Abstract

A ⁸⁵Kr measurement system for continuous monitoring based principally on the BfS-IAR method (activity measurement of ⁸⁵Kr by gas counting coupled with gas chromatographic separation, using pure CH₄ as carrier and counting gas) was implemented for the first time in Japan. In this paper, a detailed description of the system and procedures is given and the inter-comparison results of our system with the BfS-IAR system are presented. A consistent temporal concentration change with high accuracy and consistency of the respective data with the BfS-IAR data (maximum difference of 5%) were achieved with the Meteorological Research Institute (MRI) system, which shows that the system is valid and reliable for the purpose of background monitoring for ⁸⁵Kr in air. Also, the ⁸⁵Kr monitoring record at the MRI during 1995–2001 is described. The record distinctively shows the Northern Hemispheric background ⁸⁵Kr concentrations at the mid-latitude and the elevated concentrations affected by the operation of a nuclear fuel reprocessing plant in Tokai-mura, Ibaraki, Japan.