1. A New High Intensity Cold Neutron Spectrometer at NIST J. A. Rodriguez 1,3, P. Brand 3, C. Broholm 2,3, J.C. Cook 3, Z. Huang 3, P. Hundertmark 3, J. Lynn 3, N.C. Maliszewsky 3, J. Moyer 3, J. Orndorf 2, D. Pierce 3, T. Pike 3, G. Scharfstein 2 and S. Smee 2 1 University of Maryland, Department of Materials Science and Engineering, CollegePark, MD 20742-2115,USA 2 The Johns Hopkins University, Department of Physics & Astronomy, 3400 N. Charles Street, Baltimore, MD 21218,USA 3 National Institute of Standards and Technology, 100 Bureau Drive, MS 8562, Gaithersburg, MD 20899-8562, USA Isometric view of the MACS cold neutron spectrometer. A novel cold neutron spectrometer is under development at the NIST Center for Neutron Research, optimized for wave vector resolved spectroscopy below 20 meV with a resolution of 0.05 meV to 2.8 meV. The versatile monochromating system, which includes radial collimator, three filters, and a variable beam aperture offers considerable flexibility in optimizing the choice of Q-resolution, energy resolution and intensity. Vertical focusing of the DXAL allows for smaller detectors for enhanced signal to noise. Post Sample collimators and filters options provide flexibility in the choice of scattered beam energy resolution A helium filled cask contains all items associated with converting a divergent white neutron beam to a monochromatic beam that converges on the sample. Energy resolution is controlled by a segmented radial collimating system. The blades of the device converge at the center of the cold source. Two in-line segments A and B are available. Segment A provides 60’ divergence of radiation incident on a volume element of the monochromator. Segment B alone reduces this divergence to 40’. Segments A and B combined provide 24’ divergence. Wave vector resolution is controlled by a variable beam aperture. It defines the envelope of the neutron beam. The fully open aperture subtends 4. 10 -3 Sr to the source. There are two degrees of freedom: Beam Width and beam height. The absorbing apertures are 10 cm thick and made from high density poly-ethylene surrounded by Hot Pressed B 4 C. Radiation hardened motors and resolvers are used. Image of the MACS doubly focusing monochromator mounted with mirrors for optical testing. The device contains 357 pieces of 1 o mosaic PG(002) for a total area of 1428 cm 2. Each of the 21 vertical aluminum blades rotates independently by individual stepping motors thus providing full flexibility for horizontal focusing. Vertical focusing is achieved by bending aluminum backing plates under compression. These plates are profiled so the front surface assumes a circular shape when bent. Top view of MACS. Horizontal monochromatic focusing requires that the monochromator is tangent to the dashed Rowland circle. The detection system consists of 20 identical channels separated by 8 o for a total of 152 o coverage in one setting. Each channel contains: (a) Cooled Be, BeO, or PG filter (b) 36’-60’-90’ or open collimation. (c) Double Crystal analyzer (see full description to the right) (d) Diffraction detector (e) Spectroscopic detector MACS flux on sample versus energy. the sample cross section was a rectangle 2 cm wide and 4 cm tall. The neutron flux for E 5 meV were taken without the Be filter and were λ/2 corrected. The solid lines are Monte Carlo Simulations. All beam line components from source to the sample position were simulated, excluding the filters. The performance of a vertically focusing double crystal analyzer was compared to a conventional analyzer on SPINS at NIST. Both the energy resolution and the overall transmission are comparable. The DXAL offers lower background and a smaller foot-print. Multiple Bragg scattering effects are more pronounced for the DXAL because neutron passage involves two reflections The sample positioning system contains no magnetic materials within 75 cm of the sample and can tilt sample environment systems weighing up to 400 kg. Shielding Helium Cold Source Shutter Cooled filters Radial Collimators Variable Aperture Focusing monochromator on translation stage Focusing monochromator on translation stage 6.2 m Focusing supermirror Aperture Attenuator Monitor Aperture Attenuator Monitor Sample position 40 channel detection system One of the 20 vertically focusing double crystal analyzers for the MACS detector system. A single stepping motor operates each device from below. The range of analyzer scattering angles is 40 o to 140 o. The PG(002) graphite has an average mosaic of 1.4 o. The area of a single vertically focusing blade is 108 cm 2. The total solid angle subtended by the detection system is approximately 0.15 Sr.