Natural Gamma Spectrometry

Background

Gamma rays are electromagnetic radiation given off by an atomic nucleus during the spontaneous decay of an unstable element (radioisotope).  Naturally-occurring gamma radiation stems from the ‘primeval’ emitters potassium (K), uranium (U) and thorium (Th) and their daughter elements.  Therefore minerals that fix K, U or Th, such as clay minerals and K-feldspar, are the principal source of natural gamma radiation.

Operating Principle

The sensor comprises of one to three 3″ x 3″ NaI(TL) crystal and SiPM pairs housed in 6″ diameter lead shields. Each detector is coupled to a Multi-Channel Analyser (MCA). Each of the detectors has a resolution of 6-8% specified at the 0.662 MeV peak of 137Cs. The data are collected by the MCAs over the energy range 0-3 MeV.

Calibration and Processing

Measurements are presented in counts per second (cps). A background reading (or spectrum, if spectral data are being collected) is required for subtraction.  Customised standards for a given detector/core geometry allow reporting in API units and potassium (K), uranium (U), and thorium (Th) concentrations.

Applications

Natural gamma measurements, in raw cps or API units, are frequently used for core-to-borehole correlation, as downhole natural gamma is a standard measurement. Natural gamma measurements can also be used for core-to-core correlation as a complementary measurement to magnetic susceptibility. As natural radioactivity is concentrated in clays and shales, as contrasted with sands, it can be used as a proxy to evaluate shale content. If full spectral data is collected, actual abundances of K, U, and Th can be estimated; these abundances can be used for more thorough core-to-borehole correlation or can be combined with other mineralogical indicators such as XRF elemental analyses or infrared spectra to map the lithology.