Atmospheric and precipitation radioactivity in India

The contribution of the major nuclear weapon test series in 1961 and 1962 to the ground-level activity in India during 1962–1965, has been estimated from fission product ratios in the ground-level air and precipitation samples collected at eight stations in India. Induced activity Mn-54 produced in the 1961 and 1962 tests has also been examined in relation to the fission product activity. Fission products Cs-137, Sb-125, Ru-106, Ce-144 and Zr-95, and induced activity of Mn-54 were estimated by gamma spectrometry. These measurements have also shown that, (a) while the well known spring increase is observed at all these stations, the time of peak values show significant differences at the various stations which could be related to the monsoon winds, (b) the gradient in activity along the latitude band of 10° N–32° N shows seasonal differences which can also be attributed to changes in surface winds, (c) Mn-54/Cs-137 activity ratios show that the radioactive debris present at high altitudes during 1962 had started descending to ground-level by the spring of 1963.

Concentrations of natural isotope Pb-210 were also measured at all these stations. Seasonal variations in Pb-210 differed markedly from the seasonal variations in Cs-137. However, both Cs-137 and Pb-210 show higher levels at the stations in the northern latitudes in India.

On the basis of the measured ground-level concentrations of fission products in 1963, 1964 and 1965, estimates of stratospheric residence time have been discussed.