Distribution of Neutrons from a ²⁵²Cf Source in Soil At Depth and Just Below the Air-Ground Interface

The slow-neutron flux as a junction of distance from a 90 µg ²⁵²Cf neutron source buried several feet below the surface of the ground was measured with BF3 detectors in damp sand and clay strata. From 1 to 7ft from the source, the expected decrease in flux was observed, However, when similar measurements were made in shallow holes near the air-soil interface, the decrease with distance was less than exponential and a peak was noted at a distance of 8ft from the source. A qualitative explanation of the peak is given indicating that the density of thermal neutrons in the high-density thermal strata located several inches below the surface is a junction of the incident neutron energy and that the optimum incident energy is ∼2 MeV. For this reason ²⁵²Cf is a good neutron source for the purpose of in situ capture gamma analysis of soil constituents when used for mineral exploration purposes.