NLL. The Neutron Lifetime logging technique employs a pulsed [[neutron source]] which is periodically actuated to produce short bursts of [[neutron|neutrons]] and is quiescent between bursts. During the interval between bursts, the neutrons (as well as the various types of [[radiation]] which always result from neutron interactions) die away. Their average lifetime can be measured by measuring the length of time required for the neutron population at a particular instant to die away to half value. The radiation intensity is measured in each of two preselected intervals and, by intercomparing these measurements, determine thc rate of neutron die-away. This measured rate has been shown both by theory and experiment to be a measure of the [[thermal neutron|thermal-neutron]] capture cross section of the medium in which the neutrons are captured. The thermal-neutron [[capture cross section]] per unit of volume of [[formation]] material is referred to as S. It is related to L, termed the lifetime of neutrons in a material, by the equation

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Thermal neutrons are captured mainly by the chlorine present. Hence the tool responds to the amount of salt in [[formation water|formation waters]]. Hydrocarbons result in longer lifetimes than salt water. Tool measurements are porosity-dependent and sensitive to clay content. Can be used in cased holes where [[resistivity log|resistivity logs]] cannot be run or to monitor reservoir changes to opt-imize production. Resembles a resistivity log with which it is generally correlatable. Neutron Lifetime Log is a Dresser Atlas registered trademark.

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