2016 Volume 65 Issue 11 Pages 645-655
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.
