Measurements of neutron fluences

The sulfur activation by neutrons above 3MeV and the europium activation by thermal neutrons show no important discrepancy with calculations. Cobalt activation, however, contradicts the calculated values by an ever-increasing factor that is five at 1000 m. If the measured cobalt activations were accepted as correct representations of thermal fluences and the assumption then made that the calculated fluences on the ground are low by a factor that applies to all energies, then the proportion of neutron kerma in the mixed radiation field beyond 1000 m at Hiroshima would change from insignificant to significant. The sulfur activation is due to neutrons that are close in energy to those that produce most of the neutron dose. The excellent agreement with calculation close to the hypocenter, however, deteriorates at 1000 m to such a degree that experimental support is weak for the calculated kerma at 1000 m and more. The europium activation is sufficiently uncertain at all ranges, but especially so at large ranges, that its support for the calculated low-energy fluences is weak. In contrast, the cobalt determinations made by the Hashizume group show small variability, good consistency, and good reproducibility for data as far as 1000 m from the hypocenter. Strong efforts to resolve the cobalt discrepancy from both the calculation and experimental sides have only served to confirm its existence. Because the calculations have been quite thoroughly 'scrubbed' and are generally based on data and procedures admitting of limited error, we believe there must be some unrecognized contribution to the measured 60Co activity that has not been included in the calculation model. However, until such a contribution is identified, it is necessary to assume that it does not exist. If it does not, it is possible that erroneous calculate fluences are only weakly related to high-energy neutrons that determine dose. There is presently no evidence for this supposition, and, in fact, those most experienced in calculational development of thermal neutrons from fast neutrons are most skeptical. It is important to recognize that neutrons that produce the kerma at one distance from the epicenter have the same energies as other neutrons that activate cobalt some hundreds of meters farther away. The transformation of the latter into thermal neutrons results exclusively from interaction with common materials (nitrogen, oxygen, silicon) that have scattering cross sections that are well known at all relevant energies and produce energy losses by known laws of classic mechanics. There is only a very limited opportunity for error in the modeling. This leaves the possibility, however unlikely in our collective expert judgment, that the calculated neutron kerma values are wrong. No known evidence contradicts this hypothesis. Therefore, the conclusion of this chapter on neutron measurements must be that the neutron doses are in doubt until further work is done. Special value would attach to additional work on measurement of thermal neutron fluences based on unmistakable physical effects occurring in sufficiently great strength as to permit good quality results beyond 1000 m from the hypocenter. One suggested candidate is fission-track density in uranium-bearing glassware or materials containing zircon fragments, but no high-uranium containing specimens have been found. (author)