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The Gamma ray spectrometer
The purpose of gamma-ray spectrometry, is to provide information about the distribution of the three radioactive elements,uranium, thorium and potassium.
The distribution of these elements in hydrocarbon exploration is significant, since they are affected by the alteration effects of hydrocarbon micro-seepage.
Radiometrics is a term applied to the measurement of the gamma ray spectrum at three specific windows, where emissions for uranium, thorium, and potassium are located.
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Bismuth 214, represents the Uranium window at 1,760 KeV
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Thallium 208, represents the Thorium window at 2,620 KeV
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Potassium has a single emission energy at 1,460 KeV
The Gamma ray spectrometer
A standard interpretation for hydrocarbon exploration consists of looking for decreases in gamma emissions from all of these windows, or, a decrease in the total count. Seepage anomalies from those are related to the lithology. Changes in radiometric response can also be attributed to road surface changes, outcrops, drainages, road cuts and road fill. Keen observation, is the key to culling the false anomalies from the seepage anomalies. Radiometrics is a first wave culling tool, for other types of geochemical survey.
A gamma ray spectrometer consists of a detector crystal and an optical sensor. The detector, typically a sodium iodide crystal, absorbs the gamma radiation and converts it to a light flash or scintillation. The NaI is doped with Thallium, which acts as an 'activator'. The Thallium doped NaI is described as NaI(Tl). The light sensor is a sensitive photomultiplier tube, which converts the light flash to a voltage, proportional to the intensity of the light flash. The magnitude of the peak voltage is indicative of the energy of the gamma ray. The larger the crystal volume (112 cubic inches or more) and the slower the aircraft (40 knots for UAV 130 for manned), the higher the number of gamma counts, that can be collected.
The above from http://www.grdc.com/radiometrics.html
Each gamma ray detector typically consists of a 4” x 4” x 16” NaI(Tl) crystal, integrally mounted with a high sensitivity photomultiplier tube (PMT). Each of these crystals weighs 15.4 Kg, and the integral unit, as supplied by Saint Gobain, costs around $15,000. A typical airborne configuration is as shown above, consisting of 12 units, to detect ground gamma rays and two upper units, to detect the level of cosmic gamma rays.
From http://gsc.nrcan.gc.ca/gamma/survey_e.php
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