JP2018141735A - Charge detection circuit and radiation detector including the same - Google Patents

Abstract

An object of the present invention is to accurately measure the crest value of a charge signal that changes with time with a simple circuit configuration. A charge detection circuit 12 is a circuit for detecting the amount of generated charge in time series, and includes a capacitor. a converting preamplifier 13, an ADC 14 generating a comparison signal by comparing the voltage signal with a predetermined threshold voltage at predetermined time intervals, and a predetermined state of the comparison signal generated by the ADC 14 at predetermined time intervals. and a counter 15 for outputting a count value obtained by counting the number of durations of , and a predetermined amount of output from the capacitor of the preamplifier 13 when the comparison signal generated by the ADC 14 at predetermined time intervals indicates a predetermined state. and a charge injection circuit 16 for sequentially extracting charges. [Selection drawing] Fig. 1

Description

  The present invention relates to a charge detection circuit that detects the amount of generated charge in time series and a radiation detection apparatus including the same.

  Conventionally, a mechanism of a detection circuit that detects a time-varying amount of charge in a time series for a current signal (charge signal) output from a radiation detector such as a semiconductor detector has been studied. For example, the following Non-Patent Document 1 proposes a detection circuit that employs a ToT (Time over Threshold) method. This detection circuit converts the current signal from the detector into a voltage signal with an amplifier, measures the time when the pulse of the voltage signal exceeds the threshold, and outputs the time as a measurement result of the pulse peak value. it can.

T. Orita, H. Takahashi and K. Shimazoe, “Dynamic Time-over-Threshold Method for Multi-Channel APD Based Gamma-Ray Detectors”, 2012 IEEE Nuclear Science Symposium and Medical Imaging Conference Record (NSS / MIC), 2012 , P. 824-826

  However, in the detection circuit employing the above-described ToT method, the time width of the pulse of the voltage signal is converted into a peak value. However, since the relationship between the time value and the peak value is not linear, The accuracy of measurement results tends to be low. In addition, since this detection circuit requires a shaping circuit such as a filter circuit for shaping the pulse waveform of the voltage signal, the circuit configuration tends to be complicated.

  The present invention has been made in view of the above problems, and a charge detection circuit capable of accurately measuring a peak value of a charge signal that changes with time with a simple circuit configuration, and a radiation detection apparatus including the charge detection circuit The purpose is to provide.

  In order to solve the above-described problem, a charge detection circuit according to an embodiment of the present invention is a circuit that detects the amount of generated charge in time series, and includes a capacitor. An amplifier that converts the charge into a voltage signal, a comparator that generates a comparison signal by comparing the voltage signal with a predetermined threshold voltage at a predetermined time interval, and a comparator that generates the signal at a predetermined time interval. A counter that outputs a count value obtained by counting the number of times the comparison signal is kept in a predetermined state, and when the comparison signal generated at a predetermined time interval by the comparator indicates a predetermined state, A charge extracting unit that sequentially extracts a predetermined amount of charge from the capacitor.

  Or the radiation detection apparatus concerning the other form of this invention is equipped with the charge detection circuit of the said structure, and the semiconductor detector which generate | occur | produces an electric charge according to incidence | injection of a radiation, and inputs an electric charge into a charge detection circuit.

  According to the charge detection circuit of the above aspect, a voltage signal is generated by accumulating the input charge in the amplifier, and a comparison signal is generated by periodically comparing the voltage signal with the threshold voltage in the comparator. Then, when the comparison signal shows a predetermined state by the charge extracting unit, a predetermined amount of electric charge is extracted from the electric charge accumulated in the amplifier. As a result, the input charge signal is converted into a continuous comparison signal converted in the time axis direction corresponding to the amount of the charge and output. According to the charge detection circuit having such a configuration, the crest value of the charge signal can be counted and output with high accuracy, and the shaping circuit is not required for shaping the voltage waveform, so that the circuit configuration is simple. It becomes. As a result, the peak value of the charge signal that changes with time can be accurately measured with a simple circuit configuration. Or according to the radiation detection apparatus of the said form, size reduction of an apparatus is easy and a highly accurate radiation detection apparatus is realizable.

  Here, the counter counts the number of times the predetermined state indicating that the voltage signal has exceeded the predetermined threshold voltage, and the charge extracting unit determines that the comparison signal has exceeded the predetermined threshold voltage. It is also possible to draw out a predetermined amount of charge when indicating a predetermined state. In this case, the peak value of the input charge signal can be measured with high accuracy, and the circuit configuration of the charge extraction unit is simplified.

  The charge extraction unit may include a capacitor for storing a predetermined amount of charge for extraction in advance, or may include a constant current source for extracting a predetermined amount of charge. In this case, the circuit configuration of the charge extracting unit is easily simplified.

  According to the present invention, it is possible to accurately measure the peak value of a charge signal that changes with time with a simple circuit configuration.

It is a block diagram which shows schematic structure of the radiation detection apparatus concerning embodiment. FIG. 2 is a diagram illustrating a specific circuit configuration of a charge extraction unit of the charge detection circuit of FIG. 1. FIG. 2 is a diagram illustrating a specific circuit configuration of a charge extraction unit of the charge detection circuit of FIG. 1. It is a figure which shows the time waveform of the various signals processed in the radiation detection apparatus of FIG. It is a block diagram which shows schematic structure of the radiation detection apparatus concerning a comparative example.

  Hereinafter, preferred embodiments of a charge detection circuit and a radiation detection apparatus according to the present invention will be described in detail with reference to the drawings. In the description of the drawings, the same or corresponding parts are denoted by the same reference numerals, and redundant description is omitted.

  First, with reference to FIG. 1, the function and structure of the radiation detection apparatus 10 which concerns on one Embodiment of the radiation detection apparatus of this invention are demonstrated. A radiation detection apparatus 10 shown in FIG. 1 is an apparatus for detecting the intensity of incident radiation that changes over time. The radiation detection apparatus 10 includes a semiconductor detector 11 composed of one detection element and a charge detection circuit 12 that processes an output from the semiconductor detector. The radiation detection apparatus 10 has a two-dimensional distribution of the intensity of incident radiation. It may be composed of a plurality of semiconductor detectors 11 arranged two-dimensionally and a plurality of charge detection circuits 12 provided corresponding to each of the plurality of semiconductor detectors 11 so that measurement is possible. Such a two-dimensional array circuit configuration can be realized by forming an integrated circuit on a semiconductor substrate.

  As shown in FIG. 1, the radiation detection apparatus 10 includes a semiconductor detector 11 and a charge detection circuit 12. The charge detection circuit 12 is a circuit that is electrically connected to the semiconductor detector 11 and receives a charge signal generated according to the intensity of incident radiation from the semiconductor detector 11, and is a preamplifier (amplifier). 13, an analog-digital converter (ADC) 14, a counter 15, and a charge injection circuit (charge extraction unit) 16. Hereinafter, the configuration of each component of the radiation detection apparatus 10 will be described.

  The semiconductor detector 11 is a detector that generates electron-hole pairs (charge pairs) with the incidence of radiation such as X-rays and converts the incident radiation into a current signal (charge signal) corresponding to the amount of energy. . Examples of such detectors include Cd (Zn) Te semiconductor detectors, Si semiconductor detectors, Ge semiconductor detectors, GaAs semiconductor detectors, GaN semiconductor detectors, TlBr semiconductor detectors, and the like.

  The preamplifier 13 of the charge detection circuit 12 is a circuit unit that receives charge as a current signal from the semiconductor detector 11 and converts the charge into a voltage signal by accumulating the charge. Specifically, the preamplifier 13 includes a capacitor, and outputs a voltage across the capacitor as a voltage signal by accumulating electric charge in the capacitor.

  The ADC 14 of the charge detection circuit 12 is a circuit that periodically compares a voltage signal with a predetermined threshold voltage at predetermined time intervals (for example, 100 nsec intervals) by an oversampling process, and outputs a comparison signal generated as a result of the comparison Part. That is, the ADC 14 is a comparator that compares the voltage signal and the threshold voltage, and outputs a high-level comparison signal when the voltage signal exceeds the threshold voltage, and when the voltage signal is equal to or lower than the threshold voltage, Outputs a low level comparison signal. That is, the ADC 14 is a 1-bit analog-digital (A / D) converter.

  The charge injection circuit 16 of the charge detection circuit 12 sequentially extracts a predetermined amount of charge from the capacitor in the preamplifier 13 when the comparison signal periodically generated by the ADC 14 at a predetermined time interval indicates a predetermined state. It is a circuit part for. The charge injection circuit 16 indicates that the comparison signal output from the ADC 14 is in a high level state, and indicates that the voltage signal has exceeded the threshold voltage in the previous operation of the ADC 14. By supplying a charge having a polarity opposite to the supplied charge, a predetermined amount of charge is extracted from the capacitor. The charge injection circuit 16 performs this charge extraction operation in synchronization with the periodic comparison operation by the ADC 14, and the voltage signal becomes the threshold value in the operation of the ADC 14 immediately before the comparison signal becomes low level, that is, by the comparison signal. Continue until it is shown that it is below the voltage.

  The counter 15 of the charge detection circuit 12 counts the number of times that the comparison signal periodically generated at predetermined time intervals by the ADC 14 is maintained, and generates and outputs the resulting count signal. It is. That is, the number of times that the comparison signal output from the ADC 14 is in the high level state is counted (counted) in synchronization with the periodic comparison operation by the ADC 14. More specifically, the counter 15 adds 1 to the count value of the count signal when the comparison signal detects a high level state at a predetermined time interval that is the operation interval of the ADC 14. The counter 15 outputs a digital signal indicating the distribution of the charge amount in the time axis direction by outputting a signal indicating the count value at a predetermined time interval (for example, 1.6 μsec interval). The counter 15 generates a count signal while resetting the count value at a predetermined time interval. For example, when the A / D conversion interval by the ADC 14 is 100 nsec and count values are output at intervals of 1.6 μsec, a 4-bit count signal can be generated. As the counter 15, a known counter circuit such as a flip-flop can be used.

  FIG. 2 shows an example of the circuit configuration of the charge detection circuit 12 described above. In the configuration shown in FIG. 2, a bias is applied to one terminal of the semiconductor detector 11, and the other terminal of the semiconductor detector 11 is connected to the input of the preamplifier 13. The preamplifier 13 includes a differential amplifier 13a and a capacitor 13b. A non-inverting input of the differential amplifier 13a is connected to the ground, and an inverting input of the differential amplifier 13a is connected to the semiconductor detector 11. A capacitor 13b is connected between the output of the dynamic amplifier 13a and the inverting input. With such a configuration, the charge input from the semiconductor detector 11 is accumulated in the capacitor 13b, and a voltage signal corresponding to the amount of the charge is generated at the output of the differential amplifier 13a. Furthermore, the charge injection circuit 16 is a switched capacitor circuit, and includes a DC power supply 16a, a capacitor 16b, and switch elements 16c to 16f. The charge injection circuit 16 having such a configuration receives the comparison signal EN from the ADC 14 and the clock signal CLOCK synchronized with the periodic comparison operation by the ADC 14, and the comparison signal EN is at the high level at a timing synchronized with the clock signal CLOCK. Is supplied to the capacitor 13b of the preamplifier 13 with a charge amount corresponding to the voltage of the DC power supply 16a stored in the capacitor 16b in advance. At this time, the polarity of the DC power supply 16a is set so that the charge supplied from the capacitor 16b to the capacitor 13b has the opposite polarity to the polarity of the charge supplied from the semiconductor detector 11 to the capacitor 13b. . Specifically, one end of the DC power supply 16a is connected to the ground, the other end of the DC power supply 16a is connected to one end of the capacitor 16b via the switch element 16c, and the other end of the capacitor 16b is connected to the capacitor via the switch element 16f. 13b is connected to the terminal on the semiconductor detector 11 side. Furthermore, both ends of the capacitor 16b are connected to the ground via the switch elements 16d and 16e, respectively. In the charge injection circuit 16 configured as described above, the switch element 16c and the switch element 16e are closed, and the switch elements 16d and 16f are opened, whereby charges are accumulated in the capacitor 16b in advance. After that, when the comparison signal EN indicates a high level at a timing synchronized with the clock signal CLOCK, the switch element 16c and the switch element 16e are opened, and the switch elements 16d and 16f are closed, thereby being stored in the capacitor 16b. Charge is supplied to the capacitor 13b.

  FIG. 3 shows an example of another circuit configuration of the charge detection circuit 12. In the configuration shown in FIG. 3, the charge injection circuit 16A is a switched current circuit, and includes a constant current source 16g and a switch element 16h. The charge injection circuit 16A having such a configuration, when the comparison signal EN indicates a high level at the timing synchronized with the clock signal CLOCK, charges the preamplifier 13 with a charge amount corresponding to the current value of the constant current source 16g. To the capacitor 13b. At this time, the polarity of the constant current source 16g is such that the charge supplied from the constant current source 16g to the capacitor 13b has the opposite polarity to the polarity of the charge supplied from the semiconductor detector 11 to the capacitor 13b. Is set. Specifically, one end of the constant current source 16g is connected to the ground, and the other end of the constant current source 16g is connected to the terminal on the semiconductor detector 11 side of the capacitor 13b via the switch element 16h. In the charge injection circuit 16A configured as described above, when the comparison signal EN indicates a high level during one cycle of the clock signal CLOCK, the switch element 16h is closed, so that the current value of the constant current source 16g and the clock signal An amount of charge determined by the product of the period of CLOCK is supplied to the capacitor 13b.

  Next, an operation example of the radiation detection apparatus 10 described above will be described in comparison with a comparative example.

  FIG. 5 shows a schematic configuration of a radiation detection apparatus 910 according to the comparative example. The radiation detection apparatus 910 according to the comparative example includes a detection circuit that employs the ToT method, and includes a waveform shaping circuit 915 and a time-to-digital converter (TDC) 914 that are connected to the subsequent stage of the preamplifier 13. Yes. The waveform shaping circuit 915 is a circuit for reducing the noise of the voltage signal output from the preamplifier 13 and shaping and outputting the voltage signal, and includes a filter circuit and an amplifier. This waveform shaping circuit 915 is required to increase the accuracy of T / D conversion by the TDC 914 in the subsequent stage. The TDC 914 compares the voltage signal shaped by the waveform shaping circuit 915 with a predetermined threshold voltage, counts the period during which the pulse exceeds the threshold voltage, and outputs the count result as a digital value. In such a radiation detection apparatus 910, the time width of the pulse is counted and output as representing the peak value of the pulse, but since the peak value and the time width are not in a linear relationship, the obtained peak value Digital values tend to be less accurate. Also, in order to ensure the accuracy of the output digital value to some extent, the waveform shaping circuit is essential, and as a result, the circuit scale tends to increase. Furthermore, in order to increase the resolution of the output digital value, it is necessary to increase the number of thresholds in the TDC 914, and the circuit scale of the TDC 914 tends to increase.

  FIG. 4 shows time waveforms of various signals processed in the radiation detection apparatus 10 and the radiation detection apparatus 910, (a) shows a pulse waveform of an input current signal, and (b) shows in the radiation detection apparatus 910. The waveform of the output voltage of the preamplifier 13, (c) is the waveform of the output voltage of the preamplifier 13 after the charge extracting operation in the radiation detection device 10, and (d) is supplied to the ADC 14 in the radiation detection device 10. (E) shows the waveform of the output voltage (comparison signal) of the ADC 14 in the radiation detection apparatus 10. As shown in these time waveforms, a voltage signal having a level corresponding to the pulse of the input current signal is output from the preamplifier 13 of the radiation detection apparatus 10, and the level of the voltage signal is the clock signal CLOCK. Decreases step by step at the same time. The output voltage of the ADC 14 of the radiation detection apparatus 10 is maintained at a high level until the output level from the preamplifier 13 of the radiation detection apparatus 10 decreases to a predetermined level. On the other hand, as shown in the waveform of (b), the output voltage of the preamplifier 13 of the radiation detector 910 of the comparative example is maintained at a constant voltage according to the pulse of the input current signal, This output voltage is processed by the waveform shaping circuit 915 and the TDC 914 in the subsequent stage.

  According to the radiation detection apparatus 10 described above, a voltage signal is generated by accumulating the electric charge input in the preamplifier 13, and the voltage signal is periodically compared with the threshold voltage in the ADC 14 for comparison. When a signal is generated and the comparison signal indicates a high level state by the charge injection circuit 16, a predetermined amount of charge is extracted from the charge accumulated in the preamplifier 13. As a result, the input charge signal is converted into a continuous comparison signal converted in the time axis direction corresponding to the amount of the charge and output. Further, the counter 15 counts the number of times that the comparison signal remains in the high level state, so that a digital signal indicating the distribution of the charge amount in the time axis direction can be output. According to the radiation detection apparatus 10 having such a configuration, since the peak value of the current signal and the count value related to the high level state of the comparison signal are in a linear relationship, the peak value of the current signal is accurately counted and output. be able to. Further, compared with the radiation detection apparatus 910 of the comparative example, a shaping circuit for shaping the voltage waveform is not required, and a 1-bit A / D converter is required, so that the circuit configuration is simplified. As a result, the peak value of the charge signal that changes with time can be accurately measured with a simple circuit configuration. In addition, it is possible to realize a highly accurate radiation detection apparatus that can be easily integrated.

  Further, the charge injection circuit 16 employs a switched capacitor configuration or a switched current circuit configuration. Thus, by adopting a configuration in which charges are directly extracted instead of processing a digital signal, an extra circuit such as an operational amplifier becomes unnecessary, the circuit configuration of the charge injection circuit 16 is simplified, and the radiation detection apparatus Is easy to integrate.

  In addition, this invention is not limited to the aspect of embodiment mentioned above. The charge detection circuit 12 of this embodiment is not limited to the use of radiation detection, and may be widely used as a detection circuit for an image sensor. For example, it may be used for detecting γ rays, α rays and the like.

  DESCRIPTION OF SYMBOLS 10 ... Radiation detection apparatus, 11 ... Semiconductor detector, 12 ... Charge detection circuit, 13 ... Preamplifier (amplifier), 13b ... Capacitor, 14 ... ADC (comparator), 15 ... Counter, 16, 16A ... Charge injection Circuit (charge extracting part), 16b ... capacitor, 16g ... constant current source.

Claims (5)

A circuit for detecting the amount of generated charge in time series,
An amplifier that includes a capacitor, receives the input of the charge, and stores the charge in the capacitor, thereby converting the charge into a voltage signal;
A comparator that generates a comparison signal by comparing the voltage signal with a predetermined threshold voltage at predetermined time intervals;
A counter that outputs a count value obtained by counting the number of times that the comparison signal generated in the predetermined time interval by the comparator counts a predetermined state;
A charge extracting unit that sequentially extracts a predetermined amount of charge from the capacitor of the amplifier when the comparison signal generated by the comparator at the predetermined time interval indicates the predetermined state;
A charge detection circuit comprising: The counter counts the number of times the predetermined state indicates that the voltage signal has exceeded the predetermined threshold voltage;
The charge extraction unit extracts the predetermined amount of charge when the comparison signal indicates the predetermined state indicating that the voltage signal exceeds the predetermined threshold voltage;
The charge detection circuit according to claim 1. The charge extraction unit includes a capacitor for preliminarily storing the predetermined amount of charge for extraction.
The charge detection circuit according to claim 1 or 2. The charge extraction unit has a constant current source for extracting the predetermined amount of charge.
The charge detection circuit according to claim 1 or 2. The charge detection circuit according to claim 1;
A semiconductor detector that generates a charge in response to incident radiation and inputs the charge to the charge detection circuit;
A radiation detection apparatus comprising:

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