Abstract
A low background gamma spectrometer with an Etruscan, 2500 years old lead shield and a muon veto detector were applied to study 22Na and 7Be activity concentration in ground level air aerosol samples collected weekly over the years 2003–2006 in Kraków. Each sample was formed with ca 100 000 m3 of passed air, collected with two parallel ASS-500 high volume air samplers. The results for 40K and 137Cs are also presented for reference and comparison. Presented frequency distributions for activity concentration and correlation between the obtained results are discussed. The activity concentration results confirmed seasonal variation of activity to be different for all the investigated radionuclides. Moreover, the seasonal variation in nucleus activity ratio was also noticed for 22Na and 7Be. Cosmogenic radionuclides being mainly of stratospheric origin, are subsequently attached to fine aerosols, via which they are transported to the ground level air. The mean aerosol transport time within the troposphere was estimated as equal to 7.5 days on average, reaching even 50 days in warm seasons. Limitations of the applied model were identified.
Highlights
The cosmogenic radionuclides present in the Earth’s atmosphere are produced mainly by secondary cosmic rays in the stratosphere or troposphere, with the relative volume of about 2/3 and 1/3, respectively [1]
Cosmogenic radionuclides being mainly of stratospheric origin, are subsequently attached to fine aerosols, via which they are transported to the ground level air
The second cosmogenic nuclide studied here, 22Na, is formed mainly in spallation of argon nuclei by a secondary cosmic ray neutron [4]. 7Be decays (T1/2 = 53.3 days) by electron capture to stable 7Li emitting gamma radiation of 477.6 keV and a monoenergetic neutrino. 22Na decays (T1/2 = 2.602 years) by a beta plus process to 22Ne emitting a photon of 1,274.53 keV, a positron and a neutrino
Summary
The cosmogenic radionuclides present in the Earth’s atmosphere are produced mainly by secondary cosmic rays in the stratosphere or troposphere, with the relative volume of about 2/3 and 1/3, respectively [1]. Both nuclides were formed due to thermonuclear explosions in the atmosphere [6] conducted in the cold war era. The most detailed study was recently published as two papers [5, 15] Such a reverse proportionality to the solar activity results from additional screening of the Earth’s atmosphere with the magnetic field generated by charged particles from the solar wind trapped in the magnetosphere (so called Forbush effect). Soon after finishing the measurements, i.e. since 2008, the laboratory was granted Polish accreditation for gamma spectrometric measurements (ISO 17025)
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