10 1.3 EFFECTS OF IONIZING RADIATION Superficial Doses of Radiation From Beta and Soft Gamma Radiation Tiere Can Be no doubt that the doses of radiation to the surface and thefirst few millimeters of the body were substantially higher than the mid-line dose of gammaradiation as a result of physical considerations of gamma energy and depth dose. In addition, the clinical observations of the skin lesions (see Chap. ITI) forcefully demonstrated that the dose to the skin varied considerably between individuals and over the surface of any given individual. As will become evident in the following discussions of surface dose, it is obvious that any numbers presented are at best only estimates and represent an approximation of some minimalvalue. In areas where lesions were severe the doses must have beensignificantly higher than in nondamaged areas. To arrive at some physical estimate of the skin dose, an attempt must be made to add up the contributions of the high energy gamma, the very soft gamma, and the higher energy beta radiation from the large planar source in which the individuals were of necessity existing. However, as alluded to above and emphasized in Chapter ITT, the largest component of skin irradiation resulted from the spotty local deposits of fallout material on exposed surfaces of the body. The dose from deposited material is impossible to estimate; however, that from the large planar source may be roughlyestimated as follows: The beta dose rate in air 3 feet above the surface of an infinite plane contaminated with mixed 24 hourold fission products is estimated to be about three times the total air gammadose. Sa r ww? ot 4a The mid-line gamma dose is approximately 60 percent of the air dose remaining after excluding that portion of the dose below 80 KV. This portion in turn is estimated to be 40 percent of the gamma dose measuredin air by the instrument. Thus the dose at the surface of a phantom exposed to mixed fission product radiation from an external plane source might "i u —= i be expected to be 3/(0.6) (0.6) or about 8 times the mid-line dose, if both are taken at 3 feet off the ground. Such a depth dose measurement has in fact been made experimentally at a previous test, using a phantom manexposed to both the initial and residual radiation (5). The depth doses for each situation are shown in Figure 1.5, with all data as percent of the 3 centimeter dose. With the diverging initial radiation from the point of explosion, the exit dose was seen to be 63 percent of the 3 cm. dose, but with the diffuse residualfield of fission products providing a semi-infinite planar source, a surface dose some8 times greater than the 3 cm. and deeper dose from the harder gamma components was observed. This is seen to be of the same order of magnitude as that estimated above. At heights above and belowthe 3 footlevel this surface dose would become lower and higher respectively, but since it is due to soft radiation of short range, it probably would not exceed 50 times the 3 foot air gammadoseor 80 times the midline dose, even in contact with the ground. An estimateof skin dose due to ground contamination for the Rongelap case would result, for example, in a figure of about 2,000 rep at the level of the dorsum of the foot, 600 rep at the hip level and 300 rep at the head if continuous eaposure with no shielding occurred. Unknown variation in dose undoubtedly resulted from shielding and movement. It thus seems probable that the external beta dose from local direct skin contamination far outweighed that from the ground in importance, since the latter was not high enough to produce the observedlesions. Clothing probably reduced the beta dose from the ground by 10 to 20 percent. 1.4 Summary Rapiarion Doses from gammarays originating externally were calculated for the 267 individuals who were accidentally exposed to fallout following the nuclear detonation at the Pacific Proving Ground in the Spring of 1954. The dose estimations were made using information resulting from radiological safety surveys on