Dictionary:Pulsed neutron-capture log

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A neutron generator or accelerator produces short bursts of high-energy (about 14 MeV) fast neutrons that are slowed to thermal energy level (about 0.025 ev), where capture by nuclei (chlorine having the greatest capture cross-section of common elements) results in gamma-ray emissions. Two time-lapse measurements (minus a ‘‘background’’ count) of capture-gamma intensity from each neutron burst define the time-rate of thermal-neutron decrease (decay lifetime, die-away time). Logs may depict count-rate curves, a curve (sigma) of macroscopic capture cross-section at a fixed time, or a curve (tau) of time to reach a ‘‘background’’ count rate. Such logs are used in cased oil wells, where rock characteristics are known from previous logging, to determine oil saturation or changes in fluid saturation during oil production (as shown by successive logs). Shallow penetration produces large uncertainty because of mud invasion. Dresser-Atlas trade name. See neutron lifetime log.