Research On Low Level Radiation Xenon Detection Technology

The verification of Comprehensive Nuclear-Test-Ban-Treaty(CTBT), beside three different waveform techniques, global monitoring of radioactive aerosols and noble gases. Several isotopes of xenon are sufficiently produced in fission and a few have suitable half-lives and radiation to be detected. These are131mXe,133mXe,133Xe and135Xe and have been selected for continuous monitoring.The goal of this thesis is to devlope a new type of detector for radioxenon detection that was based on the investigation of low level radioxenon detection technology and relevant experiments. The works mainly focused on developing the phoswich detector for radioxenon while considering the decay law of radioxenon and the development trend of radioxenon detctors in recent years. These works includes structure design and simulation of the phoswich detector, electronic readout system development and coincidence pulse analysis, some preliminary experiments were carried out with phoswich detector such as performance test of phoswich detector, research on background reducing, energy calibration using standard source and measurement of xenon irradiation sample. The principal conclusions of this article are as follows.(1) Phoswich dector design. It was the first application of phoswich detector on radioxenon detection in china. The detector consists of two different scintillators coupled to a single photomultiplier tube; the fluorescence was collect by the same PMT. The simulation was carried on using MCNP5toolkit for futher design. Only0.8%of β particles (135Xe910keV) can break through the BC404while the thickness of the BC404scintillator was set to1.5mm and they absorption was less than6.5%for81keVγray(133Xe).(2) Data acquisition and analysis. Data acquisition system was developed based on NI digitizer. The system integrates various functions like data acquisition and data format conversion, pulse discrimination, multi-channel analysis for detector calibration and coincidence pulse analysis for β-γ measurements. Rectangular filter window method was brought out in order to figure out the contribution of each component from the coincidence pulse. The calculation of beta and gamma energy was more accurate compared with region of interest method as the position of the pulses was not fixed. Rectangular filter window method is also efficient in neutron gamma discrimination. A better separation of neutron and gamma pulses is achieved while comparing with compared charge method.(3) Preminlary experiments were carried out based on phoswich detector and data acquisition and analysis system. The special coincidence pulse which stands for a coincidence event was acquired. By selecting the coincidence pulses from the whole background signals, a background reducing factor was calculated by94.6±1.0. The low energy region of CsI(T1) was calibrated by the characteristic gamma ray of standard source241Am and133Ba. Compton electrons would deposite in BC404while compton event was occurred next to the BC404layer. The energy calibration of BC404was done using the compton scattering electron spectrum of137Cs. Minimum detectable concentration(MDC) of133Xe and135Xe was figured out using the coincidence spectrum. The xenon irradiation sample was measured using radioxenon phoswich detector, the half time of135Xe is9.56h by experiment data and the relative deviation is4.6%compared with theoretical value of9.14h.