Natural gamma-ray spectral log. Unstable [[isotopes]] emit particulate (alpha, beta) and electromagnetic (gamma) [[radiation]]. Penetrating [[gamma ray|gamma rays]] are suitable for borehole detection. Isotopes of specific elements radiate gamma rays exhibiting specific energy levels within the energy spectrum. Identification of the specific energy level(s) and the amount of gamma radiation (at the specific level) provides a means of identifying the isotope and the quantity of the element. Compare [[Induced spectral gamma ray log|induced spectral gamma-ray log]].