Technical Note

Fundamentals of Airborne Gamma-Ray Spectrometry

Radioactivity

Radioisotopes are unstable isotopes that change to more stable nuclei by the emission of radiation. The three main types of radiation arising from radioactive decay are alpha, beta, and gamma rays. Alpha particles (2 protons and 2 neutrons) have a discrete energy, and both a charge and mass. They are easily absorbed by a few centimetres of air. Beta particles (electrons) do not have a characteristic energy, and carry a negative charge. They are less easily absorbed than alpha particles and can travel up to a metre in air. The emission of an alpha or beta particle usually leaves the new nucleus in an excited state, and the surplus energy is radiated as gamma rays. These are quanta, or photons, of energy which are very penetrating because they possess neither charge nor mass. Gamma rays can penetrate up to 30 cm of rock and several hundred metres of air, making this form of radiation suitable for remote sensing via low-flying aircraft. Each gamma-ray photon has a discrete energy that is characteristic of the source isotope. The gamma-ray spectrometric method is the measurement of gamma-rays and their energies to determine both the source of the radiation (using the measured energies), and the concentration of the source isotopes (using the number of gamma rays detected per unit time).

Sources of Gamma Radiation

Almost all gamma radiation emanating from the earth’s surface are due to just 3 elements – potassium (K), uranium (U) and thorium (Th). K40 is the only radioactive isotope of potassium. It occurs as a fixed proportion (0.012 %) of K in the natural environment and can thus be used to estimate the total amount of K present. It gives rise to a single gamma-ray emission at 1.46 MeV. Uranium occurs naturally as the radioisotopes U238 and U235 which give rise to decay series that terminate in the stable isotopes Pb206 and Pb207 respectively. The vast majority of gamma-ray emissions due to uranium are from the U238 decay series. Thorium occurs naturally as the radioisotope Th232 which gives rise to a decay series that terminates in the stable isotope Pb208. Neither U238 nor Th232 emit gamma rays, and we thus rely on the gamma-ray emissions from their radioactive daughter products to estimate their concentrations.

The main gamma-ray emissions used by the gamma-ray spectrometric window method are the K40 emissions at 1.46 MeV for potassium, the Bi214 emissions at 1.76 MeV for uranium, and the Tl208 emissions at 2.61 MeV for thorium.

Disequilibrium

Disequilibrium in the uranium decay series is a serious source of error in gamma-ray spectrometer surveying. Disequilibrium occurs when one or more of the isotopes in a decay series are completely, or partially, removed from the environment. Thorium rarely occurs out of equilibrium in nature, and there are no disequilibrium problems with potassium since it only exhibits a single photo-peak. However, in the uranium decay series, disequilibrium is common in the natural environment. A simplified U238 decay series is shown in Table 1, with arrows showing the main links in the decay chain where disequilibrium can occur. U238 can be selectively leached relative to U234 (and vice versa). In fact, both uranium and radium are soluble, and thus transportable. In an oxidising environment, uranium is preferentially leached relative to radium, and in a reducing environment radium may be preferentially leached relative to uranium. Radon gas (Rn222) in the U238 decay chain is a large source of disequilibrium, as it can escape from rocks and soils through diffusion. Since it occurs above the main gamma-ray emitters in the U238 decay chain (Bi214 and Pb214), its removal is a large source of error in the estimation of U concentrations using gamma-ray spectrometry. For this reason, U concentration estimates are usually reported as "equivalent uranium" (eU) – i.e. the concentration estimates are based on the assumption of equilibrium conditions. Thorium is also usually reported as "equivalent thorium" (eTh), although the thorium decay series is almost always in equilibrium.

Table 1. Simplified U238 decay series

Isotope	Emissions	Half-life	Disequilibrium
U238	α	4.507 x109 y
Th234	β	24.1 d
Pa234	β	1.18 m
U234	α	2.48 x105 y	*
Th230	α	7.52 x104 y	*
Ra226	α	1600 y	*
Rn222	α	3.825 d	*
Po218	α	3.05 m
Pb214	βγ	26.8 m
Bi214	βγ	19.7 m
Po214	α	1.58 x10-4 s
Pb210	β	22.3 y
Bi210	β	5.02 d
Po210	α	1.384 d
Pb206	stable

Background Radiation

Any radiation not originating from the ground is regarded as "background". There are four sources of natural background radiation flux: atmospheric radon, cosmic background, aircraft background, and fallout products from atomic explosions and nuclear accidents. Atmospheric Rn222 and its daughter products, specifically Bi214 and Pb214, are the major contributors to the background. Rn222 (radon gas) is mobile and can escape into the atmosphere from soils and rock fissures. Primary cosmic radiation from outside our solar system and from the sun reacts with the air, aircraft and detector to produce the measured "cosmic" gamma-ray background. Aircraft background refers to radiation due to trace amounts of K, U and Th in the aircraft and equipment, as well as in the detector itself. This component of background is constant.

Attenuation of Gamma Rays by Matter

Gamma rays interact with matter in the earth, air, and in the detector. The main interaction is “Compton scattering”, whereby a gamma-ray photon interacts with an electron – it loses part of its energy to the electron and is “scattered” at an angle to its original direction of travel. Typically, gamma rays are scattered once or twice before being absorbed through either the photoelectric effect (energy absorbed by the bound electron of an atom) or pair production (energy absorbed by nucleus of an atom with the creation of an electron-positron pair) – see Figure 1.

Figure1. Interaction of gamma rays with matter.

The degree of attenuation of gamma rays in matter with distance from the source depends on both the density (electron density) of the matter and the energy of the gamma-ray emissions. Table 2 (after Grasty, 1979) shows the “half-thicknesses” (the thickness of an attenuating material that will reduce a beam of gamma radiation to half its intensity) for various earth materials and typical gamma-ray energies used in gamma-ray surveying.

Table 2. Half thicknesses (HT) for various gamma-ray energies in air, water and concrete (rock)

Source	Isotope	Energy (MeV)	Air HT (m)	Water HT (cm)	Rock HT (cm)
K	K40	1.46	102	11.8	5.25
U	Bi214	1.76	112	13.0	5.75
Th	Tl208	2.61	137	16.0	7.00

From this it is clear that while just 50 cm of water, or 30 cm of rock, will attenuate almost all gamma radiation, the gamma rays can travel significant distances in air before being attenuated. The gamma ray method thus only measures the concentrations of radioelements in the top 30 cm, or so, of the earth’s crust. On the other hand, gamma rays are quite penetrating in air, and for an aircraft measuring gamma radiation at a height of 100 m above the ground, over 20% of the radiation measured will be from sources at lateral distance greater than 300 m. The “field of view” of airborne spectrometers is thus quite large.

Airborne Surveying

Typically, airborne geophysical survey would be flown with line spacing between 100 m and 400 m, and at a flying height of between 40 m and 100 m. The speed of the aircraft is usually about 50-60 m/s for fixed-wing surveys, but can be appreciably slower for helicopter surveys. Airborne gamma-ray spectrometer detectors usually comprise at least 32 litres of NaI crystals and record at least 256 energy channels between 0 and 3 MeV. The crystals have the special property of converting the energy of absorbed gamma rays into scintillations of light. The intensity of the light flashes is proportional to the energy of the absorbed photon. The spectrometer thus measures the number of gamma ray photons, and their energies, detected during a given sampling period – typically 1 s for airborne surveying. The output is a gamma-ray spectrum which can be thought of as a histogram of the number of counts per energy channel. A typical 256-channel spectrum from a 32-litre detector, measured at 60 m height and a sample interval of 1 s is shown in Figure 2.

Figure 2. Typical 1-s spectrum from a 32 litre detector recorded at 60 m above ground level.

The 1-s spectrum is noisy. This is because of the statistical nature of radioactive decay. It is impossible to predict when particular atoms will spontaneously disintegrate. However, over a long period of time, the average number of gamma rays detected over a source distribution per unit time will be proportional to the concentration of the source. Also, the longer the sample integration time, the smaller the fractional errors in the measured mean count rate. Figure 3 show a spectrum recorded with a long sample integration time. The fractional errors are now much smaller, and the photopeaks used to estimate K, U and Th element concentrations are now evident.

Figure 3. A typical gamma-ray spectrum recorded at 100 m altitude with a large integration time and showing prominent photo peaks and the positions of the conventional 3-channel windows.

The conventional approach to the acquisition and processing of airborne gamma-ray spectrometric data is to monitor four relatively broad spectral windows (Table 3). The K, U, and Th windows centred on the 1.46 MeV (potassium), 1.76 MeV (uranium) and 2.62 MeV (thorium) photo peaks (Figure 3) have been generally accepted as the most suitable for the measurement of potassium, uranium and thorium. The total-count window gives a measure of total radioactivity.

Table 3. IAEA-recommended windows for conventional 3-channel airborne gamma ray spectrometry (IAEA, 1991).

Element	Isotope	Emission energy (MeV)	Energy window (MeV)
Potassium	K40	1.46	1.370–1.570
Uranium	Bi214	1.76	1.660–1.860
Thorium	Tl208	2.61	2.410–2.810
Total Count			0.410–2.810

Calibration and data processing for airborne gamma-ray spectrometry are described in Minty Geophysics Technical Note 2. Briefly, the spectra are processed to reduce noise. They are then corrected for equipment live time and energy calibrated to correct for energy drift in the spectrometer. The spectra are then summed over the conventional windows to obtain window counts. These window count rates are corrected for background radiation, and the interaction between the energy windows (channel interaction correction or ‘stripping’). The background-corrected and stripped count rates are then corrected for variations in the terrain clearance of the detector (height correction) and reduced to elemental concentrations on the ground (sensitivity correction). For many of these corrections, the approach taken to calibration is empirical. We view the source and detector as a single system and measure its response, for example, to changes in aircraft altitude (to obtain height attenuation coefficients) and sources of known geometry and concentration (to obtain sensitivity coefficients).

References

Grasty, R.L., 1979. Gamma-ray spectrometric methods in uranium exploration – theory and operational procedures. Paper 10B in Geophysics and geochemistry in the search for metallic ores. Geological Survey of Canada, Economic Geology Report 31.

IAEA, 1991. Airborne gamma ray spectrometer surveying. Technical Report Series, No. 323. International Atomic Energy Agency, Vienna, 1991.

Minty, B., 1997. Fundamentals of airborne gamma-ray spectrometry. AGSO Journal of Australian Geology and Geophysics, 17 (2), 39-50.

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