DS86 Neutron Dose: Monte Carlo Analysis for Depth Profile of ¹⁵²Eu Activity in a Large Stone Sample

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The depth profile of ¹⁵²Eu activity induced in a large granite stone pillar by Hiroshima atomic bomb neutrons was calculated by a Monte Carlo N-Particle Transport Code (MCNP). The pillar was on the Motoyasu Bridge, located at a distance of 132m (WSW) from the hypocenter. It was a square column with a horizontal sectional size of 82.5cm× 82.5cm and height of 179cm. Twenty-one cells from the north to south surface at the central height of the column were specified for the calculation and ¹⁵²Eu activities for each cell were calculated. The incident neutron spectrum was assumed to be the angular fluence data of the Dosimetry System 1986 (DS86). The angular dependence of the spectrum was taken into account by dividing the whole solid angle into twenty-six directions. The calculated depth profile of specific activity did not agree with the measured profile. A discrepancy was found in the absolute values at each depth with a mean multiplication factor of 0.58 and 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.