| Portable α/β surface contamination monitor is an important apparatus in the field of nuclear equipment monitoring,environmental radioactive pollution investigation and evaluation and monitoring of nuclear accident emergency events,and its probe is composed of detector and CSP.In the probe of the traditional portable α/β surface contamination monitor,the detector is usually composed of PMT and composite scintillator.PMT can lead to the shortcomings of the detector,such as large size,high operating voltage,poor anti-jamming performance and resistance to mechanical properties,thus limiting the application of the probe.CSP usually uses single-ended input junction field effect transistor(JFET)as the input stage,and uses a number of bipolar junction transistor(BJT)as the amplification stage and output stage,thus resulting in the complex structure of CSP and poor stability of static operating point,and increasing the difficulty of debugging the circuit and affecting the output characteristics of the probe.It can be seen that the traditional probe is not in line with the development trend of the current portable α/β surface contamination monitor.It is urgent to achieve the improvement of the traditional portable α/β surface contamination monitor under the strong promotion of nuclear safety facilities and radioactive pollution monitoring.Accordingly,this paper has carried out the improvement work on the traditional portable α/β surface contamination monitor probe,proposing a novel αβ detector structure composed of composite scintillator coupling SiPM,and using a differential input JFET twin tube and an integrated operational amplifier to form a new CSP.In order to further study the performance of the probe,this paper designs the test scheme of the α/β detector and the probe adaptation circuit(include plastic filter circuit,integral circuit,screening circuit,bias power supply and the main control system),and tests the performance of the α/β detector,CSP,probe adapter circuit and probe,and finally achieves the improvement of the traditional portable α/β surface contamination monitor,so that it has the advantages of smaller volume,higher portability,better output characteristics,lower operating voltage,better anti-jamming performance and better stability lower working voltage,more resistant to mechanical properties and better stability,and can complete the effective measurement of α particles and β particle activity.The main research results of this paper are depicted as follows:(1)In the improvement of detector design,this paper adopts composite scintillator composed of ZnS(Ag)scintillator and plastic scintillator to achieve the simultaneous measurement of α-particles and β-particles,and uses SiPM to realize photoelectric conversion for α/β composite scintillator and form a new α/β detector,thus achieving the improvement on the detector of the traditional portable αβ surface contamination monitor.In order to further study the performance of α/β detector,this paper designs the test scheme of the detector parameters,sets up the test platform,and completes the detector test according to the unique variable principle.The test results are analyzed and the reasonable suggestions are put forward for the practical application,so that the basic characteristics of α/β detector are further grasped,and the rationality of the improved scheme is verified.The experimental results show that the dark current increases with increasing temperature and bias.At a temperature of 20°C and a bias voltage of 24.4V,the dark current is 608nA;at a temperature of 20°C and within the normal operating voltage range(24.2V~24.8V),the maximum variation of the dark current is 3.3%(compared by 608 n A).The junction capacitance does not change significantly with temperature and bias voltage.When the bias voltage is 24.4V and the temperatures respectively are-20°C and 40°C,the junction capacitances respectively are 3368 p F and 3375 pF.Within the normal operating voltage range and the temperatures respectively are-20°C and 40°C,the maximum changes in junction capacitance are 0.2%(compared by 3368pF)and 1.4%(compared by 3375 p F).(2)In the improvement of CSP,the new CSP is designed with differential input JFET twin tube and integrated operational amplifier against the shortcomings of the traditional CSP,and its working principle is described in detail,so as to realize the improvements on CSP of the traditional portable α/β surface contamination monitor.In order to further study the performance of the CSP,the main performance indexes of CSP and the output waveform of different radioactive sources were tested,and the excellent performance of CSP was verified.The experimental results show that the zero capacitance noise of CSP is 51.09 fC,the noise slope is 1.96fC/pF,the signal to noise ratio is up to 33: 1,the rise time is 86 ns,the time drift is 0.112%,and the α and β output characteristics of different radioactive sources are excellent.(3)The combination of the detector and the CSP constitutes a new α/β probe,which improves the traditional portable α/β surface contamination monitor.In order to further study the performance of the probe and the activity measurement of α particles and β particles,this paper has designed the probe adaptation circuit,including plastic filter circuit,integral circuit,screening circuit,bias power supply and main control system,and tested the output waveforms of the shaping filter,integral and screening circuit and the ripple voltage and stability of the bias voltage.The experimental results show that the output characteristics of the 239Pu-α source and 90Sr-β source are good,and the analog signal output from the probe can be converted into a digital signal to realize the activity measurement of α-particle and β-particle.Bias power supply ripple voltage is about 2mV,ripple factor is less than 1‰,and the stability is 0.40%/5h.(4)In the test of the performance of the probe,the paper mainly tests the detection efficiency,cross-channel ratio,temperature effects and humidity effects.The experimental results show that the detection efficiency of α particle is 56.05%,the detection efficiency of β particle is 26.85% at a temperature of 20°C,the ratio of α channel to β channel is 11.88%,and the ratio of β channel to α channel is 0.95%.When the temperature is in the range of-20°C to 40°C,the variation of the detection efficiency of α particle is less than 7.2%(compared by 20℃),the variation of the detection efficiency of β particle is less than 8.9%(compared by 20℃).When the temperature is 40℃ and the humidity is 80%,the variation of the detection efficiency of α particle is 11.7%(compared by 20℃)and the variation of the detection efficiency of β particle is 13.9 %(compared by 20℃),which is in line with national standards GBF5202-2008 regulations. |
