A method to determine the response coefficient of environmental gamma spectrometer to airborne radionuclide concentration

[Background] In the last decade, a large number of environmental gamma spectrometers have been installed to monitor ambient radionuclide in the environmental radiation monitoring stations in China. However, the response coefficients to airborne radionuclide concentration of most facilities were unknown. [Purpose] This study aims to establish an accurate and practical method for determining the response coefficient to airborne radionuclide concentration of the environmental gamma spectrometers. [Methods] Based on two assumptions on the spatial distribution of the facility's detection efficiency, a theoretical model was proposed and simplified. Subsequently. Monte Carlo simulation method was utilized to analyze the deviation contributed by the two assumptions for several common radionuclides quantitatively. Simultaneously, the detection efficiency spatial distribution of a self-developed gamma spectrometer which consists of a typical NaI(Tl) crystal (76.2 mm × 76.2 mm) was measured in an ¹³⁷Cs expose experiment to verify the accuracy of Monte Carlo simulation method. Lastly, the method for determining the response coefficient was established by using the simplified theoretical model, and applied to two environmental gamma spectrometers in field. [Results] The relative difference between the experimental and simulated detection efficiency of the self-developed gamma spectrometer was less than 6% at every measurement position. According to the Monte Carlo simulation results of the typical gamma spectrometer, relative deviations of response coefficients to several common airborne radionuclides contributed by the two assumptions were less than 0.1% and 10% respectively. In the field test, the changing trend of the detection efficiency was similar to the simulated results, and the response coefficients to airborne ¹³⁷Cs of the two facilities were 6.49 min^-1 ∙ Bq^-1 ∙ m³ and 2.62 min^-1 ∙ Bq^-1 ∙ m³ respectively. [Conclusion] The results of Monte Carlo simulation indicated that the simplified model was acceptable for environmental radiation monitoring field, and the applicability of this method had been validated through field test. (authors)