An activity predictor software was previously developed to foresee activities, exposure rates and gamma spectra of activated
samples for Radiation Science and Engineering Center (RSEC), Penn State Breazeale Reactor (PSBR), Neutron Activation Analysis
(NAA) measurements. With Activity Predictor it has been demonstrated that the predicted spectra were less than satisfactory.
In order to obtain better predicted spectra, a new detailed model for the RSEC NAA spectroscopy system with High Purity Germanium
(HPGe) detector is developed using Geant-4. The model was validated with a National Bureau of Standards certified 60Co source and tree activated high purity samples at PSBR. The predicted spectra agreed well with measured spectra. Error in
net photo peak area values were 8.6–33.6%. Along with the previously developed activity predictor software, this new model
in Geant-4 provided realistic spectra prediction for NAA experiments at RSEC PSBR.