Numerical simulation of photon response in portal monitors

The adjoint technique has been applied to accurately and economically predict the response of a portal monitor to photon emissions below about 1.5 MeV, thus encompassing those sources generally of interest in nuclear safeguards applications. The adjoint source was defined as the product of the total attenuation coefficient and an experimentally determined efficiency factor, which accounts for the performance characteristics of the signal-processing system. The efficiency factor was determined from a combination of data obtained from a single NE-102 scintillator and results from corresponding three-dimensional forward MORSE calculations. A prototype walk-through portal was then fabricated with four identical NE-102 scintillators. Adjoint MORSE calculations were performed to obtain net count rates for various sources within this portal. These results were compared to experimental data and were found to agree to well within 10%. The photon response within the portal detection volume was then characterized by a series of MORSE calculations