RADIOISOTOPES

The certified reference material of brown rice grain was developed for validation in measurements of massic activities ¹³⁴Cs and ¹³⁷Cs. The homogenized brown rice harvested from north east area of Japan at 2011 was used as raw material of the certified reference material. The certified values of ¹³⁴Cs and ¹³⁷Cs in brown rice grain certified reference material were determined using the calibrated Ge semiconductor detectors. The counting efficiencies of ¹³⁴Cs and ¹³⁷Cs on the Ge semiconductor detectors were calibrated by standard solutions of ¹³⁴Cs and ¹³⁷Cs traceable to the national standard. The certified values of the certified reference material is close to the radioactive caesium standard value in general foodstuff set by Ministry of Health, Labour and Welfare of Japan. The associated expanded uncertainties of the certified values were calculated from standard uncertainties related ¹³⁴Cs and ¹³⁷Cs measurements and homogeneities of ¹³⁴Cs and ¹³⁷Cs. The relative expanded uncertainties of massic activities of ¹³⁴Cs and ¹³⁷Cs are 5.9％ and 5.5％, respectively.