A DYNAMIC-RANGE UPGRADE FOR NEUTRON BACKSCATTERING SPECTROSCOPY

We report on an instrumental development of the cold neutron backscattering spectrometer IN10 at the Institut Laue-Langevin which has led to a significant increase in its dynamic range. Thermal expansion of a variety of neutron monochromator crystals is used instead of a mechanical oscillation of the monochromator, yielding an increase in the energy transfer range by nearly two orders of magnitude in an elastic wave vector transfer range of 0.07 less-than-or-equal-to Q (angstrom -1) less-than-or-equal-to 2.0. Using this new configuration, first inelastic measurements have been performed using the (200) reflections from KCl and NaCl monochromators with crystal temperatures between 80 K and 700 K. The thermal expansion of these crystals in this temperature range gives rise to energy transfer ranges (neutron energy gain) of -16 < HBAR-omega (mu-eV) < + 83 for KCl and -530 < HBAR-omega (mu-eV) < -420 for NaCl with energy resolution (FWHM) of around 0.6 and 1.4-mu-eV for KCl and NaCl respectively. These figures represent the highest energy resolution currently available at these energy and wave vector transfers.