BRIEF SUMMARY OF GAMMA RADIATION SPECTRA FROM RESIDUAL RADIATION SOURCES FOLLOWING A NUCLEAR DETONATION By R. L. Marner U.S. Naval Radiological Defense Laboratory, San Francisco, California Introductory Note—The following brief summary is extracted froin research carried out by members of the Navel Radiological Defense Laboratory, including Dr. C. 8. Cook, Mr. F. M. Tomnovec, Mr. W. E. Thompson, Lt. R. F. Johnson, Mr. L. A. Webb, Mr. F. L. Bouquet and the author. The research has been supported by the Bureau of Ships, Navy Department, and in part by the Armed Forces Special Weapons Project. In the progress of a nuclear detonation both fission product and induced activities are produced in ratios which may depend onthe details of the weapon construction and of its environment. Following the detonation these activities are dispersed and fractionated by physical and chemical phenomena influenced by terrain and meteorological conditions. These activities come to rest and create a residual radiation field which can be controlled by shielding. The effectiveness of the shielding will depend on the nature of this radiation field. This Laboratory has been gathering empirical data on the nature of the radiation fields following various weapon detonations of the past several years from which one can say what the usually observed effects are and can say something about their customary variability. The distribution of residual activities is typically in two parts, one symmetrical about ground zero and due to activities induced in the soil by the bomb neutrons and to activities deposited there by the fireball, the second elon- gated and downwind due to fallout from the bomb cloud. The total gamma radiation intensity from mixed fission products decays with time in a fashion which is the sum of the exponential decays of the various nuclides in the mixture. The decay is usually empirically fitted by a negative power function of the time after detonation. The power is usually observed to be one and a fraction with some vi riation from shot to shot, from sample to sample of the same shot, from time to time on the same sample, and on the definition of the measure of intensity. A group of us has been applying gamma-ray scintillation spectroscopy to samples of residual activities from a dozen or so shots exploded in the last three years [1, 2, 3, 4, 5, 7]. A sample of someof our recent data is shown in Figure 1 which is a pulse height spectra of pulses from a 4-inch diameter by 4 inches long Nal(Tl) crystal detector but which, for purposes of this summary, Maybe called a gamma ray photon spectra. Beneath this spectrumare the spectra of 5 nuclides or nuclide chains which are often identifiable in these spectra. The first 3 are induced activities and the last 2 are fission products. There are, of course, many other isotopes present most of which seem to con- tribute unidentifiable lines in the region of 200 to 800 kev. Thefirst two induced activities are prominent in the soil around ground zero, The third can be formed from bomb materials which are in35