Design And Development Of An On-line Detecting Instrument For The Radioactivity Of Noble Gases

During usual operation or spent nuclear fuel reprocessing,nuclear reactors can generate radioactive noble gases with 85Kr,133Xe,135Xe and 41Ar as the main representatives.In order to evaluate the level of internal irradiation dose produced by radioactive noble gases to the staff working on site,a portable on-line detecting instrument to measure and classify the radioactivity of these noble gases was designed and developed.The chief structures of this instrument consist of a sampling gas chamber,a double-layer phoswich detector,front-end electronics,and data acquisition system.In this paper,the research of optimum structure simulation study,mechanical structure design and electronics design were carried out.In the meantime,the systematic work such as detector machining and assembly,basic performance tests were also completed.The major research contents and results are presented as follows:(1)The phoswich detector adopts a cylindrical structure design,which can detect the particles in total 4? direction and increase the detection efficiency as well.The outer layer of CsI(T1)scintillators are applied to detect the gamma rays.The inner layer of plastic scintillators can also act as the gas sampling chamber while detecting beta rays.Due to the great difference in the luminescence attenuation time between two kinds of scintillators,the resolution and radioactivity measurement of different radionuclides can be realized by detecting and classifying beta,gamma as well as beta-gamma coincidence events of various energies.The excitation illuminating signals emitted from scintillators are read out simultaneously by a couple of terminal photomultipiler tubes(PMT).The pulse shape discrimination algorithm based on on-line digital pulse analysis technique is used for beta-gamma discrimination.(2)Based on the Monte Carlo simulation program Geant4(Geometry and Tracking 4),the key factors including the thickness of scintillators,the geometric size of sampling chamber which can affect the detection efficiencies of beta and gamma particles emitted from different nuclide were discussed The Geant4 simulation study plays a highly important role in determining the optimal geometric sizes of phoswich detector.The design of detector shell and front-end electronics follows the principles of lightweight and modular design,which is convenient for nuclear power field staff to carry.In the research of the front-end electronics system,the amplitude of pulse signals generated by the CsI(T1)scintillator were depressed by adjusting the RC time constant in the preamplifier,so that the system can acquire the luminescence signals from two different scintillators simultaneously.In order to realize the on-line analysis of digital pulse,the full waveform coincidence signals from dual-terminal readout circuit can be acquired based on a multi-channel digital pulse analyzer with built-in FPGA module.In data acquisition,a multi-channel digital pulse analyzer based on built-in FPGA module can realize on-line analysis of digital pulse by acquiring full-waveform coincidence signal of dual-end readout circuit.(3)After the development of the on-line detecting device for the radioactivity of noble gases,few preliminary tests were carried out by standard radioactive source 137Cs.The test results show that both the double-layer phoswich detector and front-end electronics have quite stable performance in particle detection.During the seven-days energy spectrum acquisitions of both ends,the peak position change rate of the main energy peak is 0.5%,and the variation range of the energy resolution is less than 0.4%.In this paper,a phoswich detector with two different scintillation layers for on-line classification and measurement of four different radionuclides:85Kr,133Xe,135Xe and 41Ar has been successfully developed.The work not only offers a feasible scheme for the radioactivity measurement of noble gases around nuclear reactors,but also can provide necessary reference data for estimating and evaluating internal radiation dose produced by radioactive noble gases to the on-site working staff.