(1) The [[thermal neutron|thermal-neutron]] decay time intrinsic to a particular material or medium. defined by (1/vS<sub>abs</sub>)where v is neutron velocity and S<sub>abs</sub> is the macroscopic [[capture cross section]] of the medium.

(2) The thermal-neutron decay time of a particular [[formation]] corrected for borehole and [[diffusion]] effects. See [[thermal decay time]], [[Thermal Decay Time Log]], and [[capture cross section]].