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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.

The spin and parity of the 3.004-MeV level in 27Al have been determined by measuring the angular correlation function of radiation emitted from levels populated by resonant absorption of polarized photons. The nuclear resonance fluorescence experiments were carried out at the High Intensity g-ray Source (HIgS) facility at Duke University using quasi-monoenergetic linearly polarized photon beams. The spin and parity of levels in 27Al were deduced from a comparison of the measured angular distribution ratios and azimuthal intensity asymmetries with theoretical predictions. The observed resonance properties were compared with shell model calculations using the universal sd interaction. The Monte Carlo shell model calculations were also performed to investigate low-energy structure of 27Al.