1. n. [Formation Evaluation]
A measurement of the spectrum of gamma rays emitted by a formation bombarded by high-energy neutrons. Neutrons are emitted by a high-energy neutron generator (14.1 MeV). The neutrons interact with different nuclei, which may emit characteristic gamma rays through inelastic neutron scattering, fast-neutron reactions and neutron capture. When pulses from a neutron generator are used, it is possible to separate the different interactions in time after each neutron pulse. Inelastic and fast-neutron interactions occur very soon after the neutron burst, while most of the capture events occur later. The two types can therefore be separated to give a so-called inelastic spectrum and a capture spectrum. The spectra are analyzed either by counting gamma rays in windows placed at the main peaks for the elements concerned, or by comparison with spectral standards, or by combining the two (alpha processing). The resultant logs are known as pulsed neutron spectroscopy logs, the most common of which are the carbon-oxygen log and the elemental capture spectroscopy log.