2.7 Natural sources of radiation

While many elements have radioactive isotopes, only potassium, and the uranium and thorium decay series have radioisotopes that are found in the terrestrial environment. The products of disintegration of these isotopes are commonly denoted as NORM (Naturally Occurring Radioactive Materials).

Neither ²³⁸U, nor ²³²Th emit gamma rays, and high energy gamma ray emissions from their radioactive daughter products are more commonly measured to estimate their concentrations.