1. n. [Formation Evaluation]

An element, or mineral, that is particularly effective in absorbing thermal neutrons (neutrons with about the same energy as the surrounding matter, typically less than 0.4 eV). The elements gadolinium, boron, chlorine, hydrogen and iron are thermal absorbers (in decreasing order of effectiveness). The effect of chlorine is used in a pulsed neutron capture log to distinguish salty water from hydrocarbons. In a thermal neutron porosity measurement, the effect of hydrogen is important, while boron and iron affect the response in shales.

See: neutron capture,  neutron interactions,  thermal diffusion length