First participation by the NMISA in a low-level comparison: CCRI(II)-S9 exercise

• First participation by the NMISA in a low-level inter-comparison using an HPGe detector.
• Measured 137Cs and 40K in rice reference material.
• Efficiency determination by comparison against water standard.

The NMISA Radioactivity Standards Laboratory participated in the CCRI(II)-S9 inter-comparison of the measurement of the activity concentration of 137Cs and 40K in rice material, piloted by the KRISS. The paper describes the equipment used, the measurement set-up and data analysis. The efficiency of the detector for 137Cs and 40K was determined by comparison against a spiked standard solution, and Monte Carlo simulations performed to estimate the difference in γ-ray escape probability between the solution standard and starch (as an approximation for milled rice) due to attenuation disparities. The uncertainty budget was estimated rather conservatively, since these were the first low-level measurements performed by the NMISA using an HPGe detector.