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Abstract
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.
Highlights
Sodium-22 (22 Na) is a β+ cosmogenic radionuclide with a relatively long half-file (2.603 years), continuously produced by nuclear spallation reactions of cosmic rays on argon-40 (40 Ar) nuclei [1].It decays into the stable isotope 22 Ne by β+ emission (90.35%) and electron capture (EC, 9.65%).Its production mainly occurs in the stratosphere and is essentially due to the high energy particles (E > 100 MeV/n) belonging to the galactic cosmic rays component (GCR, galactic cosmic rays)
Since the last atmospheric nuclear weapon detonation occurred in 1980 (Lop Nor, China), the 22 Na levels returned to the typical pre-Cold War values: atmospheric concentrations usually well below 1 μBq/m3 at ground level [5]
22 Na can be measured by γ spectrometry with hyperpure germanium detectors (HPGe): it emits a strong γ line at 1274.5 keV with a yield close to unity (99.94%) in a region of the spectrum only slightly influenced by the Compton background of the 40 K high energy γ emission
Summary
Sodium-22 (22 Na) is a β+ cosmogenic radionuclide with a relatively long half-file (2.603 years), continuously produced by nuclear spallation reactions of cosmic rays on argon-40 (40 Ar) nuclei [1]. It decays into the stable isotope 22 Ne by β+ emission (90.35%) and electron capture (EC, 9.65%). A sudden increase of the ground level concentrations of the cosmogenic radionuclides, for example, can be used as indicator of the intrusion of air masses of stratospheric origin [6,7,8,9] In this respect, the study of the ratio 22 Na/7 Be could give very interesting information, as was recently pointed out in a recent study (Hoffmann, 2018, [10]). The availability of reliable time series of this radionuclide is very important and of great scientific relevance, allowing the gathering of some very interesting information that cannot be obtained using only easier-to-measure radionuclides, for example, 7 Be, usually present in larger concentrations
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