MEDUSA system

Gamma-rays are detected by means of a scintillation system comprising, amongst others, a CsI(Na) crystal (15 cm long, 7 cm diameter), a photo-multiplier tube (PMT), a Cockcroft Walton high-voltage generator and a spectroscopic amplifier. In order to record spatial co-ordinates of the detector a GPS signal receiver (Garmin 76) is located above the MEDUSA scintillator. Data from this receiver are interfaced with the gamma-ray data and recorded onto a laptop computer. The MEDUSA system is normally operated in a mode in which a gamma-ray spectrum is recorded every 2 seconds while all other data (e.g. GPS data, and count rate) are written to file every second.

Activity concentrations of the primordial radionuclides are determined from the measured MEDUSA spectra using the full-spectrum analysis procedure (Hendriks et al., 2001). The FSA technique involves the (least-squares) fitting of a gamma-ray spectrum with a linear combination of so-called standard spectra after proper background subtraction. A standard spectrum represents the response of the detector per second, for a particular geometry, to a volume source containing 1 of a particular radionuclide, or series of radionuclides (in the case of 238U and 232Th). Standard spectra are calculated using from Monte Carlo simulations. These simulations produce the shape of the spectra, but not the absolute magnitude that is influenced by light conversion and other electronic efficiencies. The fit yields the optimal coefficients to use in reconstructing the measured sample spectrum from the standard spectra.

The MEDUSA technique could be of benefit on a number of issues, for example:

  • prospecting: making a surface map as a guidance for making drill holes and assessing the uranium content of the tailings dump;
  • ore processing: making an on-line analysis in order to streamline processing; and
  • radiation control: it can be used to map activity concentrations on, amongst others, tailings dams, and on dam/river beds. These data can then be used for risk assessments.


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