A well log of the capture cross-section of thermal neutrons; similar to the thermal-decay-time log. A Van de Graaf neutron generator in the sonde periodically releases a burst of neutrons that enter the formation and begin to lose energy in collisions. The neutron-lifetime log measures the thermal neutrons at two discrete time intervals after a neutron burst. Measurements are made of the gamma rays that result from the capture of neutrons by nuclei in the thermal-decay-time log. The quantity plotted is sometimes (a) the reciprocal of the percentage that decay per unit of time, called the thermal decay time τ ; (b) the time for the thermal neutron population to fall to half value, called the neutron lifetime L; or (c) the macroscopic capture cross-section Σ that is derivable from the foregoing (Σ=4.55/τ=3.15/L). Thermal neutrons are captured mainly by the chlorine present and hence this log responds to the amount of salt in formation waters. Hydrocarbons result in longer decay times than salt water. Log readings are porosity-dependent and sensitive to clay content and permeability changes. This log is used in cased holes where resistivity logs cannot be run or to monitor reservoir changes to optimize production. It resembles a resistivity log with which it is generally correlatable. Dresser Atlas trade name. See Figure N-2c and pulsed neutron capture log.