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A High Performance Hybrid Spectrometer for the Single Crystal Studies at the Pulsed SNS

A High Performance Hybrid Spectrometer for the Single Crystal Studies at the Pulsed SNS. Igor Zaliznyak. Outline. Scientific case, design objectives, and background for the proposed hybrid spectrometer for the SNS

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A High Performance Hybrid Spectrometer for the Single Crystal Studies at the Pulsed SNS

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  1. A High Performance Hybrid Spectrometer for the Single Crystal Studies at the Pulsed SNS Igor Zaliznyak Outline Scientific case, design objectives, and background for the proposed hybrid spectrometer for the SNS General layout of the proposed spectrometer and its place in the SNS instrument suite Comparison of different spectrometer concepts and evolution of our thinking Analysis of the instrument performance, current questions, plans

  2. Scientific case for a proposed instrument for the SNS Neutron spectrometer for studies of the coherent low-energy states in single crystals. • Coherent collective excitations in single crystals: • lattice dynamics (phonons) • spin dynamics (magnons, critical scattering) • Structure and dynamics of partially ordered and glassy states • spin glasses • charge glasses • correlated amorphous phases • Study of the microscopic physical properties of samples in a variety of extreme environments: • magnetic field • pressure • temperature • Characterization of spin-dependent cross-sections by means of polarization analysis Neutron Scattering Group

  3. What are the typical samples we want to study? CuGeO3 sample used by M. Arai group for detailed measurement of the excitation dispersion on MAPS in 2000 CuGeO3 sample used by L.-P. Regnault for the original measurement in 1993 Neutron Scattering Group

  4. General requirements for a single crystal neutron spectrometer • Transmission of both primary (monochromator) and secondary (analyzer) spectrometers should be close to 1 within the resolution acceptance range, and vary smoothly over a substantial energy interval, typically from 2.5 meV to 60 meV. • Both spectral (energy resolution) and angular (~ wavevector resolution) acceptances of the monochromator and analyzer should be flexible and easily adjustable, typical resolutions are 1% to 5%. • Scattering volume seen by a detector should be well defined and easily adjustable depending on the sample size to minimize the background. • Efficient use of the large incident neutron beam by focusing it on the sample is very important, and should be previewed. Neutron Scattering Group

  5. What are our main design goals? Aiming at great achievements, “step on the shoulders of giants” : 40+ years of experience in 3-axis spectroscopy, honored by the Nobel prize • Use most of the monochromatic neutron intensity produced by the source, while providing the highest possible signal-to-background ratio in the detector for good and moderate energy and wavevector resolution • Use the pulsed time structure of the neutron beam at the SNS • Avoid the direct view of the moderator and the source by the sample • Make solid angle accepted by the secondary spectrometer as large as necessary • Make range of the scattering angles accessible to the secondary spectrometer as large as possible • Preview an easy setup of the polarized beam option • Trade-off of the resolution for intensity should not degrade the instrument performance Neutron Scattering Group

  6. How does the instrument we propose fit SNS inelastic instruments suite? • High energy transfer • 10-1000 meV Fermi Chopper Spectrometer • E = 10 - 1000 meV • Q = 0.1 – 22 Å-1 • High intensity at moderate resolution and medium energy transfer + polarized beam • Crystal Monochromator Hybrid Spectrometer • E = 2.5 - 60 meV • Q = 0.1 – 8 Å-1 • High resolution and low energy transfer • 10-100 meV Multichopper Spectrometer • E = 2 - 20 meV • Q = 0.1 - 4 Å-1 Neutron Scattering Group

  7. FOCUS, hybrid TOF direct deometry spectrometer at SINQ@PSI. “Off-the-shelf” system? Monotonous limited brains converge. Layout of the SINQ experimental hall FOCUS spectrometer Neutron Scattering Group

  8. FOCUS concept: mode of operation and technical details. Concept so attractive, they even chop a CONTINUOUS beam! Neutron Scattering Group

  9. FOCUS technical characteristics and first results. S. Janssen et al. Physica B 283 (2000) Monochromator Bragg angle Ei [meV] Reselast [meV] Qmax[Å] PG002 17.5 20.0 1.00 5.6 PG002 36.6 5.1 0.14 2.8 PG002 63.4 2.3 0.04 1.9 PG004 36.6 20.0 0.50 5.6 Mica 002 65.4 0.3 0.01 0.6 Neutron Scattering Group

  10. So what is the difference? We will do better! • FOCUS : • large segmented monochromator with independently variable vertical and non-monochromatic(!) horizontal focusing • designed for cold neutrons, PG crystals with rather large isotropic mosaic ~ 50’ + mica • long, 3 m guide-to-monochromator and 1.5 m - 3 m monochromator-to-sample distances • 2.5 m secondary flightpath • Hybrid Spectrometer for the SNS (HYSPEC?): • monochromator immediately at the end of the guide, <1.8 m from the sample • thick PG crystals with anisotropic mosaic • Heusler crystals for polarized beam • guide tube before sample • 4-5 m secondary flightpath Most important: pulsed beam allows using 1/4 to >1/2 of the total amount of neutrons produced => gain factor ~100 Neutron Scattering Group

  11. Choice of the moderator “Benchmark” the instrument performance at Ei = 15 meV Ei = 10 meV Ei = 20 meV Intensity Intensity decoupled H2 decoupled H2 coupled H2 coupled H2 t t Figure of merit is the integral flux within 30-50 s time window, defined by the length of the secondary flight-path Coupled supercritical H2 moderator Neutron Scattering Group

  12. Schematics of the proposed primary spectrometer. Neutron Scattering Group

  13. Schematic layout of the proposed Hybrid Spectrometer Tentative placement: beamline 14. • Monochromator shielding + beam stop • Guide and analyzer shieldings Important but not shown: Neutron Scattering Group

  14. One of the possibilities for T2 chopper: counter-rotating “drum” pair. The closer is T2 chopper to the sample, the better is analyzer resolution. Could we put it after the monochromator? Advantages: • Light and compact. • No gyroscopic forces from horizontal displacement. • No restriction on beam height. • Fast to speed up, slow down, re-phase, similar to modern Fermi choppers. Problems: • High rotation rate, up to 900 Hz is necessary for 50s pulse. • Nested vertical magnetic bearings. Neutron Scattering Group

  15. How does the instrument we propose fit SNS floor layout? Needs rather short, 15 to 25 m primary flight path, but large, ~ 6-8 m radius floor area for the moving analyzer bank and the sample table. Beamline 14? • Use the space following a shorter instrument? • Occupy a “tangential” position with no other instruments on one side? Neutron Scattering Group

  16. Pro’s and Con’s of a Crystal Monochromator • Advantages of a crystal monochromator • beam may be compressed on the sample using focusing Bragg optics, beam height is not limited • sample and its environment are exposed to reflected monochromatic beam only: reduced background • works for wide beams • resolution is easily adjusted by putting the collimators • (Heusler) may provide a polarized beam for energies up to ~60 meV • Disadvantages of a crystal monochromator • lower than one and energy-dependent reflectivity of the crystal, is not well-suited for providing high-energy incident neutrons • energy and wavevector resolutions are coupled and depend on the instrument position • sample table and secondary spectrometer need to be moved when incident neutrons energy is changed Neutron Scattering Group

  17. Secondary spectrometer (TOF analyzer) and its operation in the polarized beam mode. All scattering angles of interest in the polarized mode are filled in by appropriately moving the detector bank! • Ei=5-60 meV incident polarized beam is produced by reflection from VF Heusler crystal monochromator • Set of 5-10 bender-type polarizers provide for the polarization analysis for Ef<15 meV. • Scattering from the BULKY SAMPLE ENVIRONMENT (aluminium, etc.) is outside analyzer acceptance. Neutron Scattering Group

  18. Secondary flight path and analyzer performance Uncertainty of the flight time in the analyzer gives largest contribution to the energy resolution. LSDt/t E/E Ef=5.0 meV 4 m 0.0098 2.0% 5 m 0.0078 1.6% Ef=14.7 meV 4 m 0.0168 3.3% 5 m 0.0134 2.6% Ef=60.0 meV 4 m 0.0339 6.8% 5 m 0.0271 5.4% Analyzer resolution for the length of the secondary flight path LSD = 4 m and LSD = 5 m and for the t=40 s burst width at the sample (FWHM) kf/ki=0.5 60° - 90° coverage of the scattering angle by the detector array gives simultaneous access to large enough interval in Q for 0.5<kf/ki<1 kf/ki=1.0 Moving the analyzer is cost-effective! Neutron Scattering Group

  19. Performance of the guide. Impact of the guide curvature (relative to the similar straight guide) Impact of the guide coating (relative to 3qc supermirror guide) • L = 20 m curved 3qc supermirror guide at a reasonable offset of 8 cm ~ 2 times the width of the guide provides ~75% transmission at 60 meV • Shorter (L = 15 m?) but narrower (3 cm?) guide may still be O.K. Neutron Scattering Group

  20. Evolution of our concept: from multi-chopper TOF to hybrid spectrometer We choose to optimize the instrument performance at Ei = 15 meV Optimized layout of the direct geometry disc chopper spectrometer: curved guide loss ~6%, tapered section gain <1.4 Optimized layout of the direct geometry hybrid spectrometer with vertically focused crystal monochromator: expected gain from vertically focusing the beam >~ 2 Neutron Scattering Group

  21. Comparative layout of the vertically focused crystal and conventional disk-chopper TOF primary spectrometers. The side view of the guide cross-section is shown schematically. Neutron Scattering Group

  22. Neutron intensity for 2 cm tall sample obtained from MC simulation using NISP package. Focusing monochromator yields a gain of factor 2 or more for Ei>15meV! • NISP algorithm seems to under-estimate gain from focusing => actual gain may be even bigger! • Decrease in intensity for hybrid spectrometer at Ei<10 meV is due to the improved energy resolution. Neutron Scattering Group

  23. Hybrid spectrometer: where the gain comes from? Taller guide! Relative performance of the hybrid instrument at lower energies will improve further if more realistic m=2 guide coating is implied. Neutron Scattering Group

  24. Hybrid spectrometer: some more MC results. High-E tail is nicely cut by the monochromator. Especially PG(004). Spectral distribution of neutrons incident on the sample Time profile of the neutron pulse on the sample * Although it nicely illustrates the trend, this preliminary calculation contained an error, which underestimated the total intensity for the hybrid model. Neutron Scattering Group

  25. Summary of the technical features of a proposed direct geometry hybrid instrument for the SNS Tall neutron guide + efficient vertical focusing provide reasonable data collection rates even for very small samples ! • Spectrometer is optimized for • energy resolution dE/E~0.03-0.05 • wavevector resolution dQ/Q~0.01-0.03 • High flexibility • continuous variation of the incident energy from 2.5 to 60 meV with no higher-order contamination • full range of energy transfers available at each Q • easily and broadly variable energy and Q resolution • possibility of time focusing/defocusing • easily adaptable for polarization analysis • Very low background • sample is 1.8 m away from the direct beam • scattering volume is well defined by collimators • High sensitivity to small scattering cross-sections (signal to BG ratio) • Can be operated at small scattering angles (near the forward direction) • User friendly: allows for easy on-line data monitoring and analysis and experiment planning Neutron Scattering Group

  26. Principal features of the proposed hybrid spectrometer. Bonus: monochromator shapes resolution function, cutting ugly high-E tail. • High flux on sample at Ei = 5 - 60 meV: tall neutron guide + efficient vertical focusing by curved crystal monochromator. • Polarized beam option: polarized incident neutron beam for Ei = 5 - 60 meV, polarization analysis of the scattered beam for Ef < 15 meV. • Low background even with bulky sample environments: collimator(s)+slit(s) define scattering volume seen by detector(s) and restrict analyzer acceptance to scattering from sample only. Scattering from cryogenic environment, magnet, pressure cell, etc., is rejected. • Continuous wavevector coverage: no “blind spots” caused by spaces in the detector array. • Flexibility: both energy and wavevector resolutions are variable and easily adjustable, typical resolutions are 1% to 5%. In addition, different crystal reflections may be user on monochromator. Neutron Scattering Group

  27. Punchline: current questions and future plans. • Questions • Placement: beamline 14? Is there enough floor space for the instrument large footprint? • Main beam stop: can it be incorporated in the monochromator shielding? How significant background source would it be? • Beamline shielding: how thick is enough and what restrictions result? • T2 chopper: can it be mobile and placed after the monochromator? • Plans • Workshop on the proposed Hybrid Spectrometer at BNL, October 12-13. • Assemble Instrument Development Team and submit formal Letter of Intent to the SNS. • Put together a comprehensive proposal and search for funding. • Find answers to the above questions. • Work on further optimization of the instrument setup. Neutron Scattering Group

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