The development of a digital gamma-gamma coincidence/anticoincidence spectrometer and its applications to monitor low-level atmospheric 22Na/7Be activity ratios in Resolute Bay, Canada

Abstract

Using a previously developed digital gamma-gamma coincidence/anticoincidence spectrometer, daily aerosol samples collected at Resolute Bay, Canada (74.71°N, 94.97°W) from May 2016 to April 2017 were analysed for activity concentrations of 22Na and 7Be. The spectrometer design allows a more selective measurement of 22Na with a significant background reduction by gamma-gamma coincidence events processing. It has been demonstrated that the improved spectrometer provides a more sensitive and effective way to quantify trace amounts of 22Na and 7Be with a critical limit of 3 mBq and 5 Bq respectively for a 20 h counting. The 7Be/22Na ratio data set obtained in this study showed significant annual variation, which has a distinct spring (typically from February to May) maximum and winter (from September to February of next calendar year) minimum, which suggest that it could be used as a radiochronometer for studying the atmospheric processes. The 7Be/22Na ratios are most likely connected to deep stratosphere-to-troposphere transport (STT) exchange events where air with a higher 7Be/22Na ratio originates from downward flow from stratosphere to the troposphere. The aerosols with lower 7Be/22Na ratios located between two oscillation peaks may have longer residence time. The correlations between 7Be and 22Na activity concentration were the high during these time periods. Compared with other studies based on weekly collected aerosol samples, the techniques greatly improve the temporal resolution of 7Be/22Na data set that will be able to provide more detailed information to study various atmospheric phenomena.