Geant4 validation of a dual-HPGe coincidence spectrometer using the 35Cl(n,γ)36Cl cascade

We develop a Geant4 detector-response model of the Dalat Nuclear Research Reactor (DNRR) dual high-purity germanium (HPGe) γ-γ coincidence spectrometer and benchmark it using the 35Cl(n,γ)36Cl prompt-γ cascade. Experimental data are acquired with a digitizer?FPGA coincidence data acquisition (DAQ) system; simulated energy deposits are folded with a data-derived resolution function and compared with measured singles spectra, E1 + E2 sum-energy spectra, and sum- peak-gated coincidence projections. Photopeak centroids agree within ±3 keV over 72-8579 keV, and relative peak ratios for 39 lines (788?8070 keV) are reproduced typically within ±30% with a small overall bias (mean −5.8%). The inter-detector ∆t distribution is used as a timing-quality check, and agreement in sum peaks and gated projections supports the modeled geometry, shielding, and cascade-source implementation. The validated model enables coincidence-e?ciency calculations and provides a baseline for optimizing future coincidence con?gurations and cascade-based metrology; remaining uncertainties are dominated by detector dead layers, unmodeled low-energy backgrounds, and neglected angular correlations.