CHAPTER 3 FALLOUT CHARACTERISTICS In order to calculate a total gamma dose received by an indivi- dual in an area where dose rate was measured at a given time, a value. for the rate of change of radiation intensity during the exposure 7!> The latter quantity has often been approxie te assume must tha period using mated well d, known way-Signer (t74 ) decay law. In this case.= however, it was known that large amounts of Np*39 and Np¢49 vere: to be. expected in the fallout of the 1 March shot, making its early decay. characteristics as well as its energy spec trum somawhat different from: G those of previous detonations, It was therefore decided, that the -, value of decay rate assumed to exist during the exposures should be based, as tar as possible, upon experimental data fran this test, Unfortunataly, no decay rates ware followed closely in any of the imuediate areas where the exposures occurred, and it 1s known that the radiochenical canposition and decay rate of the fission product mixture usually vary both with place and time. However, carly decay rates in he Bikini lagoon itself had been measured in a series of fallout samples taken at other points nearer the site of the detonation (keference 5). Since these values were the best data available, they were used in the calculations and wera assumed to hold HO the fallout on each of the islands, The early samples showed a consistent pattern among various loca- tions and a decay exponent (n) of tetween 0,8 and 0.9 in Equation 3.1, A= ay(t/t)-n wheres: (3.1) A= activity (d/m) at time t, | This decay exponent (n) was found experimentally to fit the data for the period H+ 5 to H+ 50 hcurs, in Reference 5. 3.2 The observed values are given CALCULATED LECAY HATES These decay ratgs were Spmpered with calculated values based on the presence of Np and Np in the fallout mixture. The calenl tions were made on the assumption that the.eeeeee as pe at am cies ” oof, . me TE RT ee, . eo . J . ve ae no Seow * moe, eo an La rs gS 39 , gaat The ne mitts EXPERIMENTAL DATA ee 3.1