gammafield of 1,000 mr/hr will correspond to a contamination of 0.356 megacurie per naut mi’, The altitude absorption factor, over land, is shown in Figure 1.2, 1.3.3 Radioactive Decay. Mixed fission products have been assumed to have a radioactive decay proportional to t-!? (Reference 6), to reduce the aerial-survey measurements to a common time, ¢ is the time since the detonation. Large amounts of Np?* may be found in the fallout from thermonuclear shots. It is possible to calculate the expected increase in the total activity, over that resulting solely from fission T , Tt T T TTT T Percent Response 200 (B) Lond Surface pf 6 Oo poy 100 200 L 300 _ L I 400 l 4 500 iL. 600 _t Too h= Altitude , Feet Figure 1.2 Radiation attenuation referred to h = 3 feet. products, from the capture-to-fission ratio of the device. The decay characteristics of the fall- out activity will be mcdified by the Np? contribution. The Np?*® can be present in amounts up to 50 percent of the total activity, 1 to 3 days after the shot, based on a mixed fission product activity described in Reference 7. Because of the low energy of the neptunium gamma emission which is predominately 120 kev, the Np?3 adds relatively small contribution to the gamma dose rate when compared to the average fission-product gamma energy. In water the mean free-path length of the lower-energy gamma ray is less than that for the mixed fission product gamma; hence, a lesser volumeat the surface of the oceancontributes to the dose rate measured above the surface. This is inversely proportional to the total absorption coefficients of water, at 120 and 500 kev, and reduces the neptunium gamma contribution to 60.6 percent. In addition, the lower-cnergy gammaflux deposits less energy per unit volume of air, and therefore contributes less to the dose rate. This is an additional reduction to 18¥, percent of the fission product dose rate (Reference 8). The aerial-survey detector response is down to 75 percent at 120 kev cnergy (Figure 2.5). Because of these factors, even with the neptunium gamma ray contribution to the total activity at 50 percent, the dose rate response in the Top Hat detector willbe increased about 4 percent. The relative attenuation, in air, for these two gamma energies, approximately 65 percent, reduces the neptunium gamma contribution to less than 2¥, percentof the fission product dose rate measuredat an aircraft at 300 feet flight altitude. It is possible that other isotopes inay be formed, depending on the type and location of the test. Primary among these is Na*4 produced by neutron activation of the sodium in sea water. This isotope has a 14.8-hour half life and emits two gamma photons, 1.38 and 2.76 Mev. 14 Refer-