Preparation of a reference fallout material for activity measurements

Since the present environmental activity level is becoming extremely low, the quality control in determining fallout activities has become more important. So far, there is no reference material for fallout activity. In this paper, preparation of such a reference material is tried by using deposition samples collected at 14 stations throughout Japan in 1963–1979, which were preserved after the γ-activity measurement. About one-third of each deposition sample was combined and pulverised in a rolling pot mill. The obtained mixture was divided into about 1000 bottles, each of which contains 4 g of the sample. A homogeneity test was done by 137Cs determination. With this reference material, 90Sr, 137Cs and Pu isotopes were determined by several independent institutions. The results showed good accordance among the individual institutions, meaning that the sample can be used as a reference material for the measurement of the fallout activity.