Primary Testing Performance of Designed Wide-dynamic Range Gamma Ray Dosimeter Based on Double GM Counters

ZHANG Jun, LI Zhongliang, LIU Shuhuan,*, MA Yong, BU Hongtao, XUE Tao, LI Yonghong, CHU Jun, CHEN Wei

(1.School of Nuclear Science and Technology, Xi’an Jiaotong University, Xi’an 710049, China; 2.Xi’an CNNC Nuclear Instrument Co., Ltd., Xi’an 710018, China; 3.Tsinghua University, Beijing 100084, China; 4.Northwest Institute of Nuclear Technology, Xi’an 710024, China)

Abstract: For locating and grasping the lost gamma ray source with a robot in a nuclear accident, a gamma ray dosimeter based on double GM (Geiger-Müller) counters (ZP1321, ZP1301) was primarily designed with wide-dynamic range. The front-end analogue circuits for detector signal processing and the microcontroller-based detector data processing and transmission programs were designed and tested primarily with 137Cs gamma ray source. The main influence factors on the designed dosimeter output signal were measured and analyzed. The primary experimental results show that the designed gamma ray dosimeter without shielding case can detect gamma ray radiation sources on-line successfully.

Key words：dosimeter; GM counter; gamma ray detection

1 Introduction

In recent years, with the development of nuclear technology, gamma ray radiation sources have been widely used in various fields. At the same time, nuclear safety has always been a topic of concern[1-2]. In the last decades and until now, several gamma ray sources were lost and several accidents happened in China and other countries[3-4]. Exposure to gamma ray and other nuclear radiation are dangerous to human life. It can cause cellular destruction and subsequently lead to DNA damage, cancer and radiation sickness if exposed in excess[5]. Therefore, it is vital to find the lost gamma ray sources in time with suitable instruments.

Robot combined with radiation detectors is usually used in nuclear emergency accidents including to find and localize the lost radiation sources[6-9]. Geiger-Müller (GM) counter is one of the detectors commonly used in gamma ray dose rate measurement due to its high sensitivity, large output pulse amplitude and good stability, etc[10-15]. In general, the radioactivity range of various lost gamma sources may be cover 6-8 orders of magnitude (μCi-Ci) due to their different applications. However, the traditional gamma ray dose rate dosimeter based on single GM counter may only cover about 3 orders of dose rate response. For enlarging gamma ray dosimeter dynamic range based on GM counter and improving its performance, a new wide dynamic range dosimeter based on dual GM counters (ZP1321, ZP1301) with different dose rate response ranges (the dose rate response ranges without shielding for ZP1301 and ZP1321 are 1×10-1-1×104mGy/h and 3×10-3-1×102mGy/h, respectively) was designed in this work.

2 Working principle of dosimeter system

The designed dosimeter system contains the double GM counters, a high voltage power supply module, signal processing circuit, single chip microcomputer system and power supply circuit. The system hardware structure block diagram is shown in Fig.1.

Fig.1 System structure diagram of designed dosimeter

After the system is powered on, the high-range GM counter ZP1301 and the low-range GM counter ZP1321 start to work and generate pulses, which are respectively input to the pins of single-chip microcomputer to count. Firstly, the high-range GM counter ZP1301 for dose rate detection will be selected by program and judged whether the dose rate is within its response range. If the measured dose rate is not within its detection range, the measured program will switch to the other low-range GM counter ZP1321. The dose rate of the GM counter within appropriate dose rate range will be selected by program and transmitted to the host computer as the current dose rate. The bias voltage of the GM counter is supplied by the designed high-voltage converting circuit. The GM counter output signal can be converted into a regular voltage pulse through the signal processing circuits connected to the GPIO pin of the single-chip microcomputer. The count rate of the output pulse signal from any of the double GM counters can be detected with the timer and external trigger function in the single-chip microcomputer and convert it into gamma ray dose rate by the calibrating factors of the dosimeter. The measured gamma ray dose rate data were then transmitted to the computer via the serial ports setting.

3 Dosimeter circuit design

The characteristics of the dosimeter circuit modules and the optimized designing techniques are respectively described in detail as follows.

3.1 Power supply circuit

Fig.2 Schematic diagram of high voltage power module circuit

A high-voltage module with an output of +550 V is used for both GM counters (ZP1301, ZP1321) due to the large overlap range of their operating voltages. The DW-P102-0.5AL1 DC/DC high-voltage module with a start-stop control function (selected from Dongwen High-voltage Power Supply Company) has the characteristics of high-stability and low-ripple. The high-voltage module shown in Fig.2 starts to work when the level of the control pin R/S of the module is lower than 0.7 V, which can output a voltage in the range of 0-1000 V adjusted by the potentiometer RP7. A high-voltage filter capacitor CY5is connected between the output pin and ground to better reduce the output ripple noise.

For avoiding the high-voltage discharge or short-circuit to the ground which may cause damage to the surroundings or the power supply, a certain insulation distance between the output port of the high-voltage power supply and the surrounding components is set in the PCB layout design.

3.2 GM counter signal acquisition circuit

The GM counter operating in the Geiger-Müller area will generate pulse signals when gamma rays interact with the GM counter and produce the ionized electrons in the GM counter sensitive region. Schematic diagram of double GM counters operating circuits is shown in Fig.3. As shown in Fig.3, the positive nuclear pulse signals from the cathode of GM counter can be obtained in the system, which need to be further conditioned and stably recognized by single-chip microcomputer.

Fig.3 Schematic diagram of double GM counters operating circuit

3.3 Signal processing circuit

Although the signal processing circuit of the GM counter is not as complicated as that of other types of detectors, the performance of the GM counters will be greatly degraded if the signal acquisition and pulse shaping cannot be optimized by the signal processing circuits[16]. The undershoot of the pulse signal and the excessive pulse width of the shaping pulse signal will lead to more missed counts.

The GM counter output signal processing procedure can be divided into two steps. In the first step, the current pulse signal produced by the GM counter is converted into a voltage pulse with appropriate pulse width and amplitude during the signal acquisition system operating. In the second step, the designed pulse shaping circuit module converts the voltage pulse generated in the first step into a regular pulse signal with a fixed amplitude. The regular pulse signal can be recognized by the single-chip microcomputer for counting.

1) Differential circuit with pole-zero cancellation

Generally, the rise time of the GM counter probe output pulse signal is relatively fast compared with the long falling time. Its top drops exponentially over time. The simplest forming circuit and CR differential circuit can make the pulses narrow and separate from each other, but undershoot will occur at the end of the pulse, which is likely to cause missed counts. Here, a differential circuit with pole-zero cancellation is designed (Fig.4).

Fig.4 Schematic diagram of differential circuit with pole-zero cancellation

The transfer function of the above circuit module can be expressed as Eq.(1).

(1)

The input signalUi(t) from GM counter is generally shown as an exponential function (Eq.(2)).

(2)

The output signalUo(t) in Fig.4 is then deduced as:

(3)

By turning the adjustable resistorR1in Fig.4 to makeτ1=τf, the Eq.(3) can be deduced as:

(4)

Eq.(4) shows that the output signal processed by the designed pole-zero cancellation circuit is a unipolar pulse without undershoot, and the decay time constant of the signal isτ2.τfcannot be calculated accurately, so it is necessary to adjustτ1to check the processed pulse signal until the pulse undershoot is eliminated. AdjustR1≫R2to reduceτ2, then a narrower pulse can be obtained.

2) Amplitude discrimination circuit

The amplitude discriminator is designed based on the voltage comparator chip (LM393). The schematic diagram of the designed circuit is shown in Fig.5. The threshold voltage is set by the resistor divider, which is set to +2.5 V here. The output pin of the LM393 chip is connected to the positive power supply via a pull-up resistor (3-15 K). When the amplitude of the input voltage pulse signalV(i) is greater than the set threshold voltage +2.5 V, the output portV(o) will output a high level of +5 V. Or else, it will output a low level of 0 V.

The signalV(i) passing through the amplitude discrimination circuit (Fig.5) will be transformed into a regular rectangular pulse signalV(o) with an amplitude of +5 V, which is input to the GPIO pin of the single-chip microcomputer. And then the rising edge of the signalV(o) triggers the interrupt function module of the microcontroller to count.

Fig.5 Schematic diagram of amplitude discrimination circuit

4 Software design

The dosimeter data acquisition, process-ing and transmission system is designed based on the single-chip microcomputer STM32F103ZET6. The program made with C language is developed with Keil MDK software. The designed program flow chart is shown in Fig.6. The counts of the output pulses from both GM counters can be recorded and accumulated with the external trigger interrupt program of the microcontroller during gamma ray irradiation. Meanwhile, the time period for recording the accumulated pulse counts can be set by the internal timer program of the microcomputer. Thus, the count rates of both GM counters are obtained and then converted into the dose rateRHandRLby multiplying the calibration factorHandL, respectively. Fi-nally,RHorRLis selected as the current dose rateRaccording to the range switching program and transmit it to the host computer.

Fig.6 Flow chart of designed program

The dosimeter system uses two types of GM counters with different dose rate response range for counting. The measuring ranges of the double GM counters should partially overlap to facilitate range switching. Schematic diagram of range switching is shown in Fig.7. As shown in Fig.7, two switching thresholdsPandQare set in the program to avoid the instability during switching. When the dose rateRLof the low-range GM counter is selected as the dose rateR, if the value of dose rateRLis greater than the high thresholdQ, the dose rateRHof the high-range GM counter will be selected. Contrarily, when the dose rateRHof the high-range GM counter is selected as the dose rateR, the dose rateRLof the low-range GM counter will be selected if the value of dose rateRHis less than the low thresholdP.

Fig.7 Schematic diagram of range switching

5 Test result and analysis

The system is initially tested with a137Cs gamma ray source. The count rate is monitored in the serial port debugging window. The output signals from some key testing ports are measured with an oscilloscope. The influence of the parameter setting on the output voltage pulse signal waveform is measured and analyzed by changing the key parameters in some circuit modules. Take the waveform of the high-range GM counter as an example for analysis and the schematic diagram of the high-range GM counter circuit is shown in Fig.8.

For analyzing the characteristics of the GM counter’s output pulse signal and the forming pulse signal, etc., the output signals from the different test points marked with A, B and C in Fig.8 are measured with the oscilloscope, as shown in Fig.9. It can be found that the width and amplitude of the output signal A from the GM cathode is larger than that of signal B which is processed by the pole-zero cancellation circuits. By reasonably configuring the parameters of resistors (R1,R2) and capacitance in the pole-zero cancellation circuit module, the overshoot of signal B can be eliminated. At point C, a regular voltage pulse with an amplitude of +5 V is obtained and then it will be input to the GPIO of the single-chip microcomputer for counting.

Fig.8 Schematic diagram of high-range GM counter circuit

Fig.9 Pulse signal measured with oscilloscope (RTE1204)

In order to optimize the performance of the GM counter, it is necessary to increase the upper limit of its count rate and reduce the missed count. Therefore, it is necessary to analyze the factors on the output voltage pulse signal waveform, and then configure reasonable parameters in the circuits to obtain the required pulse waveform, thereby increasing the upper limit of the count rate. In this experiment, the GM counter’s output pulse signal A and forming pulse signal B are measured, compared and analyzed under different parameter configuring conditions.

Fig.10 Changing trend of FWHM and peak amplitude for experimental GM counter cathode output pulse signal with capacitance CY1 and resistance R3

For analyzing the main factors on the waveform of the GM counter’s output signal A, the full width at half-maximum amplitude (FWHM) and the peak amplitude of signal A are measured and compared when the values of the cathode capacitance CY1and the cathode resistanceR3are altered respectively with other parameters unchanged. The changing trends of the peak amplitude and FWHM of the signal A with CY1andR3are shown in Fig.10, displaying that the larger the cathode capacitance CY1, the smaller the peak amplitude of signal A and the larger the FWHM. The larger the cathode resistanceR3, the larger the peak amplitude of the cathode pulse and the larger the FWHM. Furthermore, the cathode resistanceR3has little effect on the peak amplitude of GM counter’s output signal A.

Similarly, in order to analyze the factors on the forming pulse signal B, while the resistance values ofR1andR2in the pole-zero cancellation circuit module are respectively adjusted with other parameters unchanged, the peak amplitude and the FWHM of signal B are measured and analyzed. The trends of the peak amplitude and FWHM of the signal B withR1andR2are shown in Fig.11, indicating that the larger theR1, the smaller the FWHM of signal B, and the larger theR2, the larger the FWHM of signal B. In addition, bothR1andR2have little effect on the peak amplitude of forming pulse signal B.

Fig.11 Changing tendency of FWHM and peak amplitude for forming pulse signal with different resistances of R1 and R2

According to the above measurement results, it can be found that the characteristics of the GM counter original output signal A from the counter cathode electrode are mainly influenced by the cathode capacitance and cathode resistance parameters under the certain bias voltage supply. As thus, the cathode capacitance and cathode resistance parameters need to be reasonably configured. Besides, the count rate of the GM counter tested may be increased by optimizing the parameter setting of resistorR1andR2to decrease the FWHM of forming pulse signal B without overshoot. After preliminary testing, the FWHM of the signal B of high-range GM counter is 4.56 μs and the amplitude is 4.15 V when the circuit parameters in Fig.8 are set to CY1=10 pF,R3=40 kΩ,R1=5 MΩ,C1=330 pF andR2=10 kΩ.

6 Summary and conclusion

The characteristics of the front-end analogue circuit modules designed for the GM counter output signal processing, power supply and the microcontroller-based detector data processing and transmission programs are introduced and tested primarily with137Cs gamma ray source. The factors on the waveforms of the key processed analogue pulse signals and the missed counts of the designed gamma ray dosimeter are tested and discussed in detail. The missed counts caused by signal overshoot and high FWHM are decreased by optimizing the parameter settings of the key components (resistors, capacitances) in the front-end analogue circuit modules. In addition, the dead time of the GM counters combined with the dosimeter front-end circuits and data processing system is another important factor on the dosimeter missed counts. The factors on the dosimeter dead time need to be further studied in future.

The dynamic range of the designed gamma ray dosimeter based on double GM counters without shielding shell may cover 7 orders of magnitude from environmental dose rate (about 3×10-3mGy/h) to high dose rate (about 1×104mGy/h) according to the data sheets of the two types of GM counters (ZP1321, ZP1301). However, the accurate dynamic range of the designed dosimeter can be determined by the calibrating experiments with standard gamma or/and X ray sources in future.