NEUTRON-SPECTRUM AND YIELD OF THE HIROSHIMA A-BOMB DEDUCED FROM RADIONUCLIDE MEASUREMENTS AT ONE LOCATION

In this paper measurements of the radionuclides of Cl-36, Ca-41, Co-60 , Eu-152 and Eu-154 samples from Hiroshima, which were exposed to neut rons of the A-bomb explosion, are interpreted. In order to calculate t he 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 reproduce d by this method using DS86 input parameters. The calculated neutron s pectrum at the sample site for non-thermal neutrons is identical to th at obtained in DS86, but contains some 50% more thermal neutrons. The influence of parameters like soil composition, source terms and air hu midity on the activation of these radioisotopes is discussed. The gran ite-covered earth at the sample site, for example, hardens the spectru m in comparison with DS86 values. Even when using a fission spectrum p ointing downward and neglecting air humidity one cannot explain our Cl -36 measurements. If the effective thermal neutron fluences, that have a similar ratio of resonance integral to thermal neutron capture cros s sections obtained from Cl-36, Ca-41 and Eu-152, are averaged, a bomb yield of about 16 kt is deduced in agreement with a bomb yield of (15 +/- 3) kt estimated in DS86.