Neutron Instruments I & II. Ken Andersen ESS Instruments Division

Transcription

1 Neutron Instruments I & II ESS Instruments Division

2 Neutron Instruments I & II Overview of source characteristics Bragg s Law Elastic scattering: diffractometers Continuous sources Pulsed sources Inelastic scattering: spectrometers Continuous sources Pulsed sources Transmitted beam: imaging Fundamental physics

3 Neutrons vs Light light neutrons λ < μm < nm E > ev > mev n θ c 90 1 Φ/ΔΩ p/cm 2 /ster/s (60W lightbulb) n/cm 2 /ster/s (60MW reactor) P left-right up-down spin 1 ½ interaction electromagnetic strong force, magnetic charge 0 0

4 Neutron Moderators ILL ISIS 2.5m 1m

5 Source brightnesses Peak brightness: ILL ~ 1-10 x ISIS Time-integrated: ILL ~ x ISIS Lightbulb ~ 100,000 x ILL

6 log(intensity) Pulsed-source time structures cold neutrons ISIS- TS1 SNS ISIS- TS2 ILL time (ms)

7 Pulsed-source time structure Intensity λ=1å λ=2å λ=5å time (μs) A

8 Distribution by Guides

9 Neutron transport by total internal reflection ~ 100m at present sources Distribution by Guides

10 Reflecting Surfaces incident θ n=1 n <1 reflected refracted critical angle of total reflection θ c cos θ n' cos θ c c 1 n' 1 n N λ 2 2π θ b 2 c n' 2 θ c λ Nb/ π for natural Ni, θ c = λ[å] 0.1

11 Neutron Supermirrors

12 Neutron Supermirrors

13 Neutron Supermirrors

14 Neutron Supermirrors

15 An Fe/Si multilayer

16 Neutron transport by total internal reflection ~ 100m at present sources Distribution by Guides

17 Focusing guide ~ 100 samples < 1 cm 2 cm 2

18 Bragg s Law

19 Bragg s Law

20 Bragg s Law

21 Bragg s Law 2d sin

22 Bragg s Law 2d sin 2θ

23 Diffraction: Bragg s Law 2d sin k i Q θ 2 d θ k f Q

24 Diffraction: Bragg s Law θ k i f k Q θ f i f i k k Q Q k k k i θ Q k f θ d Q 2

25 Diffraction: Bragg s Law f i f i k k Q Q k k k i θ Q k f θ 2 sin 2 sin 2 k d k Q k k k f i d Q 2

26 Diffractometers Measure structure (d-spacings) Assume k i =k f Measure k i or k f : Bragg diffraction Time-of-flight Velocity selection Samples: Crystals Powders Liquids Large molecules or structures Surfaces

27 Powder diffractometers Measure crystal structure Large single crystals rarely available Polycryst al Q 2 d 2d sin

28 Time-of-flight method distance detector sample h / mv [Å] /v [m/ms] time

29 and for The POWTEX project is funded by the BMBF. Time-of-flight method

30 Time-of-flight method

31 Crystal Monochromators Graphite 002 Copper 200 Germanium 333 Copper 200 Silicon 111 Graphite 002 d-spacing Å Å Å Å

32 Monochromator Focusing A θ A B θ B Q 4 sin /

33 Powder Diffraction at a Cts Source 2d sin

34 Powder Diffraction at a Cts Source 2d sin

35 Powder Diffraction Determining the structure Rietveld refinement Measuring strain Engineering applications

36 Diffuse Scattering Q 2 d

37 Resolution in diffraction d 2 sin d 2 2 d d 2 Q Q d d 0.2% Q Q d d 2%

38 Mosaic-crystal Monochromators 2 d sin <hkl> cot fwhm < 10-4 Perfect crystal fwhm > 0.1 cot B Mosaic crystal

39 Time-of-flight Resolution d distance 2 sin d 2 2 d d 2 h mv [Å] 4 v[m/ms] 4 t[ms] L[m] L 4 t[ms] 3 B Data 1 L[m] Δt A time

40 Time-of-flight Resolution distance To improve resolution, increase the length: long guides move to a different moderator L B 3 Data Δt A time

41 Single-Crystal Diffraction Availability of large (~mm 3 ) crystal No loss of information from powder average Direct and unambiguous structural determination Complex structures

42 Cts-Source Single-Crystal Diffractometer

43 Laue Diffraction White-beam method No prior knowledge of k i or k f (110 (310 ) λ(310) ) Peak position depends only on angle of crystal plane, (120 ) not on d-spacing λ(110) Good for crystal orientation, and looking for odd reflections λ(120)

44 Laue Diffraction

45 Single-Crystal with TOF TOF determination of k i, k f Large solid-angle coverage Lower flux than Laue method

46 Small-Angle Scattering

47 Small-Angle Scattering Probing the longest length scales available to neutrons Monochromator Δλ/λ Sample Detector 10 d 2d sin 2 sin 8 Collimator I - 2θ Q I d 2 λ d 2θ 4-20 Å Å I - +I + I - -I +

48 Small-angle scattering Access to smallest angles: remove direct beam Good collimation required

49 Small-angle scattering Access to smallest angles: remove direct beam Good collimation required Soller collimator Pin-holes separated by distance 5 cm < 30 m 5 cm > 0.1

50 Continuous-source SANS Δλ/λ 10%

51 Continuous-source SANS d 2 sin 2 d d Direct beam spot ~ 10% of detector I size - Δθ/θ>10% I Δλ/λ 10% 2 I - +I + I - -I +

52 Log Reflectometry Reflection from surfaces and interfaces in out Specular: θ in =θ out Off-specular: θ in θ out Depth profile of the scattering-length density

53 Specular reflectometry Monochromatic λ fixed θ θ scan through θ Time-of-flight scan through λ θ θ θ fixed

54 Specular reflectometry 2 m Horizontal sample geometry all samples (including liquids) Vertical sample geometry solid samples, e.g. magnetic straightforward to vary θ straightforward to build

55 Off-specular reflectometry Replace single detector with position-sensitive detector Measure in-plane correlations

56 Neutron Instruments I: Summary Neutron sources Very weak: neutrons are precious Pulsed and continuous Instrument components & concepts Time-of-flight method Guides Monochromators Elastic scattering: diffractometers Powder diffractometers: single-peak, Laue, TOF SANS Reflectometers: specular & off-specular Oxford, 2/9/09 Ken

57 Neutron Instruments I & II Overview of source characteristics Bragg s Law Elastic scattering: diffractometers Continuous sources Pulsed sources Inelastic scattering: spectrometers Continuous sources Pulsed sources Transmitted beam: imaging Fundamental physics

58 Neutron Spectroscopy Excitations: vibrations and other movements Structural knowledge is prerequisite Measure diffraction first k i k f Measure k i and k f : Bragg diffraction Time-of-flight Resonant absorption Larmor precession Methods Fix k i and scan k f direct geometry Fix k f and scan k i indirect geometry Energy scales: < μev > ev

59 Scattering triangle onservation of energy & momentum r r Initial: E i,h Final: E f,h k i E i E f h r r r k f Q k i k f r Q Momentum transfer r k i r k f Q 2 k i 2 k f 2 2 k i k f cos 2 Energy transfer h h 2 2m n 2 k i k f 2 Accessible kinematic range given by scattering triangle Q ki 2θ kf

60 Chopper Spectrometers

61 Chopper Spectrometers distance Direct geometry: fix k i by chopper phasing scan through k f by time-of-flight detector sample chopper Pulsed Source 0 time

62 Chopper Spectrometers distance detector sample chopper 2 Continuous Source chopper 1 0 time

63 Crystal-Monochromator Chop. Spec. distance detector sample chopper monochromator Continuous Source 0 time

64 Choppers Fermi Disk choppers choppers f < 600 f < 300 Hz Hz Δt > 1μs Δt > 10μs

65 Chopper Spectrometers General-purpose spectrometers Energy ranges from 1 mev to 1 ev covered Huge position-sensitive detector arrays Single crystals

66 Detectors 3 He gas tubes n + 3 He 3 H + 1 H MeV >1mm resolution High efficiency Low gamma-sensitivity 3 He supply problem Scintillators n + 6 Li 4 He + 3 H MeV <1mm resolution Medium efficiency Some gamma-sensitivity Magnetic-field sensitivity

67 Energy transfer Direct-geometry kinematics ki 2θ E i o Q kf 0 hq 2 2m n E i E f 2 E i E f cos 2 0 Momentum transfer Q

68 Alternative to direct geometry distance Indirect geometry: fix k f scan through k i by time-of-flight detector analyser sample 0 time

69 Energy transfer Indirect-geometry kinematics hq 2 2m n E i E f 2 E i E f cos 2 0 Q ki 2θ kf E f o 0 Momentum transfer Q

70 ev spectroscopy Use resonant absorption to define k f. TOF defines k i. 1) Measure with absorber in and out. Count neutrons. Take difference 2) Measure with absorber in. Count gammas. 238 U

71 Chemical spectroscopy Density-ofstates measurements

72 High Resolution 1: Backscattering 2d sin d d cot cot cos sin 2 0 Use single crystals in as close to backscattering as possible to define k f. Scan through k i with as good energy resolution.

73 Pulsed-Source Backscattering detectors High k i resolution: long instrument on sharp moderator analyser crystals exact backscattering near-backscattering

74 Backscattering

75 Continuous-Source Backscattering

76 Continuous-Source Backscattering Fix k f by backscattering analysers Scan k i by Doppler-shifting backscattering monochromator Energy resolution < 1μeV Energy range ~ ± 15 μev

77 High Resolution 2: Neutron Spin Echo High energy resolution < 1 μev Larmor precessions encode energy transfer B 0 B 1 P z -flipper L 0 L 1

78 Triple-axis Spectrometers Only at continuous sources Very flexible Measures a single point Q in -E space at a time Scans: Q Constant : Scan E at constant k i or k f Q Constant E: Scan in any direction

79 TAS with Multiplexing IN20 flat-cone multi-analyser Top view Side view sample 31 channels 75º angular range k f = 3 Å -1 k f = 1.5 Å -1

80 Imaging: Neutron Radiography Neutro ns X- rays

81 Fundamental Physics

82 Neutron Instruments 2: Summary Instruments for measuring excitations Energy scales : < μev > ev Instrument components & concepts Direct and indirect geometry Choppers Detectors Inelastic scattering: spectrometers Chopper spectrometers ev spectroscopy Chemical spectroscopy Backscattering Spin-echo Triple-axis spectrometers Imaging & Fundamental physics