Passive gamma-radiometrics utilise the gamma radiation naturally emitted through the decay of radioactive isotopes. The discrete energies of gamma-ray photons are characteristic of the radioactive isotopes from which they originate. By collecting a gamma-ray energy spectra, the concentration of elements with radioactive isotopes that have sufficient gamma-ray energy and intensity can be measured (e.g. potassium, uranium and thorium).
Typically four broad spectral windows or regions of interest (ROI) corresponding to potassium, uranium and thorium, and the total count are monitored (e.g. [1]).
Commercial.
1. Wong, M. T. F. & Harper, R. J. (1999) Use of on-ground gamma-ray spectrometry to measure plant-available potassium and other topsoil attributes. Australian Journal of Soil Research, 37, 267-277.