The Disk Chopper Spectrometer at NIST: a new instrument for quasielastic neutron scattering studies

We describe the Disk Chopper Spectrometer (DCS) at the NIST Center for Neutron Research (NCNR). The maximum measured flux at the sample, averaged over the 3 cm width and 10 cm height of the beam, with all choppers turning at 20,000 revolutions per minute and phased to transmit neutrons through the widest slots on each disk, is approximately 3 x 10(4) n cm(-2) s(-1). In this configuration the elastic resolution FWHM (full width at half maximum height) with an incident wavelength of 6 Angstrom is 67 mueV. In an alternative configuration, achieved by changing the chopper phases so that neutrons pass through somewhat narrower slots, the elastic resolution FWHM at 6 A, is 34 mueV. Useful wavelengths range from about 1.5 Angstrom to at least 12 Angstrom. The DCS has been used for a variety of measurements, including quasielastic neutron scattering (QENS) studies of systems such as water in confined geometries and biological molecules in solution. (C) 2003 Elsevier Science B.V. All rights reserved.