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Gamma-ray Spectrometry

Gamma rays are the most penetrating radiation from natural and man-made sources, and gamma ray spectrometry is a powerful tool to monitor and assess environmental radioactivity.

Radiometric surveys and maps are applicable in several fields of science. They retain their geological and geophysical information for mineral prospecting and structural geology, and enable the comparison of geological features over large regions.

Although the gamma-ray method was originally developed for geoscience, it is also successfully applied in emergency situations for mapping the contamination from nuclear fallout and for the location of radioactive material.

Naturally occurring radioactive isotopes can be found in most rocks and soils. All rocks and soils are naturally radioactive, containing various proportions of a variety of radioactive elements.
The most abundant of these are potassium (40K), uranium (238U), and thorium (232Th). The radioactive decay of these elements results in the emission of gamma-rays of sufficient intensity that they can be measured using instruments used on the ground or in aircrafts flying close to the ground.

Whereas airborne methods provide valuable, systematic coverage of large areas, ground spectrometry greatly improves the resolution of individual radioelement sources. The airborne measurement technique is known as aerial gamma-ray spectrometry and developed quantitative methods allow data to be expressed in terms of element concentrations.

Figure 1

As the concentration of these different radioelements varies between different rock types, we can use the information provided by a gamma-ray spectrometer to map the rocks. Radiometric surveys can be useful for mapping certain rock types. Granites, for example, which are potassium-rich, produce a strong potassium signature. Potassium may also be associated with alteration zones related to certain mineral deposits, and can be a useful fingerprint for the presence of ore. Potassium signatures and/or Thorium/Potassium ratio signatures have been documented in association with gold, copper porphyry and sulphide deposits.

Radiometric data may provide also information on:

  • baseline levels of natural radioactivity that give a background to assess future contamination and assist in epidemiological studies
  • distribution of 137Cs (Caesium) (e.g. Chernobyl fallout)
  • identification of radioactive industrial waste
  • identification of areas prone to high levels of radon (222Rn)
  • information on soil types
  • baseline information for mineral exploration.

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