The efficiency calibrations measurements of the system were done at radiation reference pads of the Nuclear Energy Corporation of South African (NECSA) according the method describe by Chiozzi
et al. (2000) and Corner et al. (1979). The facility comprises of 11 surface standard sources, each consisting of a solid circular pad with a 2 m diameter. Calibration spectra were acquired by positioning the detector 20 cm above the standard and then recording for a time period of 10 min. The detector was exposed to three standards that contained 4400 ppm uranium oxide (U3O8),
14,200 ppm thorium oxide (ThO2) and 12.2% potassium oxide K2O, respectively. These calibration spectra were used to extract efficiency parameters for each of the nuclides according to the method described in Paragraph 2.3. The nuclide concentrations were consequently expressed in Becquerel per kilogramme.