The Nanoscale Ordered MAterials Diffractometer NOMAD at the Spallation Neutron Source SNS

Abstract

The Nanoscale Ordered Materials Diffractometer (NOMAD) is neutron time-of-flight diffractometer designed to determine pair dist ribution functions of a wide range of materials ranging from short range ordered liquids to long range ordered crystals. Due to a large neutron flux provided by the Spallation Neutron Source SNS and a large detector coverage neutron count-rates exceed comparable instruments by one to two orders of magnitude. This is achieved while maintaining a relatively high momentum transfer resolution of a $delta Q/Q sim 0.8%$ FWHM (typical), and an achievable $delta Q/Q$ of 0.24% FWHM (best). The real space resolution is related to the maximum momentum transfer; A maximum momentum transfer of 50AA$^{-1}$ can be achieved routinely and the maximum momentum transfer given by the detector configuration and the incident neutron spectrum is 125 AA$^{-1}$. High stability of the source and the detector allow small contrast isotope experiments to be performed. A detailed description of the instrument is given and the results of experiments with standard samples are discussed.