Study On Efficiency Calibration Technology Of Carborne NaI(T1) γ Spectrometry System

The carbone Nal(Tl) γ spectrometer system is mainly applied in real-time measurement of the artificial radionuclide in on-site inspection (OSI) and nuclear emergency monitoring. In order to obtain the accurate concentration of radionulide, the efficiency calibration has to be performed. Because the project of this system is measuring the radionuclide in the air, which means that the radionuclide approximates distributing in an infinite space, it is difficult to perform the efficiency calibration correctly by the general methods such as experimental method, Monte Carlo (M-C) simulation method and numerical calculation method based on the Beck formula. To solve this problem, a kind of sourceless efficiency calibration method is proposed, which is a combined method of numerical calculation and simulation. The source efficiency can be obtained by numerical calculation, as well as the peak-to-total ratio can be done by numerical simulation, so the source-peak efficiency can be obtained by these two parameters. The primary contents on the study are as follows:(1) Calculation of source efficiencyAccording to the characteristic of interaction between gamma-ray and crystal of detection, the measured space is divided into26subspaces. And then the formulas on source efficiency in different subspace are deduced respectively.(2) Calculation parametersThe parameters used in the formulas were acquired through point source experiments, numerical simulation and regression analysis.(3) Calculation of peak-to-total ratioThe peak-to-total ratios of point source were acquired by numerical simulation at180different positions. And the result indicated that the variation of peak-to-total of a given gamma-ray at different positions is less than3%, so it can be treated as a constant.(4) Verification of the methodThe proposed method was verified by point-source experiments at41different positions and plane-source experiment. To the point source and plane source, the deviation between the calculation results and the experiment results were less than10%and6%respectively.By the method, the full-space efficiency calibration of carbone Nal(Tl) γ spectrometer system can be performed accurately.