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 radioisotopes with sufficiently long lives that produce significant amounts of gamma rays are potassium (40K), with a half life of 1.3 x 109 years; uranium (238U), with a half life of 4.5 x 109 years; and thorium (232Th), with a half life of 1.4 x 1010 years. K, U and Th are known as the ‘primeval’ emitters, and each gives off characteristic energies. Minerals that fix K, U and Th, such as clay minerals, are the principal source of natural gamma radiation.

Sensor Operating Principles:
The entire sensor assembly comprises at least two, but generally three to four, 2″ x 2″ or 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 which then strikes 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 multi channel 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. Total counts per second for a given detector/core geometry can be converted to API units with customized standards. If elemental data are desired, exported spectral data can be processed through third-party software.

Applications:
Natural gamma measurements have three purposes:

  • core/core and core/borehole correlation
  • evaluation of clay/shale content
  • abundance estimates for K, U, Th

Bulk gamma emissions (total counts) are often sufficient for the first two items. Elemental analysis requires spectral data acquisition and much longer sampling times.

For further information please contact us.

MSCL Systems