A record of elemental concentrations derived from the characteristic energy levels of gamma rays emitted by a nucleus that has been activated by neutron bombardment. In the context of production logging, the term normally refers to the activation of silicon and aluminum to determine the quality of a gravel pack. Silicon and aluminum are activated by a neutron source to produce isotopes that decay with a half-life of 2. 3 minutes emitting a 1. 78 MeV gamma ray. These gamma rays are counted in a detector placed below the source, with a high count indicating a high quantity of silicon in a sand pack, or aluminum in a bauxite pack. The log is run slowly so that oxygen and other activated elements have decayed before the detector crosses the activated interval.

The carbon-oxygen log, elemental-capture spectroscopy log, pulsed-neutron spectroscopy log, aluminum-activation log and the oxygen-activation log are also examples of neutron-activation logs.