products formed per KT of fission yield, plus 1.1 x 10° curies of beta- active products, for a total of 4.1 x 10° curies of fission product activity. The average effective gamma ray energy in a fission product field is about 0.7 Mev, so that if 1 million curies of mixed fission products are spread uniformly over a one square mile plane surface, the gamma radiation intensity measured 3 feet above that surface would be about 4 r/hr. The specific yields of radioactive materials have been studied in % a variety of ways. The Hunter-Ballou studies?! . on slow neutron fission, and Coryell-Sugarman compilation2/ ean be used to obtain percentage values. Zinc-72 is the atom of lowest mass and gadolynium-160 the greatest of those found to result from fission. Figure 1 indicates the variation of fission yield with mass number, with fission yield exe pressed as a per cent, for y> uv, and Pu’? , The total adds up to 200% since each fission gives two fission products. Fast fission of uranium-238, plutonium-239, and uranium-235 results in approximately the same fission yield values with the largest difference in the middle zone where the value varies from 0.01% for uranium-235 to 0.05% for uranium-238. Thus, the various mixtures of fissionable material and uranium-238 tamper material which may be present in a weapon have relatively little effect percentage-wise on the relative amounts of radioactive isotopes formed. The first peak in Figure 1 includes strontium- -89 and strontium-90 and the second peak contains iodine-131, which are ¥ — Hunter-Ballou studies are an accurate measurement of the percentage of radioisotopes obtained from a laboratory bombardment of uranium and plutonium by slow neutrons. The results of the observations in the laboratory are compared with the percentages of radioisotopes found in fall-out particles and variations between the labcratory and test percentages are an indication of fractionation. This approach is considered the only accurate method possible to calculate percentage yield. ‘The Coryell-Sugarman studies provide similer information about the percentages of isotopes produced in the laboratory from the bombardment of fissionable materials with fast neutrons. a 2/ Hunter, H.F., Ballou, N.E., Simultaneous Slow Neutron Fission of U-235 Atoms, I. Individual and Total Rates of Decay of the Fission s, USNRDL ADC-65, 1949. 3/ Coryell, C.D., Sugarman, N., Radiochemical Studies, The Fission Products, ine National Nuclear Energy Series, Book 2; 6 Part V, 1951.