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CERN, 12/5/09

Neutron Optics. Optics reflection refraction diffraction polarization Neutron Instruments source transport focusing, divergence wavelength encoding polarization encoding. CERN, 12/5/09 . Ken Andersen. Neutrons vs Light. Neutron scattering. 10 barns. 1 barn.

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CERN, 12/5/09

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  1. Neutron Optics • Optics • reflection • refraction • diffraction • polarization • Neutron Instruments • source • transport • focusing, divergence • wavelength encoding • polarization encoding CERN, 12/5/09 Ken Andersen

  2. Neutrons vs Light

  3. Neutron scattering 10 barns 1 barn

  4. About 10 neutron facilities worldwide Fission (continuous) Spallation (pulsed) User Facilities ILL: 40 instruments 700 experiments/year mainly solid-state physics, but also fundamental physics, chemistry, biology Neutron Sources ILL ISIS

  5. ILL & ESRF in Grenoble

  6. Neutron Moderators 2.5m

  7. Neutron Moderators

  8. Source Optics 200cm 20cm 5cm 30cm reflective internal surfaces Max angle ≈ 1°

  9. Reflecting Surfaces incident reflected n=1 critical angle of total reflection θc θ refracted n’<1 for natural Ni, θc = λ[Å]0.1

  10. l c l 1 l 2 l 3 l 4 } d1 } d2 } d3 } d4 Increasing the Critical Angle Interface reflection: θc(Ni) = λ[Å]0.1 Equivalent Bragg diffraction: λ = 2dsinθ  d = λ/2θ = 200 Å

  11. An Fe/Si multilayer

  12. Multilayers with up to several 1000s of layers feasible by magnetron sputtering 4×θNi is commercially available Layer thicknesses > 20 Å Interlayer roughness < 3 Å limited by roughness of substrate (1-8 Å) < 0.5 m2 deposition in one batch Technology transfer from neutron labs to industry Neutron guides up to >100m Neutron Supermirrors

  13. Instrument Example: Powder Diffraction λ = 2dsinθ d = λ/2sinθ Structure Determination

  14. Monochromating by time-of-flight Choppers distance 300 Hz ~μs burst time Δλ/λ≈ 1% time

  15. Monochromating by time-of-flight Velocity selector Δλ/λ≈ 10%

  16. h <hkl> 2h Single-crystal Monochromators Bragg’s law: λ = 2dsinθ fwhm < 10-4° fwhm > 0.1 ° Dl/l = cotqBDq Mosaic crystal Dl/l → 0 ! Perfect crystal

  17. Single-crystal Monochromators

  18. Focusing samples < 1 cm2 guide ~ 100 cm2

  19. Focusing Devices Crystal monochromators Supermirror optics Kirkpatrick-Baez mirrors Copper 200 Graphite 002 Focusing guides

  20. A θB B 2 Monochromator Focusing

  21. A θB B 2 Monochromator Focusing θA

  22. Liouville’s theorem: phase-space density is constant Increase in spatial density implies a reduction in angular density worse resolution Limitations of focusing

  23. Liouville’s theorem: phase-space density is constant Increase in spatial density implies a reduction in angular density worse resolution Source brightness ~ 1014 n/cm2/ster/s ΔΩ≈ 10-3 ster Δλ/λ ≈ 1% Flux impinging on sample < 108 n/s strongly limited by Poisson statistics Sometimes S/N can be more important Limitations of focusing

  24. Magnetism neutron magnetic moment interacts with that of unpaired electrons magnetic scattering depends strongly on relative orientation of neutron spin, electron spin and momentum transfer unambiguous separation of magnetic and nuclear scattering Precession techniques polarization vector precesses around field direction frequency ~ B phase measurement gives time spent in field  neutron speed   Polarization Optics B “Spin-up” (+) +½ħ -½ħ “Spin-down” (-)

  25. B Polarizing Supermirrors B=0 B • with Si Fe Si B

  26. Polarizing Crystals Cu2MnAl (Heusler) crystal ILL is the only producer

  27. MEOP Metastability Exchange Optical Pumping Buffer 2,5l Polarised 3He Cells Hydraulic piston 5.2 liter compressor B0 Discharge Optical polarimeter OPC OPC Mirrors Optical pumping Cells Capillary Yb fiber laser Purifier 3He bottle Polarized-3He Spin-filters

  28. 2008 2002 Polarized-3He Spin-filters where O(λ)= 7.28×10-2×P[bar] ×t[cm] ×λ[Å]

  29. 3He Polarization < 80% World leaders Supplying 3He filling stations, spin-filter cells & magnetic-field environments to neutron labs in UK, Australia, Germany, Taiwan, USA Applications: magnetic structures in single crystals magnetic domain structures in thin films disorder in frustrated magnetic systems magnetic excitations in high-Tc superconductors Medical applications functional lung imaging (MRI) Polarized-3He Spin-filters

  30. User facilities experiments performed mainly by outside groups ILL: 200 days/yr, 40 instruments, 700 experiments/year, 1200 users/year mainly solid-state physics, but also fundamental physics, chemistry, biology Sources thermalized Maxwellian spectrum low brightness, large sources beam distribution by guides Monochromatization time-of-flight: velocity selectors, pulsing choppers crystal monochromators: Bragg formula, perfect crystals, mosaic crystals Focusing sample size vs source/guide size, resolution degradation crystal monochromators guides Polarization crystal monochromators supermirrors polarized 3He Summary

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