Progress on the development of a polychromatic beam neutron reflectometer at the NCNR

Charles F. Majkrzak (NIST Center for Neutron Research)

Progress on the general development of the polychromatic beam reflectometer CANDOR (for Chromatic Analyser Neutron Reflectometer Or Diffractometer) being constructed at the NIST Center for Neutron Research is reported. This includes the evolution of an energy-dependent neutron detector which incorporates pyrolytic graphite analyser crystals (54 separate elements in series) in conjunction with 6LiF/ZnS(Ag) scintillation detectors and silicon photomultiplier (SiPM) devices to form an array which simultaneously detects neutrons within a 4 to 6 Angstrom bandwidth with a fractional wavelength resolution of approximately one percent. The general design of this energy dispersive detector is applicable to other instruments as well, for example, a materials diffactometer (Simmons et al. J. Appl. Cryst. 46, 2013). How 30 such energy-dependent detector arrays are to be configured within the reflectometer is described, particularly in regard to achieving a focusing condition in the wavevector transfer Q for specular reflectivity measurements. For non-specular reflectivity measurements, an order of magnitude gain over a conventional monochromatic beam instrument at a continuous source is in principle possible, whereas for specular reflectivity measurements -- in which a broader angular divergence of the incident polychromatic beam can be used in addition -- a gain of several orders of magnitude may be realized under certain conditions. Other components of this instrument, including polarizers, focusing optics, and filters to suppress undesirable portions of the incident spectrum are described.

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