Recent Advances in Backscattering Spectroscopy

Over the last years we have extended the possibilities of neutron backscattering spectroscopy with respect to both energy resolution by using the GaAs 200 reflection (75 neV FWHM) and energy transfer range by integrating an inverted ToF setup into IN16B at the ILL (1000 µeV as compared to 32 µeV presently available).

Particular challenges for the GaAs extension are e. g. a careful choice and handling of the crystals and a gravity compensation by a thermal gradient along the height of the analysers. The ToF extension builds upon a chopper system located 34 m upstream from the sample. It features two pairs of counter rotating disks made from oriented carbon fibre reinforced polymer discs with a diameter of 750 mm and attaining 19,000 rpm. Several cut outs in the disks with openings between 90 mm and 10 mm allow to change the energy resolution within minutes in 7 steps between 9.8 and 1.4 µeV, where 6 cut outs are smaller than the guide width of 90 mm. In consequence, the count rate in BATS mode may appear low at present in particular for the high resolution setups. This will be overcome by a focusing element before the sample and additionally by a variable focusing/defocusing guide section of 10 m near the choppers.