Tehran University of Medical Sciences Acta Medica Iranica 0044-6025 27 1-4 1985 12 15 41 46 HASSAN ASKARI SHIRAZI 2015 09 28 Following a nuclear explo sion, the radioactive materials descend t o the earth. Most of the act i v i t y results from fis sion fragments. Therefore, new atoms created when heavy a toms such as uranium and plutonium are fissioned. Atoms made radioactive by neutron capture are a:50 important. If the explosion is sufficiently high in the air, it takes months to years for most of t he resulting finely dispersed par t icl e to settle (1& 2). By t his time the gr eater pa r t o f "t he initial radioactivity has de caye d away. This delayed f allo ut covers the entire wor~d. Rain and snow help t o bring it down . It tends t o be gr ea t er in wet c l i ma t e and seasons. In t h is a r t i cle the da t a obtaine d during Russi an nuclear air t e sts has been a nalys e ( 1961) . https://acta.tums.ac.ir/index.php/acta/article/view/183 https://acta.tums.ac.ir/index.php/acta/article/download/183/179