Calculating off-axis efficiency of coaxial HPGe detectors by Monte Carlo simulation.

In beam geometries where a directed γ -ray beam hits the surface of a coaxial high purity germanium detector (HPGe), the detector efficiency is sensitive to the position where γ -rays initially hit the detector surface because the structure of the detector is nonuniform. This may cause inaccuracy of the detector efficiency when measured using standard sources that are point-like sources emitting γ -rays isotropically. Obtaining a precise estimation of the full energy peak efficiency of the coaxial HPGe detector in the beam geometry for on-axis and off-axis measurements requires a Monte Carlo simulation. We performed Monte Carlo simulations that calculate the detector efficiency in the beam geometry. The effects of the off-axis distance and γ -ray beam size on the efficiency are quantitatively analyzed. We found that the intrinsic efficiency in the beam geometry is maximized when the beam hits the detector at specific off-axis distances. Our Monte Carlo calculations have been supported by nuclear resonance fluorescence experiments using laser Compton scattering γ -ray beams. • Calculating efficiency of coaxial HPGe detector for directed γ-ray beams by Geant4. • Revealing the sensitivity of efficiency to γ-ray beam size and off-axis distance. • Off-axis efficiency is higher than on-axis efficiency, depending on γ-ray energy. • Maximizing efficiency by selecting the optimum off-axis distance. • LCS γ-ray beams up to 3 MeV validate calculations experimentally. [ABSTRACT FROM AUTHOR]