a see . seer eee oe . . ae woe soe fF = = ae . oo. so oF + « eo. eof Fourth: doses calculated on the long fallout hypothesis are lower than those dueto a short fallout, since a short fallout quickly deposits a large amount of activity. On Rongerik, a set of film bade readings covered the range listed below. Several badges worn both outdoors and inside buildings on the island read 50--65 r, and one badve which remained outdoors over the 98.5 hr period read 98 r. Another group kept indoors inside a refrigerator read 381. These dose values represent a variety of conditions, but considering the shielding and attenuation ; ficturs, are consistent with the assumption that the dose reached the calculated upper limit outside, again favoring the shorter fallout hypothesis. The upper limit of 98 r will result if it is assumed that the fallout lasted one hour during which the intensities rose from zero to the maximum dose rate which then decayed to values observed later. A long fallout will not produce such a high dose of radiation, Fifth: on Utirik, only «a short falfout time is consistent with the later dose rates observed, provided the fallout began as late as was estimated [rom wind and distance factors. hour duration* of fallout appears likely. A one On the other islands the actual fallout time is known to have exceeded one hour; however, since the approximate dose discussed above was seen to fit the film data on Rongerik, it was used for the other islands as listed in the calculations in Table 1.1. The hour limit is thus “an effective value.” If the long fallout case is also considered, 2 lower limit for the dose may also be estimated, though the upper limit is taken as most probable. The ranges are then as follows: Rongerik 50 r—104 r; Rongelap 102 r—175 r; Ailinginae 53 r—69 r; and Utirik —14 r. The dose value for Rongerik given in Table 1.1 is 75 per cent of the short fallout case value, averaged for 28.5 and 34 hour exposures. This best expresses the average air dose received by personnel who spent roughly half their time inside structures where the dose rate was later found to be roughly half that outdoors. On the other islands no such shielding was 4 present. oa is ts we << = ~ hem : | «<¢ } rR cS | | aj 2 Figure 1.3, for the Rongelap atoll, illustrates the cumulative dose as a function of time after the detonation. It can be seen that the rate of delivery of the dose varied continuously, the major portion being received at the higher dose rate prevailing in the early portion of the exposure period. By the time that 90 per cent of the dose had been received, for example, the dose rate had fallen to less than 30 per cent of its initial value. Thus the dose rate of exposure differed markedly from that usually encountered using x-ray units. : = tN S = oe “a s A third difference between the type of exposure encountered here and other external exposures lay in the geometry of the source, These doses were delivered from a plane source, = so that the radiation fietd did not follow the narrow beam geomelry usually employed experi- ot ‘| | “ . = ! ee / 4 ES SS Geometry of the Exposures mentally. In such a diffuse 360° field, the decrease of dose with depth in tissue is less pror 9} t NESS 1.4.4 6onounced than that resulting from a unilateral or bilateral exposure to an X-ray beam, so that fora given energy, the dose at the cenler of the abdomenis approximately 50 per cent higher than a given air dose would imply for the narrow beam case. Figure 1.4 illustrates an experimental simulation of the field geometry using a spherically oriented group of Co™ sources with a phantom placed at their center, compared with a conventional depth dose curve obtained witha single source. It would appear under the circumstances that the midline dose, rather than dose measured in air, would be the better parameter in terms predicting biological ef- fects. On this basis, the air dose values stated in Table 1.1 should be multiplied by approximately 1.5 in order to compare their effects to those of an exposure using a narrow beam geometry, If this is done, assuming a fast fallout of one hour, the following doses in terms of anair dose under laboratory conditions result: Rongelap 260 1; Ailinvinae 190 r; Rongerik 120 r; and Utirik 21 r. “While it is obvious that the fallout lasted longer than one hour, calculations of dose are based on an ussumedone hour fallout as explained in the text. 20 | i: t ' 1 . .- tae . . . aa ee ote + one aNeB as