Bryan van der Ende (Atomic Energy Canada Ltd.)
A Monte-Carlo simulation via the MCNP computer program is a convenient method of indirect detector calibration when a direct calibration is not feasible. This is the case when very-short-lived gaseous radioactive sources are involved. Before being relied on, the MCNP model is normally validated with commercially available calibrated radioactive sources. This presentation will discuss the steps to compare an MCNP output with a real energy spectrum acquired with a Multi-Channel Analyzer (MCA). The detector in this case is a 1.5”x1” NaI scintillator. The source used for the validation test is Co-60. A large discrepancy between the MCNP-predicted output and the MCA measurement was noticed during the validation. There is no data from MCNP above the 1.33 MeV photo-peak from Co-60. Analysis indicates that the discrepancy is caused by the coincidence summing effect which becomes significant when the detector has a large solid angle for the source. After assuming homogeneous spatial distribution of the gamma photons, a method is developed to simulate the coincidence summing effect. Starting with a point source case, a volume source compensation method is developed. The compensated MCNP output is compared with the real measurement. The MCNP output range is extended to 2.51 MeV in good agreement with the MCA spectrum. The result confirmed the correctness of the geometry and material configuration of the MCNP model.
Liqian Li (Atomic Energy of Canada Limited)