ence 6 may be use | to deduce the dose rate contribution for an amount, in curic , eyial to thet for th: mixed fission products. ‘The dose-rate mcasurement at 300 feet is more Sensitive ta this isotope by a factor of 3.6 because of its increased roentgen conversion from curies, and the larger volume of water contributing to the surface radiation flux. 1.3.4 Distribution of Fallout. To estimate the distribution of fallout, the equation relating gana doserate abovc the surface to contamination density ina volume of sea water may be used in conjunction with the isodose distribution charts and depth of mixing measurements. The contamination density in a thin layer at the surface may be estimated fromthe average gamma dose rate in the various isodose defined areas. Summation of the estimated contamination would 7 @ c 2 = -" | Outer Boundary A ane N > B = c Distance —> Minimom ~~ Detectable Limit 0 Outer Boundary Figure 1.3 Determination of estimated outer boundary. yield an estimate of the megacuries of surface radioactivity in the fallout area. This may be correlated with the depth of mixing and the total fallout activity computed. If the fallout is deposited in the sea, the equation in Section 1.3.1 indicates that a contamination density of 1 megacurie per naut mi? would produce a gamma dose rate of 2.5 mr/hr at 3 feet from the surface. The same contamination density, on land, would produce 2,800 mr/hr (Section 1.3.2). For rough estimates, 1 mr/hr at 3 feet over water is equivalent to a 1,000:1 increase in activity per naut mi? when compared to 1 mr/hr on land. The calculations for land and water are summarized as follows: on land, 1 mr/hr at 3 fect is equivalent to 2.1 x 107 (dis/min)/ft? or 3.56 x 10°! megacuries/naut mi?; on water, 1 mr/hr at 3 feet is equivalent to 4.43 x 10°(dis/min)/liter or 4.04 x 107! megacuries/naut mi? where depth of mixing is 60 meters. When the fission product falls into the sea, the outer boundary of the contaminated area will be indicated by gamma-radiation readings that are only slightly above the background gamma dose rate. Figure 1.3 illustrates the radiation profile across a contaminated area. The estimated outer boundary (EOB) from a shot with a high-fission yield is indicated at A and D. A shot with the same total energy yield, but producing a smaller quantity of fission products, will have an EOB at B and C. Both shots may have the same actual outer boundary, yet the minimum detectable limit of radiation of the instrumentation will result in a low estimate for the area. For material-balance calculations, the quantity of radioactivity outside the EOB will be small in relation to the quantity located in the higher-intensity areas. 15