1. n. [Formation Evaluation]

With reference to pulsed neutron logging, the characteristic time for the decay of the epithermal neutron population. The slowing-down time of a formation is strongly dependent on the porosity. In openhole pulsed neutron logging, it is also dependent on the standoff between tool and borehole wall. Epithermal neutrons have energies above that of the surrounding matter, between about 0.4 eV and 10 eV.

See: epithermal neutron porosity measurement,  slowing-down length