WO2015146534A1

WO2015146534A1 - Radiation measuring apparatus and method

Info

Publication number: WO2015146534A1
Application number: PCT/JP2015/056581
Authority: WO
Inventors: Ikuo Watanabe
Original assignee: Canon Kabushiki Kaisha
Priority date: 2014-03-28
Filing date: 2015-02-27
Publication date: 2015-10-01
Also published as: JP2015190965A

Abstract

A radiation measuring apparatus according to the present invention includes: a detecting unit 201 which detects a scattered radiation that has been generated by Compton scattering which is caused in a scatterer by an incident radiation; and a detecting unit 208 which amplifies a secondary electron that has been generated by a recoil electron which has been generated by the Compton scattering, and detects a trajectory thereof. A measuring unit determines an incoming direction of the incident radiation, from information on a detection result of the scattered radiation and the detection result of the trajectory of the secondary electron. An operation of a power supply 210 is stopped which supplies a voltage for causing an electron avalanche phenomenon to the electron trajectory detecting unit 208, for a predetermined time period which is needed for detecting the trajectories of the secondary electrons.

Description

DESCRIPTION

Title of Invention

RADIATION MEASURING APPARATUS AND METHOD

Technical Field

[0001] he present invention relates to a radiation measuring apparatus and a radiation measuring method which can obtain information on a recoil electron while reducing an influence of a power supply noise, in an electron tracking Compton camera (ETCC) and the like.

Background Art

[0002] In a gamma ray detector, a technology has been proposed which identifies a detector that has output a noise signal, and does not use the data (see Patent Document 1) . In addition, among gamma ray detectors using

Compton scattering, there is proposed a detector which uses MSGC (micro strip gas chamber) (see Non-patent Document 1 ) .

Citation List

Patent Literature

[0003] PTL 1: Japanese Patent Application Laid-Open No. 2011- 237457

Non Patent Literature

[0004] NPL 1: History of development of MSGC Kobe University Atsuhiko Ochi MPGD study group Dec. 3, 2004

Summary of Invention

Technical Problem

[0005] A Compton camera which is one type of a gamma camera measures an intensity distribution of gamma rays that are generated from a radiation source, and displays the intensity distribution as an image. The Compton camera uses Compton scattering which occurs between incident gamma rays and a scatterer, and detects an incident direction of the incident gamma rays. Here, in order to calculate an incident direction of the incident gamma rays, the energy and a vector of a recoil direction of a recoil electron which has been generated by the Compton scattering are necessary, in addition to the energy and a vector of a scattering direction of the scattered gamma rays which have been generated by the Compton scattering. In order to detect the recoil electron, a radiation measuring apparatus is used which can detect both the energy and the position of the trajectory. The radiation measuring apparatus is provided with a gas which is a scatterer, an electric field applying unit . for drifting a secondary electron, and an MSGC which is an electron detecting unit formed of two-dimensionally arranged electrodes. The recoil electron which has been generated by the Compton scattering of the incident gamma rays and an electron in a molecule of the gas flies in the gas while

continuously ionizing the molecules of the gas, and generates a large number of gas electrons which are formed of secondary electrons, on the trajectory.

These gas electrons are drifted toward the MSGC by a force which the gas electrons receive from an electric field formed by the electric field applying unit, while keeping the same distribution state as the trajectory of the recoil electron. The MSGC amplifies the

incident electrons, and simultaneously detects

positions (X, Y coordinate) of the gas electrons which have been focused onto a two-dimensional plane. On the other hand, a distance (Z coordinate) between the MSGC and the trajectory is detected from a difference between a time point at which the scattered gamma rays have been detected by the gamma ray detector,

specifically, a time point at which the Compton

scattering has occurred, and a time point at which the secondary electrons have been detected by the MSGC, and from a drifting speed of the secondary electrons. The speed of the scattered gamma rays is very fast, and accordingly the time point at which the Compton scattering has occurred can be considered to be the same time as the time point at which the scattered gamma rays have been detected. Thus, the three- dimensional position of the trajectory of the recoil electron can be calculated.

[0006] Generally, the amplification ratio of a secondary

electron which has been ionized by the recoil electron occasionally varies due to a power supply noise which is overlapped on a voltage applied to a gas electron amplifier when the gas electron is amplified. Because of this, the determination accuracy of the position (direction) of the incident gamma rays is occasionally lowered, which is calculated from the detected energy of the recoil electron, and the like.

[0007] The above described Patent Document 1 and Non-patent Document 1 do not disclose a countermeasure for the noise which is overlapped on the power supply voltage. An object of the present invention is to reduce the power supply noise, and thereby enhance the

determination accuracy of the position (direction) of the recoil electron.

Solution to Problem

[0008] A radiation measuring apparatus according to the

present invention includes: a radiation detecting unit which detects a scattered radiation that has been generated by Compton scattering which is caused in a scatterer by an incident radiation; an electron

trajectory detecting unit which detects a trajectory of a secondary electron which has been generated by a recoil electron that has been generated by the Compton scattering; and a measuring unit which determines an incoming direction of the incident radiation, from a detection result of the scattered radiation and a detection result of the trajectory of the secondary electron, wherein an operation of a power supply is stopped which supplies a voltage to the electron trajectory detecting unit, in a predetermined time period which is needed for detecting the trajectory of the secondary electron.

[0009] Further features of the present invention will become apparent from the following description of exemplary embodiments (with reference to the attached drawings) .

Brief Description of Drawings

[0010] Fig. 1 is a flow chart illustrating an operation of a measuring apparatus according to the present invention.

Fig. 2 is a block diagram illustrating one example of the measuring apparatus according to the present invention .

Fig. 3 is a time chart for describing an operation of the measuring apparatus according to the present invention .

Fig. 4 is a schematic view for describing a gas

electron multiplication unit in the measuring apparatus according to the present invention.

Fig. 5 is a graph for describing a relationship between an applied voltage and a gas amplification ratio, according to the present invention.

Description of Embodiments

[0011] In the present invention, an operation of a power

supply is stopped which^" supplies^" a voltage for causing an electron avalanche phenomenon, to an electron trajectory detecting unit that detects a trajectory of a secondary electron, in a predetermined time period which is needed for detecting the trajectory of the secondary electron that has been generated by Compton scattering. The above described predetermined time period is a predetermined time period, for instance, after the arrival of the scattered radiation has been detected in a radiation detecting unit which detects the scattered radiation that has been generated by the Compton scattering which is caused in a scatterer by an incident radiation. This predetermined time period shall be set so as to be D/V or more, when a space between a first electrode of an electron detecting unit and a drift plane (second electrode) which will be described later is represented by D, and a drifting speed of the gas electron is represented by V.

[0012] The above described power supply may be a power supply containing an LPF (low pass filter) which will be described later, a power supply having characteristics such as a distributed stray capacitance, and the like. In other words, the power supply may have such

properties that when the operation of the power supply has been stopped, the voltage does not suddenly fall but gradually falls while spending a fixed period of time. If such a power supply is used, the secondary electrons which are generated from the recoil electron that has been generated by the Compton scattering are amplified and are accurately detected, in the state that the power supply noise is reduced within the above described predetermined time period in which the operation of the power supply is stopped, and the detection accuracy of the energy of the secondary electrons and the like is enhanced.

[0013] In the following description, the radiation will be described as the gamma rays, but even though the radiation is another radiation, the measurement

principle is the same. In addition, in the present specification, the electron trajectory detecting unit may be referred to as an electron detecting unit, a gas electron amplifier, a gas electron multiplication detecting unit and the like, as terms considered from different viewpoints. The power supply may also be referred to as a high-voltage power supply, a power supply circuit and the like.

[0014] (Embodiment)

The embodiment of the present invention will be

described below with reference to Fig. 2. A gamma ray event signal 301 which is output from a gamma ray detecting unit 201 is a thin pulse having positive polarity. When the gamma ray event signal 301 is input to a monomulti-vibrator 202, an oscillation stop signal having a predetermined width of approximately 6 μΞ is output, and is input into an oscillator 203 in a high- voltage power supply 210. The output of the oscillator 203 is connected to a booster circuit 205, and an electric power is supplied to the booster circuit 205 from a low-voltage power supply 204. An AC pulse output of the booster circuit 205 is rectified in a rectifier circuit 206, the pulsing component is removed by an LPF 207, and the output becomes a high-voltage output. The high-voltage output is supplied to a gas electron multiplication detecting unit 208 which is an electron trajectory detecting unit. By the high- voltage output which has been applied to the gas electron multiplication detecting unit 208, an electron avalanche phenomenon is caused, and the secondary electrons are amplified.

[0015] An operation in the present embodiment will be

described below with reference to Fig. 1 to Fig. 3. In Fig. 1, after the start 100, a high-voltage power supply 210 starts the output at the start 101 of the power supply oscillation, and a high-voltage is applied to the gas electron multiplication detecting unit 208. In determination 102 "Whether gamma rays are detected or not?", the operation passes through the path of No, and is looped.

[0016] In Fig. 3, the state before the gamma ray event signal is output is a portion of 300. The high-voltage power supply 210 is operated by the oscillation of an

oscillator 203, and a noise is overlapped on the high- voltage output. The oscillation of the oscillator 203 is shown by a portion of the output of the oscillator. Here, at the time point at which the arrival of the gamma rays has been detected in the gamma ray detecting unit 201, the gamma ray event signal 302 is input into the monomulti-vibrator 202. At this time, the

operation passes through a path of Yes in the

determination 102 "Whether gamma rays are detected or not?" in Fig. 1, and a process of a power supply

oscillation stop 103 is performed. Determination 104 "Has predetermined time period passed after detection of gamma rays?" is No, and the operation is looped here. Subsequently, an oscillation stop signal having a predetermined time width is output from the monomulti- vibrator 202 to the oscillator 203. Specifically, the monomulti-vibrator 202 outputs the oscillation stop signal to the oscillator 203, for the time period of the maximum value of time periods or longer which are spent before the secondary electrons reach the electron trajectory detecting unit (anode strip and back strip in electron detecting unit) after the gamma ray event signal 302 has been input. Alternatively, the

monomulti-vibrator 202 outputs the oscillation stop signal to the oscillator 203, for the time period of the maximum value of time periods or longer which are spent before the detection of the trajectory of the secondary electrons is completed. Thus, a switching operation of the power supply circuit is stopped. Time periods which are spent before each portion of the secondary electrons reach the detecting unit are

different in many cases, and all of the portions of the secondary electrons need to be detected in the

detecting unit. Accordingly, the oscillation stop signal is set in this way. When a processing time in a processing unit in the latter stage of the detecting unit is considered, the oscillation stop signal is output to the oscillator 203, for the time period of the maximum value of time periods or longer which are spent before the detection of the trajectory of the secondary electrons is completed after the gamma ray event signal 302 has been input, in order that all of the portions of the secondary electrons are more surely and accurately detected. However, it is also

acceptable to generate a trigger signal after the rise of a signal corresponding to the front end of the gas electrons, and output the trigger signal to the

monomulti-vibrator 202.

[0018] he time period during which the oscillation stop

signal is output is an interval between 302 and 304 in Fig. 3; and as for the high-voltage output, the noise 301 is not overlapped thereon, and the voltage

gradually slightly drops, as is illustrated in 303.

However, this voltage drop can be controlled by a time constant of an LPF 207 in the high-voltage power supply 210, and actually does not cause a problem. In the period during which this noise is not overlapped, the gas electron amplifier such as an MSGC which is

sensitive to a voltage noise operates at high accuracy. In addition, a mode of the voltage drop can be

calculated from the configuration of the power supply, and a relationship between the voltage and the

amplification ratio can be also calculated as is

illustrated in Fig. 5 which will be described later.

Accordingly, if the detection result in the detection unit is corrected with the use of these calculation results, a more accurate detection result can be also obtained.

[0019]After that, when a predetermined time period has passed, the determination 104 of "Has predetermined time period passed after detection of gamma rays?" in Fig. 1 turns into Yes, a process of the restart 101 of the power supply oscillation is performed, and the high-voltage power supply 210 restarts the operation. This

operation is performed at the timing of 304 in Fig. 3.

[0020] ext, the operation of the Compton camera and the mechanism of the gas electron amplification by the gas electron multiplication detecting unit 208 which is the electron trajectory detecting unit will be described below, while the case where an MSGC is used in Fig. 4 is taken as an example. A space between the drift plane 401 and the cathode strip 405 is approximately 10 cm, and a potential difference therebetween is set to be approximately -3,000 V. The cathode strip 405 is a first electrode, and the drift plane 401 is a second electrode which is arranged at a position facing the electron trajectory detecting unit through a gas portion. The electric field application unit sets the second electrode as a negative potential with respect to the first electrode, and applies an electric field for drifting the secondary electron in a direction toward the first electrode. In Fig. 4, only

approximately five anodes 404 are illustrated, but actually the space between the anodes 404 is

approximately 200 μπι, and there are 200 or more anodes. The width of the cathode 405 which has been arranged with respect to each of the anodes 404 is approximately 100 μπι. The space between the back strips 406 is also approximately 200

A space between the anode 404 and the cathode 405 on a substrate 403 is approximately 50 μιη, a potential difference between the anode and the cathode is

approximately 500 V, and an electric field of

approximately 100,000 V/cm is applied therebetween. A gas portion 402 having a thickness of approximately 10 cm in the inside of a gas chamber accommodates a gas therein in which approximately 10% of methane or ethane is contained as a quencher gas in argon. There are gamma ray detecting units 410 to 412 in the lower part thereof, and a scintillator 410, a photomultiplier tube 411 and a head amplifier 412 are mounted in this order from the top, in a form of a plurality of matrices. The matrix size is approximately 8*8 to 24*24, and the size of the element of the matrix is approximately 3 mm square to 6 mm square.

[0022] When gamma rays 413 have entered, a part of the

incident gamma rays is compton-scattered at a Compton scattering point 414, and becomes scattered gamma rays

416 and a recoil electron 415. The scattered gamma rays 416 are converted into a plurality of visible rays

417 in the scintillator 410, and are photoelectrically converted and are further amplified to an electron having an energy of approximately 1 million times in a photomultiplier tube 411. The electron is amplified by the head amplifier 412, and is converted into an electric signal. The electric signal is sent to the gamma ray detector 201. When the gamma ray detection signal is input from any one of the plurality of head amplifiers 412 to the gamma ray detector 201, the gamma ray detector 201 outputs the gamma ray event signal 302 as has been previously described. At the same time, the arrival position of the scattered gamma rays is detected, because the gamma ray detecting unit is arranged in a matrix form.

[0023] On the other hand, the recoil electron 415 recoiled at the Compton scattering point 414 emits a secondary electron, and the secondary electron moves in a

direction toward the anode strip 404 and the cathode strip 405, due to the negative potential of the drift plane 401. Then, the secondary electron is submitted to gas electron amplification and are amplified to tens of thousands times of secondary electrons and ions in a strong electric field between the anode 404 and the cathode 405, in a single burst. The trajectories are detected as a back strip readout signal 408 and an anode readout signal 409, and the arrival positions of the secondary electrons are detected from the

intersections. The sampling time cycle of these signals is approximately 10 ns to 40 ns .

[0024] The Compton scattering formula is inversely calculated from the pattern of the detected electron trajectory, the arrival position of the scattered gamma rays and the like, and thereby the Compton scattering position 414 and a direction of the incident gamma rays 413 are determined. Specifically, the electron detecting unit calculates the energy of the recoil electron 415, the position of the Compton scattering point, and the vector of the recoil direction, from the detected data on the recoil electron 415. In addition, the gamma ray detecting unit calculates the energy of the scattered gamma rays and the vector of the scattering direction, from the detected data on the scattered gamma rays. A gamma ray incident direction calculating unit in a measuring unit which determines the incoming direction of the incident gamma rays calculates the incoming direction of the incident gamma rays for each

individual Compton scattering event, based on the energy and the vector of the recoil direction of the recoil electron, and the energy and the vector of the scattering direction of the scattered gamma rays. An image reconfiguration apparatus converts the intensity distribution of a plurality of Compton scattering events into image data from the incident direction of the incident gamma rays, and a display apparatus can display the intensity distribution also by a difference between concentrations and/or between colors and the like. However, in the above described calculation processing, the energy of the recoil electron 415 is important for the calculation of an angle of the incident gamma rays, and a measurement error of this energy gives a large influence on the angle error of the direction of the incident gamma rays 413.

[0025] The inverse calculation according to the Compton

scattering formula is as the following Expression 1. Specifically, a vector s of the incident direction of the incident gamma rays (unit vector of incident

direction) is calculated by the following expressions (1), (2) and (3). Incidentally, in the expressions, the energy of the recoil electron for each individual Compton scattering event is represented by Ke, the energy of the scattered gamma rays is represented by Εγ, the vector of the recoil direction of the electron is represented by e, and the vector of the scattering direction of the gamma rays is represented by g. In addition, the static mass of the electron is

represented by m, and the velocity of light is

represented by c. The angle formed by the vector e of the recoil direction of the electron and the vector g of the scattering direction of the gamma rays is

represented by a, and the angle formed by the vector of the incident direction of the incident gamma rays and the vector g of the scattering direction of the gamma rays is represented by φ .

[0026] [Math. 1]

( , 3ΐη 4Λ . sin φ

S = ( COS φ - ) S +

\ ^ !i « / ^a —- e

sir. a (1)

φ - cos M(_Λl - - mc² e \ (2)

^ V E—y+—e ^■—Ey^)

[0027] Fig. 5 illustrates a relationship between an applied voltage and an amplification ratio of a proportional counter which is the base of the MSGC. It is

understood that the amplification ratio is

approximately proportional to the exponentiation of the applied voltage. Parameters of the proportional counter are an anode radius of 25 μπι, a cathode radius of 25 mm, 1 atmosphere and Gas=Ar+10%CH₄. At this time, the electric field of the periphery of the anode becomes approximately 100,000 V/cm. As has been described above, the gas electron amplification ratio is proportional to the exponentiation of the electric field or a potential difference between the anode strip 404 and the cathode strip 405, and accordingly the noise which is overlapped on the electric field or the potential difference gives a large error to the

calculation of the angle of the incident gamma rays, compared to a noise which has been overlapped on another portion. Such an error is reduced in the present invention. Specifically, the energy

amplification ratio of the electrons which have been ionized by the recoil electron results in dispersing due to an influence of a noise of an electric system during the gas amplification, in the electron tracking Compton camera and the like, and accordingly the determination accuracy for the position (direction) of the incident radiation is lowered, which is calculated from the detected energy of the recoil electron and the like. According to the present invention, the noise is reduced which is overlapped when the electrons are amplified, thereby the detection accuracy of the energy of the secondary electrons and the like is enhanced, and the determination accuracy for the incoming

direction of the incident radiation such as the

incident gamma rays is enhanced, which is calculated from the detection accuracy.

(Other embodiment) An object of the present invention can.be achieved also by the embodiment of the following radiation measuring method. Specifically, the method includes supplying a storage medium to a radiation measuring apparatus, a gamma camera or the like, which stores a program code of a software that achieves the functions of the above described embodiment (functions such as gamma ray incident direction calculating unit in measuring unit, and controller for controlling power supply and the like) . The program code of the software can be

supplied also through a network. Then, a computer (or CPU, MPU or the like) of the controller reads out the program code which is stored in the storage medium, and performs the above described functions. In this case, the program code itself which is read out from the storage medium results in achieving the functions of the above described embodiment, and the computer program for measuring the radiation and the storage medium which stores the computer program therein constitute the present invention. In detail, the radiation measuring method of the present embodiment includes at least the following steps of: detecting a scattered radiation which has been generated by Compton scattering that is caused in a scatterer by an incident radiation; amplifying a secondary electron which has been generated by a recoil electron that has been generated by the Compton scattering by using an

electron avalanche phenomenon, and detecting the trajectories of the secondary electrons; and

determining an incoming direction of the incident radiation from information on a detection result of the scattered radiation and a detection result of the trajectories of the secondary electrons. In the trajectory detecting step, an operation of a power supply which supplies a voltage for causing the

electron avalanche phenomenon is stopped, for a predetermined time period which is needed for detecting the trajectories of the secondary electrons.

[0029] he technology of the radiation measurement according to the present invention can be used in a gamma camera and the like which perform environmental radiation measurement, nuclear medicine diagnosis and the like. Advantageous Effects of invention

[ 0030 ] According to the present invention, the noise is

reduced which is overlapped when the secondary

electrons that are generated from the recoil electron which has been generated in the Compton scattering are amplified, and thereby the detection accuracy of the energy of the secondary electrons and the like is enhanced, and the determination accuracy for the incoming direction of the incident radiation such as the incident gamma rays is enhanced, which is

calculated from the energy of the secondary electrons.

[0031] While the present invention has been described with reference to exemplary embodiments, it is to be

understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest

interpretation so as to encompass all such

modifications and equivalent structures and functions.

[0032] This application claims the benefit of Japanese Patent Application No. 2014-070617, filed March 28, 2014, which is hereby incorporated by reference herein in its entirety .

Reference Signs List

[0033] 201 .. gamma ray detecting unit (radiation detecting unit) ,

202 .. monomulti-vibrator,

203 .. oscillator,

204 .. low-voltage power supply,

205 .. booster circuit,

207 .. LPF, 208 .. gas electron multiplication detecting (electron trajectory detecting unit) ,

210 .. power supply

Claims

[Claim 1]A radiation measuring apparatus comprising:

a radiation detecting unit which detects a scattered radiation that has been generated by Compton scattering which is caused in a scatterer by an incident radiation;

an electron trajectory detecting unit which detects a trajectory of a secondary electron which has been generated by a recoil electron that has been generated by the Compton scattering; and

a measuring unit which determines an incoming direction of the incident radiation, from a detection result of the scattered radiation and a detection result of the trajectory of the secondary electron, wherein

an operation of a power supply is stopped which supplies a voltage to the electron trajectory detecting unit, in a predetermined time period which is needed for detecting the trajectory of the secondary electron.

[Claim 2 ] The radiation measuring apparatus according to claim

1, wherein the predetermined time period is a predetermined time period after the arrival of the scattered radiation has been detected in the radiation detecting unit.

[Claim 3] The radiation measuring apparatus according to claim

2, wherein the predetermined time period is a maximum value or more of time periods which are spent before the secondary electrons arrive at the electron trajectory detecting unit after a time point at which the arrival of the scattered radiation has been detected .

[Claim 4 ] The radiation measuring apparatus according to claim

2, wherein the predetermined time period is a maximum value or more of time periods which are spent before the detection of the trajectories of the secondary electrons is completed after a time point at which the arrival of the scattered radiation has been detected .

[Claim 5] The radiation measuring apparatus according to any one of claims 1 to 4, wherein the stop of the operation of the power supply is a stop of a switching operation of a power supply circuit.

[Claim 6] The radiation measuring apparatus according to any one of claims 1 to 5,

further comprising an electric field application unit which applies an electric field for drifting the secondary electron in a direction toward a first electrode, wherein

the scatterer has a gas portion, the first electrode is formed of a part of the electron trajectory detecting unit, a second electrode is arranged at a position facing the electron trajectory detecting unit through the gas portion, and the second electrode is set at a negative potential with respect to the first electrode.

[Claim 7 ] The radiation measuring apparatus according to any one of claims 1 to 6, wherein the electric power supply includes a low pass filter.

[Claim 8] The radiation measuring apparatus according to any one of claims 1 to 7, wherein the incident radiation is gamma rays.

[Claim 9] A radiation measuring method comprising the steps of:

detecting a scattered radiation which has been generated by Compton scattering that is caused in a scatterer by an incident radiation;

amplifying a secondary electron which has been generated by a recoil electron that has been generated by the Compton scattering by using an electron avalanche phenomenon, and detecting trajectories of the secondary electrons; and determining an incoming direction of the incident radiation, from a detection result of the scattered radiation and a detection result of the trajectories of the secondary electrons, wherein

in the trajectory detecting step, a supply of a voltage for causing. the electron avalanche phenomenon is stopped, for a predetermined time period which is needed for detecting the trajectories of the secondary electrons.

[Claim 1.0] The radiation measuring method according to claim 9, wherein the predetermined time period is a

predetermined time period after the arrival of the scattered radiation has been detected in the radiation detecting step.

[Claim 11] A computer program comprising making a computer

perform the radiation measuring method according to claim 9 or 10.