25 as a function of time, t, for a given shot, D(A), curies of mass chain A for 1-kt fission yield as a function of time, calculated by the ORIGEN computer program with the 2334 235 yy 238) and 2392405, data omitted (table in computer program). KT 5. kilotons of fission yield (input data). External gamma-ray exposure rates Assume that the fallout is distributed uniformly over 1 square kilometer. The external gamma-ray exposure rate for each individual nuclide measured 1 meter above ground level is given by MR(A), = CIA), * C(A), where MR(A),. = mR/hr measured 1 meter above ground level due to mass chain A as a function of time. C(A) = conversion factor (Be80), (mR/hr) /(Ci/km*), (table in computer program), the value is defined by specifying the relaxation length* (input data). * According to H. L. Beck, (Be80) the concentration of fallout varies exponentially with soil depth, z; C = Coe4, He defines relaxation length as 1/a.