DS86 neutron dose: Monte Carlo analysis for depth profile of Eu-152 activity in a large stone sample

The depth profile of(152)EU activity induced in a large granite stone pilla r by Hiroshima atomic bomb neutrons was calculated by a Monte Carlo N-Parti cle Transport Code (MCNP). The pillar was on the Motoyasu Bridge, located a t a distance of 132 m (WSW) from the hypocenter. It was a square column wit h a horizontal sectional size of 82.5 cm x 82.5 cm and height of 179 cm. Tw enty-one cells from the north to south surface at the central height of the column were specified for the calculation and Eu-152 activities for each c ell were calculated. The incident neutron spectrum was assumed to be the an gular fluence data of the Dosimetry System 1986 (DS86). The angular depende nce of the spectrum was taken into account by dividing the whole solid angl e into twenty-six directions. The calculated depth profile of specific acti vity did not agree with the measured profile. A discrepancy was found in th e absolute values at each depth with a mean multiplication factor of 0.58 a nd also in the shape of the relative profile. The results indicated that a reassessment of the neutron energy spectrum in DS86 is required for correct dose estimation.