Neutron Op+cs. P. Courtois. Service for Neutron Op4cs. Cremlin May 2018 INSTITUT LAUE LANGEVIN

Transcription

1 Neutron Op+cs INSTITUT LAUE LANGEVIN P. Courtois Service for Neutron Op4cs Cremlin May 2018 INSTITUT MAX VON LAUE - PAUL LANGEVIN 1 THE EUROPEAN NEUTRON SOURCE

2 Neutron Optics Neutrons have wave-like properties Basic concepts and op+cal components Ø Reflec+ng Op+cs Mirrors & Super-Mirrors Ø Diffrac+ng Op+cs Mosaic crystals Ø Filters Crystals, polycrystalline materials Ø Polarizing Op+cs Super-Mirrors, crystals, 3 He spin filters Ø Some examples of applica+ons at I.L.L. INSTITUT MAX VON LAUE - PAUL LANGEVIN 2

3 Why do we need neutron optics Neutron Optics are key components for neutron instrumentation Mirror & Super-Mirror are used to construct efficient neutron guides Collimator determines incident and scarered neutron direc+ons - Angular resolu+on Filter selects out unwanted neutrons Crystal Monochromator/Bragg Mirror determines incident wavelengths (Energies) Crystal Analyzer determines scarered wavelengths (Energies) Wavelength resolu+on (Energy resolu+on) Crystal/ Super-Mirror/ 3 He spin filter are used to polarize neutron beams Orienta+on of the neutron spin Ø Instrument Performances are then determined by Neutron Op7cs! INSTITUT MAX VON LAUE - PAUL LANGEVIN 3

4 Neutron Mirrors Reflection/Refraction at Surfaces incident reflected Total reflec7on for θ < θ c θ Material refracted θ = λ c Nb (x /m 2 ) Nb π coh Nb coh = ScaRering Length Density N = atoms/cm 3 b coh = coherent sca<ering length θ c (mrad) 58 Nickel Nickel Iron Copper Silicon Aluminium reflectivity for natural Ni: θ c ( ) = 0.1 x λ (Å) λ = 5 Å θ [ ] INSTITUT MAX VON LAUE - PAUL LANGEVIN 4 θ c Ni mirror

5 Periodic Multilayers Reflection/Refraction at Surfaces θ substrate (glass, Si ) d Total reflec+on for θ < θ c + addi+onal Bragg peak at θ B λ / 2d Total Total reflec+on Bragg peak Neutron Reflec+vity R 4N 2 d 4 ( N 1 b 1 - N 2 b 2 ) 2 π 2 n 4 Wavelength band Nb coh = ScaRering Length Density N : number of bilayers θ c θ Β λ / 2d Δλ λ = 2d 2 N 1 b 1 N 2 b 2 π n : order of the reflec4on INSTITUT MAX VON LAUE - PAUL LANGEVIN 5

6 Super-Mirrors How to increase angular acceptance of mirrors θ substrate (glass, Si ) d 3 d 2 d 1 Sequence of bi-layers of variable thicknesses d Total reflec+on for θ < θ c + addi+onal Bragg peaks reflectivity regime of total reflec+on m - value regime of supermirror 0.1 λ = 5 Å θ [ ] significant increase in cri7cal angle (θ c = λ / 2d min ) m Super-Mirror Gain in neutron flux m = θ SM c Ni θ c G = θ c sm θ c Ni 2 m 2 INSTITUT MAX VON LAUE - PAUL LANGEVIN 6

7 Ni/Ti Super-Mirrors Properties Neutron Reflec7vity R (N 1 b 1 -N 2 b 2 ) 2 High contrast high reflec4vity! Ni Nb = 9.40 (10-6 Å -2 ) Ti Nb = (10-6 Å -2 ) m=4 Performances R > 80% for m = 4 Ni/Ti Ø Gain factor / Ni mirror = 16 (2D) Ø but transmission T R n!! eg : for a 100 m long guide, at least ten Reflec4vity profiles of Ni/Ti super-mirrors for 4 m 7 (sources : reflec+ons Transmission < 10%... INSTITUT MAX VON LAUE - PAUL LANGEVIN 7

8 Ni/Ti Super-Mirrors Deposition : Reactive DC Magnetron Sputtering Produc7on Ø m= 4 : 1600 layers! Ø Substrate : 0.5 cm thick Si wafers or 0.2 cm thick Glass/Si/Sapphire Ø Ø Ø Ø Applica7ons Neutron guides (previous presenta+on) Collima+on Filters (SANS instruments) Large Band monochromators (Laue Diffrac+on) Spu<ering machine (ILL) - Produc4on 0.8 m 2 / day INSTITUT MAX VON LAUE - PAUL LANGEVIN 8

9 m Super-Mirrors Long wavelength cut-off filters on D33 at I.L.L. (SANS instrument) to select out unwanted neutrons of wavelengths > λ c (3 available wavelength bands) m=1.1 SM NiV/Ti on 0.5 cm thick Si substrate mounted at an angle θ in a neutron guide White beam Borofloat glass m=0.65 θ λ c = mθ Ni c Θ = 2.2 reflected λ >λ C λ c = 20 Å Transmi<ed λ <λ C 0.84 m INSTITUT MAX VON LAUE - PAUL LANGEVIN 9

10 m Super-Mirrors Long wavelength cut-off filters on D33 at I.L.L. (SANS instrument) to select out unwanted neutrons of wavelengths > λ c (3 available wavelength bands) m=1.1 SM NiV/Ti on 0.5 cm thick Si substrate mounted at an angle θ in a neutron guide White beam λ = Borofloat glass m=0.65 c θ mθ Ni c Θ = 2.2 reflected λ >λ C Θ = 1.32 λ c = 20 Å Transmi<ed λ <λ C λ c = 12 Å 0.84 m INSTITUT MAX VON LAUE - PAUL LANGEVIN 10

11 m Super-Mirrors Long wavelength cut-off filters on D33 at I.L.L. (SANS instrument) to select out unwanted neutrons of wavelengths > λ c (3 available wavelength bands) m=1.1 SM NiV/Ti on 0.5 cm thick Si substrate mounted at an angle θ in a neutron guide White beam λ = Borofloat glass m=0.65 c θ mθ Ni c Θ = 2.2 reflected λ >λ C Θ = 1.32 λ c = 20 Å Transmi<ed λ <λ C λ c = 12 Å 0.84 m INSTITUT MAX VON LAUE - PAUL LANGEVIN 11

12 m Super-Mirrors Long wavelength cut-off filters on D33 at I.L.L. (SANS instrument) Neutron Flux at the sample posi4on on D33 λ c = 20 Å λ c = 12 Å no filter C. D. Dewhurst et al., J. Appl. Cryst. (2016). 49, 1-14 INSTITUT MAX VON LAUE - PAUL LANGEVIN 12

13 Bragg Mirrors Large wavelength band Monochromator on LADI (Laue diffractometer for Protein crystallography) Ø to produce a «monochroma7c» beam at 3.5 Å with a bandwidth Δλ/λ = 20% guide Θ L mono-sample = 2135 mm monochromator sample Stacked Multilayer monochromator Beam width = 20 mm θb = > d0 = 80 Å Δλ/λ = 20 % -> graded d-spacing Å Beam size >> Sample size (1 mm) -> Focusing device (fan geometry) I N S T I T U T mirror length = 25 mm thickness 0.5 mm 40 mirrors Ni/Ti Si substrate (low a<enua4on factor) M A X V O N L A U E - P A U L L A N G E V I N 13

14 Bragg Mirrors Large wavelength band Monochromator on LADI (Laue diffractometer for Protein crystallography) guide Θ monochromator monochromator neutrons TOF spectra on LADI direct beam reflected beam sample λ Å Δλ/λ 20% Reflec+vity 75% INSTITUT MAX VON LAUE - PAUL LANGEVIN 14

15 Polarizing Co/Ti Super-Mirrors Analyzer for wide angle spin echo spectrometer (WASP) Co/Ti m= 2.8 substrate substrate 0.2 cm thick Borated glass absorbing layer P Co/Ti SM R + R - C bender C bender - op7mized for λ = [4-12 Å] Neutrons propagate into air Teta ( ) Curvature R = 7m to avoid direct line of sight : at least on reflec+on! Double sided mirrors (h 141 x w 254 mm 2 ) INSTITUT MAX VON LAUE - PAUL LANGEVIN 15

16 Polarizing Co/Ti Super-Mirrors Analyzer for wide angle spin echo spectrometer (WASP) WASP spectrometer (coil diameter 6 m) 90 analyzer CoTi Super Mirror Analyzer 1 = 37 CoTi SMs INSTITUT MAX VON LAUE - PAUL LANGEVIN 16

17 Polarizing Fe/Si Super-Mirrors at I.L.L. S bender Polarizer for neutron reflectometer D17 Ø to produce a polarized cold neutron beam 200 µm Si channel Reflectivity P R + m=θ / θ c (Ni) m=3.8 Fe/Si SMs on 0.2 cm thick Si substrate Spin > transmired Stack of 60 blades (l 50 mm; h 160 mm; w 10 mm) B Polarizing supermirrors > 0.2 R Omega m = 3.8 Fe/Si R+ > 80% P neutrons > 95% (5.5 Å) T ~ 40% of the good neutrons neutrons INSTITUT MAX VON LAUE - PAUL LANGEVIN 17

18 Mosaic Crystals Bragg Diffraction Monochromators & Analyzers Use of single crystals to select a given wavelength band according to the Bragg s Law polychromatic beam crystal λ 0 monochromatic beam 2 d hkl sinθ Β = nλ 0 d= distance of the lasce (hkl) planes θ Β = Bragg angle The rela+ve bandwidth Δλ/λ is given for Gaussian distribu+ons by Δλ/λ = (α 2 + β 2 ) 1/2 cotθ B α : divergence of the primary beam β : full width at half maximum of the neutron rocking curve or neutron mosaic spread If α ~β, the resolu+on and intensity are said to be op+mised Ø Mosaic Crystal, i.e. crystal with defects such as disloca+ons, must be used to match the divergence α of primary beam which is typically n/s INSTITUT MAX VON LAUE - PAUL LANGEVIN 18 β θ

19 Mosaic Crystals Good Materials should have -W 2 3 q High neutron reflec.vity sca<ering power Q Q = (Fhkle /V) λ /sin2θ B Ø Large structure factor F hkl Ø High coherent scarering length b coh (F hkl b coh ) Ø Small unit cell Volume & compact structures = Cubic, Diamond Ø d-spacing op+mized for a given wavelength (d ~ λ ) q low incoherent scarering length low background q low absorp+on cross sec+ons small a<enua4on q No higher orders Ø Availability of large single crystals with suitable width and uniform mosaic block distribu4on! INSTITUT MAX VON LAUE - PAUL LANGEVIN 19

20 Mosaic crystals for neutron monochromators Crystal orienta7on Crystal Mosaic Neutron Energy Applica7on Supplier HOPG (C) d 002 = 3.35 Å HOPG(002) Cold Thermal High flux Panasonic, Op4graph, Momen4ve Cu d 111 = 2.08 Å (111) (220) (200) Hot Thermal High resolu+on or high flux I.L.L. (or?) Si d 111 = 3.13 Å (111) (113) Perfect Cold Thermal High resolu+on many! Ge d 111 = 3.26 Å (111) (113) < 0.4 Cold Thermal High resolu+on I.L.L. (or?) KC 8 d 002 = 5.3 Å Heusler Cu 2 MnAl (002) 2-5 Cold High flux I.L.L. (111) Hot Thermal Polarized neutrons I.L.L. INSTITUT MAX VON LAUE - PAUL LANGEVIN 20

21 Mosaic Crystals Control of the mosaic distribution by plastic deformation (I.L.L.) Perfect crystal Ge111 P = 1 MPa FWHM=0.4 Peak reflec+vity at λ = 3.4 Å R exp = 50-55% (FWHM=0.4 ) Hea4ng element T = 800 C R exp 70 % of R th crystal Anvil as-grown crystal Cu200 P,T Cu200 FWHM=0.15 Peak reflec+vity at λ = 1.1 Å R exp = 40-45% (FWHM=0.3 ) FWHM=0.45 FWHM=1 R exp % of R th INSTITUT MAX VON LAUE - PAUL LANGEVIN 21

22 Mosaic Crystals Bent perfect Silicon crystals (I.L.L.) Stack of thin wafers to allow bending Perfect Si crystal wafer thickness = 1 mm 10 wafers to get t = 10 mm (or more) Curvature : flat to RH 2 m θβ R <hkl> Effec+ve mosaic δ (rad) t = total crystal thickness R = radius of curvature θb = Bragg angle neutrons δ = cot(θb) t / R I N S T I T U T M A X V O N L A U E - P A U L L A N G E V I N 22

23 Monochromators HOPG D19 Large effective area to cover the direct beam Assembly of mosaic crystals FWHM ~ beam divergence 0.2 < FWHM < 0.6 Crystal size ~ Sample size (1 to 10 cm 2 ) Crystals fixed onto specific mechanics 10 B 4 C is used to reduce background and activation Orientation accuracy ± 0.03 crystal B 4 C Al Vertical Focusing Horizontal Focusing (Triple axis spectrometers) crystal + support INSTITUT MAX VON LAUE - PAUL LANGEVIN 23

24 Focusing devices Divergent beam flux ~ n /cm 2 /s lost neutrons! Neutron source (guide, beam tube) α= 2 θ c Beam size S ~100 cm 2 Sample S ~1 cm 2 Ø Focusing devices are used to increase the neutron flux at the sample posi+on. However, the increase of neutron flux implies a degrada+on of the angular resolu+on. (Liouville s theorem!) H H 2 1 Divergence α H 1 α 1 = H 2 α α INSTITUT MAX VON LAUE - PAUL LANGEVIN 24

25 Focusing Devices Principles Ver7cal Focusing = + 1 R v 2sinθ L 1 L 2 Source θ R v Image size ~ crystal size (height) Gain in flux compression factor Ø Increase of ver7cal angular divergence Ø R v λ Horizontal Focusing 1 = sinθ R H 2 L 1 L 2 Horizontal focusing affects Q resolu7on R h 1/λ Source L 1 θ L 1 L 2 R H L 2 Sample Sample Sample INSTITUT MAX VON LAUE - PAUL LANGEVIN 25

26 Focusing Devices Monochromators for neutron Diffractometers Production of a fixed neutron wavelength at a given take-off angle D20 Ge monochromator is well suited for high resolu7on neutron diffractometers D20 (high intensity difffratometer with variable resolu+on) - D2B & D1B (powder diffractometers) Large dimensions Heigth 300 mm Width mm no λ/2 contamina7on (odd reflec7ons) D2B Ge (335) - λ = 1.6 Å crystal mosaic = 0.2 High Resolu4on D20 Ge (111)- λ = 3.15 Å crystal mosaic = 0.4 High Resolu4on Fixed ver+cal focusing R v = 2 L MS sinθ B D20 : R v = 3200 mm D2B : R v = 4600 mm INSTITUT MAX VON LAUE - PAUL LANGEVIN 26

27 Focusing Devices Monochromators for neutron Diffractometers D19 Single crystal diffractometer HOPG (002) - λ = 2.4 Å crystal mosaic = 0.5 High Flux D10 Single crystal diffractometer Cu(200) - λ = 1.2 Å crystal mosaic = 0.2 High Resolu4on HOPG monochromator Provides high neutron flux (thermal & cold D20 neutrons) λ/2 contamina+on (002 reflec+ons) Ø Use of HOPG Filter! Fixed ver+cal focusing Cu monochromator provides high neutron flux or high resolu+on (hot & thermal neutrons) λ/2 contamina+on Ø use of HOPG Filter! Ø thermal neutron guide cut-off INSTITUT MAX VON LAUE - PAUL LANGEVIN 27

28 Focusing Devices Monochromators for Triple Axis Spectrometers Ø Optimization of instrument performances for a wide energy range Ø Variable horizontal and vertical curvature Ø Focusing monochromator is used in combination with focusing guide (virtual source) see previous presentation from J. Kulda! Cremlin May 2018 THE EUROPEAN NEUTRON SOURCE 28

29 Focusing Devices Monochromators for Triple Axis Spectrometers D20 HOPG(002) monochromator crystal mosaic = 0.5 cold neutrons Thales IN8C IN1 Cu(200) monochromator crystal mosaic = 0.3 High Flux Thermal neutrons Si(113) monochromator bent perfect crystals Hot neutrons INSTITUT MAX VON LAUE - PAUL LANGEVIN 29

30 Monochromator for polarized neutrons Heusler Cu 2 MnAl mosaic crystals Fixed Ver+cal Curvature Variable Horizontal Curvature (111) reflec+on F 111N = -F 111M Mosaic 0.2 < fwhm < 0.6 Reflec+vity R exp R theor Polariza+on P > 92 % IN20 (Thermal neutron 3 axis spectrometer) THALES (3-Axis for low energy spectroscopy) INSTITUT MAX VON LAUE - PAUL LANGEVIN 30

31 Neutron Filters Neutron filters are used to select out unwanted neutrons Cut-off T ~ 80% θ T ~ 5% Polycristalline Beryllium at 77K is used to select out neutrons of λ< 4 Å (Cut-off at λ = 2d Be max ) n λ < λ cut Perfect Saphirre crystal is used to select out fast neutrons of λ < 0.5 Å Ø Reduc+on of background λ > λ cut INSTITUT MAX VON LAUE - PAUL LANGEVIN 31

32 Neutron Filters Neutron filters are used to select out unwanted neutrons transmission θ HOPG filter, 4 cm thick λ= 0.8 Å λ= 1.2 Å λ= 2.4 Å HOPG crystal (Highly Oriented Pyroli7c Graphite) is commonly used for elimina+ng higher-order c o n t a m i n a + o n o f a monochromated neutron beam PG filter has strong arenua+on at 1.2 Å but passes 2.4 Å λ 0 = 2.4 Å + λ 0 /2 = 1.2 Å Wavelength λ (Å) λ 0 / 2 λ 0 = 2.4 Å T > 80 % at 2.4 Å T < 1% at 1.2 Å INSTITUT MAX VON LAUE - PAUL LANGEVIN 32

33 Neutron Filters Polarized Neutrons 3 He spin filters at I.L.L. Absorp7on cross sec7on of 3 Helium nuclei Ø If the nuclear spin of He and the neutron spin are parallel, σ a 0 Ø If the nuclear spin of He and the neutron spin are an+-parallel, σ a 6000 barns For fully polarized 3 He (P He = 1), one spin state goes through the filter with zero absorp+on. The other spin state is almost fully absorbed since σ = 6000 barns polarized neutron beam P B 3 He spin filter cell INSTITUT MAX VON LAUE - PAUL LANGEVIN 33

34 Neutron Filters Polarized Neutrons 3 He spin filters at I.L.L. Ø Polariza+on of neutron beams (hot, thermal, cold) Ø 3 He spin filters provide an unique tool to perform XYZ polariza+on analysis Polariza+on of 3 He: P(t)=exp(-t/T 1 ) and 750 T d [h] = Tw h P[b] T m [h] = P[b] " B / r[cm] % $ ' 7000 # B & To perform polarized neutrons experiments : T 1 > 100h Ø High quality 3 He spin filters (Polariza+on, T w ) Ø Homogeneous magne+c field 1 = T 1 T d T w T m 2 INSTITUT MAX VON LAUE - PAUL LANGEVIN 34

35 Production of 3 He spin filters at I.L.L. Metastability Exchange Optical Pumping (MEOP) 3 He filling sta4on at I.L.L. Produc+on rate > 1 bar-litre/h Final pressure up to 4 bar Polariza+on of 3 He 80 % Cremlin May 2018 Development of 3 He cells Banana shaped Coverage angle up to 125 Si windows : Reduc+on of background Cs coa+ng T w > 200 h THE EUROPEAN NEUTRON SOURCE 35

36 Conclusion Neutron Op7cs define beam proper7es - Direc+on, Divergence, Wavelength, Energy, Polariza+on - Angular Resolu+on, Wavelength resolu+on, Energy resolu+on Ver+cal focusing devices allow the op+miza+on of the neutron flux at the sample posi+on Double variable focusing devices allow the op+miza+on of instrument performances for a wide energy range Ø Since the power of the source is low, neutron op7cal components must be of high quality and properly designed Ø Neutron Op+cs obey to Liouville s Theorem : It costs flux to increase resolu+on and it costs resolu+on to increase flux The op7miza7on of instrument performances is always a compromise between flux and resolu7on THE EUROPEAN NEUTRON SOURCE 36

37 Thank You for your aren+on Cremlin May 2018 THE EUROPEAN NEUTRON SOURCE 37