1. INSTITUT MAX VON LAUE - PAUL LANGEVIN CERN, 12/5/09 Ken Andersen Neutron Optics Optics –reflection –refraction –diffraction –polarization Neutron Instruments –source –transport –focusing, divergence –wavelength encoding –polarization encoding

2. INSTITUT MAX VON LAUE - PAUL LANGEVIN lightneutrons λ< μm< nm E> eV> meV n1→41→4 0.9997→1.0001 θcθc 90°1°1° Φ/ΔΩ 10 19 p/cm 2 /ster/s (60W lightbulb) 10 14 n/cm 2 /ster/s (60MW reactor) Pleft-rightup-down spin1½ interaction electromagneticstrong force, magnetic charge00 Neutrons vs Light

3. INSTITUT MAX VON LAUE - PAUL LANGEVIN Neutron scattering 10 barns 1 barn

4. INSTITUT MAX VON LAUE - PAUL LANGEVIN 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 ILL ISIS Neutron Sources

5. INSTITUT MAX VON LAUE - PAUL LANGEVIN ILL & ESRF in Grenoble

6. INSTITUT MAX VON LAUE - PAUL LANGEVIN 2.5m coldthermalhot moderatorliquid D 2, H 2, CH 4 liquid H 2 O graphite moderator temperature 15–25K300K2000K neutron wavelength 3→20Å1→3Å1→3Å0.3→1Å sample lengthscale 1Å→100 nm 0.3→5Å0.1→2Å sample timescale 1kHz→1 THz 0.1→10 THz 1→100 THz Neutron Moderators

7. INSTITUT MAX VON LAUE - PAUL LANGEVIN Neutron Moderators

8. INSTITUT MAX VON LAUE - PAUL LANGEVIN

9. 20cm 5cm 30cm 200cm reflective internal surfaces Max angle ≈ 1° Source Optics

10. INSTITUT MAX VON LAUE - PAUL LANGEVIN Reflecting Surfaces n=1 n’<1 incident refracted reflected θ critical angle of total reflection θ c for natural Ni, θ c = λ[Å]  0.1 

11. INSTITUT MAX VON LAUE - PAUL LANGEVIN Increasing the Critical Angle d1d1 }     cc d2d2 } d3d3 } d4d4 } Interface reflection: θ c (Ni) = λ[Å]  0.1  Equivalent Bragg diffraction: λ = 2dsinθ  d = λ/2θ = 200 Å

12. INSTITUT MAX VON LAUE - PAUL LANGEVIN An Fe/Si multilayer

13. INSTITUT MAX VON LAUE - PAUL LANGEVIN Neutron Supermirrors Multilayers with up to several 1000s of layers feasible by magnetron sputtering 4×θ Ni is commercially available Layer thicknesses > 20 Å Interlayer roughness < 3 Å –limited by roughness of substrate (1-8 Å) < 0.5 m 2 deposition in one batch Technology transfer from neutron labs to industry Neutron guides up to >100m

14. INSTITUT MAX VON LAUE - PAUL LANGEVIN

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

16. INSTITUT MAX VON LAUE - PAUL LANGEVIN Monochromating by time-of-flight 300 Hz ~μs burst time distance time Choppers Δ λ/λ ≈ 1%

17. INSTITUT MAX VON LAUE - PAUL LANGEVIN Monochromating by time-of-flight Velocity selector Δ λ/λ ≈ 10%

18. INSTITUT MAX VON LAUE - PAUL LANGEVIN Single-crystal Monochromators fwhm < 10 -4 °  →  Perfect crystal  cot  B  Mosaic crystal fwhm > 0.1 °   Bragg’s law: λ = 2dsinθ

19. INSTITUT MAX VON LAUE - PAUL LANGEVIN Single-crystal Monochromators d-spacing Germanium 333 1.089 Å Copper 111 2.087 Å Silicon 111 3.135 Å Graphite 002 3.355 Å stage 1 K- intercalated graphite 002 5.35 Å stage 2 K- intercalated graphite 002 8.74 Å

20. INSTITUT MAX VON LAUE - PAUL LANGEVIN Focusing guide ~ 100 cm 2 samples < 1 cm 2

21. INSTITUT MAX VON LAUE - PAUL LANGEVIN Focusing Devices Crystal monochromators Copper 200 Graphite 002 Supermirror optics Kirkpatrick-Baez mirrors Focusing guides

22. INSTITUT MAX VON LAUE - PAUL LANGEVIN θBθB A B  Monochromator Focusing

23. INSTITUT MAX VON LAUE - PAUL LANGEVIN θAθA Monochromator Focusing θBθB A B 

24. INSTITUT MAX VON LAUE - PAUL LANGEVIN Limitations of focusing Liouville’s theorem: phase-space density is constant –Increase in spatial density implies a reduction in angular density –worse resolution

25. INSTITUT MAX VON LAUE - PAUL LANGEVIN Limitations of focusing Liouville’s theorem: phase-space density is constant –Increase in spatial density implies a reduction in angular density –worse resolution Source brightness ~ 10 14 n/cm 2 /ster/s –ΔΩ ≈ 10 -3 ster –Δλ/λ ≈ 1% Flux impinging on sample < 10 8 n/s –strongly limited by Poisson statistics Sometimes S/N can be more important

26. INSTITUT MAX VON LAUE - PAUL LANGEVIN Polarization Optics 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 +½ħ -½ħ “Spin-up” (+) “Spin-down” (-) B  

27. INSTITUT MAX VON LAUE - PAUL LANGEVIN Polarizing Supermirrors B Fe Si with B B B=0

28. INSTITUT MAX VON LAUE - PAUL LANGEVIN Cu 2 MnAl (Heusler) crystal Polarizing Crystals ILL is the only producer

29. INSTITUT MAX VON LAUE - PAUL LANGEVIN MEOP Metastability Exchange Optical Pumping B0B0 OPC 3He bottle Purifier Capillary Optical pumping Cells Yb fiber laser OPC Buffer 2,5l 5.2 liter compressor Hydraulic piston Polarised 3He Cells Optical polarimeter Mirror s Discharge Polarized- 3 He Spin-filters

30. INSTITUT MAX VON LAUE - PAUL LANGEVIN Polarized- 3 He Spin-filters where O(λ) = 7.28×10 -2 ×P[bar] ×t[cm] ×λ[Å] 2002 2008

31. INSTITUT MAX VON LAUE - PAUL LANGEVIN Polarized- 3 He Spin-filters 3 He Polarization < 80% World leaders –Supplying 3 He 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-T c superconductors Medical applications –functional lung imaging (MRI)

32. INSTITUT MAX VON LAUE - PAUL LANGEVIN 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 3 He Summary