Subsurface thermal neutron production rates

Abstract

Ground water in excess of one million years old may now be accurately age dated by using the radionuclide Chlorine 36 (36Cl), which has a half-life of 3.01 x 105 years. To maintain a high degree of accuracy in the resultant age it is necessary to take into account the buildup of 36Cl, which is due to thermal neutron activation of 35Cl to 36Cl. The purpose of this research is to determine the thermal neutron flux in various geochemical subsurface environments by conducting field measurements of thermal neutron production rates at discrete locations. These data are then compared with the theoretical thermal neutron flux calculated for each location. The field measurements were conducted from the surface to a maximum depth of 44 meters in a copper, silver and zinc mine. The measured thermal neutron flux was found to be larger than the theoretical thermal neutron flux by a factor of from three to six when below 17 meters depth.