Airborne radiometrics

BGR helicopter at Hadelner Marsch Source: BGR

Airborne radiometric methods are used to determine the natural radioactivity (gamma radiation) of near-surface rocks and soils using a gamma ray spectrometer installed in an aircraft. The measurements reveal information on rock and soil properties, particularly their contents of natural radionuclides. Natural radionuclides are mainly the three elements Potassium, Uranium and Thorium. They occur in various concentrations in the minerals of crustal rocks such as granite, gneiss, shale and basalt, as well as in their weathering products. An airborne radiometric survey gives insight in the distribution of radio elements on the Earth's surface and is an important tool for geological mapping. The results of the survey are presented in the form of maps indicating total radiation and the concentrations of Potassium, Uranium and Thorium.

Principle sketch of the gamma-ray spectrometry

Gamma ray detectors are composed of large Sodium-Iodine crystals that emit weak light pulses when hit by gamma quantums. Depending on the radio element, the pulses show characteristic energy levels (measured in Mega-Electronvolts, MeV) which are determined for each pulse by photo multiplier tubes installed in the detector.

The system used by BGR is composed of five crystals with a total crystal volume of 21 litres and a weight of approximately 100 kg. The detector and the associated electronics console are mounted inside the BGR helicopter. The system records one complete gamma spectrum per second, each spectrum containing 255 channels covering the energy range between 0 and 3 MeV.

During data processing the recorded gamma spectra are converted to ground concentrations of Potassium (in %), Uranium and Thorium (in ppm). In Summer 2013, BGR replaced the spectrometer (GR-820), which had been successfully used for several decades, by a modern RS-500 spectrometer with crystal detector RSX-5.