Results of a Collaborative Experiment on a Correction of the True Coincidence Summing Effect for ¹³⁴Cs

Abstract

The results of the International Inter-Comparison Exercise (IICE) of a fish certified reference material suggested that true coincidence summing (TCS) correction by a method widely used in Japan is insufficient for ¹³⁴Cs. Thus, the committee for the preparation of reference materials for radioactivity analysis of JSAC conducted a collaborated experiment for the determination of ¹³⁴Cs and ¹³⁷Cs radioactivity of standard sources of those nuclides to validate the TCS correction from 4^th January to 4^th April 2017. A total of twelve Japanese laboratories participated in this experiment. Two standard sources of ¹³⁴Cs, and a mixture of ¹³⁴Cs and ¹³⁷Cs with cylindrical shape were passed along the laboratories to determine the radioactivity of ¹³⁴Cs and ¹³⁷Cs of the sources. The metrological traceability is ensured in terms of the radioactivity of the standard sources. The laboratories analyzed the standard sources using a gamma-ray spectrometer, and reported the analysis results together with measured raw data and their related information to the organizer. The measured data were analyzed by the organizer using two Japanese gamma-ray spectrum analysis programs independently. The analysis results between the laboratories and the organizer almost agreed. The analysis results of ¹³⁴Cs and ¹³⁷Cs of two standard sources were lower by 2.3 % to 5.2 % than the reference values. While the –2.3 % difference for ¹³⁷Cs is still within the rage of the standard uncertainty of the reference value, the –2.7 % to –5.2 % differences for ¹³⁴Cs were out of the standard uncertainty range of the reference values concluded that the differences for ¹³⁴Cs is created due to the insufficient correction of TCS.