Neutron and Gamma-Ray Die-Away in a Heterogenous System

The die-away of thermal neutrons and capture gamma rays from a pulsed source has been measured in a heterogeneous rock-fluid system for a variety of physical conditions. The system was a cubical lattice about one meter on a side, consisting of vertical calcium carbonate rods and empty channels. The channels were filled with either calcium carbonate rods or fluid, thus giving a variable volume ratio of rock and fluid. Measurements with a ³He counter and NaI detector were carried out in a 7-in. (18-cm) diameter hole through the middle of the cube. Experimentally measured neutron lifetimes were compared to values calculated from a three-group time- and space-dependent computer code. A theoretical gamma-ray decay curve was calculated from a spatial integration over the computed neutron distribution. Unattenuated and singly scattered radiation were included.