1. adj. [Formation Evaluation]

A measurement based on the slowing down of neutrons between a source and one or more detectors that measure neutrons at the epithermal level, where their energy is above that of the surrounding matter, between approximately 0.4 and 10 eV. The slowing-down process is dominated by hydrogen, and is characterized by a slowing-down length. By measuring the neutrons at the epithermal level, rather than the thermal level, the response is a purer estimate of hydrogen index, unaffected by thermal absorbers. On the other hand, the count rate is smaller for the same source and source-detector spacing. Epithermal measurements have been made with both the compensated neutron technique and by using a pad pressed against the borehole wall with detectors focused into the formation.

See: chemical neutron source,  compensated neutron log,  excavation effect,  hydrogen index,  limestone porosity unit,  limestone-compatible scale,  neutron interactions,  slowing-down length,  thermal neutron porosity measurement