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).
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).
1: The electromagnetic spectrum, highlighting the useful parts for
obtaining information on soil and proximal sensing (according to
McBratney et al. 2003).
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.
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
occurs naturally as the radioisotopes 238U and 235U
which give rise to decay series that terminate in the stable isotopes 206Pb
respectively. The half-lives of 238U
and 235U are
years, respectively. Thorium occurs naturally as the
which gives rise to a decay series that terminates in the stable
The half life of 232Th
is 1.39X1010 years. Neither 238U,
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
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).
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
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
gamma-ray spectrometry in soil and regolith mapping
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).
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,
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.
B.R.S., 1997. The fundamentals of airborne gamma-ray spectrometry. AGSO
Journal of Australian Geology & Geophysics, 17(2), 39-50.
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.
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,