Neutron Instruments for Materials Research

1
Neutron Instruments for Materials Research

• T.E. Mason
• Experimental Facilities Division
• Spallation Neutron Source
• Acknowledgements Doug Abernathy, John Ankner,
  Ken Herwig, Frank Klose, Jinkui Zhao, Xun-Li Wang

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A Brief Aside on What You Actually Measure
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Neutron Scattering Cross-Section
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CW vs Pulsed Instrumentation

• In general continuous sources work with fixed
  wavelengths (dl, all t) and pulsed sources work
  with a wavelength band (dt, all l using time
  and distance to determine velocities and hence
  wavelengths)
• If the useful wavelength band is dispersed over
  the full time between pulses then the pulsed
  instrument counts useful neutrons all of the time
  and the figure of merit is the peak flux
• If the full time between pulses is not useful
  then the figure of merit is reduced by the duty
  cycle
• For fixed wavelengths, counting continuously the
  integrated flux is the figure of merit
• Variations (e.g. pulsed instrument at CW source)
  can, and do exist

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Example Reactor - SANS

• As an example consider the static approximation
  in a
• homogeneous system

Fourier transform of scattering length density
for an object
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cAMP-Dependent Protein Kinase (PKA) Combining
Neutrons with X-rays

• PKA catalyzes a variety of cellular activities,
  ranging from gene induction to color change in
  pigment cells.
• PKA serves as the prototype for a class of
  enzymes which catalyzes protein phosphorylation,
  the major mechanism of cellular regulation.
• The combination of neutron studies and x-ray
  structures of PKA subunits has provided insights
  into the quaternary structure of PKA, which is
  key to the understanding of PKA function.

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Neutron Contrast Data of PKA and RC

• RC with deuterated R-subunit

• PKA (R2C2) with deuterated R-subunits

• Free C and Truncated R with x-ray

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Momentum Resolution
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Having Made Some Neutrons WeWant A Monochromatic
Beam!
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Pinhole SANS

• NIST/NSF 30 m SANS
• NG-3 cold neutron guide
• 20 MW reactor, hydrogen cold source

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NIST SANSCharacteristics and Performance

• Source neutron guide (NG-3), 60 mm x 60 mm
• Monochromator mechanical velocity selector with
  variable speed and pitch
• Wavelength Range 0.5 nm-2.0 nm
• Wavelength Resolution 9-30 (FWHM)
• Source-to-Sample Dist. 4 m to 16 m in steps via
  insertion of neutron guide sections
• Sample-to-Detector Dist. 1.3 m to 13 m
• Collimation circular pinhole collimation
• Sample Size 0 to 25 mm diam
• Q-Range 0.015 nm-1 to 6 nm-1
• Detector 650 mm x 650 mm 3He position-sensitive
  proportional counter (10mm resol.)

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Neutron Scattering and Spin Fluctuations

• excitations characterized by c(Q,w) è a measure
  of absorption at (Q,w).
• neutron scattering measures
• S(Q,w) c(Q,w) n(w)1.
• note Q è0, recover uniform susceptibility.
• the proportionality constant involves magnetic
  moment direction and form factor.

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co(Q,w) for Metals

• Excitations are electron-hole pairs
• Lindhard susceptibility
• As T è0 states near eF dominate
• Note NMR relaxation rate

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Normal State Energy Dependence

• As the frequency is increased the peaks become
  less well defined.
• The response is qualitatively quite similar to
  that of the spin density wave system Cr, above TN.

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Example Reactor Triple Axis

• RITA (Re-Invented Triple Axis) at Risø, DR-3
  Denmark
• 10 MW reactor, supercritical hydrogen cold source

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Having Made Some Neutrons WeWant A Monochromatic
Beam!
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For Inelastic Scattering You AlsoNeed Energy
Analysis
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Crystal Monochromator (continued)
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Triple-Axis Spectrometer Resolution

• There are analytical methods for calculating the
  resolution
• (Cooper Nathans, Nielsen Bjerrum-Møller,
  Popovici)
• These are obtained from the program RESCAL for
• collimation 60' 60' 60' 60'
• 30' PG002 monochromator and analyzer
• 10' sample mosaic
• Ef 5meV (l 4.04 Å)
• hw 0 q 1 Å-1

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Incident Beam Optics

• Supermirror guide in front of the monochromator
  increases flux
• Sapphire filter reduces gamma and fast neutron
  background and protects guide
• Velocity selector in front of monochromator
  remove higher order

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Effect of the Guide
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Secondary Spectrometer

• Area detector and analyser crystal array permits
  flexible focussing

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SPINS at NIST

• A similar (on the back end) instrument is SPINS
  at NIST

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Time-of-Flight Inelastic Instruments

• Two basic types direct geometry fixed Ei
  (e.g. HET chopper)
• Indirect geometry fixed Ef (e.g. IRIS
  backscattering)

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Chopper Spectrometers

• Operate in the thermal to epithermal energy range
  
• 5 meV lt Ei lt 1000 meV
• Use fast, magnetic bearing Fermi choppers to
  select Ei
• Maximal Q range with continuous coverage to large
  scattering angles
• Need room at least on one side for scattering
  chamber

Consider two choppers should be placed on the
bottom upstream moderator at end positions - BL9
BL18 at SNS
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Target Layout
BL17
BL18
X 5 m
X 6.5 m
BL9
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Geometric constraints
2?
Lf
Li
X
?m
Accessible regions in Lf lie below a straight
line in (Li,Lf) plane
BL18 X 5 m ?m 35 For 2? gt125
Lf 2.5 m , Li 13.0 m For 2? 60 Lf 6.0
m , Li 13.1 m
BL9 X 6.5 m ?m 35 For 2? gt125
Lf 2.5 m , Li 15.7 m For 2? 60 Lf
6.0 m , Li 15.8 m
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Flux on sample at 200meV - 5 elastic resolution
Optimum flux not accessible for any L3
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Flux on sample at 200meV - 1 elastic resolution
Can optimize flux for a given L3 that is not too
large
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Proposed two spectrometer layout
BL17 - high resolution BL18 - high flux
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Instrument parameters
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Instrument parameters
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Resolution and flux calculations
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Resolution and flux calculations
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Q-w space accessible with proposed spectrometers
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Comparison to current chopper spectrometers
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High Resolution Backscattering Spectrometer

• Crystal analyzer (Si) with 84 m incident flight
  path
• Achieves 2.2 meV resolution at the elastic
  position with
• 250 meV bandwidth
• Can operate up to 18 meV energy transfer with 10
  meV resolution
• Unprecedented capabilities

• Performance gains over comparable reactor
  backscattering instruments gt100 (depending on
  bandwidth needed)
• High-Q option (with Si 311) 500x IN13 and 18x
  IRIS (with 3 times Q range and better resolution!)

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Instrument Resolution and Q-w Range

• Elastic Resolution 2.2 meV (fwhm)

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Scattering Chamber
beam stop
detector for top analyzer
evacuated sample chamber
scattering vessel
supermirror funnel
detector for bottom analyzer
top analyzer
bottom analyzer
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Guide Design

• 84 m total flight path moderator face to sample
• Curved guide
• radius of curvature 4.325 km
• 10 cm horizontal x 12 cm vertical
• Natural Ni (outer radius horizontal needs higher
  index)
• begins 1 m from moderator face
• Supermirror funnel, ends 30 cm from sample
• horizontal, 3 x qc for Ni, 10 cm to 3 cm over 5 m
• vertical, 4 x qc for Ni, 12 cm to 3 cm over 6 m
• Gains
• 1200 l 6.3 Å
• 400 l 3.2 Å

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Shutter and Core Insert Regionsfor the Standard
Shutters
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Major Spectrometer Components

• Source/Moderator decoupled, supercritical
  poisoned H2, TU
• Incident flight path - 84 m moderator face to
  sample position
• Chopper System - 3 bandwidth/frame overlap
  choppers
• Sample - dimensions 3 x 3 cm2
• Analyzer crystals, Bragg angle 88 deg
• Si (111) l 6.267 Å, 2.9 ster, 26 m2, dd/d
  3.5 10-4
• Si (311) l 3.273 Å, 1.45 ster, 13 m2, dd/d
  4.0 10-4
• Final flight path - 3 m sample-analyzer, 2.5 m
  analyzer-detector
• Detectors
• Backscattering for diffraction
• 7040 cm2 PSD 1 x 1 cm2 spatial resolution

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Melittin in Alkanethiol/Phospholipid Hybrid
Bilayer Membranes - NIST
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Melittin in Alkanethiol/Phospholipid Hybrid
Bilayer Membranes - NIST
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NG1 Reflectometer Polarized Beam - NIST
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MBE Chamber for In-situ Neutron Scattering on the
NG1 Reflectometer
UHV Techniques ? Protective Environment ?
Epitaxial Thin Film Growth ?Gas Loading (e.g.
H) ?Sputter Etching of Surface Material ?RHEED
Analysis ?Mass Spectrometry Scattering
Techniques ?Specular Reflectometry ?Off-Specular
Scattering ?Grazing Angle Diffraction ?High Angle
Diffraction ?SANS Phenomena ?Adsorption /
Desorption ?Diffusion ?Segregation ?Morphology ?Cr
ystallography ?Magnetism ?Superconductivity
Joe Dura NIST-NCR
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Reflectometry

• In addition to providing a unique probe for
  magnetic surfaces and multi-layers polarized
  neutrons permit direct inversion to obtain the
  scattering length density profile - no phase
  problem
• a magnetic reference layer buried in the
  substrate can have magnetization wrt neutron
  polarization varied
• for a weak absorbtion probe (valid for the
  neutron) three known references lead to unique
  solution
• drawback is the price paid in sensitivity for
  polarized beam
• Off-specular reflection for in-plane structure

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SNS Reflectometers

• 2 reflectometers sharing a single beamport
• Requires new multi-channel shutters in the target
  station
• Allows for both vertical sample (magnetism) and
  horizontal sample (liquids) studies

• Novel beam bender optics allows multiplexing and
  reduces background
• Reflectivities lt10-9, 10-50 times faster than any
  existing instrument

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Instrument features

• Views coupled 20-K H2 moderator from beamline 4TD
• Shares beamline with magnetism reflectometer
• Multi-channel beam bender eliminates all l lt 1.5
  Å
• Three bandwidth choppers allow clean operation in
  0.5-4.5, 5-9, or 9.5-13.5 Å wavelength frames
• Tapered guide delivers angular bandwidth that can
  be sampled by slits at 0 lt q lt 7 relative to the
  horizontal
• 1-mm2-resolution PSD permits study of
  off-specular and grazing-incident small-angle
  scattering
• For liquid samples 0 lt Q lt 0.5 Å-1 is accessible
  by tilting a solid surface, 0 lt Q lt 1.0 Å-1 (Rmin
  410-10)

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Schematic
Microguide Bender
Tapered Guide
Source
Slits
Detector
Bandwidth Choppers
14.5 m

• Bender/tapered guide combination eliminates
  source line-of-sight
• Tapered guide delivers angular bandwidth that
  allows multiple angles of incidence

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Microguide bender

• Bender acts as high-pass wavelength filter
• Multiple channels n transmit higher flux

Left Phase-space acceptance at 5-channel
bender exit for 9-Å neutrons after 0-5 bounces
(red-green). Right Integrated acceptance for
n-channel benders.
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Reflectometer beam benders
Bender inside wide shutter deflects the
beam downward to a 4.75 angle onto sample
surface.
Sample
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Tapered guide
12 cm
4gcNi
1.75 cm
Contours represent ratio of actual to maximum
optical acceptance (X / Xmax) at tapered guide
exit.
Tapered guide provides angular bandwidth for
liquid measurement. Slits select sample incident
angle about q 4.75 centerline.
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Slits
q 2.06 dq 0.0014 X / Xmax 0.89 mavg
3.3
q 4.75 dq 0.0061 X / Xmax 0.86 mavg
1.3
Slit settings off the q 4.75 centerline
exhibit higher average number of bounces mavg.
Judicious selection of incident angle q ensures
optimal acceptance X.
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Liquids reflectometer
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Performance comparison
SNS reflectometers will accumulate specular
reflectivity data 10-50 times faster than the
best existing instruments. Improved Qmax will
yield near-atomic-scale layer-thickness
sensitivity.
qi Sti (s)
Simulated data from 10-Å SiO2 layer atop Si.
SNS-L utilizes 12 incident angles qi to measure
Qmax gt 0.9 Å-1 in St lt 5 hours (18,000 s). Arrows
indicate reflectivities measured in 12-24
hours (40-80,000 s) by existing instruments.
POSY-II
SURF
ADAM, NG-1
MURR
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SNS Powder Diffractometer
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Detector Array
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Characteristics of Major Components

• Source/Moderator - decoupled poisoned ambient
  water
• dt0 10 ms at l 1 Å
• Incident Flight Path - 60 m moderator-sample
  distance
• curved supermirror guide with 3qcNi coating, 1.5
  cm wide x 3 cm tall
• 8 m moderator-guide distance
• adjustable 9 m guide-sample distance
• Chopper System - To and 2 bandwidth/frame-overla
  p choppers
• Collimators - variable aperture for incident
  beam
• inside scattering chamber - oscillating radial
  collimator
• outside scattering chamber - fixed radial
  collimators
• Detectors - type TBD
• 10 170 in-plane, 30 out-of-plane coverage
  (45 at 90)
• 40 mm tall x 5 mm wide pixel size
• 6 ster solid angle coverage, 47 m2 area
• 1 6 m variable distance from sample

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Narrow Bandwidth Concept

• Previous TOF diffractometers used detector
  "banks" at a few angles, along with a broad
  wavelength range to produce a limited number of
  data sets of intensity vs. wavelength.
• A new idea put forward by Paolo Radaelli
  proposes the use of wide angular coverage and
  full 60 Hz operation to collect a single data set
  of intensity vs. angle and time-of-flight. These
  spectra would then be combined "appropriately"
  after-the-fact to produce a single histogram with
  intensity vs. d-spacing.
• This new data collection/analysis concept can be
  applied to any detector geometry. However, it
  appears optimally suited to a continuous detector
  locus, with this locus chosen to optimize the
  resolution as a function of d-spacing.

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D-spacing-Lambda Space Coverage
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GEM Powder Diffractometer at ISIS
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Detector Locus
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D-spacing Coverage of POW-GEN3
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D-spacing Coverage of POW-GEN3
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Instrument Resolution Function
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Neutron Source
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Neutron Source
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Simulated Diffraction Experiment
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Simulated Diffraction Experiment
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Another Variation on Powder Diffraction Residual
Strain
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SNS SANS
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Schematic Layout
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Bender System guide-bender-guide
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Bender System Performance
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Choppers
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Soller Collimators Configuration
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Soller Collimators Direct Beam
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Soller Collimators Resolution and Flux
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Low Angle Detector
Needed 1 x 1 m2 ? 5mm resolution ? 1Å 103
n/s/pixel 4 x 107 n/s/detector low background
Available (3He) 1 x 1 m2 7 mm resolution ?
5Å 104 n/s/wire 106 n/s/detector Low background
1.25 ?2.5 bar (concave structure) counting
efficiency resolution counting rate 5mm wire
spacing.
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High Angle Detectors
20 x Standard 3He PSD
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Performance Flux at the End of the Bender-Sytem
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Performance Flux at Sample (14m)
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Performance Flux vs. Collimation
Max. counting rate 700n/s/pixel at 1m
collimation
100n/s/pixel at 4m collimation
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Q-Coverage
Huey Huang Rice University
3rd Frame
1st Frame
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Performance (low angle detector)
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Performance
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Performance MC simulation