Efficiencies of NaI(Tl) and Ge(Li) Detectors for Large Cylindrical Sources

The calculation of efficiencies of NaI(Tl), Ge(Li) detectors for large cylindrical sources using Monte Carlo method with weighed technique is presented. The calculating method was briefly described, and the efficiencies of a 3' x 3' NaI(Tl) crystal for a 76.2 mm aqueous cylindrical source were computed by means of Monte Carlo method in a mini-computer PDP 11/34A and the results were in good agreement with the results obtained by M.I.Verherijke et al. within 1% in the range of 80 to 3000 keV. The efficiencies of a real Ge(Li) detector for NBS reference material and for large aqueous cylindrical sources of which radius were larger than that of the Ge(Li) crystal were computed separately. The correction of N layer and packing cover of the detector was considered in calculation. After the ratios of peak-to-total for point sources was measured, the peak efficiencies of the Ge(Li) detector were obtained. The comparison between the computed peak efficiencies of the Ge(Li) and the efficiencies experimentally measured using 152Eu, 60Co, 137Cs, U-Ra and Th cylindrical standard sources showed that both of them were in agreement within 4%. Finally, the relationship between minimal detectable specific activity and sample volume was discussed