Gamma ray spectrometry
Gamma rays are the most penetrating radiation from natural and man-made sources, and gamma ray spectrometry is a powerful tool for the monitoring and assessment of the radiation environment. Gamma ray surveys are carried out by aircraft, field vehicles, on foot, in boreholes, on the sea bottom and in laboratories. Ground and airborne gamma ray measurements cover large areas of the earth's surface, and many national and regional radiometric maps have been compiled and published (IAEA, 2003).
Gamma rays are produced as result of a previous radioactive decay (e.g. alpha or beta-decay) of an atomic nucleus. The newly formed daughter nucleus is in an energy excited state, and the surplus energy is radiated as gamma rays. Gamma rays typically have frequencies above 1019 Hz, and therefore have energies above 0.1 MeV and a wavelength of less than 10-11 m (Figure 1).

Figure 1: The electromagnetic spectrum, highlighting the useful parts for obtaining information on soil and proximal sensing (according to McBratney et al. 2003).

Natural resources of radiation
While many naturally occurring elements have radioactive isotopes, only potassium, and the uranium and thorium decay series, have radioisotopes that produce gamma rays of sufficient energy and intensity to be measured by gamma ray spectrometry. This is because they are relatively abundant in the natural environment. Average crustal abundances of these elements quoted in the literature are in the range 2.0-2.5 % K, 2-3 ppm U and 8-12 ppm Th.
40K is the radioactive isotope of potassium, and occurs as 0.012% of natural potassium. This isotope decays to 40Ar with the emission of gamma rays with energy 1.46 MeV. Since 40K occurs as a fixed proportion of K in the natural environment, these gamma rays can be used to estimate the total amount of K present. The half-life of 40K is 1.3x109 years.
Uranium occurs naturally as the radioisotopes 238U and 235U which give rise to decay series that terminate in the stable isotopes 206Pb and 207Pb respectively. The half-lives of 238U and 235U are 4.46x109 and 7.13x108 years, respectively. Thorium occurs naturally as the radioisotope 232Th which gives rise to a decay series that terminates in the stable isotope 208Pb. The half life of  232Th is 1.39X1010 years. Neither 238U, nor 232Th emit gamma rays, and gamma ray emissions from their radioactive daughter products are used to estimate their concentrations (IAEA, 2003). Therefore, distinct emission peaks associated with 208Tl and 214Bi, are used to calculate the abundance of Th and U, respectively (Minty, 1997). Consequently, U and Th are usually expressed in equivalent parts per million (eU and eTh), which indicates that their concentrations are inferred from daughter elements in their decay chain, whereas, because of its higher crustal abundance, K is typically expressed as a percentage (K%) (Wilford et al. 1997).
However, the estimation of U and Th in this manner assumes that the daughter products in the Th and U decay series are in equilibrium. In some cases the measured isotopes in the U and Th decay series (i.e. 208Tl and 214Bi) may not perfectly quantify the parent elements, because of disequilibrium in the decay chain. Disequilibrium occurs when one or more of the daughter products in the decay chain is removed or concentrated. For example, in some salt lakes and groundwater discharge sites, high eU and eTh values may be due to the accumulation of radium isotopes that are mobile in acid saline solutions (Dickson, 1985). Disequilibrium should therefore be considered when interpreting gamma ray data.

Application of gamma-ray spectrometry in soil and regolith mapping
Radiometric surveying is a non-invasive, fast method for identifying and quantifying radionuclide concentration and distribution in the landscape. The technique does not require extensive laboratory analysis, and hence the cost is lower than equivalent standard field surveying and mapping methods (Pracilio et al., 2003).
Radiometric response is closely associated with soil texture (Talibudeen, 1964), and can be used to identify and/or differentiate landscape processes. The influence of other soil characteristics, such as soil pH, Eh and structure, on radionuclide distribution is more difficult to identify unless they have direct influence on the radiometric response (Beckett, 2007).     

Beckett, K.A., 2007. Multispectral analysis of high spatial resolution 256-channel radiometrics for soil and regolith mapping. Phd thesis, Curtin University of Technology, Australia.

Dickson, B.L., 1997. Radium isotopes in saline seepages, south-western Yilgarn, Western Australia. Geochimica et Cosmochimica Acta, 49, 361-368.

IAEA, 2003. Guidelines for radioelement mapping using gamma ray spectrometry data. International Atomic Energy Agency.

McBratney, A.B., Mendonça Santos, M.L., Minasny, B., 2003. On digital soil mapping. Geoderma 117, 3-52.

Minty, B.R.S., 1997. The fundamentals of airborne gamma-ray spectrometry. AGSO Journal of Australian Geology & Geophysics, 17(2), 39-50.

Pracilio, G., Adams, M.L., Smettem, K.R.J., 2003. Use of airborne gamma radiometric data for soil property and crop biomass assessment, northern dryland agricultural region, Western Australia, in J.v. Stafford and A. Werner, eds., Precision Agriculture '03, Proceedings of the 4th European Conference: Wageningen Academic Publishers, 551-557.

Talibudeen, O., 1964. Natural radioactivity in soils. Soils and Fertilizers, 27: 347-359.

Wilford, J.R., Bierwirth, P.N., Craig, M.A., 1997. Application of airborne gamma-ray spectrometry in soil/regolith mapping and applied geomorphology. AGSO Journal of Australian Geology & Geophysics, 17(2) 201-216.

© Ulrich Schuler 2008 -