Laue lens: the challenge of focusing gamma rays

The development of a Laue lens to focus soft gamma rays appears today as the only solution to improve significantly telescopes’ sensitivity and angular resolution in the 100 keV – 1 MeV domain. A Laue lens makes use of diffraction in the volume of a large number of crystal tiles carefully orientated to concentrate incident radiations from a large collecting area in a small focal spot. Various type of crystal have been measured and show excellent results, with angular spread of diffracting planes in the range 15-45 arcsec, and reflectivities up to 31 % at 600 keV. Precision of orientation of crystals is demanding because of the long focal length involved by small angles of diffraction. A prototype currently being built by a French collaboration including the CESR, the CNES and Thales Alenia Space aims to achieve 10 arcsec of tolerance.