The structural and electronic properties of  and semiconductor detectors at various concentrations x = 0, 0.25, 0.5, 0.75 and 1 of Selenium (Se) were determined by using the full potential-linearized augmented plane wave (FP-LAPW) based on the density functional theory (DFT). The compositional dependence of such properties was analysed and discussed. The concentration dependence of lattice parameter and bulk modulus show nonlinearity. All the investigated alloys have a direct bandgap (Γ-Γ) which decreasesnonlinearly with increase in Se concentration. On the other hand, Geant4 simulations have been performed for studying the absolute and full-energy peak detection efficiencies and energy resolution at 1.5”×1.5” of these alloys as semiconductor detectors in the 511-1332 keV gamma-ray energy range. Ours findings are in a good agreement with the available theoretical and experimental data. We hope that our results serve as are reference for future theoretical and experimental researches.