Activity measurement of 85Kr diluted by a large volume balloon technique

The National Metrology Institute of Japan (NMIJ) participated in the CCRI(II) Key-comparison of 85Kr activity measurement. The absolute internal gas-counting method was used, with a set of proportional counters of different length. The original 85Kr gas was diluted by a factor of 2×105 in order to achieve the suitable activity concentration for the proportional counters. A new dilution method was introduced, based on a large-volume balloon and gravimetric determination of the dilution volume with small uncertainties.