| Installed environment monitoring gamma-ray dosimeter is mainly used in routineenvironment monitoring, accident alarming and accident emergent monitoring. Theinstruments should be calibrated by metric organization in order to ensure theaccuracy and precision of the instruments. The problems, such as too difficult toremove, long calibration period and need to keep instruments working continuously,make field calibration be brought forward. High Pressure Ionization Chamber (HPIC)is selected to be standard because it has the advantages as low measurement limit andlow inherent background, wide measure range and small statistical fluctuation,besides it has good energy response characteristic with energy compensation method.Accuracy and precision of the HPIC are the key factors for high quality fieldcalibration. The purpose of this paper is to study the measurement method of HPIClow-level current signal, and design a suit of precise electrometer system for HPICused in field calibration.HPIC create low-level current signal, it is usually to be10-14A and even10-15Ain environment background. It is not only a kind of low level signal but also easilydisturbed by all kinds of noise, so it is difficult for low-level current measurement.Besides, accuracy and precision should be guaranteed during field calibration. Aboveall, it is necessary to study on the method of low-level current measurement.Townsend method, vibrate-cap method, feedback integration method and I-Vconversion method are popular methods for low-level current measurement. First ofall, the paper analyses the basic theories of the methods, and compare the advantagesand disadvantages of the methods. And then select I-V conversion method to be thesolution of field calibration HPIC based on the actual situations and circumstances during field calibration. It analyzes the basic theory of I-V conversion and key factorsof inherent noise and response speed. It introduces the ways of improving the inherentnoise and response speed, and other factors need to be paid attention.Design a low-level current measurement system based on I-V conversion methodwith AVR Atmega32MCU. The amplifier circuit of electrometer system consists oftwo mega-resistors which are1T（1012）Ω and100M（108）Ω., with relay switches itcan achieve7orders of magnitude, wide range low-level current measurement. Ahigh-precision16-bit sigma-delta ADC is used in the system. Besides, the system hasa temperature sensor to acquire environment temperature; the measurement data issent by a serial port to host computer software. Additional precautions are adopted tocurb the impact of external interference to the system.A series tests is taken on the low-level current measurement system with acertificated Keithley6430SourceMeter, the test results show the measurement systemharbors high accuracy, precision and resolution: when a input current is1pA, therelative error is better than0.6%, the standard deviation is about0.01%and theresolution is1fA as it can response to the current changes in1fA. Additional test ishold with JLZ-I HPIC which is designed by China Institute of Atomic Energyconnected: the relative error is about0.83%compared to true value in a radiationreference field under a dose rate of2.276μ Gy/h by Cs-137; the relative error is lessthan-0.80%compared with a model RSS-131ER HPIC which is made in USA inenvironment background. The test result shows the system is compatible with HPICand the accuracy and precision are guaranteed with a HPIC. Above all, the low-levelcurrent measurement system fulfills the needs of field calibration HPIC. Thelow-level current measurement system has been used in the study on calibrationmethod of install environment monitoring gamma-ray dosimeter. |
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