SPWLA Glossary

All stable isotopes are capable of capturing thermal neutrons. In well logging, neutron bombardment of a formation and the subsequent capture of thermal neutrons causes excitation of certain elements. Following the capture of a thermal neutron by a stable isotope, the compound nucleus de-excites by the prompt emission of one or more gamma-ray photons. If the resulting product nucleus is a radioisotope its later decay to a stable state can be detected and the energy level of emitted gamma rays is characteristic of the specific element. The analysis of the energies of the decay gamma rays is neutron activation analysis.