1. Conceptual Design: MACS is designed to maximize the efficiency for energy resolved surveys of Q-space in the sub-thermal energy range. Two innovations should lead to performance enhancements of up to two orders of magnitude over current instrumentation. Incident beam line principles: The NG0 port of the NIST center for neutron research where MACS will be situated, affords an unusually large 10 -2 Steradian view of the NIST cold source. MACS will take advantage of the large solid angle through a doubly focusing and monochromating Bragg lens. While this implies a greater angular divergence at the sample, the longer wave lengths employed by MACS ensures that Q-resolution remains comparable to a thermal triple axis machine with 60’ collimation. The combination of a bright cold source and a large divergence angle yields a flux on the sample position that exceeds 10 8 n/cm 2 /s at 0.2 meV energy resolution. On a conventional triple axis spectrometer, energy and wave vector resolution are coupled. Not on MACS, where energy resolution is controlled by radial collimators that define the active source area and wave vector resolution is controlled by the variable aperture before the monochromator. MACS –a New High Intensity Cold Neutron Spectrometer at NIST C. Broholm 1,2, P. C. Brand 2, C. Brocker 2, J. W. Lynn 2, R. Barkhouser 1, J. D. Orndorff 1, T. D. Pike 1,2, Y. Qiu 1, T. Reeves 1, G. Scharfstein 1, S. A. Smee 3 1 Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218 2 NIST Center for Neutron Research, Gaithersburg, MD 20899 3 Spallation Neutron Source, ORNL, Oak Ridge, TN 37830 Introduction: Inelastic neutron scattering is a unique probe of nano-scale dynamic phenomena in solids. Unfortunately, current instrumentation often limits applicability to cases where large crystalline samples can be produced. The Multi Axis Crystal Spectrometer (MACS) now under development at NIST, aims to broaden the range of materials that can be analyzed with this powerful technique. Two orders of magnitude improvement in efficiency is achieved by focusing cold neutrons with a Bragg lens and using a multiplexing detection system. Science: Dynamic short range order is important in many topical condensed matter systems. While MACS will be a general purpose spectrometer for energies less than 20 meV, it will be particularly well suited for probing dynamic nano-scale structure. In a matter of hours the instrument will deliver a map of the wave vector dependence of inelastic neutron scattering from which real space short range order can be extracted by Fourier inversion. Frustrated Magnets: The crystalline lattice can define a frustrating pattern of interactions between magnetic atoms that cannot be satisfied by any spin configuration. MACS will be particularly well suited for probing short range order resulting from frustration. Fig. 1 shows elastic diffuse scattering from geometrically frustrated, SrCr 9p Ga 12-9p O 19. While samples close to full concentration on the magnetic sub-lattice are of greatest interest, the experiment was on magnetically diluted materials (p=60%) where sizeable single crystals are available. MACS will help to probe the materials of greatest scientific interest even when only milligram samples are available. Colossal Magneto-resistance: The ferromagnetic transition in La 1-x Ca x MnO 3 is accompanied by a dramatic reduction in resistivity, which could be useful for magnetic sensing. The reason is strong spin-lattice-charge coupling that leads to polarons whose structure is revealed by anomalous diffuse scattering. MACS will be ideally suited for probing the structure of such coupled degrees of freedom. Quantum Impurities: In quantum systems with a macroscopic singlet ground state impurities can have counterintuitive and potentially useful properties. For example a hole in a spin-1 chain generates a complex spin polaron, the structure of which can be probed by inelastic magnetic neutron scattering. SrCr 9p Ga 12-9p O 19 Figure 1. Crystal structure of SrCr 9p Ga 12-9p O 19. Where geometrical frustration suppress long range magnetic order. Cr O Figure 3. Diffraction from lattice polarons in La 0.7 Ca 0.3 MnO 3 in the resistive phase. Figure 4. Temperature dependence of lattice and spin polaron scattering indicating that polaron localization is responsible for the T-dependent resistivity. Figure 2. Coherent diffuse elastic scattering from a geometrically frustrated magnet. Figure 5. Chain structure of a hole doped quantum spin liquid. Y 2-x Ca x BaNiO 5 Figure 6. Wave vector dependence of inelastic scattering that reveals the structure of a complex spin polaron surrounding holes in Y 2-x Ca x BaNiO 5. Figure 8. Flux on sample versus incident energy calculated using Monte Carlo Simulation (MCSTAS). Numbers for IN14 are from the ILL web page. Figure 9. Energy resolution versus incident energy for three collimation configurations. Detection System Principles: The conventional triple axis spectrometer has a single detection channel that can rotate around the sample to map inelastic scattering. MACS will have 21 channels in simultaneous operation to increase efficiency for surveys by more than an order of magnitude. With a different data acquisition protocol, MACS will be complementary to the advanced pulsed neutron instrumentation now being developed for the Spallation Neutron Source. The ability to probe excitation spectra over a wide range of energies at the SNS and to zoom in on specific energy ranges of interest on MACS will be a powerful combination that will help scientists and engineers to understand and design the materials that will fuel the technologies of the twenty first century. Details on Incident beam line: The reflecting surface of the monochromator is curved for vertical focusing. Focusing in the horizontal plane is accomplished by varying the projection of the monochromator surface normal so it bisects the angle formed by lines from the given point on the monochromator to the sample and source respectively. The distribution of scattering angles varies as the monochromator is rotated around a vertical axis. The distribution is minimized, as is the range of incident energies, when the monochromator is tangent to the Rowland circle shown on Fig. 11. The mean incident energy is varied by translating the monochromator along the beam tube and rotating the cylindrical drum and sample to intercept the reflected beam. A super-mirror guide with adjustable sides increases the angular size of the sample as viewed from the monochromator to match that of the cold source for a 20% intensity gain. The Double focusing monochromator: The MACS monochromator has been built and it has passed optical testing. The reflecting surface consists of 357 Pyrolytic Graphite (002) platelets with a total area of 1428 cm 2. These are mounted on 21 aluminum fingers that can rotate under stepping motor control about parallel vertical axes for horizontal focusing. The fingers can be bent so as to form the arc of a circle with variable radius for vertical focusing. Figure 12. Picture of the MACS doubly focusing monochromator mounted with mirrors for optical testing. Figure 13. Top view of the MACS 21 channel detection system. The emphasis in the design is high reliability and efficiency and ultra low background. Shielding thickness averages 33 cm of moderating and absorbing material. Figure 7. Isometric view of the MACS cold neutron spectrometer. Along the beam line are seen shutter, cooled Be, PG, and Al 2 O 3 filters, 60’ and 40’ radial collimators, variable aperture, monochromator, super-mirror guide, cryostat and detector system. Shutter Cryo filters Collimators Variable aperture Figure 11. top view of MACS. The detector system is shown in a single setting while the monochromator is indicated in three different positions. The dashed line is the Rowland Circle that goes through the neutron source, the monochromator and the sample. Horizontal monochromatic focusing requires that the monochromator is tangent to this circle. kiki kiki kfkf kfkf Q Q Figure 10. Sketch of the region of wave vector space that can be mapped using MACS at Ef=3.7 meV and 1 meV energy transfer. The ellipses show the areas probed by the 21 detection channels in one setting of sample and detectors. Details on MACS detection System: The 21 channels of the MACS detector are separated by 8 degrees. Each channel has a vertically focusing double bounce analyzer with 2 o by 8 o maximum. acceptance. There is also a “two-axis” detector in each channel providing high intensity diffraction and energy integrated data. Each channel will also have three cooled filter options (PG, Be, and BeO) to suppress higher order contamination and elastically scattered neutrons from the sample during inelastic experiments. There are also three collimation options to allow variation of detection system energy and wave vector resolution.