The concept of transforming the detector efficiency of ϕ 75mm×20mm 13737Cs gas sample to a virtual 137Cs point source simulation, in order to facilitate efficiency evaluations for different source location, was proposed for the HPGe detector. The virtual point source usually has spot diameter, so, analysis of the influence of spot diameter of point source was studied in the present work. It was found that the point source model does not seem to fit too well to the experimental data, for the small source-to-detector distances. A multi-parameter polynomial expression describing the dependence of the count rate versus the source-to detector distance was fitted to the experimental data. The point source (ϕ 4.0mm) and small disk source (ϕ 20mm) were used in this experimental simulation, and their result discrepancy was 5.6%. Monte Carlo simulations were also used to study above experiment process, the discrepancy was 2.6%. For another simulation, the point source (ϕ 4.0mm) and ideal point source (ϕ 0.0mm) were used in Monte Carlo simulation, and their result discrepancy was zero. So, for the whole gas sample simulation, the influence of spot diameter of point source can be neglected in this simulation method.

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