An element is identified by the number of protons in its nucleus, while varying numbers of neutrons indicate different atomic isotopes (nuclides) of the same element (review delayed gamma theory). The element measured in the Bodycomp Lab via prompt gamma neutron activation is nitrogen. Most nitrogen (>99%) exists as ¹⁴N.

As in delayed gamma activation, incoming neutrons combine with an atom, in this case nitrogen, to form a nuclide of the same element. The newly formed nuclide, ¹⁵N, is unstable.

¹⁵N promptly releases energy in the form of an 11.3 MeV gamma ray, resulting in a stable form of ¹⁵N. About 0.4% of nitrogen exists as this stable nuclide.