1. n. [Formation Evaluation]
A parameter used to characterize thermal neutron interactions in bulk material. Thermal diffusion length (Ld) is the characteristic distance between the point at which a neutron becomes thermal and the point of its final capture. It is related to the quantity of thermal absorbers in the formation, and therefore is an important factor in the thermal neutron porosity measurement. Thermal neutrons have about the same energy as the surrounding matter, typically less than 0.4 eV (0.025 eV at room temperature).
See: chemical neutron source, neutron capture, neutron interactions
