Monte Carlo Calculations of Skyshine Neutron Doses from an 18 MeV Medical Linear Accelerator

Neutron skyshine doses were calculated using a Monte Carlo code, FLUKA, by changing the heights and openings of shield walls. In the calculation geometry, the distance from the floor of the room to the roof was varied from 3 to 10 m. The target into which electrons were injected was located at a height of 2.3 m. The distance between the shield wall and target was varied from 1.5 to 5.79 m. Further, the height of the atmosphere was varied from 10 to 40 m. Consequently, the dose outside the shield wall increased with the height of the atmosphere and became saturated at 30 m; the neutrons reaching the ground were observed to be scattered mostly below 30 m. For neutron skyshine, NCRP Report No. 51 provided an expression stating that the neutron dose at ground level was proportional to the solid angle of shield walls and inversely proportional to the square of the distance from the target to the ceiling plus 2 m. In the Monte Carlo results, the doses conformed to the expression below a height of 5 m and above a distance of 2.6 m from the target to the shield wall. Under other conditions, the doses became higher than those predicted by the expression, which indicated the large contribution of neutrons emitted immediately above the target to the skyshine doses.