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 entire sensor assembly comprises at least one, but generally three, 3″ x 3″ NaI(Tl) detectors housed in 6″ diameter lead shields. Each detector unit has a NaI(Tl) crystal optically coupled to a photomultiplier tube and connected to an integrated bias base and MCA. Emitted gamma rays hit the NaI(Tl) crystals which produces a pulse of light. These photons strike the photomultiplier tube, producing a small electrical current to give a voltage pulse. The peak height of the voltage pulse is related to the energy of the gamma emission which is recorded by the multichannel analyser in one of 1024 channels.
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.
