Specific Activities of 60Co and 152Eu in Samples Collected from the Atomic-Bomb Dome in Hiroshima

Neutron-induced activities 60Co and 152Eu have been measured for samples collected from the Atomic-Bomb Dome locating at 161 m from the hypocenter of the Hiroshima Bomb. Specific activities 60Co/ Co and 152Eu/Eu at the time of the detonation have been determined as 10.0±1.0 Bq mg' (steel sample S4) and 80±9 Bq mg－1 (granite sample G1), respectively. Detailed measurements of 60Co and 152Eu activities for samples collected from various locations of the Dome show almost no directional dependence whether the sample faced to the epicenter or not, nor vertical height dependence between 17 m height and the ground level. In addition, 152Eu was not detected in the sample collected from the basement. It has been shown that the present 60Co activity value, the nearest steel one to the hypocenter, as well as other short distance data are systematically lower than the calculated values based on the neutron fluence of the DS86. Reevaluation of Hiroshima and Nagasaki atomic-bomb dosimetry has been completed in 1987 with a final report1) of a new dosimetry system called DS86. Conceming to the evaluation of the low energy neutron nuence, 60C activation data taken by Hashizume et al. 2, 3)both at Hiroshima and Nagasaki, systematically deviated from the calculated values by Loewe4). Nevertheless detailed discussion on the 60Co activation, the discrepancy was still in open question. Other 60Co data taken by Saito5), by Hoshi and Kato6) and by Nakanishi et al. 7) were not considered in the above discussion. No detailed consideration was made on 152Eu activation data in the final report.