| With the development of nuclear energy,the society pay more and more attention to the safety of utilization of nuclear energy.The monitoring of radionuclides not only provide assessment standards for nuclear emergencies,but also effectively monitor the operation safety of nuclear facilities,thus it has became an indispensable part in the process of ensuring the safe use of nuclear energy.In the monitoring of many radionuclides produced by nuclear reactions,radioactive noble gases represented by nuclides such as 85Kr and 133Xe are important nuclear reaction products,and their monitoring had received more and more attention.At present,the monitoring of noble gases were usually analyzed by high-purity germanium gamma spectrometer.The gamma spectrometer measurement method is a relative measurement method,the key of measurement reliability is the accurate detection efficiency,and to make the measurement results more accurate,the detection efficiency is often needed to trace back to the corresponding national standards.Up to now,a complete traceable system for measuring the radioactivity(or activity concentration)of noble gas such as 85Kr and 133Xe has not been established in China,so the problem of the standard transfer of response is urgently need to be solved.Under this background,the internal gas proportional counter is used as the detection system,and the measurement principle of the length-compensated method is used to carry out the research on the absolute measurement and the standard transfer method of the 85Kr activity concentration.The research results will provide the technical foundation for establishing the tracing system of activity value of radioactive noble gas.The main research of this topic are as follows:1.The absolute measurement method of activity concentration of noble gas(85Kr)was studied by using length-compensated method on the radioactive gas measuring device set up by the National Institute of Metrology of China(NIM).Including:through the counter high-voltage flat curve,measurement of ? spectrum of 85Kr nuclide decay,optimize the proportional counter system operating high-voltage,pulse molding amplification,non-extending dead time and threshold and other experimental parameters.The counting losses of end effect was corrected by the length-compensated method.The method of extrapolation of ? energy spectrum is used to correct the loss counting below the threshold of electronics.The effects of dead time and background counting rate on the measured results of 85Kr activity concentration in the counter were studied.2.A system was established for quantitative sampling of 85Kr noble gas.The controllable activity concentration of 85Kr gas in the transfer vessel could achieve by accurately measuring the temperature and pressure of gas sampling.With the reference volume bottle,of which volume traceable to the National Institute of Metrology of China(NIM),was used as the volume standard,the volume of transfer vessel and the volume of the absolute measuring device(including the counter)were accurately measured by the method of gas diffusion.Thus,the dilution ratio of 85Kr activity concentration in the counters and the transfer vessel was determined,and the absolute measurement of 85Kr gas activity concentration in the transfer vessel was obtained.In order to ensure the accuracy and reliability of the measured results of 85Kr gas activity concentration,the sample volume of 85Kr gas activity was controlled by changing the gas pressure in the transfer vessel,and the measured results of 85Kr activity concentration under different sampling conditions were normalized to the standard temperature and pressure conditions(100 kPa&273.15k).The results show that the 85Kr gas activity concentration measured under standard temperature and pressure conditions is 766.6 Bq/mL(Reference data,September 6,2018),which is-0.25%relative deviation from the measurement results traced to National Physical Laboratory(NPL).Through the quantitative analysis of the influence on the measured results of 85Kr activity concentration during the experiment,the relative standard uncertainty of the measured results of 85Kr gas activity concentration is 0.61%in this work.3.A set of stainless steel containers suitable for the measurement of high purity germanium spectrometer were designed and developed,the measurement of containers' volume and 85Kr gas sampling were completed on the gas quantitative sampling system.The ? spectrum of 85Kr was obtained on the spectrometers in the National Institute of Metrology of China(NIM)and the National Institute for Radiological Protection,Chinese Center for Disease Control and Prevention,the detection efficiency of 85Kr y characteristic ray(514keV)was obtained,with relative extended uncertainty(k=2)of the detection efficiency results was 1.34%.The feasibility of the method of quantity transfer was verified by experiments,which provided technical support for the traceability of 85Kr gas activity concentration measured by HPGe spectrometer. |
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