Radiometric Methods of Prospecting

The presence of radioactive elements can be determined by the familiar geiger counter. The instrument measures the energy released during the process of radioactive decay. As a uranium molecule decays, for instance, three kinds of rays are given off: alpha beta and gamma. Of these, the gamma ray is the most penetrating and is therefore the most likely to be detected by the geiger counter. The geiger, because it has a low operating efficiency and a low sensitivity, has in many applications been supplanted by an instrument known as the scintillation coi.inter or scintillometer.

Gamma-ray spectrometers are an even more advanced version of the scintillation counters. They can distinguish between radiation from the three main radioactive elements that occur in nature — uranium, potassium and thorium — by measuring the energy of the radiation. Radiometric surveying can also be done from aircraft.

Ground radiometric surveys are most useful to detect showings of radioactive minerals directly. Airborne radiometric surveys are often used for geological mapping, because the radioactive elements occur in greater abundance in granitic rocks. Down-hole probes are frequently used to measure the radioactivity of rock units encountered in drill holes.