorescent light generated according to a proton beam 103 from each of a plurality of directions intersecting the travel direction of the proton beam 103. A computer 13 acquires a three-dimensional beam distribution being the distribution of the proton beam 103 in the three-dimensional space on the basis of the plurality of photographed images. A computer 14 calculates variance and covariance due to the position and momentum of the proton beam 103 in the two directions intersecting the travel direction of the proton beam 103 on the basis of the three-dimensional beam distribution.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a beam monitor device, an accelerator, a radiation therapy device, and a beam measurement method. [Background technology]

[0002] To stably supply beams from accelerators used in radiation therapy equipment, etc., it is necessary to monitor the beam in real time. Particularly for high-current charged particle beams, the impact of space charge in the space through which the charged particle beam passes is significant, making it important to monitor the state of the charged particle beam.

[0003] Non-Patent Document 1 discloses a technique for measuring the envelope and two-dimensional emittance of a charged particle beam by detecting fluorescence generated by the interaction between the charged particle beam and residual gas on its orbit using a CCD (Charge-Coupled Device) camera installed parallel to the orbit of the charged particle beam. This technique enables real-time monitoring of the charged particle beam by non-contact measurement, in which the measuring instrument does not come into contact with the charged particle beam, thereby enabling real-time monitoring without affecting the charged particle beam. [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] Welsch, Carsten P. "Non-destructive beam profile monitors." Proceedings of the 8th International Particle Accelerator Conference IPAC. 2017, p. 1234-1239. Summary of the Invention [Problem to be solved by the invention]

[0005] In the technology described in Non-Patent Document 1, emittance and the like are measured by projecting the particle distribution of the beam onto a single plane on which a CCD camera is installed, so the particle distribution of the beam cannot be detected three-dimensionally. This makes it impossible to grasp the three-dimensional structure of the beam, and in particular, there is a problem in that it is impossible to measure variance, covariance, emittance, and the like in a four-dimensional phase space consisting of positions and momenta in two different directions that intersect with the traveling direction of the beam.

[0006] During the beam transport process, the interaction between the electric charge of the beam itself and the magnetic field during transport can cause the particle distribution of the beam to become correlated or the beam topology to change in a plane perpendicular to the beam axis. Therefore, in order to accurately understand the beam state, it is important to understand the three-dimensional structure of the beam, especially to measure the variance, covariance, and emittance in four-dimensional phase space.

[0007] An object of the present disclosure is to provide a beam monitor device, an accelerator, a radiotherapy device, and a beam measurement method that are capable of grasping the three-dimensional structure of a beam. [Means for solving the problem]

[0008] A beam monitor device according to one aspect of the present disclosure is a beam monitor device that monitors a charged particle beam, and includes: an imaging unit that generates a plurality of captured images by capturing fluorescence generated in response to the charged particle beam from each of a plurality of directions intersecting the traveling direction of the charged particle beam; an acquisition unit that acquires a three-dimensional beam distribution, which is the distribution of the charged particle beam in three-dimensional space, based on the plurality of captured images; and a calculation unit that calculates variance and covariance due to the position and momentum of the charged particle beam in two directions intersecting the traveling direction, based on the three-dimensional beam distribution. [Effects of the Invention]

[0009] According to the present invention, it is possible to grasp the three-dimensional structure of the beam. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 illustrates a boron neutron capture therapy system according to an embodiment of the present disclosure. [Figure 2] 10 is a flowchart illustrating an example of a beam monitoring process. [Figure 3] FIG. 10 is a diagram showing an example of a three-dimensional beam image. DETAILED DESCRIPTION OF THE INVENTION

[0011] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings.

[0012] Figure 1 is a diagram showing a boron neutron capture therapy system according to an embodiment of the present disclosure. The boron neutron capture therapy system 999 shown in Figure 1 is a type of radiation therapy device that irradiates a patient with radiation to treat an affected area of ​​the patient, such as cancer. Specifically, the boron neutron capture therapy system 999 irradiates a thermal neutron beam as radiation to cancer cells of the patient in which boron has been accumulated by a drug.

[0013] The boron neutron capture therapy system 999 is arranged throughout the accelerator room 1000, accelerator cab 1001 and treatment room (not shown).

[0014] The accelerator room 1000 is a room whose interior is a radiation-controlled area, and is surrounded by a thick concrete shielding wall 1000a to prevent radiation leakage. Access to the accelerator room 1000 is restricted. The accelerator room 1000 also contains, as components of the boron neutron capture therapy system 999, an accelerator 100, a lithium target 107, and a CCD camera 11 and a motor-driven rotating gantry 12 that constitute the beam monitor 1.

[0015] Accelerator operator room 1001 is a room in a non-radiation controlled area located near accelerator room 1000. Accelerator operator room 1001 is equipped with computers 13 and 14, a display device 15, a recording device 16, and a speaker 17 that constitute beam monitor 1 as components of boron neutron capture therapy system 999. Note that accelerator operator 1002, who operates boron neutron capture therapy system 999, stays in accelerator operator room 1001 during radiation therapy. Accelerator operator 1002 grasps the state of the beam based on visual information from display device 15 and auditory information from speaker 17, and performs the operation of accelerator 100.

[0016] The accelerator 100 accelerates and extracts a particle beam. In this embodiment, the accelerator 100 is a proton accelerator that extracts a proton beam 103, which is a charged particle beam, as the particle beam, and accelerates the proton beam 103, which has a current of 25 mA and a kinetic energy of 30 keV, to a kinetic energy of 2.5 MeV and extracts it to a lithium target 107.

[0017] The accelerator 100 includes an ion source 110, a low energy beam transport line 105, and a radio frequency quadrupole linear accelerator 106.

[0018] The ion source 110 is an extraction unit that generates and extracts a proton beam. In the example of Fig. 1, the ion source 110 is an electron cyclotron resonance (ECR) type ion source, and includes an internal plasma chamber (not shown), an extraction electrode 102, and a beam extraction power supply 104.

[0019] In the plasma chamber, hydrogen gas is ionized by a high-frequency voltage to generate hydrogen plasma. Protons in the hydrogen plasma are extracted to the outside of the plasma chamber by a voltage applied to an extraction electrode 102 and emitted as a proton beam 103 into a low-energy beam transport line 105. The proton beam 103 is a collection of protons with momentum. In this embodiment, the proton beam 103 extracted from the plasma chamber has a current of 25 mA and a kinetic energy of 30 keV.

[0020] Specifically, the extraction electrode 102 has two plate electrodes arranged opposite each other, and when a voltage of 30 kV is applied between these plate electrodes, protons in the hydrogen plasma generated in the plasma chamber are accelerated to 30 keV and emitted as a proton beam 103.

[0021] The beam extraction power supply 104 is a high-voltage power supply, and applies a high voltage of 30 kV to the extraction electrode 102. The voltage applied by the beam extraction power supply 104 is controlled by the computer 14 via a cable 54. The cable 54 is, for example, a BNC (Bayonet Neill Concelman) cable.

[0022] The low-energy beam transport line 105 is a transport line whose interior is evacuated and through which a low-energy beam passes. In this embodiment, the low-energy beam transport line 105 transports the proton beam 103 extracted from the ion source 110 and makes the proton beam 103 enter the radio-frequency quadrupole linear accelerator 106. The low-energy beam transport line 105 includes solenoid-type electromagnets 1051 and 1052 and a measurement point 101.

[0023] The solenoid electromagnets 1051 and 1052 have electric wires wound in a spiral shape along the traveling direction of the proton beam 103, and are electromagnets that induce a magnetic field parallel to the traveling direction of the proton beam 103 by currents supplied to the electric wires from solenoid electromagnet power supplies 1053 and 1054. The solenoid electromagnets 1051 and 1052 apply a focusing force to the proton beam 103 by the induced magnetic field, shape the proton beam 103 into a shape that can be accelerated by the radio frequency quadrupole linear accelerator 106, and emit the beam into the radio frequency quadrupole linear accelerator 106.

[0024] The solenoid electromagnet power supplies 1053 and 1054 are large current output power supplies that supply currents in the range of 20 A to 100 A to the solenoid electromagnets 1051 and 1052. The currents supplied by the solenoid electromagnet power supplies 1053 and 1054 are controlled by the computer 14 via cables 56 and 57, and the value of the currents is changed over time to shape the proton beam 103 into a shape that can be accelerated by the radio frequency quadrupole linear accelerator 106. The cables 56 and 57 are, for example, BNC cables.

[0025] The measurement point 101 is a portion of the low-energy beam transport line 105 whose sidewall is formed with an optically transparent window 108. The window 108 is formed of, for example, lead glass. The window 108 is a portion that enables fluorescence generated in the low-energy beam transport line 105 in response to the proton beam 103 to be measured from outside the low-energy beam transport line 105. In this embodiment, the measurement point 101 is a point immediately before the proton beam 103 enters the radio-frequency quadrupole linear accelerator 106 from the low-energy beam transport line 105, but it may be located elsewhere. For example, the measurement point 101 may be provided in the radio-frequency quadrupole linear accelerator 106.

[0026] The radio frequency quadrupole linear accelerator 106 is an accelerator that accelerates a particle beam along a straight line using a radio frequency voltage, which is an acceleration voltage supplied from an accelerating radio frequency source 1055. In this embodiment, the radio frequency quadrupole linear accelerator 106 uses the radio frequency voltage to accelerate the proton beam 103 while applying a focusing force to the proton beam 103 until the kinetic energy reaches 2.5 MeV, and then emits the proton beam 103 onto the lithium target 107.

[0027] The acceleration RF source 1055 is a RF source equipped with a vacuum tube that generates microwaves, and supplies a RF voltage for accelerating the proton beam 103 to the RF quadrupole linear accelerator 106. The RF voltage supplied by the acceleration RF source 1055 is transmitted to the computer 14 via a cable 58. At this time, the computer 14 can modulate the acceleration frequency, etc. The cable 58 is, for example, a BNC cable.

[0028] The lithium target 107 is a cone-shaped target mainly made of lithium (Li), and is arranged so that the bottom surface faces the radio frequency quadrupole linear accelerator 106. The lithium target 107 has a heat removal function using cooling water. The lithium target 107 is heated by the protons in the proton beam 103 supplied from the accelerator 100. 7 Li(p,n) 7 Thermal neutrons are generated by the Be reaction, and are emitted as a thermal neutron beam toward the patient in the treatment room.

[0029] The CCD camera 11, motor-driven rotating stand 12, computer 13, computer 14, display device 15, recording device 16, and speaker 17 constitute the beam monitor 1. The beam monitor 1 is a beam monitoring device that monitors the proton beam 103 in real time and detects any abnormalities in the beam based on the monitoring results.

[0030] The CCD camera 11 and the motor-driven rotating gantry 12 constitute an imaging unit that captures, through a window 108, images of fluorescence emitted in response to the proton beam 103 from each of a plurality of imaging directions intersecting the traveling direction of the proton beam 103 at the measurement location 101, and generates a plurality of captured images corresponding to each imaging direction. The imaging directions are preferably perpendicular to the beam axis direction, which is the traveling direction of the proton beam 103, and in this embodiment, are approximately perpendicular to the beam axis direction.

[0031] The CCD camera 11 is a camera that uses a CCD, which is a semiconductor element, as an imaging element, and measures the position and intensity of fluorescence generated in response to the proton beam 103 at the measurement location 101. The CCD camera 11 is fixed to a motor-driven rotating gantry 12 with bolts or the like. The CCD camera 11 is also communicatively connected to a computer 13 installed in the accelerator cab 1001 via a cable 50. The cable 50 is, for example, an RJ45 cable.

[0032] The motor-driven rotating gantry 12 is a drive unit that rotates the CCD camera 11 around the proton beam 103 by rotating the CCD camera 11 around the low-energy beam transport line 105 with the beam axis direction of the proton beam 103 as the rotation axis direction. The motor-driven rotating gantry 12 has guide rails 18, a plate 19, and a motor (not shown).

[0033] The guide rail 18 is a rail made up of two aluminum rings joined together, and is arranged in a ring shape to surround the window 108. The plate 19 is supported by the guide rail 18. In addition, the CCD camera 11 is fixed to the plate 19 with bolts or the like. The plate 19 is driven by a motor and moves along the guide rail 18. This causes the plate 19, together with the CCD camera 11 fixed to it, to rotate around the proton beam 103, and the CCD camera 11 can capture images of fluorescence corresponding to the proton beam 103 from each of a plurality of imaging directions substantially perpendicular to the beam axis direction of the proton beam 103.

[0034] The computer 13 is a computer that performs various information processing such as input / output, calculation, and conversion of digital data using electronic circuits. The computer 13 is installed in the accelerator cab 1001 and is communicatively connected to the computer 14 via a cable 51. The cable 51 is, for example, an RJ45 cable.

[0035] The computer 13 functions as an acquisition unit that acquires a three-dimensional beam image that shows the distribution of fluorescence in three-dimensional space corresponding to the proton beam 103 as a three-dimensional beam distribution, which is the distribution of the proton beam 103 in three-dimensional space, based on multiple captured images acquired by the CCD camera 11.

[0036] The computer 14 is a computer that performs various information processing such as input / output, calculation, and conversion of digital data using electronic circuits. The computer 14 is communicably connected to the display device 15 via cable 52, the recording device 16 via cable 53, and the speaker 17 via cable 55. The cables 52 and 53 are, for example, RJ45 cables, and the cable 55 is, for example, a coaxial cable.

[0037] The computer 14 functions as a calculation unit that calculates, based on the three-dimensional beam image generated by the computer 13, a variance-covariance matrix that indicates the variance and covariance due to the position and momentum of the proton beam 103 in two directions that intersect (specifically, are approximately perpendicular to) the beam axis direction, and the emittance of the proton beam 103.

[0038] Furthermore, the computer 14 determines whether or not an abnormality has occurred in the proton beam 103 based on the calculation results. Specifically, the computer 14 determines whether or not each element of the variance-covariance matrix exceeds a threshold, which is each element of a predetermined threshold matrix, and whether or not the emittance has exceeded an emittance threshold. The computer 14 determines whether or not an abnormality has occurred in the proton beam 103 based on the determination results. If it determines that an abnormality has occurred, the computer 14 executes feedback processing to control the outputs of the display device 15, the speaker 17, the beam extraction power supply 104, the solenoid electromagnet power supplies 1053 and 1054, and the acceleration high frequency source 1055.

[0039] In addition, the computers 13 and 14 may be computer systems having a memory for recording a computer program and a processor for reading the computer program recorded in the memory and executing the read computer program to realize the above functions.

[0040] The variance-covariance matrix is ​​a 4-by-4 real symmetric matrix that represents the correlation between position and momentum in two different directions that intersect with the beam axis. Emittance is an index (numerical value) that represents the quality of the beam, and is the value calculated from the determinant of the variance-covariance matrix. The threshold matrix is ​​a 4-by-4 real symmetric matrix, and the element in the i-th row and j-th column of the threshold matrix is ​​the threshold that corresponds to the element in the i-th row and j-th column of the variance-covariance matrix. A more detailed explanation of the variance-covariance matrix and emittance will be given later.

[0041] The display device 15 is a device that displays various information such as characters, figures, and graphics, and is installed in the accelerator operator's cab 1001. The display device 15 displays, for example, a three-dimensional beam image acquired by the computer 13 and a variance-covariance matrix and emittance calculated by the computer 14 in real time, and notifies the accelerator operator 1002 of the same.

[0042] The recording device 16 has a recording medium (not shown) such as a magnetic tape and writes information to the recording medium. In this embodiment, the recording device 16 sequentially writes and records the 3D beam image acquired by the computer 13, and the variance-covariance matrix and emittance calculated by the computer 14, onto the recording medium.

[0043] The speaker 17 is an audio output device that converts an electrical signal into audio, and is installed in the accelerator cab 1001. When an abnormality is detected in the proton beam 103, the speaker 17 receives an electrical signal from the computer 14 and outputs an alarm sound corresponding to the electrical signal, thereby functioning as a notification unit that notifies the accelerator operator 1002 of the alarm (abnormality in the proton beam 103).

[0044] FIG. 2 is a flowchart for explaining an example of a beam monitoring process for monitoring the proton beam 103.

[0045] In the beam monitoring process, first, the computer 14 receives a threshold matrix and an emittance threshold, which is a threshold for emittance, from the accelerator operator 1002, and sets the threshold matrix and emittance threshold to itself (step S1).

[0046] Next, the computer 14, in accordance with an instruction from the accelerator operator 1002, starts the beam extraction power supply 104, starts the generation and extraction of the proton beam 103 by the ion source 110, and controls the solenoid electromagnet power supplies 1053 and 1054 and the acceleration high frequency source 1055 to accelerate the proton beam 103 (step S2). As a result, the proton beam 103 extracted from the ion source 110 is accelerated through the low energy beam transport line 105 and the high frequency quadrupole linear accelerator 106, and is irradiated onto the lithium target 107. As a result, the lithium target 107 and the proton beam 103 7 Li(p,n) 7 The Be reaction occurs, producing a thermal neutron beam that is irradiated onto the patient in the treatment room.

[0047] The CCD camera 11, in accordance with instructions from the computer 13, captures the fluorescence corresponding to the proton beam 103 emitted from the ion source 110 to generate a captured image, and transmits the captured image to the computer 13 (step S3).

[0048] Specifically, the proton beam 103 generates fluorescence in the low-energy beam transport line 105 when the proton beam 103 itself or nitrogen excited by the proton beam 103 captures electrons. The CCD camera 11 captures images of this fluorescence through a lead-glass window 108 provided at the measurement location 101. The CCD camera 11 is fixed to a plate 19 of the motor-driven rotating platform 12 and moves along a guide rail 18 arranged in a ring shape so as to surround the window 108. The CCD camera 11 captures images of the fluorescence corresponding to the proton beam 103 from multiple directions while rotating around the proton beam 103. The captured images are sequentially transmitted to the computer 13. In this embodiment, the CCD camera 11 captures images of the fluorescence from at least four different imaging directions.

[0049] The computer 13 receives a plurality of captured images from the CCD camera 11, generates a three-dimensional beam image based on the plurality of captured images, and transmits the three-dimensional beam image to the computer 14 (step S4). At this time, the computer 13 can generate the three-dimensional beam image by, for example, performing an inverse Radon transform on the plurality of captured images.

[0050] Computer 14 receives the three-dimensional beam image from computer 13 and calculates the variance-covariance matrix and emittance of proton beam 103 based on the three-dimensional beam image. Computer 14 transmits the calculated variance-covariance matrix and emittance to display device 15 and recording device 16 (step S5). Upon receiving the calculation results, display device 15 displays the calculation results, and upon receiving the calculation results, recording device 16 records the calculation results (step S6).

[0051] Furthermore, the computer 14 compares each element of the variance-covariance matrix with a threshold, which is each element of the threshold matrix, and also compares the emittance with the emittance threshold, to determine whether or not an abnormality has occurred in the proton beam 103 (step S7). In this embodiment, the computer 14 determines that an abnormality has occurred in the proton beam 103 if any value exceeds the threshold (including the emittance threshold), and determines that no abnormality has occurred in the proton beam 103 if all values ​​do not exceed the threshold.

[0052] If no abnormality has occurred (step S7: No), the process returns to step S3. On the other hand, if an abnormality has occurred (step S7: Yes), the computer 14 transmits a beam stop signal to the speaker 17 and the beam extraction power supply 104. Upon receiving the beam stop signal, the speaker 17 outputs an alarm sound notifying that the beam will be stopped due to an abnormality in the proton beam 103. Upon receiving the beam stop signal, the beam extraction power supply 104 stops supplying power to the extraction electrode 102, stopping the extraction of the proton beam 103 from the ion source 110 (step S8), and then the process ends.

[0053] Next, the processing of step S5 by the computer 14 will be described in more detail.

[0054] 3 is a diagram showing an example of a three-dimensional beam image used by the computer 14. The three-dimensional beam image 201 shown in FIG. 3 is an image showing the three-dimensional distribution of fluorescence corresponding to the proton beam 103, and in this embodiment, is considered to be an image showing the three-dimensional distribution of the proton beam 103.

[0055] The example in Figure 3 shows a state in which the proton beam 103 is transported from left to right. In Figure 3, the beam axis direction of the proton beam 103 is the s-axis direction, the horizontal leftward direction as viewed from the beam axis direction is the positive direction of the x1 axis, and the vertical upward direction perpendicular to the beam axis direction is the positive direction of the x3 axis. In addition, the brightness center (x1, x3, s) of the proton beam 103 calculated from Equation 1 using the brightness distribution ρ(x1, x3, s) of the three-dimensional beam image 201 is center is the origin.

number

[0056] The variance-covariance matrix Σ(s) of the proton beam 103 is defined by Equation 2, and the emittance ε(s) is defined by Equation 3.

number

number

[0057] The variance-covariance matrix Σ(s) is a real symmetric matrix, and emittance ε(s) is the determinant of the variance-covariance matrix Σ(s). In Equations 2 and 3, x1(s) is the position of the particle (proton) constituting the proton beam 103 in the x1-axis direction, x3(s) is the position of the particle in the x3-axis direction, x2(s) is the gradient of the particle's trajectory in the x1-axis direction, and x4 is the gradient of the particle's trajectory in the x2-axis direction, all expressed as a function of the beam axis direction s. The gradient x2(s) corresponds to the particle's momentum in the x1-axis direction, and the gradient x4(s) corresponds to the particle's momentum in the x3-axis direction. Therefore, the variables x1(s) to x4(s) correspond to coordinates in a four-dimensional phase space consisting of the position and momenta in two different directions that intersect (specifically, are perpendicular to) the beam axis direction s of the proton beam 103. Also, the diagonal components of the variance-covariance matrix Σ(s) <x i 2 (s)> is the variable x i (s) and the off-diagonal components <x i (s)x j (s)> is the variable x i (s) and variable x j (s) indicates the covariance of x i (s) squared ({x i (s)} 2 ) to x i 2 It is written as (s).

[0058] The element Σ in the i-th row and j-th column (i≠j) of the variance-covariance matrix Σ(s) that does not include the slopes x2(s) and x4(s) ij As shown in Equation 4, (s) is a function x with the brightness distribution ρ(x1, x3, s) of the three-dimensional beam image 201 as a weight. i (s)x j The luminance distribution ρ(x1, x3, s) and the positions x1(s) and x3(s) are calculated from the three-dimensional beam image 201.

number

[0059] Also, the i-th row and j-th column element Σ, which contains the slopes x2(s) and x4(s) in the variance-covariance matrix Σ(s), ij (s) is calculated in the following steps S11 to S13.

[0060] Step S11: The computer 14 calculates the variances of the variables x1(s) and x3(s) at predetermined intervals Δs along the s-axis based on the three-dimensional beam image 201. <x1 2 > and <x3 2 The interval Δs is determined, for example, by Δs=Ls / NpH, where Ls is the total length of the three-dimensional beam image in the s-axis direction and NpH is the number of pixels of CCD camera 11 in the s-axis direction, and is, for example, about 0.05 mm to 0.1 mm.

[0061] Step S12: The calculator 14 calculates the <x1 2 > and <x3 2 > to the envelope equation shown in Equation 5, the variances of the variables x1(s) and x3(s) are <x1 2 (s)> and <x3 2 (s)> is obtained as a function of the variable s. Note that the envelope equation shown in Equation 5 is an envelope equation that takes into account only the air charge of the proton beam itself in free space. In Equation 5, p1 to p4 are fitting parameters.

number

[0062] Step S13: The computer 14 calculates the variance as shown in Equation 6. <x1 2 (s)> and <x3 2 By taking the first derivative of (s)> in the s-axis direction, the covariance<x1(s)x2(s)> and<x3(s)x4(s)> Calculate.

number

[0063] Also, the variance of the slopes x2(s) and x4(s) <x2 2 (s)> and <x4 2 For (s)>, the computer 14 calculates the variance as shown in Equation 7. <x1 2 (s)> and <x3 2 It is calculated by taking the second derivative of (s)> in the s-axis direction.

number

[0064] Also, the covariance of the positions x1(s) and x3(s) and the slopes x2(s) and x4(s)<x1(s)x4(s)> and<x2(s)x3(s)> As shown in Equation 8, the variance of the positions x1(s) and x3(s) is calculated using the angular velocity ω of the rotation about the s-axis direction. <x1 2 (s)> and <x3 2 (s)>. Also, the angular velocity ω is related to the covariance of the positions x1(s) and x3(s), as shown in Equation 9.<x1(s)x3(s)> It is related to the first derivative of the s-axis.

number

number

[0065] For this reason, the calculator 14 calculates the covariance<x1(s)x4(s)> and<x2(s)x3(s)> , the covariance<x1(s)x3(s)> It is calculated based on the dependence of the s-axis direction.

[0066] Specifically, the calculator 14 first calculates the covariance of the variables x1(s) and x3(s) at predetermined intervals Δs along the s-axis. <x1x3>Next, the calculator 14 calculates the calculated <x1x3>, <x1 2 (s)> and <x3 2 (s)> is fitted with the differential equation of Equation 9 to obtain ω as a fitting parameter. Then, the calculator 14 uses the fitting parameter ω and Equation 8 to calculate the covariance<x1(s)x4(s)> and<x2(s)x3(s)> Also, the variance <x1 2 (s)> and <x3 2 As (s)>, the function calculated in step S12 above is used.

[0067] The calculator 14 also calculates the covariance at predetermined intervals Δs along the s axis. <x1x3>The change in Δ <x1x3> / Δs change Δ(Δ <x1x3>) / Δs 2 is calculated at intervals of Δs. Since this value satisfies the relationship between the covariance <x2(s)x4(s)> between slopes and that shown in Equation 10, the computer 14 calculates <x2(s)x4(s)> using Equation 10.

Equation

[0068] In this way, all elements of the variance-covariance matrix can be calculated, and furthermore, the emittance is calculated as the determinant of the variance-covariance matrix.

[0069] Next, the process of step S7 by the computer 14 will be described in more detail.

[0070] The computer 14 substitutes a predetermined value (here, 0) for the variable s of the variance-covariance matrix and the emittance. Further, since both the variance-covariance matrix and the threshold matrix are real symmetric matrices, the computer 14 calculates, for example, a difference matrix by subtracting the variance-covariance matrix from the threshold matrix after setting the component in the i-th row and j-th column (where i < j) in the variance-covariance matrix and the threshold matrix to 0. The computer 14 makes a positive / negative determination for each element in the i-th row and j-th column (where i ≥ j) in the difference matrix. When a negative determination is made, the computer 14 determines that an abnormality has occurred. Similarly, the computer 14 makes a positive / negative determination for the difference value obtained by subtracting the emittance from the emittance threshold, and when the difference value is negative, determines that an abnormality has occurred.

[0071] The configurations, functions, and operations described above are merely examples and are not limited thereto. For example, in this embodiment, the beam monitor 1 monitors the beam of the accelerator 100 for the boron neutron capture therapy system 999, but the accelerator for beam monitoring is not limited to this example, and may be, for example, an accelerator for nuclear conversion / fusion or an accelerator for particle beam therapy. Further, although the accelerator 100 is a linear accelerator, the accelerator for beam monitoring may be a circular accelerator or the like.

[0072] Furthermore, although a CCD camera 11 is used as the camera of the beam monitor 1, for example, a CMOS (Complementary Metal Oxide Semiconductor) camera, a multi-channel photomultiplier tube, or a silicon photomultiplier (SIPM) array may be used instead of the CCD camera 11. Furthermore, although images are acquired from each of a plurality of directions using the CCD camera 11 and the motor-driven rotating stand 12, images may be acquired from each of a plurality of directions by arranging a plurality of cameras to surround the window 108 of the measurement location 101. Furthermore, the functions of the computers 13 and 14 may be realized by one computer or by three or more computers.

[0073] Furthermore, if an abnormality occurs in the proton beam 103, the computer 14 may adjust the state of the proton beam 103 by controlling the solenoid electromagnet power supplies 1053 and 1054 and the accelerating radio frequency source 1055. For example, the computer 14 controls the solenoid electromagnet power supplies 1053 and 1054 and the accelerating radio frequency source 1055 by transmitting control signals indicating the value and cycle of electromagnet current supplied from the solenoid electromagnet power supplies 1053 and 1054 to the solenoid electromagnets 1051 and 1052, and the value and cycle of acceleration voltage supplied from the accelerating radio frequency source 1055 to the radio frequency quadrupole linear accelerator 106.

[0074] At this time, the computer 14 determines the electromagnet current and acceleration voltage indicated by the control signal based on the variance-covariance matrix and emittance. For example, the computer 14 stores a lookup table showing the relationship between each element of the variance-covariance matrix and emittance and the electromagnet current and acceleration voltage, and determines the electromagnet current and acceleration voltage according to the element of the variance-covariance matrix or emittance that exceeds a threshold based on the lookup table.

[0075] Furthermore, the captured images may be acquired from two different directions. In this case, a three-dimensional beam image can be generated by applying symmetry to the shape of the proton beam 103.

[0076] As described above, according to this embodiment, the imaging unit generates a plurality of captured images by capturing fluorescence emitted in response to the proton beam 103 from each of a plurality of directions intersecting the traveling direction of the proton beam 103. The computer 13 acquires a three-dimensional beam distribution, which is the distribution of the proton beam 103 in three-dimensional space, based on the plurality of captured images. The computer 14 calculates the variance and covariance due to the position and momentum of the proton beam 103 in two directions intersecting the traveling direction of the proton beam 103, based on the three-dimensional beam distribution. Therefore, the variance and covariance due to the position and momentum of the proton beam 103 represent the three-dimensional structure of the beam, making it possible to grasp the three-dimensional structure of the beam.

[0077] Furthermore, in this embodiment, the emittance of the proton beam 103 is calculated based on the above variance and covariance, so that the three-dimensional structure of the beam can be grasped more accurately.

[0078] Furthermore, in this embodiment, images are acquired from multiple directions by rotating the CCD camera 11 around the proton beam 103. This eliminates the need to prepare multiple CCD cameras 11, making it possible to grasp the three-dimensional structure of the beam while reducing costs.

[0079] Furthermore, in this embodiment, the computer 14 determines whether or not an abnormality has occurred in the proton beam 103 based on the variance and covariance. This makes it possible to accurately determine the abnormality of the beam based on the three-dimensional structure of the beam.

[0080] Furthermore, in this embodiment, if an abnormality occurs in the proton beam 103, an alarm is issued, so that the accelerator operator 1002 can grasp the abnormality in the proton beam 103.

[0081] Furthermore, in this embodiment, if an abnormality occurs in the proton beam 103, the extraction of the proton beam 103 is stopped, so that it is possible to prevent an abnormal proton beam 103 from being emitted.

[0082] Furthermore, in this embodiment, if an abnormality occurs in the proton beam 103, the state of the proton beam is adjusted based on the variance and covariance, making it possible to prevent the abnormal proton beam 103 from being emitted.

[0083] The above-described examples of the present disclosure are illustrative examples of the present disclosure and are not intended to limit the scope of the present disclosure to only these embodiments. Those skilled in the art can implement the present disclosure in various other forms without departing from the scope of the present disclosure. [Explanation of symbols]

[0084] 1: Beam monitor 11: CCD camera 12: Motor-driven rotating stand 13: Computer 14: Computer 15: Display device 16: Recording device 17: Speaker 18: Guide rail 19: Plate 100: Accelerator 101: Measurement point 102: Extraction electrode 103: Proton beam 104: Beam extraction power supply 105: Low energy beam transport line 106: High frequency quadrupole linear accelerator 107: Lithium target 108: Window 110: Ion source 999: Boron neutron capture therapy system 1000: Accelerator room 1000a: Shielding wall 1001: Accelerator operation room 1002: Accelerator operator 1051: Solenoid electromagnet 1053: Solenoid electromagnet power supply 1054: Solenoid electromagnet power supply 1055: High frequency source for acceleration

Claims

1. A beam monitor device for monitoring a charged particle beam, comprising: an imaging unit that generates a plurality of captured images by capturing fluorescence generated in response to the charged particle beam from a plurality of directions intersecting a traveling direction of the charged particle beam; an acquisition unit that acquires a three-dimensional beam distribution, which is a distribution of the charged particle beam in a three-dimensional space, based on the plurality of captured images; a calculation unit that calculates variance and covariance due to the position and momentum of the charged particle beam in two directions intersecting the traveling direction based on the three-dimensional beam distribution.

2. The beam monitor device according to claim 1 , wherein the calculation unit calculates an emittance of the charged particle beam based on the variance and the covariance.

3. The imaging unit a camera that captures the fluorescent light and generates the captured image; 2. The beam monitor device according to claim 1, further comprising: a drive unit that rotates the camera around the charged particle beam with the traveling direction of the charged particle beam as a rotation axis direction.

4. The beam monitor device according to claim 1 , wherein the calculation unit determines whether or not an abnormality has occurred in the charged particle beam based on the variance and the covariance.

5. 5. The beam monitor device according to claim 4, further comprising a notification unit that issues an alarm when an abnormality occurs in the charged particle beam.

6. An accelerator that accelerates and extracts a charged particle beam, an extraction unit for generating the charged particle beam; a transport line for transporting the charged particle beam generated at the extraction unit; The beam monitor device according to claim 1, the transport line has a measurement point whose wall portion is formed with a light-transmitting window; The imaging unit of the beam monitor device images the fluorescence through the window.

7. 7. The accelerator according to claim 6, wherein the calculation unit of the beam monitor device determines whether or not an abnormality has occurred in the charged particle beam based on the variance and the covariance, and when an abnormality has occurred in the charged particle beam, stops extraction of the charged particle beam from the extraction unit.

8. 7. The accelerator according to claim 6, wherein the calculation unit of the beam monitor device determines whether or not an abnormality has occurred in the charged particle beam based on the variance and the covariance, and if an abnormality has occurred in the charged particle beam, adjusts a state of the charged particle beam based on the variance and the covariance.

9. The accelerator according to claim 6; an irradiation device that irradiates the charged particle beam from the accelerator.

10. A beam measurement method using a beam monitor device that monitors a charged particle beam, comprising: generating a plurality of captured images by capturing fluorescence generated in response to the charged particle beam from a plurality of directions intersecting the traveling direction of the charged particle beam; acquiring a three-dimensional beam distribution, which is a distribution of the charged particle beam in a three-dimensional space, based on the plurality of captured images; A beam measurement method for calculating variance and covariance of the position and momentum of the charged particle beam in two directions intersecting the traveling direction based on the three-dimensional beam distribution.

Images