Chlorine-39 in rainfall at a temperate latitude (40°N)

Rainwater samples were collected and measured for the radioactivity to confirm the presence of Chlorine-39 (T1/2 = 56.2 min) formed by the interaction of cosmic-ray slow negative muons, μ− with Argon-40 in the lower atmosphere. The presence of 39Cl was confirmed by estimating the half-life of a radionuclide present in the rainwater ranging from 58 to 63 min and averaging 60.4 min. The disintegration rate of this radionuclide varied from 2.1 to 2.7 Bq L−1 (125–160 dpm/L) of rainwater and averaged 2.3 Bq L−1 (140 dpm/L) of rainwater in Thessaloniki (40°38′N, 22°58′E), Northern Greece. This concentration is indicative of mid or temperate latitudes. This is clearly less than that reported in the literature, 1.7 Bq L−1 (100 dpm/L) of rainwater, for a southern latitude 31°54′N, 34°49′E (Rehovot, Israel), due to the latitudinal effect of cosmic radiation.