All rocks and soils contain radioactive isotopes; the decay of these unstable isotopes gives rise to the natural background radiation at the Earth's surface. Some isotopes change to a more stable state by the emission of ionising radiation (e.g., gamma-rays). These are the so-called radioactive isotopes or "radioelements". Almost all gamma radiation detected near or at the Earth's surface is derived from the natural radioactive decay of just 3 elements - potassium (K), thorium (Th) and uranium (U). Gamma-ray spectrometric surveys map the distribution of these elements at the Earth's surface. You will learn more about the fundamentals and application of gamma-ray spectrometric surveys in this interactive module.
Gamma-rays emitted from the natural decay of K, Th and U can penetrate some 35 cm of rock and several hundred metres of air. Gamma-rays can thus be used for the remote sensing of terrestrial radioelement concentrations. Airborne gamma-ray spectrometry was originally developed as a uranium exploration tool. However, the method is now widely used for geological and environmental mapping.
Gamma-ray photons have an associated energy; this energy is diagnostic of the source isotope. Some of the gamma-rays from radioisotope disintegrations near the Earth's surface penetrate through the Earth and lower atmosphere and can be recorded in a survey aircraft carrying a gamma-ray detector. Most airborne gamma-ray detectors used today consist of crystals of thallium-activated sodium iodide (NaI). Gamma-rays absorbed in the crystals result in the emission of a scintilla of light - the intensity of which is proportional to the energy of the absorbed gamma-ray photon. Airborne detectors measure a gamma-ray spectrum (Figure 1), i.e., both the number of gamma-rays recorded during a specific sample period and the energy of each gamma-ray photon. The number of gamma-rays recorded is proportional to the concentration of the radioisotopes in the source, and the energies of the gamma-rays can be used to determine the composition of the source isotopes.
Figure 1 (above). An airborne gamma-ray spectrum (averaged over a long period of time) showing the diagnostic photopeaks and the positions of the K, U and Th windows used in airborne gamma-ray spectrometry.
Potassium abundance is measured using the 1.46 MeV gamma-ray photons emitted when 40K decays to 40Ar (argon). Uranium and Th abundances are measured from daughter nuclides in their respective decay chains (Figure 2). Distinct emission peaks associated with 214Bi (bismuth, a daughter product in the 238U decay series) and 208Tl (thallium, a daughter product in the 232Th decay series) at 1.76 MeV and 2.61 MeV (Figure 1) are used to estimate the concentrations of U and Th, respectively.
The estimation of U and Th using daughter isotopes in their respective decay series is based on the assumption that their radioactive decay series are in equilibrium. However, disequilibrium is common in the 238U decay series and this should be taken into consideration when interpreting estimated U abundances. For example, U anomalies can be caused by the accumulation of radium (226Ra) in groundwaters (Giblin and Dickson 1984). Uranium and Th concentrations derived from gamma-ray spectrometry are normally expressed in units of "equivalent" parts per million (eU and eTh) as a reminder that these estimates are based on the assumption of equilibrium in their respective decay series.
Figure 2. Radioactive decay series for K, Th and U values for various rock types (Dickson and Scott 1997).