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Home , Elastic scattering, Inelastic scattering, Length scale, Neutron, Neutron cross section, Neutron diffraction

by

Roger Pynn

Los Alamos National Laboratory

LECTURE 1: Introduction & “Theory” Overview

1. Introduction and theory of 1. Advantages/disadvantages of 2. Comparison with other structural probes 3. and definition of the , S(Q) 4. Coherent & incoherent scattering 5. 6. Magnetic scattering 7. Overview of science studied by neutron scattering 8. References 2. Neutron scattering facilities and instrumentation 3. 4. 5. Small angle neutron scattering 6. Inelastic scattering Why do Neutron Scattering?

• To determine the positions and of in condensed – 1994 Nobel Prize to Shull and Brockhouse cited these areas (see http://www.nobel.se/physics/educational/poster/1994/neutrons.html)

• Neutron advantages: – Wavelength comparable with interatomic spacings – Kinetic comparable with that of atoms in a – Penetrating => bulk properties are measured & sample can be contained – with matter aids interpretation of scattering data – Isotopic sensitivity allows contrast variation – Neutron couples to B => neutron “sees” unpaired spins

• Neutron Disadvantages – Neutron sources are weak => low signals, need for large samples etc – Some elements (e.g. Cd, B, Gd) absorb strongly – Kinematic restrictions (can’t access all energy & transfers) The 1994 Nobel Prize in – Shull & Brockhouse Neutrons show where the atoms are….

…and what the atoms do. The Neutron has Both -Like and Wave-Like Properties

-27 • : mn = 1.675 x 10 kg • Charge = 0; = ½

• Magnetic dipole moment: mn = - 1.913 mN -27 -1 • : mN = eh/4pmp = 5.051 x 10 J T • Velocity (v), (E), wavevector (k), wavelength (l), temperature (T). 2 2 • E = mnv /2 = kBT = (hk/2p) /2mn; k = 2 p/l = mnv/(h/2p)

Energy (meV) Temp (K) Wavelength (nm) Cold 0.1 – 10 1 – 120 0.4 – 3 Thermal 5 – 100 60 – 1000 0.1 – 0.4 Hot 100 – 500 1000 – 6000 0.04 – 0.1

l (nm) = 395.6 / v (m/s) E (meV) = 0.02072 k2 (k in nm-1) Comparison of Structural Probes Note that scattering methods provide statistically averaged information on structure rather than real- pictures of particular instances

Macromolecules, 34, 4669 (2001) Thermal Neutrons, 8 keV X-Rays & Low Energy :- Absorption by Matter

Note for neutrons: • H/D difference • Cd, B, Sm • no systematic A dependence Interaction Mechanisms

• Neutrons interact with atomic nuclei via very short range (~fm) . • Neutrons also interact with unpaired electrons via a magnetic dipole interaction. Brightness & Fluxes for Neutron & X-Ray Sources

Brightness dE/E Divergence Flux (s-1 m-2 ster-1) (%) (mrad2) (s-1 m-2) Neutrons 1015 2 10 x 10 1011

Rotating 1016 3 0.5 x 10 5 x 1010 Anode Bending 1024 0.01 0.1 x 5 5 x 1017 Wiggler 1026 0.01 0.1 x 1 1019

Undulator 1033 0.01 0.01 x 0.1 1024 (APS) Cross Sections

F = number of incident neutrons per cm2 per second s = total number of neutrons scattered per second / F s measured in barns: ds number of neutrons scattered per second into dW = 1 barn = 10-24 cm2 dW F dW d 2s number of neutrons scattered per second into dW & dE = Attenuation = exp(-Nst) dWdE F dW dE N = # of atoms/unit t = thickness Scattering by a Single (fixed) Nucleus

• range of nuclear (~ 1fm) is << neutron wavelength so scattering is “point-like” • energy of neutron is too small to change energy of nucleus & neutron cannot transfer KE to a fixed nucleus => scattering is elastic • we consider only scattering far from nuclear where neutron absorption is negligible

If v is the velocity of the neutron (same before and after scattering),the number of neutrons passing through an area dS per second after scattering is :

2 2 2 2 v dS y scat = v dS b /r = v b dW 2 Since the number of incident neutrons passing through unit areas is : F = vy incident = v ds v b2 dW = = b 2 so s = 4pb2 dW FdW total Adding up Neutrons Scattered by Many Nuclei

r r r i k 0 .R i At a nucleus located at R i the incident wave is e é ù r r - b r r r i k0 .Ri ê i ik '.(r -Ri ) ú so the scattered wave is y scat = åe r r e ê r - R ú ë i û 2 2 vdSy r r r r r ds scat dS ik '.r 1 i (k0 -k ').Ri \ = = bie å r r e dW vdW dW r - R i

2 If we measure far enough away so that r >> Ri we can use dW = dS/r to get r r r r r r r ds i(k0 -k ').(Ri -R j ) -iQ.(Ri -R j ) = åbibje = åbibje dW i, j i, j r r r where the wavevector transfer Q is defined by Q = k'-k0 Coherent and Incoherent Scattering

The scattering length, bi , depends on the nuclear , spin relative to the neutron & nuclear eigenstate. For a single nucleus:

bi = b +dbi where db averages to zero 2 bibj = b + b (dbi +dbj ) +dbidbj

but db = 0 and dbidbj vanishes unless i = j

2 2 2 2 dbi = bi - b = b - b r r r ds 2 2 \ = b åe-iQ.(Ri -R j ) + ( b2 - b )N dW i, j

Coherent Scattering Incoherent Scattering (scattering depends on the (scattering is uniform in all directions) direction of Q) Note: N = number of atoms in scattering system Values of scoh and sinc

Nuclide scoh sinc scoh sinc

1H 1.8 80.2 V 0.02 5.0

2H 5.6 2.0 Fe 11.5 0.4

C 5.6 0.0 Co 1.0 5.2

O 4.2 0.0 Cu 7.5 0.5

Al 1.5 0.0 36Ar 24.9 0.0

• Difference between H and D used in experiments with soft matter (contrast variation) • Al used for windows • V used for sample containers in diffraction experiments and as calibration for energy resolution • Fe and Co have nuclear cross sections similar to the values of their magnetic cross sections • Find scattering cross sections at the NIST web site at: http://webster.ncnr.nist.gov/resources/n-lengths/ Coherent Elastic Scattering measures the Structure Factor S(Q) I.e. correlations of atomic positions

r r r ds 2 r r 1 -iQ.(R -R ) = b N.S(Q) for an assemblyof similar atoms where S(Q) = e i j dW N å i, j ensemble

r r r r r r r r r r r Now e-iQ.Ri = dr.e-iQ.r d(r -R ) = dr.e-iQ.r r (r) where r is the nuclear number å ò å i ò N N i i r 1 r r r r 2 so S(Q) = dr.e-iQ.r r (r) N ò N r 1 r r r r r r r 1 r r r r r r r or S(Q) = dr' dr.e-iQ.(r -r ') r (r)r (r') = dR dre-iQ.R r (r)r (r - R) N ò ò N N N ò ò N N r r r r r ie S(Q) =1+ ò dR.g(R).e-iQ.R

r r r r r where g(R) = å d(R- Ri + R0 ) is a function of R only. i¹0

g(R) is known as the static pair correlation function. It gives the probability that there is an , i, at distance R from the origin of a coordinate system at t, given that there is also a (different) atom at the origin of the coordinate system S(Q) and g(r) for Simple

• Note that S(Q) and g(r)/r both tend to unity at large values of their arguments • The peaks in g(r) represent atoms in “coordination shells” • g(r) is expected to be zero for r < particle diameter – ripples are truncation errors from of S(Q) Neutrons can also gain or lose energy in the scattering process: this is called inelastic scattering Inelastic neutron scattering measures atomic motions

The concept of a pair correlation function can be generalized: G(r,t) = probability of finding a nucleus at (r,t) given that there is one at r=0 at t=0

Gs(r,t) = probability of finding a nucleus at (r,t) if the same nucleus was at r=0 at t=0 Then one finds: æ d 2s ö k' r ç ÷ = b2 NS(Q,w) ç ÷ coh è dW.dE ø k (h/2p)Q & (h/2p)w are the momentum & coh energy transferred to the neutron during the æ d 2s ö k' r ç ÷ 2 scattering process ç ÷ = binc NSi (Q,w) è dW.dE øinc k where r 1 r r r r r 1 r r r r S(Q,w) = G(r,t)ei(Q.r -wt)drdt and S (Q,w) = G (r,t)ei(Q.r-wt)drdt 2ph òò i 2ph òò s

Inelastic coherent scattering measures correlated motions of atoms Inelastic incoherent scattering measures self-correlations e.g. diffusion Magnetic Scattering

• The magnetic moment of the neutron interacts with B fields caused, for example, by unpaired electron spins in a material – Both spin and orbital of electrons contribute to B – Expressions for cross sections are more complex than for nuclear scattering • Magnetic interactions are long range and non-central – Nuclear and magnetic scattering have similar magnitudes – Magnetic scattering involves a form factor – FT of electron spatial distribution • Electrons are distributed in space over distances comparable to neutron wavelength • Elastic magnetic scattering of neutrons can be used to probe electron distributions – Magnetic scattering depends only on component of B perpendicular to Q – For neutrons spin polarized along a direction z (defined by applied H ):

• Correlations involving Bz do not cause neutron spin flip • Correlations involving Bx or By cause neutron spin flip – Coherent & incoherent nuclear scattering affects spin polarized neutrons • Coherent nuclear scattering is non-spin-flip • Nuclear spin-incoherent nuclear scattering is 2/3 spin-flip • Isotopic incoherent scattering is non-spin-flip Magnetic Neutron Scattering is a Powerful Tool • In early Shull and his collaborators: – Provided the first direct evidence of antiferromagnetic ordering

– Confirmed the Neel model of ferrimagnetism in magnetite (Fe3O4) – Obtained the first magnetic form factor (spatial distribution of magnetic electrons) by measuring paramagnetic scattering in Mn compounds – Produced polarized neutrons by Bragg (where nuclear and magnetic scattering scattering cancelled for one neutronspin state) – Determined the distribution of magnetic moments in 3d alloys by measuring diffuse magnetic scattering – Measured the magnetic critical scattering at the Curie point in Fe • More recent work using polarized neutrons has: – Discriminated between longitudinal & transverse magnetic fluctuations – Provided evidence of magnetic solitons in 1-d – Quantified electron spin fluctuations in correlated-electron materials – Provided the basis for measuring slow dynamics using the neutron spin-echo technique…..etc Ne utro ns in Co n de ns e d Matte r Re s e arc h

10000

S pPaSllSa t-i oCnh o- pCpheorp Sppeer ctro meter N eutron ILL - w itho u t s pin -ec ho Induced Itine rant Hyd rog en Excitations ILL - w ith s p in -e ch o Magnets 100 Modes Crystal Momentum Elastic Scattering Fields Distributions Molecular Coherent Spin Vibrations M odes in Wa ves Glasses La ttice Vibra tions ) and Liquids and V Elec tron-

e 1 Molecular A nharmonicity

m Intera ctions

( M otions [Larger Objects Resolved] Accurate Structu res r Metallu rgical C rystal and Mem branes e Micelles Polymers Systems Magnetic Amorphous Precision f Systems s Proteins in Solution Structures Anharmonicity n

a Critic al r Sca ttering Slower T 0.01 M otions y

g Res olved r

e Diffusive

n Aggregate Modes E Polymers Molecular and Reorie nta tion 10-4 Biological Tunneling Systems

Surface Effects ?

10-6 0 .0 01 0.01 0 .1 1 10 100 -1 Q (Å ) Neutron scattering experiments measure the number of neutrons scattered at different values of the wavevector and energy transfered to the neutron, denoted Q and E. The phenomena probed depend on the values of Q and E accessed. Next Lecture

2. Neutron Scattering Instrumentation and Facilities – how is neutron scattering measured? 1. Sources of neutrons for scattering – reactors & sources 1. Neutron spectra 2. Monochromatic-beam and time-of-flight methods 2. Instrument components 1. Crystal monochromators and analysers 2. Neutron guides 3. Neutron detectors 4. Neutron spin manipulation 5. Choppers 6. etc 3. A zoo of specialized neutron References

• Introduction to the Theory of Thermal Neutron Scattering by G. L. Squires Reprint edition (February 1997) Dover Publications ISBN 048669447

• Neutron Scattering: A Primer by Roger Pynn Los Alamos Science (1990) (see www.mrl.ucsb.edu/~pynn) by

Roger Pynn

Los Alamos National Laboratory

LECTURE 2: Neutron Scattering Instrumentation & Facilities Overview

1. Essential messages from Lecture #1 2. Neutron Scattering Instrumentation and Facilities – how is neutron scattering measured? 1. Sources of neutrons for scattering – reactors & spallation sources 1. Neutron spectra 2. Monochromatic-beam and time-of-flight methods 2. Instrument components 3. A “zoo” of specialized neutron spectrometers Recapitulation of Key Messages From Lecture #1

• Neutron scattering experiments measure the number of neutrons scattered by a sample as a function of the wavevector change (Q) and the energy change (E) of the neutron

• Expressions for the scattered neutron intensity involve the positions and motions of atomic nuclei or unpaired electron spins in the scattering sample

• The scattered neutron intensity as a function of Q and E is proportional to the space and time Fourier Transform of the probability of finding two atoms separated by a particular distance at a particular time

• Sometimes the change in the spin state of the neutron during scattering is also measured to give information about the locations and orientations of unpaired electron spins in the sample What Do We Need to Do a Basic Neutron Scattering Experiment?

• A source of neutrons • A method to prescribe the wavevector of the neutrons incident on the sample • (An interesting sample) • A method to determine the wavevector of the scattered neutrons – Not needed for elastic scattering • A neutron detector Detector

Incident neutrons of wavevector k0 Scattered neutrons of

wavevector, k1 Sample

k0 Remember: wavevector, k, & k Q 1 wavelength, l, are related by:

k = mnv/(h/2p) = 2p/l Neutron Scattering Requires Intense Sources of Neutrons

• Neutrons for scattering experiments can be produced either by in a reactor or by spallation when high-energy strike a heavy target (W, Ta, or U). – In general, reactors produce continuous neutron beams and spallation sources produce beams that are pulsed between 20 Hz and 60 Hz – The energy spectra of neutrons produced by reactors and spallation sources are different, with spallation sources producing more high-energy neutrons – Neutron spectra for scattering experiments are tailored by moderators – or liquids maintained at a particular temperature – although neutrons are not in thermal equilibrium with moderators at a short-pulse spallation sources

• Both reactors and spallation sources are expensive to build and require sophisticated operation. – SNS at ORNL will cost about $1.5B to construct & ~$140M per year to operate

• Either type of source can provide neutrons for 30-50 neutron spectrometers – Small science at large facilities About 1.5 Useful Neutrons Are Produced by Each Fission Event in a Whereas About 25 Neutrons Are Produced by spallation for Each 1-GeV Incident on a Tungsten Target

Artist’s view of spallation

Spallation

Nuclear Fission Neutrons From Reactors and Spallation Sources Must Be Moderated Before Being Used for Scattering Experiments • Reactor spectra are Maxwellian • Intensity and peak-width ~ 1/(E)1/2 at high neutron at spallation sources • Cold sources are usually liquid (though is also used at reactors & methane is some- used at spallation sources) • Hot source at ILL (only one in the world) is , heated. The ESRF* & ILL* With Grenoble & the Beldonne Mountains

*ESRF = European Facility; ILL = Institut Laue-Langevin The National Institute of Standards and Technology (NIST) Reactor Is a 20 MW With a Peak Thermal Flux of 4x1014 N/sec. It Is Equipped With a Unique Liquid-hydrogen Moderator That Provides Neutrons for Seven Neutron Guides Neutron Sources Provide Neutrons for Many Spectrometers: Schematic Plan of the ILL Facility A ~ 30 X 20 m2 Hall at the ILL Houses About 30 Spectrometers. Neutrons Are Provided Through Guide Tubes Los Alamos Neutron Science Center

Manuel Lujan Jr. Neutron Scattering Center

Proton Radiography Proton

Weapons 800-MeV Linear Neutron Research Accelerator

LANSCE Visitor’s Center Neutron Production at LANSCE

• Linac produces 20 H- (a proton + 2 electrons) pulses per second – 800 MeV, ~800 msec long pulses, average current ~100 mA • Each pulse consists of repetitions of 270 nsec on, 90 nsec off • Pulses are injected into a Proton Storage Ring with a period of 360 nsec – Thin foil strips electrons to convert H- to H+ (I.e. a proton) – ~3 x 1013 protons/pulse ejected onto neutron production target Components of a Spallation Source A half-mile long proton linac…

…and a neutron production target

….a proton accumulation ring…. A Comparison of Calculations for the ESS SPSS (50 Hz, 5 MW) & LPSS (16 Hz, 5W) With Measured Neutron Fluxes at the ILL

17 10 peak flux 1017 average flux poisoned m. poisoned m. 10 16 decoupled m. 1016 decoupled m. coupled m. coupled m. 15 10 long pulse 1015 and long pulse

14 10 1014 /s/str/Å] /s/str/Å] 2 2 13 10 1013

12 10 1012 Flux [n/cm Flux [n/cm 11 ILL hot source 11 10 10 ILL hot source ILL thermal source ILL thermal source 10 ILL cold source 10 10 10 ILL cold source

0 1 2 3 4 0 1 2 3 4 Wavelength [Å] Wavelength [Å]

2.0x1014 l = 4 Å Pulsed sources only make sense if poisoned moderator

14 decoupled moderator /s/str/Å]

2 1.5x10 coupled moderator long pulse one can make effective use of the 1.0x1014 flux in each pulse, rather than the

5.0x1013 average neutron flux Instantaneous flux [n/cm

0.0 0 500 1000 1500 2000 2500 3000 Time [ms] Simultaneously Using Neutrons With Many Different Wavelengths Enhances the Efficiency of Neutron Scattering Experiments

I Neutron ki Source Spectrum kf Dlres = Dlbw I

l

Dlbw a T

ki I kf Dlres a DT l l

Potential Performance Gain relative to use of a Single Wavelength is the Number of Different Wavelength Slices used The Time-of-flight Method Uses Multiple Wavelength Slices at a Reactor or a Pulsed Spallation Source

DT Detector

Dl res= 3956 dTp / L

e Dlbw = 3956 DT / L c n a t s i

D L

dTp

DT Moderator

Time

When the neutron wavelength is determined by time-of-flight, DT/dTp different wavelength slices can be used simultaneously. The Actual ToF Gain From Source Pulsing Often Does Not Scale Linearly With Peak Neutron Flux

kf ki low rep. rate => large dynamic range Q short pulse => good resolution — Q neither may be necessary or useful

I ki k f

l large dynamic range may result in intensity changes across the spectrum

ki I k f

l

at a traditional short-pulse source the wave- ki length resolution changes with wavelength k f

l A Comparison of Reactors & Spallation Sources

Short Pulse Spallation Source Reactor

Energy deposited per useful neutron is Energy deposited per useful neutron is ~20 MeV ~ 180 MeV

Neutron spectrum is “slowing down” Neutron spectrum is Maxwellian spectrum – preserves short pulses

Constant, small dl/l at large neutron Resolution can be more easily tailored energy => excellent resolution to experimental requirements, except especially at large Q and E for hot neutrons where monochromator crystals and choppers are less effective Copious “hot” neutrons=> very good for Large flux of cold neutrons => very measurements at large Q and E good for measuring large objects and slow dynamics Low background between pulses => Pulse rate for TOF can be optimized good signal to noise independently for different spectrometers Single pulse experiments possible Neutron polarization easier Why Isn’t There a Universal Neutron Scattering ?

Mononchromatic Detector incident beam Elastic Scattering — detect all scattered neutrons

Inelastic Scattering — detect neutrons as a function of scattered energy (color)

• Conservation of momentum => Q = kf – ki 2 2 2 2 • Conservation of energy => E = ( h m/ 8 p ) (kf - ki ) • Scattering properties of sample depend only on Q and E, not on neutron l

Many types of neutron scattering spectrometer are required because the accessible Q and E depend on neutron energy and because resolution and detector coverage have to be tailored to the science for such a signal-limited technique. Brightness & Fluxes for Neutron & X-ray Sources

Brightness dE/E Divergence Flux (s-1 m-2 ster-1) (%) (mrad2) (s-1 m-2) Neutrons 1015 2 10 x 10 1011

Rotating 1016 3 0.5 x 10 5 x 1010 Anode Bending 1024 0.01 0.1 x 5 5 x 1017 Magnet Wiggler 1026 0.01 0.1 x 1 1019

Undulator 1033 0.01 0.01 x 0.1 1024 (APS) Instrumental Resolution

• in the neutron wavelength & direction of travel imply that Q and E can only be defined with a certain precision

• When the box-like resolution in the figure are convolved, the overall resolution is Gaussian (central limit theorem) and has an elliptical shape in (Q,E) space

• The total signal in a scattering experiment is proportional to the phase space volume within the elliptical resolution volume – the better the resolution, the lower the count rate Examples of Specialization of Spectrometers: Optimizing the Signal for the Science

• Small angle scattering [Q = 4p sinq/l; (dQ/Q)2 = (dl/l)2 + (cotq dq)2] – Small diffraction angles to observe large objects => long (20 m) instrument – poor monochromatization (dl/l ~ 10%) sufficient to match obtainable angular resolution (1 cm2 pixels on 1 m2 detector at 10 m => dq ~ 10-3 at q ~ 10-2)) • Back scattering [q = p/2; l = 2 d sin q; dl/l = cot q +…] – very good energy resolution (~neV) => perfect crystal analyzer at q ~ p/2 – poor Q resolution => analyzer crystal is very large (several m2) Typical Neutron Scattering Instruments

Note: relatively massive shielding; long flight paths for time-of-flight spectrometers; many or multi-detectors

but… small science at a large facility Components of Neutron Scattering Instruments

• Monochromators – Monochromate or analyze the energy of a neutron beam using Bragg’s law • Collimators – Define the direction of travel of the neutron • Guides – Allow neutrons to travel large distances without suffering intensity loss • Detectors – Neutron is absorbed by 3He and caused by recoiling is detected • Choppers – Define a short pulse or pick out a small band of neutron energies • Spin turn coils – Manipulate the neutron spin using Lamor precession • Shielding – Minimize background and radiation exposure to users Most Neutron Detectors Use 3He

• 3He + n -> 3H + p + 0.764 MeV

• Ionization caused by triton and proton is collected on an electrode

• 70% of neutrons are absorbed when the product of gas pressure x thickness x neutron wavelength is 16 atm. cm. Å

• Modern detectors are often “position sensitive” – charge division is used to determine where the ionization reached the cathode. A selection of neutron detectors – thin-walled stainless steel tubes filled with high-pressure 3He. Essential Components of Modern Neutron Scattering Spectrometers

Pixelated detector covering a wide range of scattering angles (vertical & horizontal)

Horizontally & vertically focusing monochromator (about 15 x 15 cm2) Neutron guide (glass coated either with Ni or “supermirror”) Rotating Choppers Are Used to Tailor Neutron Pulses at Reactors and Spallation Sources • T-zero choppers made of Fe-Co are used at spallation sources to absorb the prompt high-energy pulse of neutrons • Cd is used in frame overlap choppers to absorb slower neutrons

Fast neutrons from one pulse can catch-up with slower neutrons from a succeeding pulse and spoil the measurement if they are not removed.This is called “frame-overlap” Larmor Precession & Manipulation of the Neutron Spin

• In a , H, the neutron spin precesses at a rate

uL = -gH / 2p = -2916.4H Hz where g is the neutron’s gyromagnetic ratio & H is in Oesteds • This effect can be used to manipulate the neutron spin • A “spin flipper” – which turns the spin through 180 degrees is illustrated • The spin is usually referred to as “up” when the spin (not the magnetic moment) is parallel to a (weak ~ 10 – 50 Oe) magnetic guide field Next Lecture

3. Diffraction 1. Diffraction by a lattice 2. Single-crystal diffraction and diffraction 3. Use of monochromatic beams and time-of-flight to measure 4. Rietveld refinement of powder patterns 5. Examples of science with powder diffraction • Refinement of structures of new materials • Materials texture • Strain measurements • Structures at high pressure • Microstrain peak broadening • Pair distribution functions (PDF) by

Roger Pynn

Los Alamos National Laboratory

LECTURE 3: Diffraction This Lecture

2. Diffraction 1. Diffraction by a lattice 2. Single-crystal compared with powder diffraction 3. Use of monochromatic beams and time-of-flight to measure powder diffraction 4. Rietveld refinement of powder patterns 5. Examples of science with powder diffraction • Refinement of structures of new materials • Materials texture • Strain measurements • Pair distribution functions (PDF) From Lecture 1: ds number of neutrons scattered through angle 2q per second into dW = dW number of incident neutrons per square cm per second r r r r r r r æ ds ö coh coh i(k0 -k ').(Ri -R j ) coh coh -iQ.(Ri -R j ) ç ÷ = åbi b j e = åbi b j e è dW øcoh i, j i, j r r r r where the wavevector transfer Q is defined by Q = k '-k0

k0

For elastic scattering k0=k’=k: Q 2q Q = 2k sin q k’ Q = 4p sin q/l Detector

Scattered neutrons of Incident neutrons of wavevector k 0 2q wavevector, k’

Sample

• Neutron diffraction is used to measure the differential , ds/dW and hence the static structure of materials – Crystalline solids – Liquids and amorphous materials – Large scale structures

• Depending on the scattering angle, structure on different length scales, d, is measured: 2p /Q = d = l / 2sin(q )

• For crystalline solids & liquids, use wide angle diffraction. For large structures, e.g. polymers, colloids, micelles, etc. use small-angle neutron scattering Diffraction by a Lattice of Atoms r r r v r r r 1 -iQ.(Ri -R j ) r S(Q) = åe with Ri = i + ui where i is the equilibrium position N i, j r of atom i and ui is any displacement (e.g. thermal) from the equilibrium position. r r r Ignoring thermal vibrations, S(Q) is only non- zero for Q's such that Q.(i - j) = 2Mp. v r r r r r r In a Bravais lattice, we can write i = m1ia1 + m2ia2 + m3ia3 where a1,a2,a3 are the primitive translation vectors of the unit . a3 r 2p r r * a2 Define a1 = a2 Ù a3 and cyclic permutations. V0 r* r Then ai .aj = 2pd ij . r v r r* r* r* r r If Q = Ghkl = ha1 + ka2 + la3 then Q.(i - j) = 2Mp.

a So scattering from a (frozen) lattice only 1 occurs when the scattering wavevector, Q, is equal to a vector, Ghkl . For Periodic Arrays of Nuclei, Coherent Scattering Is Reinforced Only in Specific Directions Corresponding to the Bragg Condition:

l = 2 dhkl sin(q) or 2 k sin(q) = Ghkl Bragg Scattering from Crystals

Working through the math (see, for example, Squires' book), we find : 3 æ ds ö (2p ) r r r 2 ç ÷ = N åd (Q - Ghkl ) Fhkl (Q) è dW øBragg V0 hkl where the unit - cell structure factor is given by r r r iQ.d -Wd Fhkl (Q) = åbd e e d

and Wd is the Debye - Waller factor that accounts for thermal motions of atoms • Using either single crystals or , neutron diffraction can be used to measure F2 (which is proportional to the intensity of a Bragg peak) for various values of (hkl). • Direct Fourier inversion of diffraction data to yield crystal structures is not possible because we only measure the magnitude of F, and not its phase => models must be fit to the data • Neutron powder diffraction has been particularly successful at determining structures of new materials, e.g. high Tc materials We would be better off if diffraction measured phase of scattering rather than amplitude! Unfortunately, did not oblige us.

Picture by courtesy of D. Sivia Reciprocal Space – An Array of Points (hkl) that is Precisely Related to the Crystal Lattice

• • • • • • • • • • • • • • • k • • • • • • • k0 • • • • • • • • • • • Ghkl a* • • • • • • • • • • • O• • • • • • • • • • • b* G = 2p/d hkl hkl (hkl)=(260)

a* = 2p(b x c)/V0, etc.

A single crystal has to be aligned precisely to record Bragg scattering Powder – A Polycrystalline Mass

All orientations of crystallites possible

Typical Sample: 1cc powder of 10mm crystallites - 109 particles if 1mm crystallites - 1012 particles

Single crystal reciprocal lattice - smeared into spherical shells Powder Diffraction gives Scattering on Debye-Scherrer Cones (220)

Incident beam (200) x-rays or neutrons Sample (111)

Bragg’s Law l = 2dsinQ Powder pattern – scan 2Q or l Measuring Neutron Diffraction Patterns with a Monochromatic Neutron Beam

Use a continuous beam of mono-energetic neutrons.

Since we know the neutron wavevector, k, the scattering angle gives Ghkl directly: Ghkl = 2 k sin q Neutron Powder Diffraction using Time-of-Flight

Lo= 9-100m Sample

2Q - fixed Pulsed F source L1 ~ 1-2m l = 2dsinQ Detector Measure scattering as bank a function of time-of- flight t = const*l Time-of-Flight Powder Diffraction

Use a pulsed beam with a broad spectrum of neutron energies and separate different energies (velocities) by time of flight. Compare X-ray & Neutron Powder Patterns

10.000 0.025 159.00 CPD RRRR PbSO4 Cu Ka X-ray data 22.9. Scan no. = 1 Lambda1,lambda2 = 1.540 Observed Profile X-ray Diffraction - CuKa

Phillips PW1710 1.5 • Higher resolution 4 X10E

• Intensity fall-off at small d 1.0 spacings • Better at resolving small

lattice distortions .5 Counts Counts .0 .0 1.0 2.0 3.0 4.0 10.0 0.05 155.9 CPD RRRR PbSO4 1.909A neutron data 8.8 D-spacing, A

Scan no. = 1 Lambda = 1.9090 Observed Profile 2.5 2.5

Neutron Diffraction - D1a, ILL X10E 3 3 X10E

2.0 2.0 l=1.909 Å • Lower resolution

1.5 1.5 • Much higher intensity at small d-spacings 1.0 1.0 • Better atomic positions/thermal

parameters

.5 .5 Counts Counts 1.0 2.0 3.0 4.0 D-spacing, A There’s more than meets the eye in a powder pattern* I = I + SY Rietveld Model c b p

Io

Ic

I

Intensity b Yp

TOF

*Discussion of Rietveld method adapted from viewgraphs by R. Vondreele (LANSCE) The Rietveld Model for Refining Powder Patterns

2 Ic = Io{SkhF hmhLhP(Dh) + Ib}

Io - incident intensity - variable for fixed 2Q

kh - scale factor for particular phase 2 F h - structure factor for particular reflection

mh - reflection multiplicity

Lh - correction factors on intensity - texture, etc.

P(Dh) - peak shape function – includes instrumental resolution, crystallite size, microstrain, etc. How good is this function?

Protein Rietveld refinement - Very low angle fit 1.0-4.0° peaks - strong “perfect” fit to shape Examples of Science using Powder Diffraction

• Refinement of structures of new materials • Materials texture • Strain measurements • Pair distribution functions (PDF) High-resolution Neutron Powder Diffraction in CMR manganates*

• In Sr2LaMn2O7 – synchrotron data indicated two phases at low temperature. Simultaneous refinement of neutron powder data at 2 l’s allowed two almost-isostructural phases (one FM the other AFM) to be refined. Only neutrons see the magnetic reflections

• High resolution powder diffraction

with Nd0.7Ca0.3MnO3 showed splitting of (202) peak due to a transition to a previously unknown monoclinic phase. The data show- ed the existence of 2 Mn sites with different Mn-O distances. The different sites are likely occupied by Mn3+ and Mn4+ respectively

* E. Suard & P. G. Radaelli (ILL) Texture: “Interesting Preferred Orientation”

Random powder - all crystallite orientations equally probable - flat pole figure

Pole figure - stereographic projection of a crystal axis down some sample direction

Loose powder

(100) random texture (100) wire texture Crystallites oriented along wire axis - pole figure peaked in center and at the rim (100’s are 90º apart)

Orientation Distribution Function - probability function for texture Metal wire Texture Measurement by Diffraction

(220) Non-random crystallite orientations in sample (200)

Incident beam Sample x-rays or neutrons (111)

Debye-Scherrer cones • uneven intensity due to texture • different pattern of unevenness for different hkl’s • intensity pattern changes as sample is turned Texture Determination of Titanium Wire Plate (Wright-Patterson AFB/LANSCE Collaboration)

Possible “pseudo-single crystal” turbine blade material Ti Wire - (100) texture Ti Wire Plate - does it still have wire texture?

ND TD RD TD

• Bulk measurement • Neutron time-of-flight data ND • Rietveld refinement of texture

• Spherical harmonics to Lmax = 16 • Very strong wire texture in plate

reconstructed (100) pole figure Definitions of and Strain

• Macroscopic strain – total strain measured by an extensometer • Elastic lattice strain – response of lattice planes to applied stress, measured by diffraction

ddhkl0- e=hkl d0 • Intergranular strain – deviation of elastic lattice strain from linear behavior sapplied eI=e-hkl Ehkl

• Residual strains – internal strains present with no applied force • Thermal residual strains – strains that develop on cooling from processing temperature due to anisotropic coefficients of thermal expansion Neutron Diffraction Measurements of Lattice Strain*

s Incident neutrons

+90° Detector measures e ^ to loading direction Text box -90° Detector measures e parallel to loading direction

s The Neutron Powder at LANSCE

Neutron measurements :- • Non destructive, bulk, phase sensitive • Time consuming, Limited spatial resolution

* Discussion of residual strain adapted from viewgraphs by M. Bourke (LANSCE) Why use Metal Matrix Composites ?

Higher Pay Loads High temperatures High pressures Reduced Engine Weights Reduced Consumption Better Engine Performance

F117 (JSF) National AeroSpace Plane MMC Applications Engine Landing Gear

Rotors Fan Blades Structural Rods Impellers Landing Gears W-Fe :- Fabrication, Microstructure & Composition

• 200 mm diameter continuous Tungsten fibers, • Hot pressed at 1338 K for 1 hour into Kanthal ( 73 Fe, 21 Cr, 6 Al wt%) – to residual strains after cooling • Specimens :- 200 * 25 * 2.5 mm3

Powder diffraction data includes Bragg peaks from both Kanthal and Tungsten.

K

10 vol.% 30 vol.% K W W Normalized Intensity

20 vol.% 70 vol.% d spacing (A) Neutron Diffraction Measures Mean Residual Phase Strains when Results for a MMC are compared to an Undeformed Standard 4000

Kanthal 3000 10 vol.% W 20 vol.% W 30 vol.% W

e) 2000 e)

m 70 vol.% W m

1000

0

-1000 Residual strain ( Residual Strain ( -2000 Tungsten 10 vol.% W 20 vol.% W -3000 30 vol.% W 70 vol.% W -4000 0 20 40 60 80 100 Angle to the fiber (deg) Symbols show data points 2 2 Curves are fits of the form =< e 11>cos a + < e 22>sin a Pressure Vessels and Piping, Vol. 373, “Fatigue, Fracture & Residual Stresses”, Book No H01154 Load Sharing in MMCs can also be Measured by Neutron Diffraction

400 Kanthal • Initial co-deformation results 350 Tungsten from confinement of W fibers 300 by the Kanthal matrix 250 • Change of slope (125 MPa) is the typical load sharing 200 behavior of MMCs: 150 – Kanthal yields and ceases to bear 100 Applied stress [MPa] further load 50 – Tungsten fibers reinforce and 0 strengthen the composite -1000 -500 0 500 1000 1500 2000 2500 3000 Elastic lattice strain [me] Deformation Mechanism in U/Nb Alloys

Atypical “Double Plateau Stress Strain Curve. Diffraction Results 500

400

300

200 U-6 wt% Nb 100 Typical Metal Applied Stress (MPa) 0 0 0.02 0.04 0.06 Macroscopic Strain NPD Diffraction Geometry

Tensile Axis + 90 ° Detector Q|| Q^

-90° Detector

• Important Programmatic Answers. 1. Material is Single Phase Monoclinic (a’’). 2. Lack of New Peaks Rules out Stress Induced . 3. Changes in Peak Intensity Indicate That Twinning is the Deformation Mechanism. Pair Distribution Functions

• Modern materials are often disordered. • Standard crystallographic methods lose the aperiodic (disorder) information. • We would like to be able to sit on an atom and look at our neighborhood. • The PDF method allows us to do that (see next slide): – First we do a neutron or x-ray diffraction experiment – Then we correct the data for experimental effects – Then we Fourier transform the data to real-space Obtaining the Pair Distribution Function*

Structure function

2 ¥ G(r) = Q[S(Q) -1]sinQrdQ p ò0 Raw data

PDF

* See http://www.pa.msu.edu/cmp/billinge-group/ Structure and PDF of a High Temperature Superconductor

The structure of La2-xSrxCuO4 looks like this: (copper [orange] sits in the middle of octahedra of [shown shaded with pale blue].)

The resulting PDFs look like this. The peak at 1.9A is the Cu-O bond. So what can we learn about charge-stripes from the PDF? Effect of on the Octahedra

• Doping holes (positive charges) by adding Sr shortens Cu-O bonds • Localized holes in stripes implies a coexistence of short and long Cu-O in- Fermi- plane bonds => increase in liquid Cu-O bond distribution width with doping. like • We see this in the PDF: s2 is the width of the CuO bond distribution which increases with doping then decreases beyond optimal Polaronic doping

Bozin et al. Phys. Rev. Lett. Submitted; cond-mat/9907017 by

Roger Pynn

Los Alamos National Laboratory

LECTURE 3: Surface Reflection Surface Reflection Is Very Different From Most Neutron Scattering

• We worked out the by adding scattering from different nuclei – We ignored double scattering processes because these are usually very weak • This approximation is called the Born Approximation

• Below an angle of incidence called the critical angle, neutrons are perfectly reflected from a smooth surface – This is NOT weak scattering and the Born Approximation is not applicable to this case • Specular reflection is used: – In neutron guides – In multilayer monochromators and polarizers – To probe surface and interface structure in layered systems This Lecture

• Reflectivity measurements – Neutron wavevector inside a medium – Reflection by a smooth surface – Reflection by a film – The kinematic approximation – Graded interface – Science examples • Polymers & vesicles on a surface • Lipids at the liquid air interface • self-diffusion • Iron on MgO – Rough surfaces • Shear aligned worm-like micelles What Is the Neutron Wavevector Inside a Medium?

Comparing our expression for S(Q) with that given by Fermi' s Golden Rule, the r 2ph2 r nucleus - neutron potential is given by : V (r ) = bd (r) for a single nucleus. m 2ph2 1 So the average potential inside the medium is : V = r where r = åbi m volume i r is called the nuclear Scattering Length Density (SLD) The neutron obeys Schrodinge r's equation : [Ñ2 + 2m(E -V ) / h2 ]y (r) = 0 r r iko .r 2 2 2 2 2 in vacuo y (r) = e so k0 = 2mE / h . Simlarly k = 2m(E -V ) / h = k0 - 4pr where k0 is neutron wavevector in vacuo and k is the wavevector in a material -6 -2 Since k/k0 = n = refractive index (by definition ), and r is very small (~ 10 A ) we get : n =1-l2 r / 2p Since generally n <1, neutrons are externally reflected from most materials. Typical Values

• Let us calculate the scattering length density for quartz – SiO2 • Density is 2.66 gm.cm-3; Molecular weight is 60.08 gm. mole-1 3 -24 • Number of per Å = N = 10 (2.66/60.08)*Navagadro = 0.0267 molecules per Å3 -5 -2 • r=Sb/volume = N(bSi + 2bO) = 0.0267(4.15 + 11.6) 10 Å = 4.21 x10-6 Å-2 • This means that the refractive index n = 1 – l2 2.13 x 10-7 for quartz • To make a neutron “bottle” out of quartz we require k= 0 I.e. 2 1/2 k0 = 4pr or l=(p/r) . • Plugging in the numbers -- l = 864 Å or a neutron velocity of 4.6 m/s (you could out-run it!) Only Those Thermal or Cold Neutrons With Very Low Velocities Perpendicular to a Surface Are Reflected

k / k0 = n The surface cannot change the neutron velocity parallel to the surface so : k0 cosa = k cosa'= k0ncosa' i.e n = cosa/cosa' Neutrons obey Snell's Law 2 2 2 2 2 2 2 2 Since k = k0 - 4pr k (cos a'+sin a') = k0 (cos a +sin a) - 4pr 2 2 2 2 2 2 i.e. k sin a'= k0 sin a - 4pr or kz = k0z - 4pr

The critical value of k0z for total external reflection is k0z = 4pr For quartz k critical = 2.05x10-3 A-1 0 zl (2p /l)sina = k critical Þ critical 0 zl o a acritical ( ) » 0.02l(A) for quartz o a’ Note : acritical ( ) » 0.1l(A) for Reflection of Neutrons by a Smooth Surface: Fresnel’s Law continuity of y & y& at z = 0 Þ aI + aR = aT (1) n = 1-l2r/2p r r r aI kI + aRkR = aT kT components perpendicular and parallel to the surface : aI k cosa + aR k cosa = aT nk cosa¢ (2)

-(aI - aR )k sina = -aT nk sin a¢ (3) (1) & (2) => Snell's Law : cosa = ncosa¢ (a - a ) sina¢ sina¢ k (1) & (3) => I R = n » = Tz (aI + aR ) sina sina kIz so reflectance is given by r = aR / aI = (kIz - kTz )/(kIz + kTz ) What do the Amplitudes aR and aT Look Like?

• For reflection from a flat substrate, both aR and aT are complex when k0 < 4pr I.e. below the critical edge. For aI = 1, we find:

2 1 1.5 0.5 1

0.5 0.005 0.01 0.015 0.02 0.025

0.005 0.01 0.015 0.02 0.025 -0.5 -0.5

-1 -1

Real (red) & imaginary (green) parts of aR Real (red) and imaginary (green) parts plotted against k . The modulus of a is 0 R of aT. The modulus of aT is plotted in plotted in blue. The critical edge is at blue. Note that a tends to unity at -1 T k0 ~ 0.009 A . Note that the reflected wave is large values of k as one would expect completely out of phase with the incident wave 0

at k0 = 0 One can also think about Neutron Reflection from a Surface as a 1-d Problem

2 V(z)= 2 p r(z) h /mn

2 2 k =k0 - 4p r(z) V(z) substrate Where V(z) is the potential seen by the neutron & r(z) is the scattering length density

z Film Vacuum Fresnel’s Law for a Thin Film k0z

0.01 0.02 0.03 0.04 0.05 • r=(k1z-k0z)/(k1z+k0z) is Fresnel’s law -1

-6 -2 • Evaluate with r=4.10 A gives the -2 red curve with critical wavevector -3 1/2 given by k0z = (4pr) -4 • If we add a thin layer on top of the substrate we get interference fringes & -5 Log(r.r*) the reflectance is given by:

r + r ei2k1z t r = 01 12 0 1+ r r ei2k1z t 01 12 1 Film thickness = t

and we measure the reflectivity R = r.r* 2 substrate

• If the film has a higher scattering length density than the substrate we get the green curve (if the film scattering is weaker than the substance, the green curve is below the red one)

• The fringe spacing at large k0z is ~ p/t (a 250 A film was used for the figure) Kinematic (Born) Approximation

• We defined the scattering cross section in terms of an incident plane wave & a weakly scattered spherical wave (called the Born Approximation)

• This picture is not correct for surface reflection, except at large values of Qz

• For large Qz, one may use the definition of the scattering cross section to calculate R for a flat surface (in the Born Approximation) as follows: number of neutrons reflected by a sample of size L L R = x y number of neutrons incident on sample (= FL xLysina) s 1 ds 1 ds dk dk = = dW = x y ò ò 2 Lx Ly sin a Lx Ly sin a dW Lx Ly sin a dW k0 sin a

because kx = k0 cosa so dk x = -k0 sin a da. From the definition of a cross section we get for a smooth substrate : ds r r r v r 4p 2 = r 2 dr dr 'eiQ.(r -r ') = r 2 L L d (Q )d (Q ) so R = 16p 2r 2 / Q4 ò ò 2 x y x y z dW Qz

It is easy to show that this is the same as the Fresnel form at large Qz Reflection by a Graded Interface Repeating the bottom line of the previous viewgraph but keeping the z -dependence 2 16p 2 2 16p 2 dr(z) of r gives : R = r(z)eiQz zdz = eiQ z zdz where the second 2 ò 4 ò Qz Qz dz equality follows after intergrating by parts.

If we replace the prefactor by the Fresnel reflectivity R F, we get the right answer for a smooth interface, as well as the correct form at large Qz dr(z) 2 R = R eiQz zdz F ò dz This can be solved analytically for several convenient forms of dr/dz such as 1/cosh2 (z). This approximate equation illustrates an important point : reflectivity data cannot be inverted uniquely to obtain r(z), because we generally lack important phase information. This means that models refined to fit refelctivi ty data must have good physical justificat . The Goal of Reflectivity Measurements Is to Infer a Density Profile Perpendicular to a Flat Interface

• In general the results are not unique, but independent knowledge of the system often makes them very reliable • Frequently, layer models are used to fit the data • Advantages of neutrons include: – Contrast variation (using H and D, for example) – Low absorption – probe buried interfaces, solid/liquid interfaces etc – Non-destructive – Sensitive to magnetism – Thickness 10 – 5000 Å Direct Inversion of Reflectivity Data is Possible*

• Use different “fronting” or “backing” materials for two measurement of the same unknown film

– E.g. D2O and H2O “backings” for an unknown film deposited on a quartz substrate or Si & Al2O3 as substrates for the same unknown sample – Allows Re(R) to be obtained from two simultaneous equations for 2 2 R1 and R2 – Re(R) can be Fourier inverted to yield a unique SLD profile • Another possibility is to use a magnetic “backing” and polarized neutrons

Si or Al2O3 substrate SiO2

Unknown film H2O or D2O * Majkrzak et al Biophys Journal, 79,3330 (2000) Vesicles composed of DMPC molecules fuse creating almost a perfect lipid bilayer when deposited on the pure, uncoated quartz block* (blue curves)

When PEI polymer was added only after quartz was covered by the lipid bilayer, the PEI appeared to diffuse under the bilayer (red curves) Scattering Length Density Neutron Reflectivities

] Profiles -8 -2 -6 10 2 10 Polymer s = 5.9 Å O 4

2 Quartz z 0

-2 10 -6 Lipid Bilayer on Quartz -4 10 -6 O Hydrogenated Quartz 2

D Tails -6 10 -6 Reflectivity, R*Q Head Lipid Bilayer on Polymer Head on Quartz -8 10 -6 s = 4.3 Å 10 -9 Scattering Length Density [Å 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.0 20.0 40.0 60.0 80.0 100.0 Q [Å-1 ] Length, z [Å] z

* Data courtesy of G. Smith (LANSCE) Polymer-Decorated Lipids at a Liquid-Air Interface*

SLD Profiles of PEG-Lipids 1.3% PEG lipid 4.5% PEG lipid 9.0% PEG lipid -6 in lipids in lipids in lipids 8 10

-6

] 6 10 -2

-6 4 10 Black - pure lipid Blue - 1.3% PEG

SLD [Å Green - 4.5% PEG -6 2 10 Red - 9.0% PEG

0 40 60 80 100 120 140 160 Length [Å]

Interface broadens as PEG mushroom-to- concentration increases - this is brush transition main effect seen with x-rays

Neutron Reflectivities X-Ray Reflectivities -7 1 10 10

Pure lipid neutrons see contrast between Pure lipid heads (2.6), tails (-0.4), 10 0 4 z D2O (6.4) & PEG (0.24) F -8 10 10 -1 R*Q R/R

10 -2 Lipid + 9% PEG x-rays see heads (0.65), but all Lipid + 9% PEG

-9 -3 10 else has same electron density 10 0 0.05 0.1 0.15 0.2 0.25 0 0.1 0.2 0.3 0.4 0.5 0.6 -1 within 10% (•0.33) Q [Å ] Q z z *Data courtesy of G. Smith (LANSCE) Non-Fickian Boron Self-Diffusion at an Interface*

Boron Self Diffusion 10 -4 10

-5 Unnanealed sample 10 2.4 Hours 5.4 Hours 0.1 9.7 Hours 10 -6 22.3 Hours 35.4 Hours

-7 10 0.001

10 -8

10-5 -9 10 Reflectivity

-10 10-7 10

10 -11 -9 0.045 0.05 0.055 0.06 0.065 0.07 0.075 0.08 10

10-11 Data requires density Fickian diffusion 0 0.02 0.04 0.06 0.08 0.1 0.12 step at interface doesn’t fit the data -1 Q (Å)

Non-Standard Fickian Model Standard Fickian Model -6 8 10 -6 8 10 11 B ~650Å -6 7 10 -6 7 10 ) ) -2

-2 -6 10 6 10 -6 6 10 B

-6 ~1350Å 5 10 -6 5 10

-6 4 10 -6 4 10

Si -6 -6 3 10 3 10

2 10 -6 2 10 -6 Scattering Length Density (Å Scattering Length Density (Å

1 10 -6 1 10 -6

0 0 -800 -600 -400 -200 0 200 400 600 800 -600 -400 -200 0 200 400 600 800 *Data courtesy of G. Smith (LANSCE) Distance from 11 B- 10 B Interface (Å) Distance from 11 B- 10 B Interface (Å) Polarized (PNR)

Non-Spin-Flip Spin-Flip

++ measures b + Mz +- measures Mx + i My - - measures b - Mz -+ measures Mx – i My Structure, & Magnetism of Fe(001) on MgO(001)*

X-Ray Neutron 0.20(1)nm 100 b = 3.9(3) 1.9(2)nm a-FeOOH b = 4.4(8) 0.44(2)nm 10-1

10-2 b = 5.2(3) 10.7(2)nm Fe b = 8.6(4) 10-3 X-Ray

10-4 0.07(1)nm X-Ray Reflectivity b = 4.4(2) 2.1(2)nm interface b = 6.9(4) -5 10 0.03(1)nm

10-6 0 0.1 0.2 0.3 0.4 0.5 -1 Q [Å] b = 3.1(3) MgO b = 6.2(5) Co-Refinement H

100

R (obs) ++ R (fit) -1 ++ R (obs) 10 -- R (fit) -- R (obs) SF R (fit) SF 10 -2 Neutron TEM

10 -3

Polarized Neutron Reflectivity a-FeOOH (1.8µB) 10 -4 H

10 -5 0.00 0.02 0.04 0.06 0.08 0.10 0.12 fcc Fe (4µ) -1 Fe(001) (2.2µ) B Q [Å ] B

*Data courtesy of M. Fitzsimmons (LANSCE) Reflection from Rough Surfaces

k2 k1 z q q z = 0 1 2

t k 1

x • diffuse scattering is caused by surface roughness or inhomogeneities in the reflecting medium • a smooth surface reflects radiation in a single (specular) direction • a rough surface scatters in various directions • specular scattering is damped by surface roughness – treat as graded interface. For a single surface with r.m.s roughness s: t 2 -2kIz k1 zs R = RF e When Does a “Rough” Surface Scatter Diffusely?

• Rayleigh criterion

g g g g h path difference: Dr = 2 h sing phase difference: Df = (4ph/l) sing boundary between rough and smooth: Df = p/2 that is h < l/(8sing) for a smooth surface

where g = 4 p h sin g / l = Qz h Qx-QzTime -of-Flight, Energy-Dispersive Neutron Reflectometry Transformation

z Raw data in qf -TOF space for a single layer. Note that large divergence does not imply poor Q resolution Raw Data fromz SPEAR ® -ki ® ® ki Q ® TOF • l . L / 4 kf qi qf x

Q 2 p ® ® ® x = l *(cos qf-cos qi) Q =kf -ki where Q 2 p z= l *(sin qf +sin qi)

For specular reflectivity qf =qi. Then,

4 psin( q) Qz= =2k l z

Vandium-Carbon Multilayer — specular & diffuse scattering

in qf -TOF space and transformed to Qx-Qz The Study of Diffuse Scattering From Rough Surfaces Has Not Made Much Headway Because Interpretation Is Difficult

The theory (Distorted Wave Born Approximation) used to describe scattering 2 2 from a rough surface, works in some cases but breaks down when xkz / k >> 1, where x is the range of correlations in the surface

In some cases (e.g. faceted surfaces) one would also expect the approximation of a using an "average surface" wavefunction for perturbation theory to break down.

t 2 -2k0k1s Rmicro-rough = Rsmooth e

-2k 2s 2 0 Rfacet = Rsmooth e Observation of Hexagonal Packing of Thread-like Micelles Under Shear: Scattering From Lateral Inhomogeneities

NEUTRON QUARTZ or SILICON BEAM

INLET HOLES

TEST SECTION

TEFLON LIP OUTLET TRENCH

RESERVOIRS

TEFLON Specularly reflected beam

z

x Quartz Single Crystal

Scattering pattern

Up to implies hexagonal Microns 46Å Flow symmetry direction H2O

Thread-like micelle by

Roger Pynn

Los Alamos National Laboratory

LECTURE 5: Small Angle Scattering This Lecture

5. Small Angle Neutron Scattering (SANS) 1. What is SANS and what does it measure? 2. Form factors and particle correlations 3. Guinier approximation and Porod’s law 4. Contrast and contrast variation 5. Deuterium labelling 6. Examples of science with SANS 1. Particle correlations in colloidal suspensions 2. bubble size distribution in steel 3. Verification of Gaussian statistics for a polymer chain in a melt 4. Structure of 30S ribosome 5. The fractal structure of sedimentary rocks

Note: The NIST web site at www.ncnr.nist.gov has several good resources for SANS – calculations of scattering length & form factors as well as tutorials Small Angle Neutron Scattering (SANS) Is Used to Measure Large Objects (~1 nm to ~1 mm)

• Complex fluids, alloys, precipitates, biological assemblies, glasses, ceramics, flux lattices, long-wave- length CDWs and SDWs, critical scattering, porous media, fractal structures, etc

Scattering at small angles probes large length scales Two Views of the Components of a Typical Reactor-based SANS Diffractometer The NIST 30m SANS Instrument Under Construction Where Does SANS Fit As a Structural Probe?

• SANS resolves structures on length scales of 1 – 1000 nm • Neutrons can be used with bulk samples (1-2 mm thick) • SANS is sensitive to elements such as H, C & N • SANS is sensitive to such as H and D What Is the Largest Object That Can Be Measured by SANS?

• Angular divergence of the neutron beam and its lack of monochromaticity contribute to finite transverse & longitudinal coherence lengths that limit the size of an object that can be seen by SANS

• For waves emerging from slit center, path difference is hd/L if d<

+a r2 h • The variation of path difference, which causes decreased visibility of Young’s fringes, is ad/L so the variation in phase -a difference is Dy = kad/L

• To maintain the visibility of the Young’s interference pattern we Dy ~ 2p so the L coherence length, d ~ l/a where a is the divergence angle a/L

• The coherence length is the maximum distance between points in a scatterer for which interference effects will be observable Instrumental Resolution for SANS

Traditionally, neutron scatterers tend to think in terms of Q and E resolution 4p dQ 2 dl 2 cos 2 q.dq 2 Q = sin q Þ = + l Q 2 l2 sin 2 q dQ 2 dq 2 For SANS, (dl/l) ~ 5% and q is small, so = 0.0025 + rms Q 2 q 2 For equal source - sample & sample - detector distances of L and equal apertures at source and sample of h, dq rms = 5/12h/L.

The smallest v alue of q is determined by the direct beam size : q min ~ 1.5h / L At this value of q , angular resolution dominates and dQ rms ~ (dq rms /q min )Q min ~ dq rms 4p / l ~ (2p / l)h / L

The largest observable object is ~ 2p/dQ rms ~ lh / L . This is equal to the transvers e coherence length for the neutron and achieves a maximum of about 5 mm at the ILL 40 m SANS instrument using 15 Å neutrons. Note that at the largest va lues of q , set by the detector size and distance from the sample, wavelengt h resolution dominates. The Scattering Cross Section for SANS

r ds 2 r r 1 r r r 2 recall that = b NS(Q) where S(Q) = dr.e-iQ.r n (r ) dO N ò nuc 2 r where b is the coherent nuclear scattering length and nnuc(r) is the nuclear density

• Since the length scale probed at small Q is >> inter-atomic spacing we may use the scattering length density (SLD), r, introduced for surface reflection and note that

nnuc(r)b is the local SLD at position r.

• A uniform scattering length density only gives forward scattering (Q=0), thus SANS measures deviations from average scattering length density.

• If r is the SLD of particles dispersed in a medium of SLD r0, and np(r) is the density, we can separate the integral in the definition of S(Q) into an integral over the positions of the particles and an integral over a single particle. We also measure the particle SLD relative to that of the surrounding medium I.e: SANS Measures Particle Shapes and Inter-particle Correlations

r ds 2 r r r r = b d 3r d 3r'n (r )n (r ').eiQ.(r -r') ò ò N N dW space space 2 r r r r r r v = d 3 R d 3R' n (R)n (R') eiQ.( R-R') (r - r ) d 3x.eiQ.x ò ò P P 0 ò space space particle ds r 2 r r r = (r - r )2 F(Q) N d 3R.G (R).eiQ.R 0 P ò P dW space where G P is the particle - particle correlation function (the probability that the re r v 2 is a particle at R if there's one at the origin) and F(Q) is the particle form factor :

2 v 2 r v F(Q) = ò d 3x.eiQ.x particle

These expressions are the same as those for nuclear scattering except for the addition of a form factor that arises because the scattering is no longer from point-like particles Scattering for Spherical Particles

2 r 2 r r r The particle form factor F(Q) = ò dreiQ.r is determined by the particle shape. V For a of radius R, F(Q) only depends on the magnitude of Q :

ésin QR-QRcosQRù 3V0 Fsphere(Q) = 3V0 ê 3 ú º j1(QR) ® V0 at Q = 0 ë (QR) û QR Thus, as Q ® 0, the total scattering from an assembly of uncorrelated spherical r v particles [i.e. when G(r) ® d (r)] is proportional to the square of the particle volume times the number of particles. 1

For elliptical particles 0.8 replace R by : 3j1(x)/x 0.6 2 2 2 2 1/ 2 R ® (a sin J + b cos J) 0.4 where J is the angle between r 0.2 x the major axis (a) and Q 2 4 6 8 10 Examples of Spherically-Averaged Form Factors

Form Factor for Cylinder with Q at angle q to cylinder axis

R 4V sin([QH / 2]cosq )J (QR sinq ) F(Q,q, H , R) = 0 1 QH cosq (QR sinq )2

R H

Fo rm Fact or fo r a Ve s icle r 2 2 R j1(QR) - r j1(Qr) F(Q, R,r) = 3V0 2 3 3 Q (R - r ) t = R - r Determining From Dilute Suspensions

• Particle size is usually deduced from dilute suspensions in which inter-particle correlations are absent • In practice, instrumental resolution (finite beam coherence) will smear out minima in the form factor • This effect can be accounted for if the are mono-disperse • For poly-disperse particles, maximum entropy techniques have been used successfully to obtain the distribution of particles sizes Correlations Can Be Measured in Concentrated Systems

• A series of experiments in the late 1980’s by Hayter et al and Chen et al produced accurate measurements of S(Q) for colloidal and micellar systems • To a large extent these data could be fit by S(Q) calculated from the mean spherical model using a to yield surface charge and screening length Size Distributions Have Been Measured for Helium Bubbles in Steel

• The growth of He bubbles under neutron is a key factor limiting the lifetime of steel for fusion reactor walls – Simulate by bombarding steel with alpha particles • TEM is difficult to use because bubble are small • SANS shows that larger bubbles grow as the steel is annealed, as a result of coalescence of small bubbles and incorporation of individual He atoms

SANS gives bubble volume (arbitrary units on the plots) as a function of bubble size at different temperatures. Red shading is 80% confidence interval. Radius of Gyration Is the Particle “Size” Usually Deduced From SANS Measurements r If we measure r from the centroid of the particle and expand the exponential in the definition of the form factor at small Q : r r r r r 1 r r F(Q) = dreiQ.r »V +i Q.rd 3r - (Q.r) 2 d 3r +.... ò 0 ò ò V V 2 V p é 2 2 3 ù ê cos q sinq.dq r d r ú 2 2 Q2r2 2 ò ò é Q r ù - g ê Q 0 V ú g 6 =V0 1- +... = V0 ê1- +...ú » V0e ê 2 p d 3 r ú 6 sinq.dq ò ëê ûú ê ò V ú ë 0 û where r is the radius of gyration is r = R 2 d 3r / d 3r. It is usually obtained from a fit g g ò ò V V to SANS data at low Q (in the so -called Guinier region) or by plotting ln(Intensity) v Q 2 . 2 The slope of the data at the lowest values of Q is rg /3. It is easily verified that the expression for the form factor of a sphere is a special case of this general result. Guinier Approximations: Analysis Road Map Generalized Guinier approximation

1; a = 0 R2 Q2 P(Q) = DMa0 exp – a ap Q–a; a = 1,2 3 – a

Derivative-log analysis

d ln P(Q) Q d P(Q) = = –a – 2 R2 Q2 d ln(Q) P(Q) dQ 3 – a a

• Guinier approximations provide a roadmap for analysis. 2 • Information on particle composition, shape and size. • Generalization allows for analysis of complex mixtures, allowing identification of domains where each approximation applies.

* Viewgraph courtesy of Rex Hjelm Contrast & Contrast Matching

O D2O

Both tubes contain borosilicate beads + pyrex fibers + solvent. (A) solvent refractive index matched to pyrex;. (B) solvent index different from both beads and fibers – scattering from fibers HO O 2 dominates * Chart courtesy of Rex Hjelm Isotopic Contrast for Neutrons

Hydrogen Scattering Length Nickel Scattering Lengths Isotope b (fm) Isotope b (fm) 1 H -3.7409 (11) 58Ni 15.0 (5) 2 D 6.674 (6) 60Ni 2.8 (1) 3T 4.792 (27) 61Ni 7.60 (6)

62Ni -8.7 (2) 64Ni -0.38 (7) Verification of of the Gaussian Coil Model for a Polymer Melt

• One of the earliest important results obtained by SANS was -1/2 the verification of that rg~N for polymer chains in a melt

• A better experiment was done 3 years later using a small amount of H-PMMA in D-PMMA (to avoid the large incoherent background) covering a MW range of 4 decades SANS Has Been Used to Study Bio-machines

• Capel and Moore (1988) used the fact that can grow when H is replaced by D to produce reconstituted ribosomes with various pairs of proteins (but not rRNA) deuterated • They made 105 measurements of inter- distances involving 93 30S protein pairs over a 12 year period. They also measured radii of gyration • Measurement of inter-protein distances is done by Fourier transforming the form factor to obtain G(R) • They used these data to solve the ribosomal structure, resolving ambiguities by comparison with electron microscopy Porod Scattering

2 Let us examine the behavior of F(Q) (QR)4 at large values of Q for a spherical particle (i.e. Q >>1/R where R is the sphere radius) 2 2 2 4 2 ésin QR -QR.cosQRù 4 2 ésin QR ù F(Q) (QR) = 9V ê 3 ú (QR) = 9V ê - cosQRú ë (QR) û ë QR û ® 9V 2 cos2 QR as Q ® ¥ = 9V 2 / 2 on average (the oscillations will be smeared out by resolution) 2 2 9V 2pA Thus F(Q) ® = where A is the area of the sphere's surface. 2(QR)4 Q 4

This is Porod's law and holds as Q ® ¥ for any particle shape provided the particle surface is smooth. Another way to obtain it is to expand G(r) =1- ar + br2 +.. [with a = A/(2pV)] at small r and to evaluate the form factor with this (Debye) form for the correlation function. Scattering From Fractal Systems

• Fractals are systems that are “self-similar” under a change of scale I.e. R -> CR • For a mass fractal the number of particles within a sphere of radius R is proportional to RD where D is the fractal dimension Thus 4pR2 dR.G(R) = number of particles between distance R and R + dR = cR D-1dR \ G(R) = (c / 4p )R D-3 r r r r 2p and S(Q) = dR.eiQ.RG(R) = dR.R.sin QR.(c / 4p )RD-3 ò Q ò c 1 const = dx.x D-2 .sin x = 2 Q D ò Q D const For a surface fractal, one can prove that S(Q) µ which reduces to the Porod Q6- Ds form for smooth surfaces of dimension 2. Typical Intensity Plot for SANS From Disordered Systems Zero Q intercept - gives particle volume if concentration is known

2 Guinier region (slope = -rg /3 gives particle “size”) ln(I) Mass fractal dimension (slope = -D)

Porod region - gives surface area and surface fractal dimension

{slope = -(6-Ds)}

ln(Q) Sedimentary Rocks Are One of the Most Extensive Fractal Systems*

Variation of the average number of SEM features per unit length with feature size.

Note the breakdown of fractality (Ds=2.8 SANS & USANS data from sedimentary rock to 2.9) for lengths larger than 4 microns showing that the pore-rock interface is a surface

fractal (Ds=2.82) over 3 orders of magnitude in length scale *A. P. Radlinski (Austr. Geo. Survey) References

• Viewgraphs describing the NIST 30-m SANS instrument – http://www.ncnr.nist.gov/programs/sans/tutorials/30mSANS_desc.pdf by

Roger Pynn

Los Alamos National Laboratory

LECTURE 6: Inelastic Scattering We Have Seen How Neutron Scattering Can Determine a Variety of Structures

X-Ray Neutron 0.20(1)nm b = 3.9(3) 1.9(2)nm a-FeOOH b = 4.4(8) 0.44(2)nm

b = 5.2(3) 10.7(2)nm Fe b = 8.6(4)

0.07(1)nm

b = 4.4(2) 2.1(2)nm interface b = 6.9(4) 0.03(1)nm

b = 3.1(3) MgO b = 6.2(5) crystals surfaces & interfaces disordered/fractals biomachines but what happens when the atoms are moving? Can we determine the directions and time-dependence of atomic motions? Can well tell whether motions are periodic? Etc.

These are the types of questions answered by inelastic neutron scattering The Neutron Changes Both Energy & Momentum When Inelastically Scattered by Moving Nuclei The Elastic & Inelastic Scattering Cross Sections Have an Intuitive Similarity

• The intensity of elastic, coherent neutron scattering is proportional to the spatial Fourier Transform of the Pair Correlation Function, G(r) I.e. the probability of finding a particle at position r if there is simultaneously a particle at r=0

• The intensity of inelastic coherent neutron scattering is proportional to the space and time Fourier Transforms of the time-dependent pair correlation function function, G(r,t) = probability of finding a particle at position r at time t when there is a particle at r=0 and t=0.

• For inelastic incoherent scattering, the intensity is proportional to the

space and time Fourier Transforms of the self-correlation function, Gs(r,t) I.e. the probability of finding a particle at position r at time t when the same particle was at r=0 at t=0 The Inelastic Scattering Cross Section

æ d 2s ö k' r æ d 2s ö k' r ç ÷ 2 ç ÷ 2 Recall that ç ÷ = bcoh NS(Q,w) and ç ÷ = binc NSi (Q,w) è dW.dE øcoh k è dW.dE øinc k

r 1 r r r r r 1 r r r r where S(Q,w) = G(r,t)ei(Q.r -wt )drdt and S (Q,w) = G (r,t)ei(Q.r -wt )drdt 2ph òò i 2ph òò s and the correlation functions that are intuitively similar to those for the elastic scattering case: r 1 r r r r r 1 r r r r r r G(r,t) = r (r,0)r (r + R,t) dr and G (r,t) = d (r - R (0))d (r + R - R (t)) dr ò N N s åò j j N N j

The evaluation of the correlation functions (in which the r's and d - functions have to be treated as non -commuting quantum mechanical operators) is mathematically tedious. Details can be found, for example, in the books by Squires or Marshal and Lovesey. Examples of S(Q,w) and Ss(Q,w)

• Expressions for S(Q,w) and Ss(Q,w) can be worked out for a number of cases e.g: – Excitation or absorption of one quantum of lattice vibrational energy (phonon) – Various models for atomic motions in liquids and glasses – Various models of atomic & molecular translational & rotational diffusion – Rotational tunneling of molecules – Single particle motions at high momentum transfers – Transitions between crystal field levels – and other magnetic excitations such as spinons • Inelastic neutron scattering reveals details of the shapes of interaction potentials in materials A Phonon is a Quantized Lattice Vibration

• Consider linear chain of particles of mass M coupled by springs. Force on n’th particle is

Fn = a 0un +a1(un-1 + un+1 ) +a 2 (un- 2 + un+2 ) +...

First neighbor force constant displacements

&& • Equation of motion is Fn = Mun i (qna-wt ) 2 4 2 1 • Solution is: un (t) = Aq e with w q = åan sin ( nqa) M n 2 2p 4p N 2p q = 0,± ,± ,...... ± w L L 2 L 1.4 1.2

1 u 0.8 a 0.6 0.4 Phonon Dispersion Relation: 0.2 qa/2p Measurable by inelastic neutron scattering -1 -0.5 0.5 1 Inelastic Magnetic Scattering of Neutrons

• In the simplest case, atomic spins in a ferromagnet precess about the direction of mean magnetization r r r r + H = å J (l - l ')Sl .Sl' = H 0 + åhwqbq bq l,l' q

exchange coupling spin waves (magnons) ground state energy

with r r r iq.l hwq = 2S(J0 - Jq ) where Jq = å J (l )e l Fluctuating spin is 2 perpendicular to mean spin hw q = Dq is the dispersion relation for a ferromagnet direction => spin-flip neutron scattering

Spin wave animation courtesy of A. Zheludev (ORNL) Measured Inelastic Neutron Scattering Signals in Crystalline Solids Show Both Collective & Local Fluctuations*

Spin waves – collective Crystal Field splittings

excitations (HoPd2Sn) – local excitations

Local spin resonances (e.g. ZnCr2O4)

* Courtesy of Dan Neumann, NIST Measured Inelastic Neutron Scattering Signals in Liquids Generally Show Diffusive Behavior

“Simple” liquids (e.g. water) Complex Fluids (e.g. SDS)

Quantum Fluids (e.g. He in porous silica) Measured Inelastic Neutron Scattering in Molecular Systems Span Large Ranges of Energy

Vibrational spectroscopy Molecular reorientation Rotational tunneling (e.g. C ) 60 (e.g. pyrazine) (e.g. CH3I)

Polymers Proteins Atomic Motions for Longitudinal & Transverse r 2p Q = (0.1,0,0) a a

Q Q Transverse phonon Longitudinal phonon r r eT = (0,0.1,0)a eL = (0.1,0,0)a r r r r r i(Q.Rl -wt ) Rl = Rl 0 + ese Transverse Optic and Acoustic Phonons

Q

Acoustic Optic r r ered = (0,0.1,0)a ered = (0,0.1,0)a r r eblue = (0,0.14,0)a eblue = (0,-0.14,0)a r r r r 0 r i (Q.Rl -wt ) Rlk = Rlk + ese Phonons – the Classical Use for Inelastic Neutron Scattering

• Coherent scattering measures scattering from single phonons r r æ d 2s ö k ' p 2 (Q.e ) 2 1 ±1 r r r ç ÷ - 2W s ç ÷ = s coh e å å (ns + )d (w m w s )d (Q - q - G ) è dWdE ø coh±1 k MV 0 s G w s 2 • Note the following features: – Energy & momentum delta functions => see single phonons (labeled s)

– Different thermal factors for phonon creation (ns+1) & (ns) – Can see phonons in different Brillouin zones (different recip. lattice vectors, G)

– Cross section depends on relative orientation of Q & atomic motions (es)

– Cross section depends on phonon frequency (ws) and (M) – In general, scattering by multiple excitations is either insignificant or a small correction (the presence of other phonons appears in the Debye- Waller factor, W) The Workhorse of Inelastic Scattering Instrumentation at Reactors Is the Three-axis Spectrometer

Q

kF kI

“scattering triangle” The Accessible Energy and Wavevector Transfers Are Limited by Conservation Laws • Neutron cannot lose more than its initial kinetic energy & momentum must be conserved Triple Axis Spectrometers Have Mapped Phonons Dispersion Relations in Many Materials • Point by point measurement in (Q,E) space

• Usually keep either kI or kF fixed • Choose Brillouin zone (I.e. G) to maximize scattering cross section for phonons • Scan usually either at constant-Q (Brockhouse invention) or constant-E

Phonon dispersion of 36Ar What Use Have Phonon Measurements Been?

• Quantifying interatomic potentials in , rare gas solids, ionic crystals, covalently bonded materials etc

• Quantifying anharmonicity (I.e. phonon-phonon interactions)

• Measuring soft modes at 2nd order structural phase transitions

• Electron-phonon interactions including Kohn anomalies

• dispersion in liquid He

• Relating phonons to other properties such as superconductivity, anomalous lattice expansion etc Examples of Phonon Measurements

Soft mode

Phonons in 36Ar – validation of LJ potential

Roton dispersion in 4He

Kohn anomalies in 110Cd

Phonons in 110Cd Time-of-flight Methods Can Give Complete Dispersion Curves at a Single Instrument Setting in Favorable Circumstances

CuGeO3 is a 1-d magnet. With the unique axis parallel to the incident neutron beam, the complete dispersion can be obtained Much of the Scientific Impact of Neutron Scattering Has Involved the Measurement of Inelastic Scattering Neutrons in Condens ed Matter Res earch

10000

SpPaSllSa t-i oCnh o- pCpheorp Sppeer ctrometer Neutron ILL - without s pin-echo Induced Itinerant Excitations ILL - with s pin-e cho Hydrogen 100 Magnets Modes Crystal Momentum Elastic Scattering Fields Distributions Molecular Coherent Spin Vibrations Modes in Waves Glasses Lattice Vibrations ) and Liquids

V Electron- and

e 1 Molecular phonon Anharmonicity

m Interactions

( Motions

[Larger Objects Resolved] Accurate Liquid Structures r Metallurgical Crystal and Membranes e Micelles Polymers Systems Magnetic Amorphous Precision Crystallography f Proteins Systems s Proteins in Solution Colloids Structures Anharmonicity n

a Viruses Critical r Slower

T Scattering 0.01 Motions y

g Resolved r

e Diffusive

n Aggregate Modes E Motion Polymers Molecular and Reorientation 10-4 Biological Tunneling Systems Spectroscopy

Surface Effects?

10-6 0.001 0.01 0.1 1 10 100 -1 Q (Å ) Energy & Wavevector Transfers accessible to Neutron Scattering Neutron

By

Roger Pynn*

Los Alamos National Laboratory

*With contributions from B. Farago (ILL), S. Longeville (Saclay), and T. Keller (Stuttgart) What Do We Need for a Basic Neutron Scattering Experiment?

• A source of neutrons • A method to prescribe the wavevector of the neutrons incident on the sample • (An interesting sample) • A method to determine the wavevector of the scattered neutrons • A neutron detector Detector

Incident neutrons of wavevector ki Scattered neutrons of

wavevector, kf Sample k 2 i E = ω = h (k 2 − k 2 ) h 2m i f Q kf We usually measure scattering as a function of energy (E) and wavevector (Q) transfer Instrumental Resolution

• Uncertainties in the neutron wavelength & direction of travel imply that Q and E can only be defined with a certain precision

• When the box-like resolution volumes in the figure are convolved, the overall resolution is Gaussian (central limit theorem) and has an elliptical shape in (Q,E) space

• The total signal in a scattering experiment is proportional to the phase space volume within the elliptical resolution volume – the better the resolution, the smaller the resolution volume and the lower the count rate The Goal of is to Break the Inverse Relationship between Intensity & Resolution

• Traditional – define both incident & scattered wavevectors in order to define E and Q accurately

• Traditional – use collimators, monochromators, choppers etc to define

both ki and kf

•NSE– measure as a function of the difference between appropriate

components of ki and kf (original use: measure ki –kf i.e. energy change)

•NSE– use the neutron’s spin polarization to encode the difference

between components of ki and kf

•NSE– can use large beam divergence &/or poor monochromatization to increase signal intensity, while maintaining very good resolution The Underlying Physics of Neutron Spin Echo (NSE) Technology is Larmor Precession of the Neutron’s Spin

• The time evolution of the expectation value of the spin of a spin-1/2 particle in a magnetic field can be B determined classically as: r γ ds r r γ = s ∧ B ⇒ L = B s dt ω = −2913 * 2π Gauss −1.sγ−1

• The total precession angle of the spin, φ, depends on the time

the neutron spends in the field: φ = ω Lt

3 -1 -1 B(Gauss) ωL (10 rad.s ) N (msec ) Turns/m for 4 Å neutrons 10 183 29 ~29 Larmor Precession allows the Neutron Spin to be Manipulated using π or π/2 Spin-Turn Coils: Both are Needed for NSE

• The total precession angle of the spin, φ, depends on the time the neutron spends in the B field

φ = ω Lt = γBd / v

d

Neutron velocity, v

B

1 Number of turns = .B[Gauss].d[cm].λ[Angstroms] 135.65 How does a Neutron Spin Behave when the Magnetic Field Changes Direction?

When H rotates with

frequency ω, H0->H1 ->H2…, and the spin trajectory is described by a cone rolling on the plane in which H moves

• Distinguish two cases: adiabatic and sudden • Adiabatic –tan(δ) << 1 – large B or small ω – spin and field remain co-linear – this limit used to “guide” a neutron spin • Sudden –tan(δ) >> 1 – large ω – spin precesses around new field direction – this limit is used to design spin-turn devices Neutron Spin Echo (NSE) uses Larmor Precession to “Code” Neutron Velocities

• A neutron spin precesses at the Larmor frequency in a magnetic field, B. ωL = γB • The total precession angle of the spin, φ, depends on the time the neutron spends in the field r H,σr d

φ = ωLt = γBd / v

B Neutron velocity, v 1 Number of turns = .B[Gauss].d[cm].λ[Angstroms] 135.65 The precession angle φ is a measure of the neutron’s speed v The Principles of NSE are Very Simple

• If a spin rotates anticlockwise & then clockwise by the same amount it comes back to the same orientation – Need to reverse the direction of the applied field – Independent of neutron speed provided the speed is constant

• The same effect can be obtained by reversing the precession angle at the mid-point and continuing the precession in the same sense –Use a π rotation

• If the neutron’s velocity, v, is changed by the sample, its spin will not come back to the same orientation – The difference will be a measure of the change in the neutron’s speed or energy In NSE*, Neutron Spins Precess Before and After Scattering & a y Polarization Echo is Obtained if Scattering is Elastic x z π/2 π/2 F S π F S

Allow spins to Rotate spins Elastic Initially, precess around z: to z and Scattering neutrons slower neutrons measure Event are polarized precess further over polarization along z a fixed path-length

Rotate spins Rotate spins into Allow spins to precess through π about x-y precession plane around z: all spins are in x axis the same direction at the φ Final Polarization, P = cos( 1 −φ2 ) echo point if ∆E = 0 * F. Mezei, Z. Physik, 255 (1972) 145 For Quasi-elastic Scattering, the Echo Polarizationω depends on Energy Transfer φ • If theφ neutron changes energy when it scatters, the precession γ δ phases before & after scattering, φ1 & φ2, will be different: γ 1 2 2 using h = m(v1 − v2 ) ≈δmv γv 2 ω γ  1 1  Bd Bd Bdm2 3 − = Bd −  ≈ v ≈ h = λ ω 1 2   2 3 2  v1 v2  v mv 2πh

• To lowest order, the difference between φ1 & φ2 depends only

on ω (I.e. v1 – v2) & not on v1 & v2 separately

• The measured polarization,λ

, is the average of cos(φ1 - φ2) over all transmitted neutronsω I.e. r φ I( )S(Q, )cos( φ− )d d ∫∫ λ 1 2 λ ω P = r ω ∫∫ I( )S(Q, )dλdω λ Neutronω Polarization at the Echo Point is a φ Measure of the Intermediateφ Scattering Function λ λ ω r ω ∫∫ I( )S(Q, )cos( λ1 −ω 2 )d d r r P = r ≈ S(Q, )cos( )d = I(Q,τ) I( )S(Q, )d d ∫ ω ∫∫ τ ωτ γ Bd ω λ τ m2 where the "spin echo time" = Bd λ3 (T.m) (nm) (ns) π 2 2 h 1 0.4 12 1 0.6 40 1 1.0 186 • I(Q,t) is called the intermediate scattering function – Time Fourier transform of S(Q,ω) or the Q Fourier transform of G(r,t), the two particle correlation function

• NSE probes the sample dynamics as a function of time rather than as a function of ω

• The spin echo time, τ, is the “correlation time” Neutron Polarization is Measured using an Asymmetric Scan around the Echo Point

Echo Point

20002000

15001500 The echo amplitude decreases

10001000 when (Bd)1 differs from (Bd)2

500500 because the incident neutron Neutron counts ( per 50 sec) counts Neutron Neutron counts ( per 50 sec) counts Neutron beam is not monochromatic. 00 λ 0123456701234567 For elastic scattering:γ Current in coil 3 Bd2 (Arbitrary Units)

 m λ P ~ I( )cos {}(Bd)1 − (Bd)2 .dλ ∫  h  Because the echo point is the same for all neutron wavelengths, we can use a broad wavelength band and enhance the signal intensity What does a NSE Spectrometer Look Like? IN11 at ILL was the First Field-Integral Inhomogeneities cause τ to vary over the Neutron Beam: They can be Corrected

• Solenoids (used as main precession fields) have fields that vary as r2 away from the axis of symmetry because of end effects (div B = 0)

B solenoid

• According to Ampere’s law, a current distribution that varies as r 2 can correct the field-integral inhomog- eneities for parallel paths

• Similar devices can be used to correct the integral along Fresnel correction coil for IN15 divergent paths Neutrons in Condens ed Matter Res earch

10000

SpallationSPSS - Chopper - Chopper Spectrometer Neutron ILL - w ithou t s pin -ec ho Induced Itinerant Hydrogen Excitations ILL - with s pin-echo Magnets 100 Modes Crystal Momentum Elastic Scattering Fields Distributions Mo le cu la r Coherent Spin Vibrations Modes in Waves Glasses Lattice and Liquids Vibrations Electron- and 1 Molecular phonon Anharmonicity Motions Int era ct io ns [Larger Objects Resolved] Accurate Liquid Structures Metallurgical Membranes Crystal and Micelles Polymers Systems Magnetic Amorphous Precision Crystallography Proteins Systems sfer (meV) sfer Proteins in Solution Colloids Structures Anharmonicity Viruses Critical Scattering Slower 0.01 Motions Resolved Diffusive Aggregate Modes Energy Tran Energy Motion Polymers Molecular and Reorientation 10-4 Biological Tunneling Systems Spectroscopy

Surface Effects?

10-6 0.001 0.01 0.1 1 10 100 -1 Q (Å ) Neutron Spin Echo has significantly extended the (Q,E) range to which neutron scattering can be applied Neutron Spin Echo study of Deformations of Spherical Droplets*

* Courtesy of B. Farago The Principle of Neutron Resonant Spin Echo

• Within a coil, the neutron is subjected to a B0 steady, strong field, B0, and a weak rf field

B1cos(ωt) with a frequency ω = ω0 = γ B0

– Typically, B0 ~ 100 G and B1 ~ 1 G B1(t)

• In a frame rotating with frequency ω0, the neutron spin sees a constant field of magnitude B1

• The length of the coil region is chosen so that the neutron spin precesses around B thru an angle π. φ 1 S exit π rotated φ S • The neutron precession phase is: exit φ B1 exit φexit entry φ entry neutron = RF + ( RF − neutron ) φ entry entry entry B1 = 2 RF − neutron +ω0d / v S entry + d CD D + d = l t 0 B * Courtesy of S. Longeville AB t Echo occurs for elastic scattering when l ½ CD B=0 l   · · Ú Ú 0 C’ Ë B t   Ð Ð µ  C   t ++ ++ · ¾ ¾ ¼ Ú

 Ð  

  Ú Ú Ø Ø  ¾ ¾     · · · Ú

B=0 · · Ú Ú Sample ¼ ¼    Ð Ú Ú     Ð Ð ´ B      Ø Ø t 0 ¾ ¾ ¾ ¾

¾ ÒÙØÖÓÒ ËÔÒ Ô × ¼ B Ø ØÓÒ ddd t Ö  NRSE spectrometer NRSE AB l B=0 ¼ ¼ ËÔÒ ÓÖÒ     ¼ ¼     Ø Ø Ø Ø A’ t

Ø

Ø

Ø È × ¬Ð Ø 0 B d AB CD Ð ½º  A  ++ ++ t Ì   ¼ ·¾ ·¾ · Ú · Ú ¼ B= Ú Ú   Ð   Ð Ú   Ú Ð Ð · · · · · ·       Ø Ø Ø Ø Ø Ø n     Neutron Spin Phases in an NRSE Spectrometer*   ¼ ¼ ¼ ¼         Ø Ø Ø Ø Ø Ø ÌÑ Ø Ø Ø  ³  ³  ³  ³ H = 0

Just as for traditional NSE, if the scattering is elastic, all neutron spins arrive at the analyzer with unchanged polarization, regardless of neutron velocity. If the neutron velocity changes, the neutron beam is depolarized The Measuredλ Polarization for NRSE is given by ω an Expression Similarωτ to that for Classical NSE λ λ ω • Assume that v’ = v + δv withω δv small and expand to lowest order, giving: λ ω r ∫∫ I( )S(Q, )cos( NRSE )d d P = r τ I( )S(Q, )d d ∫∫ γ m2 where the "spin echo time" = 2 B (l + d) λ3 NRSE 0 π 2 2 h • Note the additional factor of 2 in the echo time compared with classical NSE (a factor of 4 is obtained with “bootstrap” rf coils)

• The echo is obtained by varying the distance, l, between rf coils

• In NRSE, we measure neutron velocity using fixed “clocks” (the rf coils) whereas in NSE each neutron “carries its own clock” whose (Larmor) rate is set by the local magnetic field An NRSE Triple Axis Spectrometer at HMI: Note the Tilted Coils Measuring Line Shapes for Inelastic Scattering

• Spin echo polarization is the FT of ω scattering within the spectrometer transmission function

• An echo is obtained when Q d (Bd) (Bd)  N N 1 − 2 = 0 ⇒ 1 = 2   2 2 dk1  k1 k2  k1 k2

• Normally the lines of constant spin- ω echo phase have no gradient in Q,ω space because the phase depends only on k Q • The phase lines can be tilted by using “tilted” precession magnets By “Tilting” the Precession-Field Region, Spin Precession Can Be Used to Code a Specific Component of the Neutron Wavevector

d k If a neutron passes through a // k rectangular field region at an χ angle, its total precession phase k ⊥

will depend only on k⊥. φ ωL ω= γB γ d KBd B = Lt = B = vsin χ k⊥ Stop precession here -1 with K = 0.291 (Gauss.cm.Å) Start precession here “Phonon Focusing”

• For a single incident neutron wavevector, kI, neutrons are scattered to kF by a phonon of frequency ωo and to kf by neighboring phonons lying on the “scattering surface”. – The topology of the scattering surface is related to that of the phonon dispersion surface and it is locally flat • Provided the edges of the NSE precession field region are parallel to the scattering surface, all neutrons with scattering wavevectors on the scattering surface will have equal spin-echo phase Q0 scattering surface

kF Precession field region kI kf

kf⊥ “Tilted Fields”

• Phonon focusing using tilted fields is available at ILL and in Japan (JAERI)….however,

• The technique is more easily implemented using the NRSE method and is installed as an option on a 3-axis spectrometers at HMI and at Munich

• Tilted fields can also be used can also be used for elastic scattering and may be used in future to: – Increase the length scale accessible to SANS – Separate diffuse scattering from specular scattering in reflectometry – Measure in-plane order in thin films – Improve Q resolution for diffraction An NRSE Triple Axis Spectrometer at HMI: Note the Tilted Coils Nanoscience & Need Structural Probes for 1-100 nm

CdSe Peptide-amphiphile nanofiber

10 nm holes in PMMA

1µ 2µ 10µ

Actin Si colloidal crystal Structures over many length Thin copolymer films scales in self-assembly of ZnS and cloned viruses “Tilted” Fields for Diffraction: SANS d χ θ χ B B π/2 π π/2 φ1 φ2

• Any unscattered neutron (θ=0) experiences the same precession angles φ (φ1 and φ2) before and after scattering, whatever its angle of incidence • Precession angles are different for scattered neutrons χ φ KBd KBd  KBd cos  = and = ⇒ cos( − ) ≈ cos 1 2 χ 1 2  2  k sin k sin( θ+ )  k sin  φ   φ  KBd cosχ  P = dQ.S(Q).cos Q Polarization proportional to  2 2  ∫  k sin χ  χ Fourier Transformθ of S(Q) χ χ Spin Echo Length, r = KBd cos /(k sin χ)2 How Large is the Spin Echo Length for SANS?

Bd/sinχ λ χ r (Gauss.cm) (Angstroms) (degrees) (Angstroms)

3,000 4 20 1,000

5,000 4 20 1,500

5,000 6 20 3,500

5,000 6 10 7,500

It is relatively straightforward to probe length scales of ~ 1 micron Spin Echo SANS using Magnetic Films: 200 nm Correlation Distance Measured Close up of SESAME (Spin Echo Scattering Angle Measurement) Apparatus Conclusion: NSE Provides a Way to Separate Resolution from Monochromatization & Beam Divergence

• The method currently provides the best energy resolution for inelastic neutron scattering (~ neV) – Both classical NSE and NRSE achieve similar energy resolution – NRSE is more easily adapted to “phonon focusing” I.e. measuring the energy line-widths of phonon excitations

• The method is likely to be used in future to improve (Q) resolution for elastic scattering – Extend size range for SANS (SESANS) – May allow 100 – 1000 x gain in measurement speed for some SANS exps – Separate specular and diffuse scattering in reflectometry – Measure in-plane ordering in thin films (SERGIS)