Patent Publication Number: US-11041965-B2

Title: Radiation-detecting device

Description:
TECHNICAL FIELD 
     The present invention relates to a radiation detection device used for detecting a radiation ray. 
     BACKGROUND ART 
     In a positron emission tomography (PET) apparatus, a substance labeled with a radioisotope (RI) that emits positrons is applied to a subject as a tracer. Then, a radiation detector measures a pair of γ-rays generated by annihilation of the positron emitted from the RI substance and the electron in the normal substance, thereby obtaining information about the subject. 
     In a measurement apparatus, such as the PET apparatus, a radiation detector used for detecting radiation rays, such as γ-rays, is appropriately configured by combining, for example, a scintillator that generates scintillation light in response to incidence of a radiation ray and a photodetector that detects the scintillation light and outputs a detection signal (see, for example, Patent Document 1). 
     CITATION LIST 
     Patent Literature 
     
         
         Patent Document 1: Japanese Patent Application Laid-Open Publication No. 2014-160042 
         Patent Document 2: Japanese Patent Publication No. 5531021 
       
    
     Non Patent Literature 
     
         
         Non Patent Document 1: Chen-Ming Chang et al., “Time-over-threshold for pulse shape discrimination in a time-of-flight phoswich PET detector”, Phys. Med. Biol. Vol. 62 (2017) pp. 258-271 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     In a radiation detector used in a PET apparatus, it is important to determine at which position the γ-ray incident on the detector has interacted with a scintillator and detected. In particular, when the γ-ray is detected at a position around the visual field of the detector (a position away from the center), a parallax error occurs and causes a problem that the spatial resolution of γ-ray detection is lowered. In order to prevent the spatial resolution of radiation detection from being lowered, a phoswich type detector has been proposed. 
     In the phoswich type radiation detector, a scintillator used for radiation detection is configured by stacking two types of scintillator units having detection signal time constants different from each other. With this configuration, it is possible to determine which scintillator unit detects the radiation ray, based on information on the time waveform of the detection signal, for example, the time constant of the time waveform. This phoswich type detector can be used as a depth of interaction (DOI) detector. 
     As a method for obtaining a parameter indicating a time waveform, such as a time constant of a detection signal output from a detector, for example, a configuration in which waveform sampling for the time waveform of the detection signal is performed can be used. However, with the configuration of performing the waveform sampling of the detection signal, while a lot of information on the detection signal is obtained, it is unsuitable for radiation measurement at a high count rate, and power consumption is hardly reduced. 
     Further, Patent Document 2 and Non Patent Document 1 disclose the configurations in which the detection signal is compared with a threshold voltage, and a time during which the voltage value of the signal exceeds the threshold voltage (Time over Threshold: ToT) is obtained. However, with these configurations, the time constant itself of the detection signal is not obtained, and it is difficult to determine the time waveform of the detection signal with sufficient accuracy. Further, the problem of acquisition and determination of such information on the time waveform of the detection signal similarly occurs in radiation detectors other than the phoswich type detector described above. 
     The present invention has been made to solve the above problem, and an object thereof is to provide a radiation detection device capable of appropriately acquiring and determining information on a time waveform of a detection signal output from a radiation detector including a scintillator and a photodetector. 
     Solution to Problem 
     A radiation detection device according to the present invention includes (1) a scintillator for generating scintillation light in response to incidence of a radiation ray, (2) a photodetector for detecting the scintillation light output from the scintillator and outputting a detection signal, (3) a first comparator for comparing the detection signal with a first threshold voltage and outputting a first digital signal having a first time width corresponding to a time during which a voltage value of the detection signal exceeds the first threshold voltage, (4) a first time width measurement device for measuring the first time width of the first digital signal, (5) a second comparator for comparing the detection signal with a second threshold voltage different from the first threshold voltage and outputting a second digital signal having a second time width corresponding to a time during which the voltage value of the detection signal exceeds the second threshold voltage, (6) a second time width measurement device for measuring the second time width of the second digital signal, and (7) an analysis unit for obtaining a time constant indicating a time waveform of the detection signal based on the first time width and the second time width. 
     In the above radiation detection device, the first comparator and the second comparator in which threshold voltages different from each other are set are provided for the detection signal output from the radiation detector including the scintillator and the photodetector. Then, the different time widths of the first and second digital signals output from the two comparators are measured by the first and second time width measurement devices, and the time constant which is a parameter indicating the time waveform of the detection signal in response to the radiation detection is obtained based on the obtained first time width and second time width. With this configuration, it is possible to appropriately acquire and determine information on the time waveform of the detection signal with a simple configuration. 
     Advantageous Effects of Invention 
     According to a radiation detection device of the present invention, by providing, for a detection signal output from a radiation detector including a scintillator and a photodetector, first and second comparators in which different threshold voltages are set, measuring time widths of first and second digital signals output from the comparators with first and second time width measurement devices, and obtaining, based on the obtained first and second time widths, a time constant indicating the time waveform of the detection signal, it is possible to appropriately acquire and determine information on the time waveform of the detection signal with a simple configuration. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram schematically illustrating a configuration of a radiation detection device of a first embodiment. 
         FIG. 2  is a flowchart illustrating a radiation detection method in the detection device illustrated in  FIG. 1 . 
         FIG. 3  is a graph illustrating a time waveform of a detection signal output from a photodetector. 
         FIG. 4  is a graph illustrating first and second time widths of a detection signal. 
         FIG. 5  is a table showing rise times and fall times of time waveforms of scintillation light output from scintillators. 
         FIG. 6  is a diagram illustrating a configuration of a PET apparatus using the detection device illustrated in  FIG. 1 . 
         FIG. 7  is a diagram schematically illustrating a configuration of a radiation detection device of a second embodiment. 
         FIG. 8  is a flowchart illustrating a radiation detection method in the detection device illustrated in  FIG. 7 . 
         FIG. 9  is a diagram illustrating a measurement experiment performed using a radiation detector illustrated in  FIG. 7 . 
         FIG. 10  is a graph illustrating the time waveform and the first and second time widths of the detection signal obtained in the measurement experiment illustrated in  FIG. 9 . 
         FIG. 11  is a graph illustrating determination of a scintillator unit based on a time constant of the detection signal obtained in the measurement experiment illustrated in  FIG. 9 . 
         FIG. 12  is a diagram schematically illustrating a configuration of a radiation detection device of a third embodiment. 
         FIG. 13  is a plan view illustrating a configuration of a photodetector in the detection device illustrated in  FIG. 12 . 
         FIG. 14  is a plan view illustrating the configuration of the partially enlarged photodetector illustrated in  FIG. 13 . 
         FIG. 15  is a flowchart illustrating a radiation detection method in the detection device illustrated in  FIG. 12 . 
         FIG. 16  is a diagram schematically illustrating a configuration of a first modification of the photodetector in the detection device illustrated in  FIG. 12 . 
         FIG. 17  is a diagram schematically illustrating a configuration of a second modification of the photodetector in the detection device illustrated in  FIG. 12 . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments of a radiation detection device according to the present invention will be described in detail with reference to the drawings. In the description of the drawings, the same elements will be denoted by the same reference signs, without redundant description. Further, the dimensional ratios in the drawings are not always coincident with those in the description. 
       FIG. 1  is a diagram schematically illustrating a configuration of a radiation detection device of a first embodiment. A radiation detection device  1 A according to the present embodiment includes a radiation detector  10 , a time waveform measurement unit  20 , and an analysis unit  30 . 
     The radiation detector  10  detects an incident radiation ray and outputs a generated electric signal (voltage signal) as a detection signal. The radiation detector  10  in this configuration example includes a scintillator  11  and a photodetector  15 . The scintillator  11  is made of a predetermined scintillation material, and generates scintillation light in response to incidence of a radiation ray to be detected. The time waveform of the scintillation light generated in the scintillator  11  is a predetermined waveform determined depending on the light emission characteristics of the scintillation material. Further, the radiation ray to be detected by the scintillator  11  is, for example, a γ ray, an X ray, an electron, a charged particle, a cosmic ray, or the like. 
     The photodetector  15  detects the scintillation light output from the scintillator  11  and outputs a detection signal. As the photodetector  15 , for example, a photomultiplier tube (PMT), a silicon photomultiplier (SiPM), a multi-pixel photon counter (MPPC), or the like can be used. Further, if the output from the photodetector  15  is a current signal, it is preferable to perform current voltage conversion with an amplifier or the like to generate a detection signal that is a voltage signal. The time waveform of the detection signal is a predetermined waveform determined depending on the time waveform of the scintillation light described above, the light detection characteristics of the photodetector  15 , and the like. A detection signal S 0  generated by the photodetector  15  is output from the output terminal  16  to the time waveform measurement unit  20  of the subsequent stage. 
     The time waveform measurement unit  20  is a measurement circuit unit that measures the time waveform of the detection signal S 0  output from the output terminal  16  of the photodetector  15 . The time waveform measurement unit  20  in the present configuration example includes a first comparator  21 , a second comparator  22 , a first time width measurement device  23 , and a second time width measurement device  24 . The detection signal S 0  output from the photodetector  15  is branched at a branch point  17 , and the branched detection signals S 0  are respectively input to the first comparator  21  and the second comparator  22 . 
     To the first comparator  21 , a first threshold voltage V 1  is applied. The first comparator  21  compares the detection signal S 0 , which is a voltage signal, with the first threshold voltage V 1 , and outputs a first digital signal S 1  having a first time width T 1  corresponding to a time during which the voltage value of the detection signal S 0  exceeds the threshold voltage V 1 . Further, to the second comparator  22 , a second threshold voltage V 2  having a voltage value different from that of the first threshold voltage V 1  is applied. The second comparator  22  compares the detection signal S 0  with the second threshold voltage V 2 , and outputs a second digital signal S 2  having a second time width T 2  corresponding to a time during which the voltage value of the detection signal S 0  exceeds the threshold voltage V 2 . 
     The first time width measurement device  23  measures the first time width T 1  of the first digital signal S 1  output from the first comparator  21 , and outputs the obtained data on the first time width T 1  to the analysis unit  30  of the subsequent stage. Further, the second time width measurement device  24  measures the second time width T 2  of the second digital signal S 2  output from the second comparator  22 , and outputs the obtained data on the second time width T 2  to the analysis unit  30 . Each of the first time width measurement device  23  and the second time width measurement device  24  is preferably configured by a time to digital converter (TDC). 
     The analysis unit (analysis device)  30  obtains, based on the first time width T 1  and the second time width T 2  respectively input from the first and second time width measurement devices  23  and  24 , a time constant τ, which is a parameter indicating the time waveform of the detection signal S 0 . The time constant τ is, for example, a fall time τd of the time waveform of the detection signal S 0  to be described later. Further, the analysis unit  30  may obtain, as the time constant τ, a parameter indicating the time waveform other than the fall time τd. Further, the analysis unit  30  may further obtain a pulse height E of the time waveform of the detection signal S 0  based on the time constant τ as necessary. As the analysis unit  30 , a computer including a CPU and a memory, a field programmable gate array (FPGA), or the like can be used, for example. 
     A display unit (display device)  31  and a storage unit (storage device)  32  are connected to the analysis unit  30 . The display unit  31  displays an analysis result of the detection signal S 0  by the analysis unit  30 , such as the time constant τ derived as described above, as necessary. The storage unit  32  stores data on the first and second time widths T 1  and T 2  input to the analysis unit  30 , data on the analysis result, such as the time constant τ derived by the analysis unit  30 , and the like. 
     The effect of the radiation detection device  1 A according to the above embodiment is described. 
     In the radiation detection device  1 A illustrated in  FIG. 1 , the first comparator  21  and the second comparator  22  in which the threshold voltages V 1  and V 2  different from each other are set are provided for the detection signal S 0  output from the radiation detector  10  including the scintillator  11  and the photodetector  15 . Then, different time widths of the first and second digital signals S 1  and S 2  respectively output from the two comparators  21  and  22  are measured by the first and second time width measurement devices  23  and  24 , and the time constant τ indicating the time waveform of the detection signal S 0  in response to the radiation detection is obtained by the analysis unit  30  based on the obtained first time width T 1  and second time width T 2 . With this configuration, it is possible to appropriately acquire and determine information on the time waveform of the detection signal S 0  with a simple configuration without performing waveform sampling or the like. 
     Further, in the above detection device  1 A, the analysis unit  30  may further obtain, based on the time constant τ, the pulse height E of the time waveform of the detection signal S 0  in addition to the time constant τ. With this configuration, it is possible to easily obtain the pulse height E of the detection signal S 0  at high speed with low power consumption without providing a pulse height measurement device, such as an analog to digital converter (ADC), separately from the time waveform measurement unit  20  including the comparators  21  and  22  and the time width measurement devices  23  and  24 . In addition, the pulse height E may not be obtained if unnecessary. 
       FIG. 2  is a flowchart illustrating a radiation detection method performed in the radiation detection device  1 A illustrated in  FIG. 1 . Further,  FIG. 3  is a graph illustrating the time waveform of the detection signal S 0  output from the photodetector  15 . Further,  FIG. 4  is a graph illustrating the first and second time widths T 1  and T 2  obtained by applying the first and second threshold voltages V 1  and V 2  to the detection signal S 0 . In the following, a radiation detection method according to the present embodiment will be described together with specific examples of the time waveform of the detection signal S 0  and a method of deriving the time constant τ and the like. 
     In the radiation detection method illustrated in  FIG. 2 , first, a radiation ray is detected by the radiation detector  10  including the scintillator  11  and the photodetector  15 , and a detection signal S 0  is output from the output terminal  16  of the photodetector  15  in response to the incidence of the radiation ray (step S 11 ).  FIG. 3  schematically illustrates an example of the time waveform of the detection signal S 0  output from the photodetector  15 . In the graph of  FIG. 3 , the horizontal axis indicates time, and the vertical axis indicates the voltage value of the detection signal S 0 . 
     In the time waveform of the detection signal S 0  illustrated in  FIG. 3 , the part before a signal peak Sp is a signal rising part Sr, and the part after the signal peak Sp is a signal falling part Sd. Further, the time waveform of the detection signal S 0  having a shape illustrated in  FIG. 3  can be expressed by, for example, the following Formula (1). 
             [     Formula   ⁢           ⁢   1     ]                       f   ⁡     (   t   )       =         E   ⁡     (     1   +       τ   d       τ   r         )           τ   r     /     τ   d         ⁢       {     1   -       (       τ   d       τ   r       )       -   1         }       -   1       ⁢       e     -     t     τ   d           ⁡     (     1   -     e     -     t     τ   r             )                 (   1   )               
Here, in Formula (1), E represents a pulse height that is a voltage value at the signal peak Sp, τr represents a rise time (rise time constant) of the signal rising part Sr, and τd represents a fall time (fall time constant) of the signal falling part Sd.
 
     The detection signal S 0  output from the radiation detector  10  is input to the first and second comparators  21  and  22  in the time waveform measurement unit  20 . The first comparator  21  compares the detection signal S 0  with the first threshold voltage V 1  and outputs the first digital signal S 1  having the first time width T 1  corresponding to a time during which the voltage value of the detection signal S 0  exceeds the threshold voltage V 1 , as illustrated in the graph of  FIG. 4 . Further, the second comparator  22  compares the detection signal S 0  with the second threshold voltage V 2  and similarly outputs the second digital signal S 2  having the second time width T 2  corresponding to a time during which the voltage value of the detection signal S 0  exceeds the threshold voltage V 2  (step S 12 ). The first and second time widths T 1  and T 2  are respectively measured by the first and second time width measurement devices  23  and  24  (step S 13 ). 
     In addition,  FIG. 3  and  FIG. 4  illustrate that the signal peak Sp of the time waveform of the detection signal S 0  is in the positive direction with respect to the voltage, but if the signal peak Sp of the detection signal S 0  is in the negative direction with respect to the voltage, the time width is only required to be, for example, a time width corresponding to a time during which the voltage value of the detection signal S 0  the positive/negative of which is inverted exceeds the threshold voltage. This corresponds to a time during which the voltage value of the original detection signal is below the threshold voltage. 
     The analysis unit  30  derives the time constant τ indicating the time waveform of the detection signal S 0  based on the first and second time widths T 1  and T 2  and the like measured by the first and second time width measurement devices  23  and  24  (step S 14 ). Further, the analysis unit  30  derives the pulse height E of the time waveform of the detection signal S 0  based on the first and second time widths T 1  and T 2 , the time constant τ, and the like, as necessary (step S 15 ). 
     Here, in the time waveform of the detection signal S 0  output from the photodetector  15 , if the rise time τr is sufficiently shorter than the fall time τd, the first time width T 1  of the detection signal S 0  for the first threshold voltage V 1  is expressed by the following Formula (2). 
             [     Formula   ⁢           ⁢   2     ]                       T   ⁢           ⁢   1     =       τ   d     ⁢   log   ⁢     E     V   ⁢   1                 (   2   )               
Further, the second time width T 2  of the detection signal S 0  for the second threshold voltage V 2  is similarly expressed by the following Formula (3).
 
     
       
         
           
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                     T 
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                     ⁢ 
                     2 
                   
                   = 
                   
                     
                       τ 
                       d 
                     
                     ⁢ 
                     log 
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                       E 
                       
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                         2 
                       
                     
                   
                 
               
               
                 
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     Thus, when the time constant τ derived by the analysis unit  30  as a parameter of the time waveform is the fall time τd of the time waveform of the detection signal S 0 , the time constant τ can be obtained with the following Formula (4). 
     [Formula 4]
 
τ=τ d =( T 1− T 2)/log( V 2/ V 1)  (4)
 
With Formula (4), it is possible to appropriately easily obtain the time constant τ of the detection signal S 0 .
 
     Further, when the analysis unit  30  obtains the pulse height E of the detection signal S 0  in addition to the time constant τ, the pulse height E can be obtained with the following Formula (5) using the fall time τd obtained as the time constant τ. 
             [     Formula   ⁢           ⁢   5     ]                     E   =       V   ⁢           ⁢   1   ⁢   exp   ⁢       T   ⁢   1       τ   d         =     V   ⁢           ⁢   2   ⁢   exp   ⁢       T   ⁢   2       τ   d                   (   5   )               
In addition, the first and second threshold voltages V 1  and V 2  in the first and second comparators  21  and  22  can be arbitrarily set and adjusted so as to easily obtain the time constant τ and the like.
 
     Further, regarding the above waveform condition that the rise time τr is sufficiently shorter than the fall time τd in the detection signal S 0 , specifically, it is preferable that, for example, the rise time τr of the time waveform of the detection signal S 0  with the fall time τd satisfies the following condition.
 
(τ r/τd )&lt;0.1
 
     Here,  FIG. 5  is a table showing rise times τr and fall times τd of time waveforms of scintillation light output from scintillators.  FIG. 5  shows rise times τr and fall times τd of time waveforms of existing scintillators of LSO, LYSO, LaBr3, GSO, and GAGG used in PET apparatuses. These scintillators are considered to sufficiently satisfy the above condition that the rise time τr is sufficiently shorter than the fall time τd. 
     The radiation detection device  1 A having the configuration illustrated in  FIG. 1  can be suitably applied to, for example, a PET apparatus.  FIG. 6  is a diagram illustrating a configuration of a PET apparatus to which the radiation detection device illustrated in  FIG. 1  is applied. A PET apparatus  2 A is configured by arranging a plurality of radiation detectors  10  each including the scintillator  11  and the photodetector  15  so as to surround a subject P. Further, for the detection signal S 0  output from each radiation detector  10 , a signal processing unit  60  including the time waveform measurement unit  20  and the analysis unit  30  illustrated in  FIG. 1  is provided. 
     In the PET apparatus  2 A, a pair of γ rays generated by annihilation of the positron inside the subject P is detected by the radiation detectors  10 . In the example illustrated in  FIG. 6 , a pair of γ rays generated at a measurement point P 1  inside the subject P is detected by radiation detectors  101  and  102 . Further, a pair of γ rays generated at a measurement point P 2  is detected by radiation detectors  103  and  104 . 
     The detection signal S 0  output from the radiation detector  10  is input to the signal processing unit  60 , and the signal processing unit  60  measures the first and second time widths T 1  and T 2  of the detection signal S 0  and derives the time constant τ of the time waveform, as described above with reference to  FIG. 1 . Further, based on the obtained time constant τ, the characteristics of the radiation detector  10 , such as the characteristics of the scintillator  11 , are derived. Information on the derived characteristic of the radiation detector  10  can be used, for example, to improve the performance of the PET apparatus  2 A. 
       FIG. 7  is a diagram schematically illustrating a configuration of a radiation detection device of a second embodiment. A radiation detection device  1 B according to the present embodiment includes a radiation detector  10 B, a time waveform measurement unit  20 , and an analysis unit  30 . Among these, the configurations of the time waveform measurement unit  20  and the analysis unit  30  are similar to those illustrated in  FIG. 1 . Further, in  FIG. 7 , a display unit  31  and a storage unit  32  connected to the analysis unit  30  are not illustrated. 
     The radiation detector  10 B in this configuration example includes a scintillator  11  and a photodetector  15 . Further, the scintillator  11  is configured by arranging a first scintillator unit  12  and a second scintillator unit  13  in this order from the photodetector  15 . 
     The first scintillator unit  12  is made of a first scintillation material, and generates scintillation light having a predetermined time waveform in response to incidence of a radiation ray. The second scintillator unit  13  is made of a second scintillation material different from the first scintillation material, and generates scintillation light having a time waveform different from that of the first scintillator unit  12  in response to incidence of a radiation ray. The photodetector  15  detects the scintillation light output from the first scintillator unit  12  or the second scintillator unit  13 , and outputs a detection signal S 0  via the output terminal  16  and an amplifier  18 . At this time, the time waveform of the detection signal S 0  output from the photodetector  15  varies depending on whether the detected radiation ray has interacted with the first scintillator unit  12  or the second scintillator unit  13 . 
       FIG. 8  is a flowchart illustrating a radiation detection method performed in the radiation detection device  1 B illustrated in  FIG. 7 . In the radiation detection method illustrated in  FIG. 8 , first, a radiation ray is detected by the radiation detector  10 B constituted by the scintillator  11  including the first and second scintillator units  12  and  13 , and the photodetector  15 , and the detection signal S 0  is output from the output terminal  16  of the photodetector  15  (step S 21 ). 
     The detection signal S 0  is input to the first and second comparators  21  and  22  in the time waveform measurement unit  20  via the amplifier  18  and the branch point  17 . The first comparator  21  compares the detection signal S 0  with the first threshold voltage V 1  and outputs the first digital signal S 1  having the first time width T 1 . Further, the second comparator  22  compares the detection signal S 0  with the second threshold voltage V 2  and outputs the second digital signal S 2  having the second time width T 2  (step S 22 ). The first and second time widths T 1  and T 2  are respectively measured by the first and second time width measurement devices  23  and  24  (step S 23 ). 
     The analysis unit  30  derives the time constant τ indicating the time waveform of the detection signal S 0  based on the first and second time widths T 1  and T 2  measured by the first and second time width measurement devices  23  and  24  (step S 24 ). Further, the analysis unit  30  determines, based on the obtained time constant τ, whether the detection signal S 0  output from the photodetector  15  is caused by scintillation light generated in the first scintillator unit  12  or the second scintillator unit  13 , that is, whether the radiation ray is detected by the first scintillator unit  12  or the second scintillator unit  13  (step S 25 ). 
     In this manner, with the configuration for obtaining the time constant τ of the detection signal S 0  based on the first and second time widths T 1  and T 2  as described above, it is possible to determine, based on the obtained time constant τ, whether the radiation ray is detected by the first scintillator unit  12  or the second scintillator unit  13 , in the case where the scintillator  11  includes the first and second scintillator units  12  and  13 . Further, it is possible to similarly perform such determination of the scintillator unit when the scintillator includes three or more scintillator units. 
     A measurement experiment on the determination of the scintillator units based on the time constant τ of the detection signal S 0  was performed.  FIG. 9  is a diagram illustrating a measurement experiment performed using the radiation detector  10 B illustrated in  FIG. 7 . In this measurement experiment, the radiation detector  10 B was placed in a thermostatic chamber  35  at a temperature of 25° C. Regarding the configuration of the radiation detector  10 B, a 5×5×5 mm 3  GSO scintillator was used as the first scintillator unit  12 , and a 3×3×10 mm 3  GAGG scintillator was used as the second scintillator unit  13 . 
     Further, S13360-3050 manufactured by Hamamatsu Photonics was used as the MPPC of the photodetector  15 . The light receiving surface size of this MPPC is 3.0×3.0 mm 2 , and the array pitch of a plurality of photodetection pixels arrayed two-dimensionally is 50 μm. Further, regarding the voltage applied to the MPPC, the voltage exceeding the breakdown voltage was set to V excess =4.0 V. Further, a  22 Na radiation source was disposed as a radiation source  36  at a position separated by 5 cm from the scintillator  11  including the first and second scintillator units  12  and  13 , and the γ rays from the radiation source  36  were detected by the radiation detector  10 B. 
     Further, in this measurement experiment, an oscilloscope  38  instead of the time waveform measurement unit  20  illustrated in  FIG. 7  was provided for the detection signal S 0  output from the output terminal  16  of the photodetector  15 , and the time waveform data measured by the oscilloscope  38  was taken into the computer (PC) of the analysis unit  30 , and then the first and second time widths T 1  and T 2 , the time constant τ and the like of the time waveform of the detection signal S 0  were analyzed by software. Further, as the oscilloscope  38 , DSO-S404A manufactured by Keysight Corporation was used. 
       FIG. 10  is a graph illustrating the time waveform and the first and second time widths T 1  and T 2  of the detection signal S 0  obtained in the measurement experiment illustrated in  FIG. 9 . Here, the analysis unit  30  performed fitting to time waveform data S 6  of the detection signal S 0  obtained by the oscilloscope  38  with a theoretical formula, and a time waveform S 7  was obtained as a fitting result. Further, numerical analysis was performed for the time waveform S 7  by setting the first and second threshold voltages V 1  and V 2  to obtain the first time width T 1  and the second time width T 2 . Further, the time constant τ of the detection signal S 0  was obtained based on the first and second time widths T 1  and T 2 . 
       FIG. 11  is a graph illustrating determination of the scintillator unit based on the time constant τ of the detection signal S 0  obtained in the measurement experiment illustrated in  FIG. 9 . In the graph of  FIG. 11 , the horizontal axis indicates the fall time τd (ns) of the detection signal S 0  obtained as the time constant τ. In the experimental result illustrated in  FIG. 11 , the detection data by the GSO scintillator distributed in a region R 1  in which the fall time τd is short and the detection data by the GAGG scintillator distributed in a region R 2  in which the fall time τd is long can be clearly determined. Such a determination function of the scintillator can be applied to, for example, determining scintillator units in a phoswich type detector configured by stacking a plurality of types of scintillator units having different time constants of a detection signal, and thus, it is possible to achieve a detection device capable of supporting a high count rate and reducing power consumption. 
       FIG. 12  is a diagram schematically illustrating a configuration of a radiation detection device of a third embodiment. A radiation detection device  1 C according to the present embodiment includes a radiation detector  10 C, a time waveform measurement unit  20 , and an analysis unit  30 . Among these, the configurations of the time waveform measurement unit  20  and the analysis unit  30  are similar to those illustrated in  FIG. 1 . Further, in  FIG. 12 , a display unit  31  and a storage unit  32  connected to the analysis unit  30 , and a scintillator  11  included in the radiation detector  10 C are not illustrated. 
     The radiation detector  10 C in this configuration example includes a scintillator  11  and a photodetector  15 . Further, as the photodetector  15 , a photodetector  50  configured as an MPPC including a plurality of photodetection pixels (photodetection units) is used.  FIG. 13  is a plan view illustrating a configuration of the photodetector  50  in the radiation detection device  1 C illustrated in  FIG. 12 . Further,  FIG. 14  is a plan view illustrating the configuration of the partially enlarged photodetector  50  illustrated in  FIG. 13 .  FIG. 14  is an enlarged view of a central region  51  of the photodetector  50  illustrated in  FIG. 13 . 
     The photodetector  50  includes N photodetection pixels (micropixels)  52  that are arranged one-dimensionally or two-dimensionally and each generate a detection signal S 0  in response to incidence of light, and a single output terminal  16  that outputs the detection signal S 0  generated in each of the N photodetection pixels  52  to the outside. Here, N is an integer of 2 or more. Further, regarding a specific configuration of the MPPC, Patent Document 1 can be referred to, for example. 
     In the configuration example illustrated in  FIG. 13  and  FIG. 14 , the N photodetection pixels  52  are two-dimensionally arranged on the detector chip of the photodetector  50 . Further, at the center of the detector chip, a common electrode  58  for collecting the detection signals S 0  from the photodetection pixels  52  is disposed. In addition, in  FIG. 13 , the photodetection pixels  52  are illustrated only in the vicinity of both ends of the detector chip in order for the common electrode  58  to be easily recognized and the like. 
     Each of the N photodetection pixels  52  of the photodetector  50  includes an avalanche photodiode (APD)  53  that operates in Geiger mode, and a quenching resistor  54  connected in series to the APD  53 . Further, the quenching resistor  54  is connected to the common electrode  58  via a signal line  59  as illustrated in  FIG. 14 . The detection signal S 0  generated by each photodetection pixel  52  is output from the output terminal  16  to the outside via the signal line  59  and the common electrode  58 . 
     Further, the N photodetection pixels  52  of the photodetector  50  are configured to output detection signals S 0  having time waveforms different from each other (time constants different from each other). Specifically, in the present configuration example, the photodetector  50  is configured such that the quenching resistors  54  that determine, in the N photodetection pixels  52 , the time waveforms and time constants of the detection signals have resistance values different from each other. 
       FIG. 15  is a flowchart illustrating a radiation detection method performed in the radiation detection device  1 C illustrated in  FIG. 12 . In the radiation detection method illustrated in  FIG. 15 , first, a radiation ray is detected by the radiation detector  10 C constituted by the scintillator  11  and the photodetector  50  including the N photodetection pixels  52 , and a detection signal S 0  is output from the output terminal  16  of the photodetector  50  (step S 31 ). 
     The detection signal S 0  is input to the first and second comparators  21  and  22  in the time waveform measurement unit  20  via the amplifier  18  and the branch point  17 . The first comparator  21  compares the detection signal S 0  with the first threshold voltage V 1  and outputs the first digital signal S 1  having the first time width T. Further, the second comparator  22  compares the detection signal S 0  with the second threshold voltage V 2  and outputs the second digital signal S 2  having the second time width T 2  (step S 32 ). The first and second time widths T 1  and T 2  are respectively measured by the first and second time width measurement devices  23  and  24  (step S 33 ). 
     The analysis unit  30  derives the time constant τ indicating the time waveform of the detection signal S 0  based on the first and second time widths T 1  and T 2  measured by the first and second time width measurement devices  23  and  24  (step S 34 ). Further, the analysis unit  30  determines, based on the obtained time constant τ, which one of the N photodetection pixels (photodetection units) has output the detection signal S 0  (step S 35 ). 
     In this manner, with the configuration for obtaining the time constant τ of the detection signal S 0  based on the first and second time widths T 1  and T 2  as described above, it is possible to determine, based on the obtained time constant τ, which one of the N photodetection pixels  52  has output the detection signal S 0 , in the case where the photodetector  50  includes the N photodetection pixels (photodetection units)  52 . 
     In addition, in this configuration example, the photodetector  50  includes the N photodetection pixels  52  as described above, and the number of the photodetection pixels (photodetection units)  52  is arbitrarily set to two or more. For example, when the photodetector  50  includes a first photodetection unit that outputs a detection signal having a predetermined time waveform, and a second photodetection unit that outputs a detection signal having a time waveform different from that of the first photodetection unit, it is possible to determine, based on the obtained time constant τ, whether the detection signal S 0  is output from the first photodetection unit or the second photodetection unit. 
     Further, regarding the configuration of the N photodetection pixels  52  of the photodetector  50  that output detection signals having time waveforms different from each other, various configurations other than the configuration illustrated in  FIG. 12  is applicable. 
       FIG. 16  is a diagram schematically illustrating a configuration of a first modification of the photodetector  15  in the radiation detection device  1 C illustrated in  FIG. 12 . In the present configuration example, the photodetector  15  is configured as a photodetector  50 A including N photodetection pixels  52  and a single output terminal  16 . 
     Each of the N photodetection pixels  52  of the photodetector  50 A includes an APD  53  that operates in Geiger mode, a quenching resistor  54  connected in series to the APD  53 , and a frequency filter  55  connected in series between the quenching resistor  54  and the output terminal  16 . 
     Further, in this configuration example, the photodetector  50 A is configured such that the frequency filters  55  of the N photodetection pixels  52  have frequency characteristics different from each other. Thus, the N photodetection pixels  52  of the photodetector  50 A output detection signals S 0  having time waveforms different from each other. The frequency filters  55  of the N photodetection pixels  52  are, for example, high-pass filters, low-pass filters, or band-pass filters having cutoff frequencies different from each other. 
       FIG. 17  is a diagram schematically illustrating a configuration of a second modification of the photodetector  15  in the radiation detection device  1 C illustrated in  FIG. 12 . In the present configuration example, the photodetector  15  is configured as a photodetector  50 B including N photodetection pixels  52  and a single output terminal  16 . 
     Each of the N photodetection pixels  52  of the photodetector  50 B includes an APD  53  that operates in Geiger mode, a quenching resistor  54  connected in series to the APD  53 , and a capacitor  56  connected in parallel to the APD  53 . 
     Further, in this configuration example, the photodetector  50 B is configured such that the capacitors  56  of the N photodetection pixels  52  have capacitance values different from each other. Thus, the N photodetection pixels  52  of the photodetector  50 B output detection signals S 0  having time waveforms different from each other. 
     The radiation detection device according to the present invention is not limited to the above embodiments and configuration examples, and can be variously modified. For example, in the configurations illustrated in  FIG. 7  and  FIG. 12 , the amplifier  18  is provided for the detection signal S 0  output from the photodetector  15 , however, this amplifier  18  may not be provided if unnecessary. 
     Further, regarding the time constant τ indicating the time waveform of the detection signal S 0  obtained by the analysis unit  30 , the fall time τd is used as the time constant τ in the above configuration example, but another parameter related to the time waveform may be obtained as the time constant τ as long as the time waveform of the detection signal S 0  can be determined. Further, regarding the time width of the detection signal S 0  used for deriving the time constant τ, the first and second time widths T 1  and T 2  are measured in the above configuration example, but three or more types of time widths may be measured, for example. 
     The radiation detection device of the above embodiment is configured to include (1) a scintillator for generating scintillation light in response to incidence of a radiation ray, (2) a photodetector for detecting the scintillation light output from the scintillator and outputting a detection signal, (3) a first comparator for comparing the detection signal with a first threshold voltage and outputting a first digital signal having a first time width corresponding to a time during which a voltage value of the detection signal exceeds the first threshold voltage, (4) a first time width measurement device for measuring the first time width of the first digital signal, (5) a second comparator for comparing the detection signal with a second threshold voltage different from the first threshold voltage and outputting a second digital signal having a second time width corresponding to a time during which the voltage value of the detection signal exceeds the second threshold voltage, (6) a second time width measurement device for measuring the second time width of the second digital signal, and (7) an analysis unit for obtaining a time constant indicating a time waveform of the detection signal based on the first time width and the second time width. 
     Here, in the above detection device, the scintillator may include a first scintillator unit for generating scintillation light having a predetermined time waveform, and a second scintillator unit for generating scintillation light having a time waveform different from that of the first scintillator unit. Further, in this case, the analysis unit may determine, based on the obtained time constant, whether the detection signal output from the photodetector is caused by the scintillation light generated in the first scintillator unit or the second scintillator unit. With this configuration, it is possible to reliably determine the scintillator unit based on the time constant of the detection signal. 
     Further, in the above detection device, the photodetector may include a first photodetection unit for outputting a detection signal having a predetermined time waveform, and a second photodetection unit for outputting a detection signal having a time waveform different from that of the first photodetection unit. Further, in this case, the analysis unit may determine, based on the obtained time constant, whether the detection signal is output from the first photodetection unit or the second photodetection unit. With this configuration, it is possible to reliably determine the photodetection unit based on the time constant of the detection signal. 
     Regarding a specific configuration of the detection device, each of the first time width measurement device and the second time width measurement device may include a time to digital converter. Thus, it is possible to appropriately measure the first and second time widths of the detection signal. 
     In the above detection device, a rise time τr in the time waveform of the detection signal with a fall time τd may satisfy a condition,
 
(τ r/τd )&lt;0.1.
 
Further, in the above detection device, the analysis unit may obtain the time constant τ with a formula,
 
τ=( T 1 −T 2)/log( V 2 /V 1)
 
where V 1  is the first threshold voltage, T 1  is the first time width, V 2  is the second threshold voltage, and T 2  is the second time width. With these configurations, it is possible to appropriately obtain the time constant τ of the detection signal.
 
     Further, in the above detection device, the analysis unit may further obtain a pulse height of the time waveform of the detection signal based on the time constant. With this configuration, it is possible to appropriately obtain the pulse height of the detection signal without providing a pulse height measurement device separately from the time waveform measurement unit including the comparator and the time width measurement device. 
     INDUSTRIAL APPLICABILITY 
     The present invention can be used as a radiation detection device capable of appropriately acquiring and determining information on the time waveform of a detection signal output from a radiation detector including a scintillator and a photodetector. 
     REFERENCE SIGNS LIST 
       1 A,  1 B,  1 C—radiation detection device,  10 ,  10 B,  10 C—radiation detector,  11 —scintillator,  12 —first scintillator unit,  13 —second scintillator unit,  15 —photodetector,  16 —output terminal,  17 —branch point,  18 —amplifier, 
       20 —time waveform measurement unit,  21 —first comparator,  22 —second comparator,  23 —first time width measurement device,  24 —second time width measurement device, 
       30 —analysis unit,  31 —display unit,  32 —storage unit,  35 —thermostatic chamber,  36 —radiation source,  38 —oscilloscope,  2 A—PET apparatus,  60 —signal processing unit, 
       50 ,  50 A,  50 B—photodetector,  51 —region,  52 —photodetection pixel,  53 —avalanche photodiode (APD),  54 —quenching resistor,  55 —frequency filter,  56 —capacitor,  58 —common electrode,  59 —signal line, 
     S 0 —detection signal, Sp—signal peak, Sr—signal rising part, Sd—signal falling part, S 1 —first digital signal, S 2 —second digital signal, V 1 —first threshold voltage, V 2 —second threshold voltage, T 1 —first time width, T 2 —second time width.