ation of 0.356 megacurie per naut mi’, 1,000 mr/hr will correspond to a conta gamma fiel: The altituae 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 t7'+ (Reference 6), to reduce the aerial-~survey measurements to a common time, t is the time since the detonation. Large amounts of Np?*8 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 Percent Response 200 (A) Woter Volume (B) Lond Surface ° 100 200 300 400 500 600 700 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 modified by the Np*°8 contribution. The Np?83 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 ac- tivity described in Reference 7. Because of the low energy of the neptunium gamma emission which is predominately 120 kev, the Np?*8 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 volume at the surface of the ocean contributes to the dose rate measured above the surface. This is inversely propor- tional 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-energy gamma flux 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 energy (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 an a contribution to less than 24, percent of the fission product dose rate measured at an airgrt HCYHIV 300 feet flight altitude. pon ™ It is possible that other isotopes may be formed, depending on the type and location of the test. Primary among these is Na*4 produced vy neutron activation of the sodium in sea water. This isotope has a 14.8-hour haif life and emits two gamima photens, 1.38 and 2.76 Mev. Refer- 14 aia if