(adj.) Formation Evaluation. Referring to a log of porosity based on the effect of the formation on fast neutrons emitted by a source. Hydrogen has by far the biggest effect in slowing down and capturing neutrons. Since hydrogen is found mainly in the pore fluids, the neutron porosity log responds principally to porosity. However, the matrix and the type of fluid also have an effect. The log is calibrated to read the correct porosity assuming that the pores are filled with fresh water and for a given matrix (limestone, sandstone ordolomite). It is presented in units of porosity (vol/vol or p. u.) for the matrix chosen. Older logs were presented in counts per second or API units. The depth of investigation is several inches, so that the log reads mainly in the flushed zone. The neutron porosity log is strongly affected by clay and gas. Hydrogen occurs in clays and hydrated minerals as well as pore fluids. Gas has a low hydrogen density, so that gas zones have a very low apparent porosity. The measurement is based on either thermal or epithermal neutron detection. Thermal neutrons have about the same energy as the surrounding matter, typically less than 0. 4 eV, while epithermal neutrons have higher energy, between about 0. 4 and 10 eV. Being a statistical measurement, the precision is greatest at high count rates, which in this case occurs at low porosity. Neutron porosity logs were introduced in the early 1940s. The first tools were known as neutron-gamma tools, since the detector measured the gamma rays emitted on capture. Neutron-neutron tools, using a thermal neutron detector were introduced in about 1950. See: Alpha Processing, Compensated Neutron Log, Epithermal Neutron Porosity Measurement, Excavation Effect, Flushed Zone, Hydrogen Index, Limestone Porosity Unit, Limestone-Compatible Scale, Thermal Neutron Porosity Measurement