ACTIVITY OF FALL-OUT

Press, Volume CII, Issue 30279, 4 November 1963, Page 13

ACTIVITY OF FALL-OUT

Rapid Decay With Time

Because nuclear fission products decay rapidly, protection from fall-out is especially important in the period immediately after the explos. ion which caused it, Mr E. J. Brown, of the National Radiation Laboratory, told the annual meeting of the New Zealand Medical Physicists’ Association yesterday. In general, the radioactivity of fallout material decreased rather faster than proportionately to time. For example, if fall-out was at a certain level of activity one hour after a nuclear explosion. seven hours after the explosion it would be only one-tenth of this value. Hie dose accumulated in tha first to seventh hours after detonation would be about the same, other things being equal, as the accumulation from the seventh hour to a week later; and the first week’s dose would be about twice as great as the entire remaining dose possible for the lifetime of the activity. “This rapid decay suggests the benefits of protection in the early periods after fallout.” he said. The rate of decay factor he had quoted was only approximate, however, Mr Brown added, as it represented the total effect ot a large number of isotopes decaying exponentially. In the detonation of a nuclear weapon, about 60 radioisotopes of 34 elements were formed. Most of these isotopes themselves gave rise to decay chains involving several radioisotopes, so there might be 170 different radioisotopes produced eventually. In addition, there were likely to be radioisotopes resulting from the reaction of neutrons from thy explosion with elements in’ the environment The detonation of a onemegaton fission bomb gave rise to about 300,000 megacuries of radioactivity measured one hour after the burst, not counting the -ctivity arising from neutron reactions with the environment. The decay curve tended to flatten out after the first six months because of the predominance after that time of long-lived isotopes such as caesium-137 and cerium-144.