JP2017096724A - Radiation detector - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a technique that allows detection of radiation such as gamma rays by a measurement of a shorter time over a wider energy range.SOLUTION: A radiation detector 1 includes: first scattered radiation detection means 15a, having a first radiation absorption member 16 around an electron drift region 14 between an electrode 12 and an electron collecting device 11; and second scattered radiation detection means 15b, having a second radiation absorption member 16. An angle θa, which a direction Da, from the center of the electron drift region to the center of the first radiation absorption member, forms with a reference incident angle Dc, is different from an angle θb, which a direction Db, from the center of the electron drift region to the center of the second radiation absorption member, forms with the reference incident angle Dc. The first radiation absorption member and the second radiation absorption member have different radiation detection characteristics.SELECTED DRAWING: Figure 4

Description

  The present invention relates to a radiation detection apparatus that detects scattered radiation and recoil electrons generated by Compton scattering and acquires the incident direction of radiation, and a Compton camera using the radiation detection apparatus.

  Conventionally, the following advanced Compton method is known as one of gamma ray detection methods. In this method, in addition to the scattered gamma ray energy and scattering direction vector generated by Compton scattering, the recoil electron energy generated by Compton scattering and the recoil direction vector at the Compton scattering point are used to determine the incident gamma ray incident direction. calculate. In addition, a Compton camera is known which is a type of gamma camera that can measure the intensity distribution of gamma rays emitted from a radiation source using this method and image it.

  Non-Patent Document 1 discloses an experiment using a TPC (Time Projection Chamber) which is a detection apparatus using such an advanced Compton method. Here, the TPC is filled with a gas serving as a scatterer, and a flat-plate electron collector (μPIC) that amplifies and detects ionized electrons is disposed. In addition, an electron drift region in which a uniform electric field is applied to the electron collector is formed in the TPC. Incident gamma rays (photons) cause Compton scattering, which is an interaction with electrons in gas molecules that are scatterers, in this electron drift region. The recoil electrons generated as a result proceed while ionizing gas molecules continuously, and generate an electron cloud composed of a large number of ionized electrons on the tracks. The electron cloud drifts to the electron collector while maintaining almost the same shape as the recoil electron track by the force received from the electric field in the electron drift region. The electron collector is a two-dimensional electron detector, and detects the position (X, Y coordinates) and energy of projection of an electron cloud (track) onto a two-dimensional plane. On the other hand, the distance from the electron collector to the track (Z) from the difference between the time of detection of scattered gamma rays by the gamma ray detector, that is, the time of occurrence of Compton scattering and the time of detection of ionization electrons by the electron collector, Coordinates) is detected. In this way, the three-dimensional position of the recoil electron track can be calculated.

  The gamma ray detection unit detects the arrival position and energy of the scattered gamma rays. Using the detected position information (vector) and energy information of scattered gamma rays and the position information (vector) and energy information of recoil electrons, the incident direction of incident gamma rays is calculated.

Kabuki S. et al., “Imaging study of a phantom and small animal with a two-head electron-tracking Compton gamma-ray camera”, Nuclear Science Symposium Conference Record (NSS / MIC 2010)

Non-Patent Document 1 reports the following. A part containing a drug containing a radioisotope I-131 (emission gamma-ray energy E0 = 365 keV) and a drug containing the radioisotope F-18 (emission gamma-ray energy E0 = 511 keV). I was able to image at the same time. In this experiment, LaBr ₃ is used as a scintillator for detecting scattered gamma rays in the gamma ray detection unit.

LaBr ₃ emits a large amount of light, and sufficient detection sensitivity can be obtained regardless of the energy level in detecting gamma rays. However, in the detection of high energy gamma rays, sufficient absorption efficiency cannot be obtained due to insufficient density. Generally, a scintillator having a high density has a small light emission amount, and conversely, a scintillator having a large light emission amount tends to have a low density. Under such circumstances, it has been difficult to achieve both sufficient absorption efficiency and sufficient detection sensitivity over a wide energy range.

  When the absorption efficiency of the gamma ray detection unit in the Compton camera is insufficient, the measurement time for obtaining necessary data becomes long. If the detection sensitivity is insufficient, the S / N of the detected value of the gamma ray energy decreases and the error increases, so that the angular resolution with respect to the incident gamma ray decreases. As a result, it has been difficult for a Compton camera equipped with a gamma ray detection unit as described above to achieve high-resolution imaging in a short measurement over a wide energy range (for example, 100 keV to 2000 keV).

  One aspect of the present invention is a radiation detection apparatus including a scatterer, an electrode, and an electron collector, wherein a first radiation absorption is formed around an electron drift region formed between the electrode and the electron collector. First scattered radiation detection means including a member and second scattered radiation detection means including a second radiation absorbing member are disposed. A direction from the center of the electron drift region to the center of the first radiation absorbing member is a first direction, and a direction from the center of the electron drift region to the center of the second radiation absorbing member is a second direction. And the direction from the center of the effective field of view of the detection device toward the center of the electron drift region is a reference incident direction, and the first angle formed by the first direction and the reference incident direction is The size of the second angle formed by the second direction and the reference incident direction is different, and the detection characteristics of the first radiation absorbing member with respect to radiation and the detection characteristics of the second radiation absorbing member with respect to radiation are different.

  According to the present invention, radiation such as gamma rays can be detected in a relatively short measurement over a wide energy range of, for example, 100 keV to 2000 keV.

The figure which shows schematic structure of an example of the Compton camera by this invention. The figure which shows the detection principle of TPC. The figure which shows the detail of an electronic detection circuit and a gamma ray detection circuit. The figure which shows the structure of the gamma ray detection apparatus in 1st Embodiment. The figure which shows the direction dependence of the scattering characteristic of a gamma ray. The figure which shows the structure of the gamma ray detection apparatus in 2nd Embodiment. The figure which shows the structure of the gamma ray detection apparatus in 3rd Embodiment.

  One aspect of the present invention is as follows. Hereinafter, “radiation” may be represented by “gamma rays”. Around the electron drift region, a first scattered radiation detecting means including a first radiation absorbing member and a second scattered radiation detecting means including a second radiation absorbing member are arranged. A direction from the center of the electron drift region toward the center of the first radiation absorbing member (first direction) and a direction from the center of the electron drift region toward the center of the second radiation absorbing member (second direction). Are different in the size of the angle formed by the reference incident direction.

  A schematic configuration of an example of a Compton camera according to the present invention will be described with reference to FIG. The gamma ray detection apparatus 1 includes a chamber 10 that is a container for sealing a gas (for example, argon, xenon, etc.) that is a scatterer and is also a housing of the apparatus. In addition, a first scattered gamma ray detector 15a, a second scattered gamma ray detector 15b, a voltage generator 13, an electrode 12 disposed in the chamber 10, and an electron collector 11 are provided, which are three-dimensional positions of electrons. It constitutes a TPC which is a detector.

  A flat electrode 12 is attached to the front end (gamma ray incident side) of the chamber 10, and an electron collector 11 is attached to the rear end (opposite side of the gamma ray incident side) so as to face the electrode 12 in parallel. . By applying predetermined voltages to the electrode 12 and the electron collector 11 by the voltage generator 13, a columnar electron drift region 14 is formed between the electrode 12 and the electron collector 11. A substantially vertical and uniform electric field acts on the electron collector 11 in the electron drift region 14.

  Individual incident gamma rays (photons) coming from the radiation source 4 and passing through the front end of the chamber 10 and the electrode 12 and entering the electron drift region 14 are interactions between electrons in the gas molecules. Compton scattering results in recoil electrons and scattered gamma rays. The recoil electrons are detected by the electron collector 11. The scattered gamma rays are detected by the first scattered gamma ray detector 15a or the second scattered gamma ray detector 15b, which is a scattered radiation detection means.

  As shown in FIG. 1, the signal processing device 2 includes an electronic detection circuit 30, a gamma ray detection circuit 31, a gamma ray incident direction calculation circuit 32, and an image reconstruction device 33. The electron detection circuit 30 acquires the recoil electron energy, the Compton scattering point position, and the recoil direction vector from the recoil electron detection data. The gamma ray detection circuit 31 acquires the energy of the scattered gamma rays and the scattering direction vector from the detection data of the scattered gamma rays. The gamma ray incidence direction calculation circuit 32, which is a radiation incident direction acquisition means, makes incident gamma rays incident on the basis of the recoil electron energy, recoil direction vector, scattered gamma ray energy, and scattering direction vector for each Compton scattering event. Calculate the direction. The image reconstruction device 33 generates intensity distribution image data from the incident direction of incident gamma rays detected for a plurality of Compton scattering events, and the display device 3 uses this image data to change the intensity distribution of the radiation according to the density and color difference. indicate.

  Next, the principle of TPC detection will be described with reference to FIG. In FIG. 2, the chamber 10 is omitted. Incident gamma rays 21 (photons) having an energy of about 100 keV to 2000 keV incident on the electron drift region 14 are subjected to Compton scattering, which is an interaction with electrons in the gas molecules. As a result, recoil electrons 23 and scattered gamma rays 22 having energy smaller than that of the incident gamma rays 21 are generated. When the energy of the incident gamma ray 21 is E0, the energy of the recoil electrons 23 is Ke, and the energy of the scattered gamma ray 22 is Eγ, the relationship E0 = Ke + Eγ is established.

  While recoil electrons 23 travel through the electron drift region 14, the recoil electrons 23 interact with gas molecules one after another to be scattered, and generate ionized electrons each time. In FIG. 2, Pc is a Compton scattering point, Pt is a passing point on a recoil electron track, Pa is an absorption point of scattered gamma rays 22 by the first scattered gamma ray detector 15a or the second scattered gamma ray detector 15b ( The position where the scattered gamma rays are photoelectrically absorbed). Further, s is an incident direction vector (incident direction unit vector) of the incident gamma ray 21, g is a scattering direction vector (scattering direction unit vector) of the scattered gamma ray 22, and e is a recoil direction vector (Compton scattering) of the recoil electrons 23. (Unit vector in the recoil direction at the point Pc).

  On the line along the track of the recoil electrons 23, an electron cloud 24 composed of a large number of ionized electrons is generated. Recoil electrons 23 gradually lose energy each time they are ionized, and stop when all energy is lost. If the energy Ke of the recoil electrons 23 is 30 keV or less, the range is smaller than the size of the electron drift region 14, and most of the recoil electrons 23 stop in the electron drift region 14. If it is larger than 30 keV, if it is necessary to measure, the electron drift region 14 is expanded, or the density of the scatterer is increased (the gas has a high molecular weight, the pressure is increased) to shorten the electron range. do it. Since the scattering interval and the scattering direction received by the recoil electrons 23 are irregular, the shape of the tracks, that is, the electron cloud 24 is also irregularly refracted as shown in the figure.

  The electron collector 11 is a plate-like two-dimensional detector such as MSGC (Micro Strip Gas Chamber), and includes a large number of anode electrode lines and cathode electrode lines arranged two-dimensionally at a minute interval. By applying a potential difference between the anode electrode line and the cathode electrode line, a strong electric field is generated on the detector surface, thereby amplifying and collecting electrons. The electron cloud 24 generated in the electron drift region 14 drifts in parallel toward the electron collector 11 due to the force received from the electric field. When the electron cloud 24 reaches the electron collector 11, it is amplified and detected by a detector located at the position (X, Y coordinates) of the projection 25. The minimum energy of electrons that can be detected by the electron collector 11 is limited by circuit noise or the like, and corresponds to an energy of recoil electrons Ke = about 5 keV. The maximum detectable electron energy is limited by the range of recoil electrons 23 with respect to the size of the electron drift region 14, and generally corresponds to about Ke = 30 keV.

  The first scattered gamma ray detector 15 a and the second scattered gamma ray detector 15 b (only two are shown) are arranged around the electron drift region 14 and include a gamma ray absorbing member 16 and an amplifier 17. For example, the gamma ray absorbing member 16 can be configured by combining a scintillator that is a photoelectric absorption material and a large number of photomultiplier tubes arranged two-dimensionally as the amplifier 17. A number of scintillator elements arranged two-dimensionally may be used as the gamma ray absorbing member 16. The scattered gamma rays 22 are photoelectrically absorbed by a scintillator and converted into visible light, and further converted into an electric signal by a photomultiplier tube and amplified, and the position and energy are output. Further, the gamma ray absorbing member 16 may be configured by combining a semiconductor element that is a photoelectric absorbing material and a large number of two-dimensionally arranged electronic amplifying elements as the amplifier 17. Here, the scattered gamma ray 22 is photoelectrically absorbed by a semiconductor element and directly converted into an electric signal, and further amplified by an electronic amplifying element to output its position and energy.

  Next, details of the operation of the signal processing apparatus 2 will be described. The detection signal of the recoil electrons by the electron collector 11 is then processed by the electron detection circuit 30, and the scattered gamma rays are detected by the first scattered gamma ray detector 15a and the second scattered gamma ray detector 15b. The signal is processed by the gamma ray detection circuit 31. FIG. 3 shows details of the electron detection circuit 30 and the gamma ray detection circuit 31. Each part of one electron cloud is detected by a plurality of electrode lines of the electron collector 11, and an electric signal corresponding to the charge (number of electrons) is generated. The electric signals from these electrode lines continue from the arrival of the front end of the electron cloud to the arrival of the rear end. The electrical signals from the plurality of electrode lines are amplified by the corresponding plurality of amplification units 40 and output to the data capturing unit 41.

  The scattered gamma ray detection signal is amplified by the amplification unit 48 of the gamma ray detection circuit 31. The amplifying unit 48 outputs a signal corresponding to the detection of gamma rays to the data capturing unit 41 as a trigger signal. The data capturing unit 41 captures all electrical signals for a predetermined time with the trigger signal as a starting point. Here, the predetermined time is set to be not less than D / V where D is the distance between the electron collector 11 and the electrode 12 and V is the drift speed of the electron cloud. As a result, all electrical signals from the electrode lines corresponding to one electron cloud are captured. A total charge Qe corresponding to one electron cloud is calculated by the total charge detection unit 46 from the electrical signal captured by the data capturing unit 41. Further, the electron energy calculation unit 47 calculates the energy Ke of each recoil electron from the total charge Qe.

  Furthermore, the electrical signal captured by the data capture unit 41 is output to the track coordinate calculation unit 42 as a set together with time information starting from the trigger signal. The track coordinate calculation unit 42 performs the following for each part of the electron cloud detected by the electron collector 11. The position in the direction perpendicular to the electron collector 11 obtained by the product of the detected position (X, Y coordinates) of the detector (pixel) of the electron collector 11 and the drift time ΔT and the drift velocity V with reference to the trigger signal. (Z coordinate) is calculated, and three-dimensional position data of each part of the electron cloud is generated based on these.

  Next, the Compton scattering point calculation unit 43 calculates the position (X, Y, Z coordinates) of the Compton scattering point Pc from the extracted three-dimensional position data of each electron cloud. As shown in FIG. 2, since gamma rays are incident from the electrode 12 side, recoil electrons 23 generated by Compton scattering in the electron drift region 14 travel in the direction of the electron collector 11. Accordingly, the position data at both ends of the electron cloud 24 that is far from the electron collector 11 (the Z coordinate is large) is defined as the Compton scattering point Pc. Further, the passing point calculation unit 44 calculates the position (X, Y, Z coordinates) of the passing point Pt on the track separated from the Compton scattering point Pc by a predetermined distance from the three-dimensional position data of the electron cloud. The recoil direction vector calculation unit 45 calculates a recoil direction vector e that is a unit vector in the direction from the Compton scattering point Pc to the passing point Pt for each electron cloud based on these data. This is sent to the gamma ray incident direction calculation circuit 32 together with the data of the Compton scattering point Pc.

  On the other hand, individual scattered gamma ray detection signals amplified by the amplification unit 48 of the gamma ray detection circuit 31 are input to the gamma ray absorption point calculation unit 50 and the gamma ray energy calculation unit 49. The gamma ray absorption point calculation unit 50 calculates the position of the absorption point Pa detected when the scattered gamma rays arrive and sends the data to the gamma ray scattering direction vector calculation unit 51. The gamma ray energy calculation unit 49 calculates the energy Eγ of the detected scattered gamma rays and sends the data to the gamma ray incident direction calculation circuit 32. The gamma ray scattering direction vector calculation unit 51 calculates the scattering gamma ray scattering direction vector g (scattering direction unit vector) from the Compton scattering point Pc data from the Compton scattering point calculation unit 43 and the scattering gamma ray absorption point Pa data. To do. Then, the data is sent to the gamma ray incident direction calculation circuit 32.

  Based on the recoil electron energy Ke, the scattered gamma ray energy Eγ, the electron recoil direction vector e, and the gamma ray scattering direction vector g for each Compton scattering event, the gamma ray incident direction calculation circuit 32 performs the following operations. I do. That is, the incident direction vector s (incident direction unit vector) of the incident gamma ray is calculated by the following equations (1), (2), and (3). In the equation, m is the stationary mass of electrons, and c is the speed of light. If the above is required, it becomes as follows. The first scattered radiation detection means and the second scattered radiation detection means respectively detect the energy and the arrival position of the scattered radiation generated by Compton scattering caused by incident radiation in the scatterer. The electron collector detects ionized electrons generated along a track of recoil electrons generated by Compton scattering. Then, the arithmetic circuit 32 obtains the incident direction of the incident radiation from the information of the scattered radiation energy and the arrival position, the recoil electron energy and the recoil direction in the track. The above procedure can be realized by, for example, a process of supplying a program that realizes one or more functions to a device via a network or a storage medium, and reading and executing the program by one or more processors in the computer of the device. is there. It can also be realized by a circuit (for example, ASIC) that realizes one or more functions.

  Further, the image reconstruction device 33 generates radiation intensity distribution image data of the radiation source 4 from the incident direction vector s of each incident gamma ray and the Compton scattering point Pc for a plurality of Compton scattering events. Using this image data, the display device 3 displays the intensity distribution of the radiation by shading or color difference.

(Embodiment 1)
A gamma ray detection apparatus 1 according to the first embodiment of a Compton camera according to the present invention will be described with reference to FIG. In the present embodiment, the chamber 10 has a columnar shape or a prismatic shape, and is disposed with its front surface facing the radiation source 4. Each of the first scattered gamma ray detector 15a and the second scattered gamma ray detector 15b is composed of a unit in which a scintillator that is a photoelectric absorbing material as the gamma ray absorbing member 16 and a photomultiplier tube as the amplifier 17 are combined. ing. A plurality of scintillators and photomultiplier tubes are arranged in a plane. Four units of the first scattered gamma ray detector 15 a are arranged in four directions facing the side surface of the chamber 10, and a unit of the second scattered gamma ray detector 15 b is arranged facing the rear surface of the chamber 10. Scattered gamma rays generated as a result of Compton scattering in the electron drift region 14 are photoelectrically absorbed by one of the scintillators and converted into visible light, and this visible light is converted into an electrical signal by a photomultiplier tube, amplified and output. .

  In FIG. 4, the right direction is the X direction, the upward direction is the Z direction, and the direction perpendicular to the paper surface is the Y direction, and the gamma ray detection apparatus 1 is symmetric with respect to the XZ plane and the YZ plane. Reference numeral 5 denotes an effective field of view (UFOV) of the gamma ray detection apparatus 1, which is defined as a range in which, for example, a sensitivity of 75% or more of the maximum sensitivity is obtained. A direction from the center C (area center of gravity) of the effective visual field to the center O (volume center) of the electron drift region 14 is defined as a reference incident direction Dc. In the present embodiment, the reference incident direction Dc is the same as the Z direction. The direction from the center O of the electron drift region 14 toward the center A of the gamma ray absorbing member 16 of each unit of the first scattered gamma ray detector 15a is defined as the first direction Da, the reference incident direction Dc, and the first direction Da. Let the angle be the first angle θa. Furthermore, the direction from the center O of the electron drift region 14 toward the center B of the gamma ray absorbing member 16 of the second scattered gamma ray detector 15b is the second direction Db, and the angle formed by the reference incident direction Dc and the second direction Db is the first direction. The angle θb is 2. However, in this embodiment, since the reference incident direction Dc and the second direction Db are the same, θb = 0 °.

  Next, the gamma ray absorbing member 16 constituting the first scattered gamma ray detector 15a and the second scattered gamma ray detector 15b will be described. When N1 of the N0 photons incident on the gamma ray absorbing member 16 are photoelectrically absorbed, the absorption efficiency is N1 / N0. The higher the absorption efficiency, the more necessary data can be obtained in a shorter time. When one photon of scattered gamma rays is photoelectrically absorbed by the gamma ray absorbing member 16 and converted to detectable energy E1, the detection sensitivity is E1. As the detection sensitivity is higher, the detection value is larger and the S / N is better, so the accuracy is improved. Here, these two characteristics of absorption efficiency and detection sensitivity are collectively referred to as detection characteristics. For the scintillator as the gamma ray absorbing member 16 constituting the first scattered gamma ray detector 15a and the second scattered gamma ray detector 15b, it is desirable to select an optimum material for absorption efficiency and detection sensitivity. This will be described below.

  FIG. 5 shows the direction dependency of the scattering characteristics of gamma rays with the Z direction as the incident direction when the scatterer is argon gas at 1 atm. Incident gamma ray energy E0 that can be detected by the Compton camera according to the present embodiment is 100 keV to 2000 keV, but the characteristics of the scattering direction differ depending on the incident gamma ray energy E0. As an example, FIG. 5A shows the characteristics of energy E0 = 871 keV (gamma rays emitted by radioisotope Tc-94), and FIG. 5B shows energy E0 = 159 keV (radioisotope I-123 emitted). (Gamma ray) characteristics. In the figure, each arrow indicates the direction of an incident gamma ray (energy E0), e1 is a recoil electron (when energy Ke = 5 kV), and g1 is a scattered gamma ray (when energy Eγ = E0-5 keV). For the same incident gamma ray g0, e2 indicates the direction of recoil electrons (when energy Ke = 30 kV), and g2 indicates the direction of scattered gamma rays (when energy Eγ = E0-30 keV). The length of the arrow represents the momentum of each gamma ray (photon) or recoil electron. The curve p is the differential scattering cross section of Compton scattering, and the length from the Compton scattering point Pc to the point on the curve p represents the probability that gamma rays will be scattered at the unit solid angle in that direction ( Klein-calculation result by Nishina formula).

  As described above, the energy Ke of recoil electrons detectable by the electron collector 11 is in the range of 5 keV to 30 keV. The effective scattered gamma ray scattering direction corresponding to this range is between the arrows g1 and g2, and is the range represented by the thick line p 'on the curve p. Gamma rays scattered outside this range are invalid because the energy Ke of the corresponding recoil electrons is smaller than 5 keV or larger than 30 keV and cannot be detected. Although only the XZ plane is shown in the figure, actual scattering occurs with equal probability around the Z axis (the curve p becomes a three-dimensional curved surface rotated around the Z axis).

  Further, by comparing (a) and (b) of the figure, the scattering angle of the scattered gamma ray with respect to the incident gamma ray (arrow g0) becomes more effective as the energy E0 of the incident gamma ray becomes larger (in the range between the arrows g1 and g2). It is understood. Incidentally, the angle formed with respect to the incident gamma ray (E0 = 871 keV, arrow g0) in (a) in the figure is 4.7 ° for the scattered gamma ray (arrow g1), and 11.7 ° for the scattered gamma ray (arrow g2). is there. In addition, the angle formed with respect to the incident gamma ray (E0 = 159 keV, arrow g0) in (b) in the figure is 26.4 ° for the scattered gamma ray (arrow g1) and 75.4 ° for the scattered gamma ray (arrow g2). is there.

  Here, the gamma ray absorbing member 16 of the first scattered gamma ray detector 15a is arranged in a direction in which a low energy incident gamma ray (for example, E0 = 159 keV) is scattered. Further, the gamma ray absorbing member 16 of the second scattered gamma ray detector 15b is arranged in a direction in which the ratio of high energy incident gamma rays (for example, E0 = 871 keV) is high. As a result, as shown in FIG. 4, the first angle θa is larger than the second angle θb. The first angle θa is an angle formed by the first direction Da in which each gamma ray absorbing member 16 of the first scattered gamma ray detector 15a is arranged and the reference incident direction Dc. The second angle θb is an angle formed by the second direction Db in which the gamma ray absorbing member 16 of the second scattered gamma ray detector 15b is arranged and the reference incident direction Dc.

  Generally, when the energy is low as the characteristics of gamma rays, the absorption efficiency in the photoelectric absorbing material is high but the detection sensitivity is low. Conversely, when the energy is high, the absorption efficiency in the photoelectric absorption material is low but the detection sensitivity is high. Therefore, in order to achieve both sufficient absorption efficiency and sufficient detection sensitivity regardless of the gamma ray energy level, it is desirable to do the following. That is, as the gamma ray absorbing member, the first scattered gamma ray detector 15a having a high rate of arrival of low energy scattered gamma rays selects a photoelectric absorbing material with high detection sensitivity. As the gamma ray absorbing member, the second scattered gamma ray detector 15b having a high rate of arrival of high energy scattered gamma rays selects a photoelectric absorbing material having high absorption efficiency. If the incident angle of the scattered gamma rays is different, the absorption efficiency changes because the length penetrating the gamma ray absorbing member changes. However, this variation is within an allowable range, and the detection sensitivity does not depend on the incident angle or the penetration distance.

  In general, the photoelectric absorption material has higher absorption efficiency as the density increases. The light emission amount of the scintillator corresponds to the detection sensitivity. Table 1 compares the characteristics of some typical photoelectric absorption materials such as a scintillator and a semiconductor material CZT (CdZnTe).

In the present embodiment, a scintillator NaI: Tl or LaBr ₃ is selected as the gamma ray absorbing member 16 of the first scattered gamma ray detector 15a. Further, as the gamma ray absorbing member 16 of the second scattered gamma ray detector 15b, a scintillator having a light emission amount smaller than the others but high density and high absorption efficiency is selected. When enumerated, it is BGO (Bi ₄ Ge ₃ O ₁₂ ), GSO (Gd ₂ SiO ₅ : Ce), LGSO (Lu _X Gd _{2 -X} SiO ₅ : Ce), or LSO (Lu ₂ SiO ₅ : Ce).

  As a result, low-energy incident gamma rays can be detected at a high rate by the first scattered gamma ray detector 15a with high absorption efficiency and high detection sensitivity. Further, high-energy incident gamma rays can be detected with high absorption efficiency and high detection sensitivity by the second scattered gamma ray detector 15b at a high rate. Thus, it is possible to achieve both sufficient absorption efficiency and sufficient detection sensitivity regardless of the energy level of the incident gamma ray. That is, even a single radiation detection apparatus can be detected in a relatively short measurement over a wide energy range of, for example, 100 keV to 2000 keV. Therefore, even with a Compton camera including this single radiation detection device, high-resolution imaging can be realized in a relatively short measurement over a relatively wide energy range.

  In the above example, the number of scattered gamma ray detectors is 2, but the number of scattered gamma ray detectors used can be 3 or more corresponding to gamma rays of 3 or more different energies. For example, if a scattered gamma ray detector is added below the first scattered gamma ray detector 15a in FIG. 4 (the angle is larger than θa), it is possible to deal with lower energy gamma rays.

(Embodiment 2)
A gamma ray detection apparatus 1 according to Embodiment 2 of the Compton camera according to the present invention will be described with reference to FIG. In the present embodiment, the configuration other than the arrangement of the first scattered gamma ray detector 15a and the second scattered gamma ray detector 15b is the same as that of the first embodiment, and thus the description thereof is omitted.

  Opposing the rear surface of the chamber 10, two units of the first scattered gamma ray detector 15a and two units of the second scattered gamma ray detector 15b are arranged. Also in this embodiment, the direction from the center O of the electron drift region 14 toward the center A of the gamma ray absorbing member 16 of each unit of the first scattered gamma ray detector 15a is the first direction Da, the reference incident direction Dc, and the first incident direction Dc. The angle formed by the direction Da is defined as a first angle θa. Furthermore, the direction from the center O of the electron drift region 14 toward the center B of the gamma ray absorbing member 16 of each unit of the second scattered gamma ray detector 15b is defined as the second direction Db, the reference incident direction Dc, and the second direction Db. Let the angle be the second angle θb.

  Here, the gamma ray absorbing member 16 of the first scattered gamma ray detector 15a is arranged in a direction in which a low energy incident gamma ray (for example, E0 = 159 keV) is scattered. Further, the gamma ray absorbing member 16 of the second scattered gamma ray detector 15b is arranged in a direction in which the ratio of high energy incident gamma rays (for example, E0 = 871 keV) is high. Accordingly, the first angle θa is larger than the second angle θb. Here, the first angle θa is an angle formed between the first direction Da in which each gamma ray absorbing member 16 of the first scattered gamma ray detector 15a is arranged and the reference incident direction Dc. The second angle θb is an angle formed between the second direction Db in which each gamma ray absorbing member 16 of the second scattered gamma ray detector 15b is arranged and the reference incident direction Dc.

Further, in this embodiment, as the gamma ray absorbing member 16 of the first scattered gamma ray detector 15a, a scintillator NaI: Tl or LaBr ₃ having a density smaller than others but a large light emission amount and high detection sensitivity is selected. In addition, as the gamma ray absorbing member 16 of the second scattered gamma ray detector 15b, a scintillator having a light emission amount smaller than the others but high density and high absorption efficiency is selected. When enumerated, it is BGO (Bi ₄ Ge ₃ O ₁₂ ), GSO (Gd ₂ SiO ₅ : Ce), LGSO (Lu _X Gd _{2 -X} SiO ₅ : Ce), or LSO (Lu ₂ SiO ₅ : Ce).

  As a result, low-energy incident gamma rays can be detected at a high rate by the first scattered gamma ray detector 15a with high absorption efficiency and high detection sensitivity. High energy incident gamma rays can be detected at a high rate with high absorption efficiency and high detection sensitivity by the second scattered gamma ray detector 15b. Thus, it is possible to achieve both sufficient absorption efficiency and sufficient detection sensitivity regardless of the energy level of the incident gamma ray.

(Embodiment 3)
A gamma ray detection apparatus 1 according to Embodiment 3 of the Compton camera according to the present invention will be described with reference to FIG. In the present embodiment, since the configuration excluding the first scattered gamma ray detector 15a is the same as that of the first embodiment, description thereof is omitted.

  In the present embodiment, the first scattered gamma ray detector 15 a is constituted by a unit including a semiconductor element as the gamma ray absorbing member 16 and a large number of electronic amplification elements two-dimensionally arranged as the amplifier 17. Four units of the first scattered gamma ray detector 15 a are arranged in four directions facing the side surface of the chamber 10. In the electron drift region 14, the scattered gamma rays generated as a result of Compton scattering are photoelectrically absorbed by the semiconductor element and directly converted into an electric signal, and further amplified and output by the electronic amplifying element.

  The second scattered gamma ray detector 15b is composed of a scintillator that is a photoelectric absorbing material as the gamma ray absorbing member 16, and a unit that includes a photomultiplier tube as the amplifier 17. A plurality of scintillators and photomultiplier tubes are arranged in a plane. A unit of the second scattered gamma ray detector 15b is arranged facing the rear surface of the chamber 10. Scattered gamma rays are photoelectrically absorbed by a scintillator and converted into visible light, and this visible light is detected by a photomultiplier tube and output as an electrical signal.

  Here, the gamma ray absorbing member 16 of the first scattered gamma ray detector 15a is arranged in a direction in which a low energy incident gamma ray (for example, E0 = 159 keV) is scattered. In addition, the gamma ray absorbing member 16 of the second scattered gamma ray detector 15b is arranged in a direction in which a high energy incident gamma ray (for example, E0 = 871 keV) is scattered. Accordingly, the first angle θa formed by the first direction Da of each gamma ray absorbing member 16 arrangement of the first scattered gamma ray detector 15a and the reference incident direction Dc is the first angle θa of the gamma ray absorbing member 16 arrangement of the second scattered gamma ray detector 15b. It is larger than the second angle θb formed by the two directions Db and the reference incident direction Dc.

Further, in the present embodiment, a semiconductor material CZT (CdZnTe), which is a photoelectric absorbing material, is selected as the gamma ray absorbing member 16 of the first scattered gamma ray detector 15a. As shown in Table 1, the density of CZT is smaller than that of LSO, GSO, etc., but it has the characteristics of high detection sensitivity and high energy resolution because it can directly convert photons of gamma rays into electrical signals. A scintillator having a high density and a high absorption efficiency is selected as the gamma ray absorbing member 16 of the second scattered gamma ray detector 15b. When enumerated, it is BGO (Bi ₄ Ge ₃ O ₁₂ ), GSO (Gd ₂ SiO ₅ : Ce), LGSO (Lu _X Gd _{2 -X} SiO ₅ : Ce), or LSO (Lu ₂ SiO ₅ : Ce).

  Therefore, low-energy incident gamma rays can be detected with a high rate by the first scattered gamma ray detector 15a with high absorption efficiency and high detection sensitivity, and high-energy incident gamma rays can be detected with a high rate by the second scattered gamma ray detector 15b. It can be detected with high efficiency and high detection sensitivity. Thus, it is possible to achieve both sufficient absorption efficiency and sufficient detection sensitivity regardless of the energy level of the incident gamma ray.

  1 .... Gamma ray detection device (radiation detection device) 2 .... Signal processing device 3 .... Display device 4 .... Radiation source 5 .... Effective field of view 11 .... Electron collector 12 .... Electrode 14 ..Electron drift region, 15a..First scattered gamma ray detector (first scattered radiation detection means), 15b..Second scattered gamma ray detector (second scattered radiation detection means), 16..gamma rays Absorbing member (radiation absorbing member), 21 .. Incident gamma ray ((incident radiation), Dc .. Reference incident direction

Claims (9)

A radiation detection device having a scatterer, an electrode, and an electron collector,
A first scattered radiation detection means including a first radiation absorbing member and a second scattered radiation detection including a second radiation absorbing member around an electron drift region formed between the electrode and the electron collector. Means is disposed, the direction from the center of the electron drift region toward the center of the first radiation absorbing member is defined as a first direction, and the direction from the center of the electron drift region toward the center of the second radiation absorbing member Is the second direction, and the direction from the center of the effective field of view of the detection device toward the center of the electron drift region is the reference incident direction, the first direction and the reference incident direction form the first direction. The size of the second angle formed by the angle, the second direction, and the reference incident direction is different, and the detection characteristics of the first radiation absorbing member with respect to the radiation and the detection of the radiation of the second radiation absorbing member with respect to the radiation. And wherein the characteristics are different. The first scattered radiation detection means and the second scattered radiation detection means detect energy and arrival position of scattered radiation generated by Compton scattering caused by incident radiation in the scatterer, respectively.
The electron collector detects ionized electrons generated along a track of recoil electrons generated by the Compton scattering, the electrode applies an electric field to the scatterer,
The apparatus according to claim 1, wherein an incident direction of the incident radiation is obtained from information on an energy and an arrival position of the scattered radiation, an energy of the recoil electrons, and a recoil direction in a track.   The said 1st angle | corner is larger than the said 2nd angle | corner, and the detection sensitivity with respect to a radiation is higher than the said 2nd radiation absorption member, The said 1st radiation absorption member is characterized by the above-mentioned. Equipment.   4. The method according to claim 1, wherein the first angle is larger than the second angle, and the radiation absorption efficiency of the second radiation absorbing member is higher than that of the first radiation absorbing member. The apparatus according to claim 1.   The first radiation absorbing member and the second radiation absorbing member are scintillators, and a light emission amount of the first radiation absorption member is larger than a light emission amount of the second radiation absorption member. Item 4. The apparatus according to Item 3.   The first radiation absorbing member and the second radiation absorbing member are scintillators, and the density of the second radiation absorbing member is larger than the density of the first radiation absorbing member. The device described in 1.   The apparatus according to claim 3 or 4, wherein the first radiation absorbing member is a semiconductor element that directly converts radiation into an electrical signal, and the second radiation absorbing member is a scintillator.   A radiation detection apparatus according to any one of claims 1 to 7, and an image reconstruction unit that generates intensity distribution image data of radiation generated from a radiation source from an incident direction of incident radiation detected by the detection apparatus. Compton camera comprising: display means for displaying the intensity distribution image based on the intensity distribution image data.   The apparatus according to claim 1, wherein the radiation is gamma rays.

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