Aberration in Laue lenses made of Ge and Si bent crystals for future gamma-ray astrophysics telescopes

Forthcoming hard X-ray (50-700 keV) telescopes designed for high-energy astrophysics will rely on concentrating radiation through diffractive systems utilizing crystals and solid-state detectors at focal plane. Our current focus involves exploring the feasibility of employing Laue lenses, made from Germanium and Silicon crystals curved to fit the lens curvature. In this study, we estimate the aberration of a X-ray Laue lens made of bent germanium and silicon crystals disposed on concentric rings on a disk of 2.5 m in diameter when incident X-rays arrive off axis on the lens. In each ring the crystals are pre-oriented in order to diffract a parallel X-ray beam in a focus positioned at 20 m from the lens. Each crystal is a tile 3x1 cm2, with the longer edge oriented to the centre of the lens. Thanks to the 40 m curvature of the crystal tiles each crystal produces a spot around 1.5x10 mm2 at the focus plane. The overlap of diffracted beams from all the tiles produces a spot with a point spread function of a few mm in diameter corresponding to only 40 arcseconds when the incident rays are parallel to the axis of the lens. To evaluate the imaging performance of Laue lenses made of bent crystals, simulation of diffraction from on-axis and off-axis X-ray sources are reported, also permitting to estimate the amount of optical aberration.