Radiometrics

Radiometrics Source: BGR

The most important method in geophysical radiometrics is the measurement of gamma radiation. Gamma radiation is a high-energy electromagnetic radiation emitted during radioactive decay processes. Naturally occurring radioactive elements are Potassium, Uranium and Thorium. Gamma radiation emitted by their isotopes or daughter elements K-40 (Potassium), Bi-214 (Uranium) and Tl-208 (Thorium) shows a characteristic energy distribution for each isotope. By measuring the gamma energy spectrum the content of the three elements in the earth's surface layer can be determined.

Principle of radiometrics Source: BGR

Radiometric measurements can be carried out from aircrafts (plane, helicopter or drone), on the ground (vehicle based or hand-held instruments) or in boreholes (borehole tool). The principle of measurement is almost the same for each of the setups. The detector is mostly comprised of one or more Sodium-Iodine crystals in which the energy of incoming gamma radiation is converted to light pulses and finally, by photo multiplier tubes, to electric pulses. The amplitude of the electric pulses is directly proportional to the energy of the incoming gamma rays. Depending on the energy level, the pulses are assigned to energy channels in a spectrum covering the range between 0 and 3 Mega-Electronvolts (MeV). If the whole system is calibrated it is possible to determine the associated concentrations of Potassium, Uranium and Thorium through an analysis of the spectra.

In geophysical applications gamma ray spectrometers are mostly used for geological mapping, for mineral exploration, particularly Uranium exploration, and for environmental problems such as identification and mapping of radioactive contaminations. Due to a strong absorption of gamma rays by matter, only the signal of the upper about 50 cm of the Earth's surface is captured.