Gamma radiation

Gamma spectrometry is used to quantify the presence of the naturally occurring radioactive isotopes; potassium (⁴⁰K), uranium (²³⁸U) and thorium (²³²Th). The radiation that originate from these isotopes has its source in the top few decimeters in the soil or bedrock. It is possible to identify bedrock units with separate occurrences of these elements in areas where the bedrock is present at the surface or close to the surface.

The distribution of potassium, uranium and thorium can also provide information on the genesis of the rocks and how they have been affected by geological processes. The information can also be used to identify areas where there is a risk of radon exposure.

SGU has performed airborne gamma-ray surveys of the natural gamma-ray emitters in the ground since the late 1960’s. These measurements allow for the amounts of naturally occurring radioactive isotopes—potassium, uranium and thorium—to be calculated. The maps below is based on measurements until 2020.

The spectrometer has been calibrated by measuring its response over concrete pads with known concentrations of the radionuclides above. This is routinely performed at the SGU calibration facility in Borlänge. Spectral-fitting is then performed to calculate the nuclide concentration. The effect of absorption in the air has been determined through experiments using wood as absorbing material. The calibration has been verified by comparing measurements on the ground with airborne surveys over the same area. There are however some uncertainties in the result, since variations in air humidity to some amount affect the measurement of all three nuclides. There is also electronic noise in the instruments and ionizing radiation from the measuring platform. By measuring the background level over large water areas it is possible to reduce these effects in processed data.

Radiometric map (K)

The map shows the distribution of potassium in the uppermost part of the bedrock and soil. The radioactive decay used in the calculation of the potassium concentration is the gamma radiation from the decay of potassium-40 to argon-40. Potassium-40 constitutes 0.0118% of the naturally occurring potassium.

Radiometric map (Th)

The map shows the distribution of thorium in the uppermost part of the bedrock and soil. The thorium concentration is given in ppm (parts per million) eTh, where eTh indicates that radiometric equilibrium was presupposed in the decay chain of thorium when calculating the concentration.

Radiometric map (U)

The map shows the distribution of uranium in the uppermost part of the bedrock and soil. The uranium concentration is given in ppm (parts per million) eU, where eU indicates that radiometric equilibrium was presupposed in the decay chain of uranium when calculating the concentration.