Analysis of ¹³⁴Cs, ¹³⁷Cs and ⁴⁰K in the certified reference material of Brown Rice by gamma-ray spectrometry using a Ge detector. Part 1. Determination of massic activity

The massic activity of ¹³⁴Cs, ¹³⁷Cs and ⁴⁰K in the certified reference material (CRM) of Brown Rice prepared by the Japan Society for Analytical Chemistry has been determined by gamma-ray spectrometry using a Ge detector. The determination was carried out by a simple comparison method that compares the gamma-ray peak counts of the sample with that of the standard source of the analyzing radionuclide, and by an efficiency curve method that uses a peak-efficiency curve obtained by measuring a multi-nuclides mixed standard source. Commercial standard sources were used for ¹³⁴Cs and ¹³⁷Cs analysis; however, a self-prepared standard source composed of reagent-grade KCl was used for ⁴⁰K analysis in the simple comparison method. A true coincidence summing (TCS) effect of cascade gamma-rays, which is significant in the efficiency curve method, was corrected by a gamma-ray spectrum analysis program using a peak-to-total (P/T) ratio curve prepared by measurements of non-cascade gamma-rays. Any difference of the self-absorption effect of measuring gamma-rays between the standard sources and the sample was examined by experiments using an external gamma-ray source, and by calculations using analysis programs. It was found to be smaller than 1%. The stability of the gamma-ray peak efficiency curve was found to be better than 2.3%, obtained by measuring the multi-nuclide mixed standard source during 2 years of the analysis period. The analysis results of the massic activity of ¹³⁴Cs, ¹³⁷Cs and ⁴⁰K in the Brown Rice CRM by the simple comparison and efficiency curve methods agreed within 4%, and with the certified values within their uncertainties. Because of an insufficient correction of the TCS effect, the analysis result of the ¹³⁴Cs by the efficiency curve method was 4% lower than that of the simple comparison method. Further development of the TCS correction for ¹³⁴Cs is required for high peak-efficiency measurements. (author)