Front-end reading circuit suitable for high-purity germanium detector

Abstract

The invention discloses a front-end reading circuit suitable for a high-purity germanium detector. The front-end reading circuit comprises a pre-amplifier, a filtering shaping circuit, an amplitude acquisition channel, a time discrimination channel and an automatic gain control circuit, the output end of the pre-amplifier is connected with the filtering and shaping circuit, the output end of the filtering and shaping circuit is connected with the amplitude acquisition channel and the time discrimination channel, and the output of the amplitude acquisition channel is input into an ADC module of the digital amplitude analyzer; the time discrimination channel is used for extracting time information of particles reaching the detector, and an obtained pulse signal is used as a control signal of the amplitude acquisition channel and is also input to the TDC module of the digital amplitude analyzer; a control signal of the automatic gain control circuit is generated by the external processor according to the output of the digital amplitude analyzer, and an output signal of the automatic gain control circuit is used as a control signal of the preamplifier. The advantages of the detector can be fully utilized, and resource waste is avoided.

Description

Technical Field

[0001] The present invention belongs to the field of integrated circuit design, and particularly relates to a front-end readout circuit applicable to high-purity germanium detectors. Background Art

[0002] Nuclear detection technology, as an important branch of nuclear technology, plays an important role in many fields such as nuclear radiation environment monitoring, biomedical imaging, cosmic ray exploration, and basic scientific research. As an important tool for nuclear detection technology, nuclear radiation detectors have a rich variety after decades of development and can meet the different needs of different working fields. At present, there are mainly three types of nuclear radiation detectors: gas ionization detectors, scintillation detectors, and semiconductor detectors using semiconductor materials such as silicon and germanium as the medium. Due to the advantages of wide energy detection range, high detection efficiency, and various types of semiconductor detectors, they are widely used in high-precision detection fields.

[0003] As a representative of new semiconductor detectors, high-purity germanium detectors have irreplaceable development advantages. One is high energy resolution, and the energy resolution for X / γ rays can reach a few thousandths, which is one order of magnitude higher than that of ordinary semiconductor detectors. The second is rich variety, with an extremely wide energy detection range (a few KeV to 10 MeV). The third is that the manufacturing process is simpler and the performance is more stable compared with other high-performance detectors.

[0004] In order to analyze nuclear information in the nuclear radiation environment, it is necessary to obtain information such as the energy, type, and radiation intensity of rays. Nuclear radiation detectors can capture information such as the energy, incident time, and position of rays in the environment, and output a series of electrical pulse signals with different amplitudes, unequal intervals, and different waveforms. This signal is very weak and the target information cannot be directly obtained. A pulse amplitude analyzer is required to process and analyze the output signal of the detector. Whether it is a traditional analog amplitude analyzer or a digital amplitude analyzer that has developed rapidly in recent years, a front-end readout circuit is required to preliminarily process the weak pulse signal output by the detector. The traditional analog amplitude analyzer completes work such as signal filtering and shaping, and pile-up recognition in the front-end readout circuit, which requires a large number of analog devices. Therefore, the circuit processing speed is slow, affected by temperature greatly, with low spectral resolution, poor counting rate, and a large dead time range. For current digital amplitude analyzers, only preliminary amplification is performed in the front-end circuit, and work such as filtering and shaping, and pile-up discrimination is performed in the digital part. Although digital amplitude analyzers have excellent performance, they have high costs and large power consumption. Summary of the Invention

[0005] Object of the Invention: In order to solve the problems existing in the above-mentioned prior art, the present invention provides a front-end readout circuit applicable to high-purity germanium detectors.

[0006] Technical solution: A front-end readout circuit applicable to a high-purity germanium detector, comprising: a preamplifier, a filter shaping circuit, an amplitude acquisition channel, a time discrimination channel, and an automatic gain control circuit;

[0007] The input end of the preamplifier is connected to the high-purity germanium detector, and the output end is connected to the filter shaping circuit. The output end of the filter shaping circuit is respectively connected to the amplitude acquisition channel and the time discrimination channel. The amplitude acquisition channel broadens the peak value of the received waveform, and takes the output of the amplitude acquisition channel as the first output of the front-end readout circuit and inputs it into the ADC module of the digital amplitude analyzer; the time discrimination channel is used to extract the time information of the particle reaching the detector, and the obtained pulse signal is used as the control signal of the amplitude acquisition channel and also as the second output of the front-end readout circuit and inputs it into the TDC module of the digital amplitude analyzer; the control signal of the automatic gain control circuit is generated by an external processor according to the output of the digital amplitude analyzer, and the output signal of the automatic gain control circuit is used as the control signal of the preamplifier.

[0008] Further, the preamplifier includes an operational amplifier, a first capacitor, and a first resistor; the input end of the operational amplifier is connected to the high-purity germanium detector, and the output end is used as the output end of the preamplifier. The first capacitor is connected across the two ends of the operational amplifier, and the first resistor is connected in parallel with the first capacitor; the control end of the operational amplifier is connected to the output signal of the automatic gain control circuit.

[0009] Further, the amplitude acquisition channel includes a sample and hold circuit. The sample and hold circuit includes a first amplifier, a buffer, a switch, a second capacitor, a first transistor, and a second transistor; the inverting input end of the first amplifier is connected to the output end of the buffer, and the non-inverting input end is connected to the output end of the filter shaping circuit. The output end of the first amplifier is connected to the drain of the first transistor, the gates of the first transistor and the second transistor; the sources of the first transistor and the second transistor are connected to the power supply voltage VDD, the drain of the second transistor is connected to one end of the second capacitor, the input end of the buffer, and one end of the switch. The other end of the second capacitor and the other end of the switch are both grounded, and the output end of the buffer is used as the output end of the sample and hold circuit.

[0010] Further, the time discrimination channel includes a discriminator and a logic stretcher. The discriminator receives the output pulse signal of the filter shaping circuit, extracts the time when the radiation particle reaches the detector, converts the analog signal into a digital signal, and then transmits it to the logic stretcher.

[0011] Further, the logic stretcher includes a first inverter, a second inverter, a third capacitor, a diode, and a second resistor; the input end of the first inverter is connected to the output end of the discriminator, the output of the first inverter is connected to the cathode of the diode, the anode of the diode is connected to the input end of the second inverter, one end of the second resistor and one end of the third capacitor, the other end of the second resistor is connected to the power supply voltage VDD, the other end of the third capacitor is grounded, and the output end of the second inverter is used as the output end of the logic stretcher.

[0012] Further, the discriminator uses a high-speed and high-resolution voltage comparator.

[0013] Further, the filter shaping circuit uses a CR-(RC) n shaper.

[0014] Beneficial effects:

[0015] 1) The front-end readout circuit structure proposed by the present invention is designed for the advantages of high-purity germanium detectors, such as large energy detection range, high resolution, and high detection efficiency. It can make full use of the advantages of the detector without wasting resources.

[0016] 2) In view of the high resolution and high detection efficiency of high-purity germanium detectors, the present invention incorporates a dual-channel design. The amplitude acquisition channel is used to obtain the energy information of radiation particles. Since the detector has a fast detection speed, in order to prevent the processing speed from falling behind, a time discrimination channel is added as an auxiliary. It can not only process the fast pulse signals output by the detector, but also ensure that the amplitude information obtained by the amplitude acquisition channel is not distorted to the greatest extent, and can further process the signals in the subsequent digital amplitude analyzer of the system.

[0017] 3) In view of the large energy detection range of high-purity germanium detectors, the present invention incorporates an automatic gain control circuit, which can automatically adjust the gain of the preamplifier according to the detected signal amplitude to ensure undistorted amplification. Similarly, it can ensure that the signal is in the best sampling linear region of the ADC during the digitization process in the subsequent digital amplitude analyzer. Description of the drawings

[0018] Figure 1 is a schematic structural diagram of the present invention.

[0019] Figure 2 is a schematic structural diagram of the preamplifier.

[0020] Figure 3 is a CR-(RC) n schematic structural diagram of the shaper.

[0021] Figure 4 is a waveform diagram of each node of the time discrimination channel.

[0022] Figure 5 It is a structural schematic diagram of a logic stretcher.

[0023] Figure 6 It is a structural schematic diagram of a sample and hold circuit. Detailed implementation manners

[0024] The accompanying drawings forming a part of the present invention are used to provide a further understanding of the present invention. The schematic embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation to the present invention.

[0025] As Figure 1 shown, in order to give full play to the advantages of high-purity germanium detectors such as high energy resolution and wide detection range, and to solve the limitations of existing front-end readout circuits in analog amplitude analyzers and digital amplitude analyzers, the present invention proposes a front-end readout circuit applicable to high-purity germanium detectors. This circuit includes a preamplifier 1, a filter shaping circuit 2, an automatic gain control circuit 3, an amplitude acquisition channel 5, and a time discrimination channel 4.

[0026] The preamplifier 1 directly receives the weak charge pulse signal from the detector. Generally, it is closely adjacent to the detector, which can effectively reduce the noise brought by environmental fluctuations. The functions of the preamplifier mainly include: 1) sufficient gain to preliminarily amplify the weak pulse signal; 2) sufficient bandwidth to ensure that the amplification of the signal is basically distortion-free; 3) low noise to ensure that small differences in the signal can be distinguished; 4) converting the charge pulse signal output by the detector into a voltage pulse signal required by the subsequent circuit; 5) the voltage terminal is controlled by the automatic gain control circuit. Finally, the voltage pulse signal output by the preamplifier is sent to the filter shaping circuit.

[0027] Currently, the structures of operational amplifiers include differential structures, inverter structures, ordinary cascode structures, folded cascode structures, etc. The differential structure has a good power supply rejection ratio, but the circuit structure is complex and the noise performance is poor; the inverter structure has good noise performance, but the open-loop gain is very limited; compared with the ordinary cascode structure, the folded cascode structure has a larger open-loop gain and bandwidth, and the noise performance can also be optimized. Therefore, the folded cascode structure is the most commonly used structure for preamplifiers.

[0028] In addition, to convert the charge pulse signal output by the detector into a voltage pulse signal required by the subsequent circuit, as Figure 2 shown, it is necessary to connect a feedback capacitor C across the two ends of the operational amplifier. As the charge accumulates on the capacitor, the output voltage of the amplifier will continuously increase. In order to prevent the amplifier from reaching the saturation state and causing distortion of the output signal, it is necessary to add a reset circuit. The simplest reset circuit is to connect a large-value resistor R in parallel across the two ends of the capacitor.

[0029] Finally, it is required that the voltage terminal of the operational amplifier be controlled by the automatic gain control circuit 3. Since the detector has a large detection energy range, in order to ensure that the preamplifier does not cause waveform amplitude distortion during the amplification of the detector output signal, it is necessary to adjust the gain of the preamplifier at any time according to the amplitude of the signal output by the preamplifier. By changing the voltage value of the preamplifier, the control of its gain is achieved.

[0030] The filtering and shaping circuit 2 receives the voltage pulse signal from the preamplifier. The main functions of the circuit include: 1) Since the useful signal output by the detector is often mixed with noise and interference, resulting in noise and interference in the voltage pulse signal output by the preamplifier, it is necessary to suppress and filter them; 2) The voltage pulse signal output by the preamplifier is an exponential decay signal with a relatively large waveform width. When the radiation frequency of the detector is too high, pulse pile-up is likely to occur, resulting in output waveform distortion. It is necessary to reduce the width of the output signal; 3) Since the energies, ranges, and positions of incidence of the radiation particles themselves are all different, the widths of the pulse signals output by the detector are not the same. It is necessary to perform waveform conversion on the output signal of the preamplifier so that all pulse signals have the same shaping time. Finally, the pulse signal output from the filtering and shaping circuit is divided into two paths, one path is sent to the amplitude acquisition channel, and the other path is sent to the time discrimination channel.

[0031] The noise in this embodiment mainly includes low-frequency flicker noise and high-frequency white noise. A band-pass filter is required to filter the noise; in addition, in order to narrow and shape the waveform, a CR-(RC) n shaper is selected in this embodiment to filter and shape the signal. The schematic diagram of the structure is as Figure 3 shown. It is a band-pass filter composed of a high-pass filter and an n-order low-pass filter. The role of the buffer in the figure is to isolate the load. By adjusting the time constants R d C d and R i C i and adjusting the order n of the low-pass filter, the width of the output waveform can be changed, and at the same time, the waveform has the same shaping time.

[0032] The automatic gain control circuit 3, as the feedback branch of the entire front-end readout circuit, generates a control signal at one end by the processor of the digital amplitude analyzer. Through the automatic gain control circuit, the control of the voltage terminal of the preamplifier at the other end is achieved, thereby changing the gain of the preamplifier. Since the detector has a large detection energy range, in order to ensure that the preamplifier does not cause waveform amplitude distortion during the amplification of the detector output signal, and subsequent in the process of digitizing the signal by the digital amplitude analyzer after passing through the front-end readout circuit, it can be in the best sampling linear region of the ADC, it is essential to introduce the automatic gain control circuit.

[0033] The methods for implementing voltage regulation control of the preamplifier 1 include analog and digital methods. The analog method has a simple circuit structure, but requires manual adjustment and cannot form a closed-loop control, which is suitable for open-loop voltage regulation occasions with low precision requirements. The digital method, although relatively complex in structure, has flexible control and better adjustment accuracy than the analog method.

[0034] The automatic gain control circuit of the digital method adopts the structure of a processor + DAC + operational amplifier. The processor is the Figure 1 processor of the digital amplitude analyzer. Set the voltage value in the processor, convert it into an analog current signal through the DAC, convert it into a voltage signal using a transimpedance amplifier, and connect the voltage signal to the voltage terminal of the preamplifier 1 to achieve gain control of the preamplifier 1. The selection of the DAC is related to the voltage regulation accuracy, so high resolution and low noise are required.

[0035] The time discrimination channel 4 receives the output pulse signal of the filter shaping circuit. This channel is used to extract the time information of the particles reaching the detector. The finally obtained pulse signal can be used both as the control signal of the amplitude acquisition channel and as the basis for pulse pile-up rejection in the subsequent digital amplitude analyzer.

[0036] In this embodiment, the time discrimination channel mainly includes a discriminator and a logic stretcher.

[0037] The discriminator 4-1 is connected to the filter shaping circuit 2, and is used to extract the time when the radiation particles reach the detector, convert the analog signal into a standard digital signal, and after being processed by the logic stretcher 4-2, it is used as the control signal of the sample and hold circuit 5-1 in the amplitude acquisition channel 5. The discriminator is implemented by a high-speed and high-resolution voltage comparator. The pulse signal waveform after passing through the discriminator is as Figure 4 the v i shown, which is a digital signal.

[0038] The logic stretcher 4-2 is connected to the discriminator 4-1, and is used to widen the peak of the signal. The signal widened by the logic stretcher can be used as the control signal of the amplitude acquisition channel 5 after being processed, or can be used as the basis for pulse pile-up rejection in the digital amplitude analyzer after passing through the Figure 1 TDC. Figure 5 is a schematic diagram of a logic stretcher. In the figure, v i corresponds to Figure 4 the v i in, which is the output signal of the discriminator 4-1. In the figure, v1 corresponds to Figure 4 the v1 in, which is the output signal after passing through the inverter. The function of the inverter is to reverse the digital signal from 0 to 1. In the figure, v2 corresponds to Figure 4The v2 in it is the output signal passing through the diode. The function of the diode is to have unidirectional conductivity, and the signal can only pass when there is a forward voltage. In the figure, v o corresponds to Figure 4 the v in o , which is the widened signal finally obtained by the logic stretcher 4-2. It can be seen from Figure 4 that the width of the output signal of the discriminator 4-1 is t w , which is widened by the logic stretcher 4-2, and finally the width of the output signal is T H . The widening time T can be changed by controlling the Figure 5 RC H value H .

[0039] Finally, one path of the output signal of the logic stretcher 4-2 is sent to the amplitude acquisition channel 5, and the other path is sent to the Figure 1 TDC of the digital amplitude analyzer, which is used as the basis for signal pile-up rejection after digitization.

[0040] The amplitude acquisition channel 5 also receives the output pulse signal of the filter shaping circuit. This channel widens the peak of the received waveform as needed, so that it can be sampled and quantified by the ADC of the digital amplitude analyzer later, and finally more accurately obtain the energy information of the particles detected by the detector.

[0041] The amplitude acquisition channel in this embodiment mainly includes a sample and hold circuit 5-1. This circuit receives the output pulse signal of the filter shaping circuit and stores the amplitude information of the pulse signal for subsequent sampling by the ADC (of the digital amplitude analyzer). For the radiation system of the present invention, whether it is the preamplifier introduced earlier or the filter shaping circuit, they are all for obtaining the pulse amplitude information as accurately as possible. In order to ensure that the amplitude does not change when the subsequent ADC samples and quantifies this pulse amplitude information, it is necessary to widen the peak of the pulse waveform, that is, store the amplitude near the peak. The functions that the sample and hold circuit needs to have include: 1) being able to store the amplitude of the pulse signal; 2) being able to track the amplitude of the newly arrived pulse; 3) being able to clear periodically to facilitate the next tracking.

[0042] Circuits that can achieve the above functions include switched capacitor circuits and analog stretchers. The switched capacitor circuit is complex in calculation and difficult in the design and control of the timing circuit. The present invention chooses to use an analog stretcher, which has high precision, fast conversion speed, and good stability. Its basic structure is as Figure 6 shown. When the signal received from the filter shaping circuit 2 is input from the IN port, since the input voltage v i of the amplifier is less than the output voltage v o , the current mirror starts to work at this time and charges the hold capacitor C HCharging; when the input voltage of the amplifier exceeds the output voltage, the current mirror is turned off and the circuit enters the holding phase. At the end of the holding phase, the switch SW is closed to discharge and reset the holding capacitor. The switch SW is a control switch, and the timing control terminal is connected to the output signal of the logic stretcher 4-2 of the time discrimination channel 4.

[0043] In addition, it should be noted that the various specific technical features described in the above specific embodiments can be combined in any suitable manner without contradiction. To avoid unnecessary repetition, the present invention will not separately describe various possible combination methods.

Claims

1. A front-end readout circuit suitable for a high-purity germanium detector, characterized in that: include: Preamplifier, filter shaping circuit, amplitude acquisition channel, time discrimination channel and automatic gain control circuit; The input end of the preamplifier is connected to a high-purity germanium detector, and the output end is connected to a filter shaping circuit. The output end of the filter shaping circuit is respectively connected to an amplitude acquisition channel and a time discrimination channel. The amplitude acquisition channel widens the peak value of the received waveform, and the output of the amplitude acquisition channel is used as the first output of the front-end readout circuit and input into the ADC module of the digital amplitude analyzer; the time discrimination channel is used to extract the time information of the particle arriving at the detector, and the obtained pulse signal is used as the control signal of the amplitude acquisition channel, and also as the second output of the front-end readout circuit, and input into the TDC module of the digital amplitude analyzer; the control signal of the automatic gain control circuit is generated by an external processor according to the output of the digital amplitude analyzer, and the output signal of the automatic gain control circuit is used as the control signal of the preamplifier.

2. A front-end readout circuit suitable for a high-purity germanium detector according to claim 1, characterized in that: The preamplifier includes an operational amplifier, a first capacitor and a first resistor; the input end of the operational amplifier is connected to the high-purity germanium detector, the output end serves as the output end of the preamplifier, the first capacitor is connected across the two ends of the operational amplifier, and the first resistor is connected in parallel with the first capacitor; the control end of the operational amplifier is connected to the output signal of the automatic gain control circuit.

3. A front-end readout circuit suitable for a high-purity germanium detector according to claim 1, characterized in that: The amplitude acquisition channel includes a sampling and holding circuit, which includes a first amplifier, a buffer, a switch, a second capacitor, and first and second transistors; the inverting input end of the first amplifier is connected to the output end of the buffer, the non-inverting input end is connected to the output end of the filter shaping circuit, the output end of the first amplifier is connected to the drain of the first transistor, the gate of the first transistor and the gate of the second transistor; the source of the first transistor and the second transistor is connected to the power supply voltage VDD, the drain of the second transistor is connected to one end of the second capacitor, the input end of the buffer and one end of the switch, the other end of the second capacitor and the other end of the switch are grounded, and the output end of the buffer serves as the output end of the sampling and holding circuit.

4. A front-end readout circuit suitable for a high-purity germanium detector according to claim 1, characterized in that: The time discrimination channel includes a discriminator and a logic stretcher. The discriminator receives the output pulse signal of the filter shaping circuit, extracts the time when the radiation particles arrive at the detector, converts the analog signal into a digital signal, and then transmits it to the logic stretcher.

5. A front-end readout circuit suitable for a high-purity germanium detector according to claim 4, characterized in that: The logic stretcher includes a first and a second inverter, a third capacitor, a diode and a second resistor; the input end of the first inverter is connected to the output end of the discriminator, the output of the first inverter is connected to the cathode of the diode, the anode of the diode is connected to the input end of the second inverter, one end of the second resistor and one end of the third capacitor, the other end of the second resistor is connected to the power supply voltage VDD, the other end of the third capacitor is grounded, and the output end of the second inverter serves as the output end of the logic stretcher.

6. A front-end readout circuit suitable for a high-purity germanium detector according to claim 4, characterized in that: The discriminator adopts a high-speed and high-resolution voltage comparator.

7. A front-end readout circuit suitable for a high-purity germanium detector according to claim 1, characterized in that: The filter shaping circuit adopts CR-(RC) n Shaper.

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