DIFFERENTIAL MEASUREMENTS OF FAST-NEUTRON AIR-GROUND INTERFACE EFFECTS: PROJECT 9.2 OPERATION HENRE

Abstract

Abstract : The effect of the air-ground interface on the scattered fast-neutron dose near the ground was measured at a distance of 1000 ft from a 14-MeV neutron source. The source was the HENRE accelerator operated at a height of 112 ft on the BREN tower at the Nevada Test Site. A horizontal slab of polyethylene 1 ft thick and 5 ft square with Hurst-type fast-neutron dosimeters mounted on its upper and lower surfaces, separated the neutrons arriving through the upper 2 pi solid angle from those from the lower 2 pi. A third detector, mounted on a boom, measured the free-field. The entire assembly was suspended by a hoist system to make measurements at 0.75 to 70 ft above the ground. The scattered dose at the top detector was essentially constant, that at the bottom detector increased by a factor of approximately 2 between 0.75 and 70 ft, and the free- field dose increased by less than 25% over the same height range. The experiment provided confirmation, both qualitative and quantitative, of the 'first-last collision model' of the air-ground interface effect.