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Neutron Instrumentation Neutron Instrumentation Oxford School on Neutron Scattering 5th September 2019 Ken Andersen Summary • Neutron instrument concepts – time-of-flight – Bragg’s law • Neutron Instrumentation – guides – monochromators – shielding – detectors – choppers – sample environment – collimation • Neutron diffractometers • Neutron spectrometers 2 The time-of-flight (TOF) method distance Δt time Diffraction: Bragg’s Law Diffraction: Bragg’s Law Diffraction: Bragg’s Law Diffraction: Bragg’s Law Diffraction: Bragg’s Law Diffraction: Bragg’s Law λ = 2d sinθ Diffraction: Bragg’s Law λ = 2d sinθ 2θ Reflection: Snell’s Law incident reflected n=1 θ θ’ refracted n’<1 Reflection: Snell’s Law incident reflected n=1 θ θ’ refracted θ’=0: critical angle of total n’<1 reflection θc Reflection: Snell’s Law incident reflected n=1 θ θ’ refracted θ’=0: critical angle of total n’<1 reflection θc cosθc = n' n = n' Nλ2b n' = 1− ⇒ θc = λ Nb/π 2π ≈ − 2 cosθc 1 θc 2 Reflection: Snell’s Law incident reflected n=1 θ θ’ refracted θ’=0: critical angle of total n’<1 reflection θc cosθc = n' n = n' 2 for natural Ni, Nλ b n' = 1− ⇒ θc = λ Nb/π 2π θc = λ[Å]×0.1° cosθ ≈ 1− θ2 2 c c -1 Qc = 0.0218 Å Neutron Supermirrors Courtesy of J. Stahn, PSI Neutron Supermirrors Courtesy of J. Stahn, PSI Neutron Supermirrors Courtesy of J. Stahn, PSI Neutron Supermirrors λ Reflection: θc(Ni) = λ[Å] × 0.10° c λ 1 Multilayer: θc(SM) = m × λ[Å] × 0.10° λ 2 λ 3 λ 4 } d 1 } d 2 } d3 } d4 18 Neutron Supermirrors λ Reflection: θc(Ni) = λ[Å] × 0.10° c λ 1 Multilayer: θc(SM) = m × λ[Å] × 0.10° λ 2 λ 3 λ 4 “m-number” Supermirror critical angle } d 1 } d 2 } d3 } d4 19 State-of-the-art Supermirrors Mirrotron 20 State-of-the-art Supermirrors Mirrotron Swiss Neutronics 21 Neutron guides WISH @ ISIS Swiss Neutronics guides for NIST DistributionNeutron Guides:by Guides Creating space for instruments Background Reduction Guides can be used to reduce background • Distance: – move away from fast- neutron source ~ 1/R2 Background Reduction Guides can be used to reduce background • Distance: – move away from fast- neutron source ~ 1/R2 • Curved Guides: – avoid direct line-of-sight – avoid gammas – avoid fast neutrons Focusing Guides can also be used to increase flux Converging guide increases flux, but increases divergence 26 Shielding • Shielding functions: – allow safe operation – keep background down – reduce activation • Radiation components: – Slow neutrons – Fast neutrons – Gammas 27 Shielding: radiation units and numbers Unit Description Examples Dose Curies Decay rate: 3.7×1010 Bq (decays/s) Dental x-ray 10 μSv Gray Energy dose: J/kg Abdominal CAT scan 10 mSv Sievert Biological effect of radiation Airline flight crew members 10 mSv/year Roentgen, rad, rem – legacy units 50% probability of death 5 Sv Average Person Dose Neutron experimental halls < 3 μSv/hour × 2000 hours: < 6 mSv/year Radon 2 mSv/year Cosmic 0.28 mSv/year Terrestrial 0.28 mSv/year Internal 0.4 mSv/year Medical x-rays 0.39 mSv/year Nuclear medical 0.14 mSv/year Consumer products 0.1 mSv/year Other 0.03 mSv/year Total 3.62 mSv/year 28 Source: Günter Muhrer, Los Alamos, New Mexico ISIS-TS2 Shielding: slow & fast neutrons • Slow neutrons – easy to absorb (few mm B, Cd, Gd, ..) – emit gammas when absorbed – easily detected • Fast neutrons cold thermal – can require several m of material for slow epithermal fast absorption – alternatively, thermalise using hydrogenous material (e.g. wax or ESS Target Monolith: 11m diameter polyethylene) then absorb – both require multiple (10s) of collisions – visualise as a gas, filling both solids and air – difficult to detect • Gammas – high-energy photons – absorbed by any material – absorption length scales inversely 3 with density: Pb 11.3 kg/m , steel29 7.9 WISH @ ISIS-TS2 kg/m3, concrete 2.4 kg/m3 -1 Wavevector Q [Å ] 10-3 10- 10-1 1 101 102 103 2 10-15 103 TOF & Crystal 10-12 1 Spectroscopy 10-9 10-3 Backscattering Energy [meV] Energy Time[s] Spin-echo 10-6 Length [m] 10-6 10-1 10-2 10-3 10-5 10-6 10-7 10-8 10-9 10-10 10-11 10-12 10-14 10-15 SANS Imaging Diffraction Particle Physics Reflectometry -1 Wavevector Q [Å ] 10-3 10- 10-1 1 101 102 103 2 UV & X-ray FEL: Pump & Probe studies 10-15 103 Inelastic X-ray Scattering X-ray Spec. Infrared -12 Light TOF & Crystal 10 Spectroscopy red 1 - Infra FTIR -9 Raman Backscattering 10 10-3 Brillouin Energy [meV] Energy Time[s] Visible Light Visible Spin-echo Dynamic Light Scattering SR NMR µ Dielectric Spec. 10-6 -6 Length [m] 10 Laser PCS X-ray PCS 10-1 10-2 10-3 10-5 10-6 10-7 10-8 10-9 10-10 10-11 10-12 10-14 10-15 SANS Imaging Diffraction Particle Physics Reflectometry X-ray Imaging SAXS, X-ray Refl. X-ray Diffraction EXAFS & XANES X-rays NMR Imaging Triangulation NMR NMR Optical Microscopy Optical Nanoscopy Electron Microscopy Microscopy SNOM AFM STM Scanning Techniques -1 Wavevector Q [Å ] 10-3 10- 10-1 1 101 102 103 2 UV & X-ray FEL: Pump & Probe studies 10-15 103 Inelastic X-ray Scattering X-ray Spec. Infrared -12 Light TOF & Crystal 10 Spectroscopy red 1 - Infra FTIR -9 Raman Backscattering 10 10-3 Brillouin Energy [meV] Energy Time[s] Visible Light Visible Spin-echo Dynamic Light Scattering SR NMR µ Dielectric Spec. 10-6 -6 Length [m] 10 Laser PCS X-ray PCS 10-1 10-2 10-3 10-5 10-6 10-7 10-8 10-9 10-10 10-11 10-12 10-14 10-15 SANS Imaging Diffraction Particle Physics Reflectometry X-ray Imaging SAXS, X-ray Refl. X-ray Diffraction EXAFS & XANES X-rays NMR Imaging Triangulation NMR NMR Optical Microscopy Optical Nanoscopy Electron Microscopy Microscopy SNOM AFM STM Scanning Techniques Diffractometers • Measure structures (d-spacings) • Very general method: – crystals – powders – polycrystalline materials – liquids – large molecules or structures – surfaces • Ideal diffractometer: – measure ki of each incident neutron – measure kf of the same neutrons 33 Diffractometers • Measure structures (d-spacings) • Very general method: – crystals – powders – polycrystalline materials – liquids – large molecules or structures – surfaces • Real diffractometer: – measure ki or kf of the beam • time-of-flight • Bragg diffraction – assume ki = kf 34 Powder diffractometers • Measure crystal structure using Bragg’s law – Rietveld refinement • Large single crystals are rarely available Polycrystal 2π Q = λ = 2d sinθ d Time-of-flight (TOF) Method distance detector λ = 2d sinθ sample Δt time Time-of-flight (TOF) Method distance detector λ = 2d sinθ sample Δt time Time-of-flight (TOF) Method distance detector λ = 2d sinθ sample Δt time Time-of-flight Resolution 2 2 λ 2 ∂d ∂d d = (∆d ) = ∆λ + ∆θ 2sinθ ∂λ ∂θ Δt time Time-of-flight Resolution 2 2 λ 2 ∂d ∂d d = (∆d ) = ∆λ + ∆θ 2sinθ ∂λ ∂θ To improve the resolution: • increase the length: long guide • move to a sharper moderator • reduce the beam divergence Δt time Time-of-flight (TOF) Method WISH @ ISIS TS2 Crystal Monochromators Graphite 002 Copper 200 d-spacing Germanium 333 1.089 Å Copper 200 1.807 Å Silicon 111 3.135 Å Graphite 002 3.355 Å Pulsed source time structures (λ=5Å) short pulse =5Å) 0 100μ ILL 57MW Intensity@λ 1 s ISIS-TS1 128kW ISIS-TS2 32kW log( 0.1 0.01 43 0 20 40 time ( m s ) 80 100 120 Pulsed source time structures (λ=5Å) short pulse =5Å) 0 100μ ILL 57MW Intensity@λ 1 s ISIS-TS1 128kW ISIS-TS2 32kW log( 0.1 0.01 44 0 20 40 time ( m s ) 80 100 120 Constant-Wavelength Diffraction D1B @ ILL Rietveld refinement Debye-Scherrer cone 2θ The Diffractometer Family • Powder diffraction – chemical crystallography – disordered materials – engineering: strain scanning • Single-crystal diffraction – magnetic ordering – diffuse scattering – large unit cells – protein crystallography • Small Angle Neutron Scattering (SANS) – soft matter – macromolecules in solution – nanomaterials • Reflectometry – surfaces and interfaces – both planar and in-plane structures 46 Sample environment • neutron penetration: good • sample volume: bad SNS pressure cell • range as varied as the science – magnetic fields – low temperatures – high temperatures – pressure cells (TiZr) – sample changers – stress rigs – in-situ chemistry – flow cells ILL 15mK dilution cryostat ILL adsorption trough – Langmuir troughs – … ISIS Cryomagnets 47 Neutron Spectroscopy • Excitations: vibrations and other motions – lattice vibrations – magnetic excitations – quasi-elastic scattering: diffusion & relaxation 48 Neutron Spectroscopy • Excitations: vibrations and other motions – lattice vibrations – magnetic excitations – quasi-elastic scattering: diffusion & relaxation • Ideal spectrometer: – measure ki of each incident neutron – measure kf of the same neutrons 49 Neutron Spectroscopy • Excitations: vibrations and other motions – lattice vibrations – magnetic excitations – quasi-elastic scattering: diffusion & relaxation • Real spectrometer: – measure ki and kf of the beam • time-of-flight • Bragg diffraction • Larmor precession – Fix ki and scan through kf – “direct geometry” – Fix kf and scan through ki – “indirect geometry” 50 Chopper Spectrometers Direct geometry: fix ki by chopper phasing distance scan through kf by time-of-flight detector sample chopper 0 time Neutron Choppers Fermi choppers Disk choppers f < 600 Hz f < 300 Hz Δt > 1μs Δt > 10μs Chopper Spectrometers • General-Purpose Spectrometers – Incident energy ranges from 1meV to 1eV • Huge position-sensitive detector arrays – Single-crystal samples Detectors 3He gas tubes Scintillators n + 3He 3H + 1H + 0.764 MeV n + 6Li 4He + 3H + 4.79 MeV >1mm resolution <1mm resolution High efficiency Medium efficiency Low gamma-sensitivity Some gamma-sensitivity
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