Thermal neutrons diffuse through the formationwith no further change of energy.The life of a thermal neutron is terminated by capture in the nucleus of an atom. At any instant,the number of capture events is proportional to the number of thermal neutrons remaining. Eachcapturing nucleus emits one or more gamma rays. Thus, the decay of thermal neutrons in theformation can be measured by counting capture gamma rays at several different times after theburst.