1. n. [Formation Evaluation]

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.

See: epithermal neutron porosity measurement,  neutron interactions,  slowing-down time,  thermal diffusion length,  thermal neutron porosity measurement