4, RESULTS Fallout patterns were routinely measured on all shots detonated at the Nevada Test Site from 1951 through 1958. The quantity (uC i/m2) /(mR/hr) was measured on the falloutfrom 13 tower shots and three balloon shots from 1953 through 1957 (Ba58, La66, Ra54). In 1957, fallout from four tower shots and one balloon shot were collected, sorted by particle size, and each size fraction analyzed radiochemically (La66). The analytical results from the tower shots provide a detailed check of the calculations described herein. DECAY OF FALLOUT The calculated external gamma-ray exposure rates* are shown in Fig. 1. Fractionation affects the shape of the decay very little, but losing half of the refractory elements reduces the relative exposure rate by “12% and losing all but one tenth reduces the rate by 25% over the first six weeks. * According to H. L. Beck, (Be80) the concentration of fallout varies exponentially with soil depth, z; C = C,e4, He defines relaxation length as 1l/a. All calculations in this work use a relaxation length of 0.16 g/cm2 unless otherwise specified. See Appendix 2.