Radiometry
Geophysical radiometry is a method for measuring the natural gamma radiation of rocks.
The most important method in geophysical radiometry is the measurement of gamma radiation, a high-energy electromagnetic radiation released during radioactive decay processes. The three radioactive elements potassium, uranium, and thorium occur naturally. The gamma radiation emitted by their isotopes or daughter products K-40 (potassium), Bi-214 (uranium), and Tl-208 (thorium) has an energy distribution characteristic of each isotope. By measuring and evaluating the gamma spectrum, the content of the elements potassium, uranium, and thorium in the soil can be determined.
Radiometric measurements can be carried out from the air (aircraft, helicopter, or drone), on the ground (measuring vehicle or portable device), or underground (borehole probe). The measuring principle is largely the same. The detector usually consists of one or more sodium iodide crystals. The energy of incoming gamma rays is converted into light pulses, which are then converted into electrical pulses via a photomultiplier tube. The amplitude of the electrical pulses is directly proportional to the energy of the incoming gamma rays. According to their amplitude, the pulses are assigned to energy channels of the spectrum in the range of 0 to 3 megaelectronvolts (MeV). Once the entire system is calibrated, the corresponding concentrations of potassium, uranium, and thorium can be determined after evaluating the spectra.
In geophysics, gamma-ray spectrometers are used primarily for geological mapping, in mineral prospecting, especially uranium prospecting, and in environmental research to detect the existence and distribution of radioactive substances. Due to the strong absorption of radioactive radiation by matter, only about the top 50 cm of the earth's surface are detected.