radioactive sources having the same range and proportion of energies as the fallout material. DISCUSSION Decline and fallout composition It has turned out in practice at Rongelap Atoll that when a large number of survey meter readings are taken and these are averaged, a pattern of decline of gamma-dose rates consistent with the theoretical decay for mixed fission products emerges. This is true even though the theoretical curve is based on the decay of mixed fission products from y735 distributed uniformly over an infinite plane and disregards differences in both the composition and distribution of fallout radionuclides in the actual field situation. The fallout at Rongelap consisted of mixed fission products and neutron-induced radionuclides from a thermonuclear device. The induced activities contributing to the gamma activity are of shorter half life than the long-lived fission product, 30-year cst37, It therefore might have been expected that the early decline in gamma dose rates at Rongelap would have been more rapid than the theoretical decay of mixed fission products alone. As the art of producing thermonuclear the fission yield per kiloton will decrease. devices progresses There will be a higher proportion of induced radionuclides to fission products