FAST-NEUTRON ACTIVATION MEASUREMENTS

Following the Hiroshima explosion, Japanese physicists made a number of measurements of the fast-neutron activation of 32S near the epicenter (Roesch 1987). The sulfur was contained in insulation material of electric-power poles. Japanese investigators made these measurements during the first few weeks after the event. The threshold for 32S activation is about 2.5 MeV, and the half-life of the 32P produced is only 14 d. The calculated and measured 32S activation tended to agree well close to the epicenter, particularly when corrections were made for the expected anisotropy due to bomb tilt (Roesch 1987). The exact degree of agreement, however, depends on the assumed yield. In fact, the comparison between 32S activation calculation and measurement was a factor in determining the yield that was used in DS86. The 32S data tended to diverge from DS86 calculations, and the calculated values appeared to be lower than measured by an amount that increased as the distance from the epicenter increased. However, the measured activities at these distances were very low, and the uncertainties very high. Because of the short half-life of 32P, the 32S activation measurements could not be repeated to confirm the original data. Documentation of similar measurements made at Nagasaki has not been found.

THERMAL-NEUTRON ACTIVATION MEASUREMENTS

When DS86 was released, a number of thermal-neutron activation measurements had been made at various slant ranges at Hiroshima and Nagasaki. Additional measurements have since been made of thermal-neutron activation of cobalt (Co) and europium (Eu) and, with a different technique, the generation of 36Cl by thermal neutrons. Those measurements have indicated that the thermal neutrons were more abundant at great distances than was predicted from the neutron spectrum calculated for the bomb explosion by DS86. It appeared that more high-energy neutrons penetrated the iron casing than was calculated. In general, the measured thermal-neutron activation of 60Co at Hiroshima appeared to be higher than the calculated values by an amount that increased as the slant range increased. Although some 152Eu thermal-neutron activation data were also reported, the data for larger distances were too few to confirm the 60Co comparisons. Measurements of 60Co and 152Eu activation at Nagasaki also suggested the possibility of a similar trend, but the data for distances greater than 1000 m was sparse. Near the epicenter at both Hiroshima and Nagasaki, 60Co and 152Eu activation data tended to be about 50% lower than calculated from the DS86 neutron fluence.

RERF has surveyed the literature and communicated with investigators directly and has created a database of all known activation measurements (see Appendix A). Many of the newer measurements were made at increasing distances from the epicenter to resolve the apparent discrepancy observed in the DS86 calculation-



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