SPEAR - ToF neutron reflectometer at the Los Alamos Neutron Science Center

This article discusses the Surface ProfilE Analysis Reflectometer (SPEAR), a vertical scattering geometry time-of-flight reflectometer, at the Los Alamos National Laboratory Lujan Neutron Scattering Center. SPEAR occupies flight path 9 and receives spallation neutrons from a polychromatic, pulsed (20 Hz) source that pass through a liquid-hydrogen moderator at 20 K coupled with a Be filter to shift their energy spectrum. The spallation neutrons are generated by bombarding a tungsten target with 800 MeV protons obtained from an accelerator. The process produces an integrated neutron flux of similar to 3.4 x 10(6) cm(-2) s(-1) at a proton current of 100 mu A. SPEAR employs choppers and frame overlap mirrors to obtain a neutron wavelength range of 4.5-16 angstrom. SPEAR uses a single 200 mm long (3)He linear position-sensitive detector with similar to 2 mm FWHM resolution for simultaneous studies of both specular and off-specular scattering. SPEAR's moderated neutrons are collimated into a beam which impinges from above upon a level sample with an average angle of 0.9 degrees to the horizontal, to facilitate air-liquid interface studies. In the vertical direction, the beam converges at the sample position. The neutrons can be further collimated to the desired divergence by finely slitting the beam using a set of two (10)B(4)C slit packages. The instrument is ideally suited to study organic and inorganic thin films with total thicknesses between 5 and 3000 angstrom in a variety of environments. Specifically designed sample chambers available at the instrument provide the opportunity to study biological systems at the solid-liquid interface. SPEAR's unique experimental capabilities are demonstrated by specific examples in this article. Finally, an outlook for SPEAR and perspectives on future instrumentation are discussed.