A parameter used to characterize neutron interactions in bulk material above the thermal region. The slowing-down length (L_s) is proportional to the root-mean-square distance from the point of emission of a high-energy neutron to the point at which its energy has decreased to the lower edge of the epithermal energy region. The slowing-down length is the physical parameter that best describes the response of an epithermal neutron porosity measurement, and describes a large part of the response of a thermal neutron porosity measurement. Thermal neutrons have about the same energy as the surrounding matter, typically less than about 0. 4 eV, while epithermal neutrons have higher energy, between about 0. 4 and 10 eV.