A measurement of the slowing down and capture of neutrons between a source and one or more thermal neutron detectors. The neutron source emits high-energy neutrons that are slowed mainly by elastic scattering to near thermal levels. Thermal neutrons have about the same energy as the surrounding matter, typically less than 0. 4 eV. The slowing-down process is dominated by hydrogen. At thermal levels, the neutrons diffuse through the material until they undergo thermal capture. Capture is dominated by hydrogen and other thermal neutron absorbers.

Typical thermal neutron measurements use a chemical neutron source and two thermal neutron detectors. An accelerator source (neutron generator) is sometimes used. Some, mainly earlier, devices measure the gamma rays emitted by thermal capture, rather than thermal neutrons.