The Hiroshima neutron dosimetry enigma: Missing puzzle piece No. 6

Abstract

More than a decade has elapsed since the serious nature of the discrepancy between neutron dosimetry 1 1 Throughout this exposition the term dosimetry is used in the context of radiation metrology rather than the context of radiobiology. The operational meaning of the term dosimetry is different in these two scientific specialities. In radiobiology, biological effects are expressed as a function of dose, which is the energy deposited per gram of tissue by a specific component of the radiation field. In contrast with this specific definition of dose in radiobiology, the term dosimetry possesses a very broad operational meaning in radiation metrology. In radiation metrology, the term dosimetry describes activities undertaken to characterize or quantify radiation field observables. These radiation field observables can change from one application or experiment to the next and include such quantities as reactions, reaction rates and specific activity as well as the intensity and energy distribution of specific radiation field components. experiments (E) and neutron transport calculations (C) for the Hiroshima site was identified. Since that time extensive efforts to resolve this Hiroshima neutron dosimetry enigma have not only failed, but now demonstrate that the magnitude of this discrepancy is much greater than initially estimated. The currently evaluated E/C ratio for thermal neutron fluence at the Hiroshima site increases rapidly with increasing slant range from the epicenter. In the slant range region beyond 1000 m, E/C exceeds unity by one to two orders of magnitude depending on the specific dosimetry data that are utilized. Principal features that characterize the Hiroshima neutron dosimetry enigma are summarized. Puzzle Piece No. 6: In-situ production and Prompt fallout of radionuclides from Little Boy is advanced as a possible contributory phenomenon to this enigma. (The atom bomb detonated over Hiroshima was called Little Boy.) Measurements of 60Co and 152Eu specific activity at the Hiroshima site are used to obtain order of magnitude numerical estimates that show this conjecture is plausible. Comparison of different 60Co measurements at the Hiroshima site reveals that the variation of E/C with slant range depends on the method used to quantify 60Co specific activity as well as the type of dosimetry samples that are employed. These 60Co comparisons lend additional qualitative credence to this conjecture. Within the limits of presently available data, these assessments show that Puzzle Piece No. 6 qualitatively satisfies the principal features that characterize the Hiroshima neutron dosimetry enigma. Nevertheless, current lack of data prevent this conjecture from being conclusively confirmed or refuted. Consequently, specific recommendations are advanced to resolve the Hiroshima neutron dosimetry enigma with emphasis on experimental tests that can quantitatively evaluate Puzzle Piece No. 6.