Neutron Spectrum and Yield of the Hiroshima A-bomb Deduced from Radionuclide Measurements at one Location

Abstract

In this paper measurements of the radionuclides of ³⁶Cl, ⁴¹Ca, ⁶⁰Co, ¹⁵²Eu and ¹⁵⁴Eu in samples from Hiroshima, which were exposed to neutrons of the A-bomb explosion, are interpreted. In order to calculate the neutron spectrum at the sample site, neutron transport calculations using Monte Carlo techniques were carried out. Activation profiles in a granite mock-up irradiated with reactor neutrons could be reproduced by this method using DS86 input parameters. The calculated neutron spectrum at the sample site for non-thermal neutrons is identical to that obtained in DS86, but contains some 50% more thermal neutrons. The influence of parameters like soil composition, source terms and air humidity on the activation of these radioisotopes is discussed. The granite-covered earth at the sample site, for example, hardens the spectrum in comparison with DS86 values. Even when using a fission spectrum pointing downward and neglecting air humidity one cannot explain our ³⁶Cl measurements. If the effective thermal neutron fluences, that have a similar ratio of resonance integral to thermal neutron capture cross sections obtained from ³⁶Cl, ⁴¹Ca and ¹⁵²Eu, are averaged, a bomb yield of about 16 kt is deduced in agreement with a bomb yield of (15 ± 3) kt estimated in DS86.