Efficiency calibration and coincidence summing correction for a 4π NaI(Tl) detector array

Efficiency calibration and coincidence summing correction were performed for an array of NaI(Tl) detectors developed for in vivo neutron activation analysis. The detector array consisted of eight large NaI(Tl)s with square cross-section and the enclosing geometry of the detection produced both extremely high efficiency and a severe coincidence summing effect. The efficiency was calibrated mainly by Monte Carlo simulation with the EGS4 code. In order to obtain the coincidence summing correction factors through the simulation the coincident emission of photons was modeled. The influence of the angular correlation was explicitly investigated by adding the angle-correlated sampling routine in the simulation code. The experimental efficiencies and correction factors were measured using 137Cs, 60Co and 22Na. For both peak and total efficiencies, experiment and simulation typically agreed within 5%. The discrepancy between simulation and experiment for the coincidence summing correction factors was 15% or less. The approximate methods adopted in the measurement were analyzed in detail and estimated to be reliable for calibrating high-efficiency detectors. The calculation was finally expanded from point sources to realistic volume sources.