  Observer models were developed to process data in list-mode format in order
to perform binary discrimination tasks for use in an arms-control-treaty
context. Data used in this study was generated using GEANT4 Monte Carlo
simulations for photons using custom models of plutonium inspection objects and
a radiation imaging system. Observer model performance was evaluated and
presented using the area under the receiver operating characteristic curve. The
ideal observer was studied under both signal-known-exactly conditions and in
the presence of unknowns such as object orientation and absolute count-rate
variability; when these additional sources of randomness were present, their
incorporation into the observer yielded superior performance.
