ation a Gorpon M. DUNNING the Pacific Islands, the winds were light and the first rainfall did not occur until about two weeks later. Graph 2 shows the gamina dose rates taken at 3 feet above the ground on the island of Rongelap > over a period of nearly a year. In the first ten days the decrease in activity, or disintegrations per unit time, is roughly consistent with the kuown radiological de- . * - . 20 DOSE (PER CENT) » a 4 oti vet consideration is the energy spectrum of the radiation. The gamma spectrum emanating from fallout material is complex. In Graph 1 is shown the gamma spectrum for fallout after the detonation of March 1, 1954, at the Pacific Proving Ground (+4), with the estimated percentage contribu“tions of the gamma quanta of differing energies (million electron volts). it is Ajcil 1956 ad. apy ay 4 586 0.6 "0.8 i.0 t.2 GAMMA ENERGY (MEV} Graph 1. Percentage of total dose contributed by gamma quanta energies shown (million electron volts). recognized that such spectra may vary and that any single value may conceal important features, but an estimate of 0.7 Mev mean energy has been quoted as a first approximation (5). WEATHERING AND SHIELDING The variable nature of the two parameters of weathering and shielding makes establishment of a precise rule, covering allsituations, impossible; yet these factors are operative in determining the total exposure received from fallout. One example will be used here to give some perspective as to weathering effects. After the detonation on March 1, 1934, in cay rate for fallout material, #.e., a slope of minus 1.2. The break between the tenth and twenty-fifth day, therefore, undoubtedly represents the effects of rain (and possibly winds), which was known to have occurred. The rest of the points fall roughly on a line of (time)~!’, reflecting principally the effects of weathering and possibly, to a smaller degree, the fact that the number of gamma quanta re“leased per disintegration decreases after the first thirty to forty days. In employing these data, however, one is faced with the problem of translating the effects from a Pacific island to larger land areas with different climatic conditions.