Response of gamma-ray spectrum during Ockhi cyclone

Abstract

Cyclone Ockhi hit Sri Lanka and southern parts of India in Nov–Dec 2017 with a devastating social impact. The present paper reports that the cyclone had a significant effect on the γ -ray flux measured by NaI (Tl) detector. An overall decrease is observed in the γ -ray flux during the passage of cyclone; however, a detailed investigation revealed that different energies show varying results. In the energy range between 250 – 450 keV, a decrease up to ∼14% is observed, whereas an increase up to ∼45% is observed in the energy range between 600 keV – 2.7 MeV. The energies above 2.7 MeV do not show any change. This is the first-ever observation of the varying effects of the cyclone with the energy bands of the γ -ray spectrum. We found that the increase observed in the energy range of 600 keV – 2.7 MeV is primarily due to the increase in the terrestrial radioactivity (peaks of 222Rn daughters), which is brought over by the rainfall accompanied with the cyclone. The study indicates that the decrease in the lower energy range of the γ -ray flux could be due to the attenuation caused by the increased tropospheric air-mass associated with the cyclone over the observation site. The high energy γ -rays are not affected due to the cyclone.