The gamma ray log is used to determine the bulk density for known grain density and porosities. A gamma ray source and a detector are mounted vertically across the core on a sensor stand that aligns them with the centre of the core. A narrow beam of gamma rays (photons emitted from the nucleus) is produced from a Cesium 137 source with energies centered principally at 0.662 Mev. These photons pass through the core and are detected on the other side. At this energy level the primary mechanism for the attenuation of gamma rays is by Compton scattering. The incident photons are scattered by electrons with a partial energy loss. The attenuation, therefore, is directly related to the number of electrons in the gamma ray beam (core thickness and electron density). By measuring the number of unscattered gamma photons that pass through the core unattenuated, the density of core material can be determined. To differentiate between scattered and unscattered photons the gamma detector system only counts those photons that have the same principal energy of the source. Conversion from counted gamma rays and bulk density is done through an equation with experimental parameters deduced during the calibration using a known density core (water and aluminium). The gamma beam is collimated through the choice of 2 collimators (5 and 2.5 mm diameter). Table 8 shows the diameter of the beam of gamma ray at the surface of the core.
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