A porosity well log that measures hydrogen density; a low hydrogen density indicates low liquid-filled porosity. Porosity calculated from the neutron log is affected somewhat by the formation matrix and by the presence of gas. Neutron log values are used in crossplots with other porosity log values to detect gas and determine lithology. Neutron logs are sometimes scaled in API units, sometimes in porosity units assuming a limestone or sandstone matrix. The neutron log can be recorded in cased holes. See Figure N-2a. Fast neutrons emitted by a source in the tool are slowed to thermal speed by collisions with (mainly) hydrogen atoms. The thermal neutrons are then captured by atomic nuclei of the surrounding material (mainly chlorine atoms) at which time a characteristic gamma ray of capture is given off. (a) The epithermal neutron tool detects neutrons at high energy level after they have been slowed down by atoms in the formation; (b) the thermal neutron tool detects neutrons with energy of about 0.025 eV. (c) The neutron-gamma tool detects gamma rays that originate from the absorption of thermal neutrons by atoms with high absorption power, such as chlorine. See also sidewall neutron log.