Analysis of 22Na using a spectral summation technique on high-volume aerosol samples

Measurements of 22Na in the Atmosphere: Ground Level Activity Concentration Values from Wet and Dry Deposition Samples

Sodium-22 (22Na, half-life 2.603 years) is a cosmogenic radionuclide mainly produced in the stratosphere by nuclear spallation reactions of cosmic rays on 40Ar. Due to the very low concentration levels normally reached in the environment, 22Na poses no significant radioprotection threats: actually, the effective doses delivered to humans can hardly exceed a few nSv per year, a very negligible value. However, the measurements of this radionuclides can be very interesting for atmospheric circulation and climatic studies. Unfortunately, the difficulty of 22Na detection, due to its very low concentration levels, has prevented the gathering of large and widespread time series of this radionuclide. In this paper, a method for the retrospective measurements of 22Na in the atmosphere, starting from the gamma spectra (hyperpure germanium detectors (HPGe) detectors) of wet and dry deposition samples stored in our databases is proposed and validated. The method was applied to spectra samples gathered in the context of the Italian National Radioactivity Monitoring Network (RESORAD) and allowed the detection of the very low atmospheric activity concentration values of 22Na present at ground level. The results obtained with the new method are discussed and compared for validation with the available experimental values. Finally, some possible applications to environmental studies are also highlighted and suggested.

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

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.

Circulation of cosmogenic 22Na using the global monitoring network of the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO)

Using a recently published global data set of ²²Na and ⁷Be from the global monitoring network of the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO), atmospheric circulation and stratosphere-troposphere interactions are examined. Cosmogenic ²²Na has a half-life well-suited to environmental processes with durations from several months to a decade. Combined with corresponding ⁷Be observations, these two cosmogenic isotopes form a useful environmental tracer and new radiochronometer to study physical interactions of air masses in the stratosphere and troposphere.