Natural gamma-ray spectral log. Unstable isotopes emit particulate (alpha, beta) and electromagnetic (gamma) radiation. Penetrating 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.