Transcript Introduction to Neutron Scattering and ORNL neutron facilities

Speaker: Xiangshi Yin
Instructor: Elbio Dagotto
Time: Mar. 4th 2010
(Solid State II project)
Outline
History
 Neutron Scattering Mechanism
 Neutron sources
 ORNL neutron facilities

History
In 1932, neutron was first discovered by J.
Chadwick
 In 1936, W. Elsasser proposed the idea of
neutron scattering by crystalline materials
 In 1936, F. Bloch predicted the feasibility of
neutron scattering by magnetic moment in
condensed materials
 In 1940s and 1950s, high flux neutron
reactor sources were built in U. S. and
Canada(Chalk River’s NRX reactor,
ORNL’s graphite reactor)


E. Wollan and C. Shull did a lot of pioneering work in
modern neutron diffraction between 1948 and 1955
(1) The existence of ferromagnetic state in Fe3O4
(2) E. Wollan and W. Koehler determined the magnetic structure in
La1-xCaxMnO3

In 1956, B. Brockhouse built the first triple-axis
spectrometer in Chalk River Laboratory

……

1994, the Nobel Prize
Bertram N. Brockhouse
Clifford G. Shull

……
Why Neutrons?
Properties
No charge
Almost no electric dipole moment
Spin-1/2
Short range nuclear force(10-15m)
λthermal ~10-10m
Disadvantages
It’s unique!
Advantages
Penetrate deep
Detect the lattice structure
Detect the magnetic structure
Weakly scattered
2·s at
intensity
1018Low
photons/mm
(104 neutrons/mm
synchrotron
source 2·s)
Signal-limited technique!
Neutron Scattering
Neutron
diffraction
Nuclear scattering
Magnetic scattering
Elastic scattering
Inelastic scattering
Small angle
neutron scattering
Surface
reflection
Q: Scattering Vector
2θ: Scattering Angle
Q: How do we distinguish
nuclear scattering from
magnetic scattering?
Neutron
Fast neutrons: >1 eV, 0.1 MeV or 1 MeV (Depending
on the definition)
Slow neutrons: ≤0.4 eV.
Scattering
Epithermal
: 0.025 eV ~ 1 eV. I(Q, E)
Hot neutrons : ~0.2 eV.
Thermal neutrons: ~0.025 eV.
Sample
Cold neutrons: 5x10-5 eV ~0.025 eV.
Very cold neutrons: 3x10-7 eV ~5x10-5 eV.
Ultra cold neutrons:Coherent
~3x10-7 eV.
Incoherent
Continuum region neutrons:
~25 MeV.
scattering 0.01 MeV
scattering
Resonance region neutrons:1 eV ~0.01 MeV.
Single Crystal
Low
energy region neutrons: <1 eV
Polycrystalline
Powders
Elastic
Equilibriumm
lattice structure
Inelastic
Phonons
Elastic
Unwanted
background
Inelastic
Atomic
diffusion
Difference between magnetic
and nuclear scattering
They normally occur at different wave
vectors
 Magnetic scattering is temperature
dependent while nuclear scattering is
not
 Using polarized neutrons we could get
spin flipping for magnetic scattering

Q: How do we distinguish
magnetic scattering and
nuclear scattering?
Neutron
I(Q, E)
Scattering
Q: How do
we measure
Q?
Sample
Coherent
scattering
Elastic
Equilibriumm
lattice structure
Inelastic
Phonons
Incoherent
scattering
Elastic
Unwanted
background
Inelastic
Atomic
diffusion
How to measure wave vector?
Reactor source
Pulsed source

Monochromator
(Powder diffraction)

Triple-axis spectrometer
(Inelastic scattering)

Time of flight technique
A triple-axis spectrometer built at the
Institute Laue Langevin in Grenoble,
France
Powder diffraction
Bragg’s Law:
2dSinθ = nλ
4Sin

Q  

d  2

Q


In practice, crystallographers generally have to
resort to modeling the structure of crystals, shifting
atoms around until they find an arrangement that
accurately predicts the measured Bragg intensities
In reality, atoms has thermal energy and oscillate
about their lattice. Since an atom can contribute to
the constructive interference of Bragg scattering
only when it is located exactly at its official position
at a lattice site, this scattering becomes weaker
the more the atoms vibrate and the less time they
spend at their official positions.
Inelastic scattering
Phonons



When a neutron is scattered by a crystalline solid, it can
absorb or emit an amount of energy equal to a quantum of
phonon energy hν
In most solids ν is a few terahertz (THz), corresponding to
phonon energies of a few meV (~4.18 meV). Because the
thermal neutrons used for neutron scattering also have
energies in the meV range, scattering by a phonon causes
an appreciable fractional change in the neutron energy,
allowing accurate measurement of phonon frequencies
The constant-Q scan(invented by B. Brockhouse).
CaFe2As2
Phys. Rev. Lett. 102, 217001(2009)
Spin waves
Phys. Rev. B64, 224429 (2001)
Neutron sources
Research reactors
 Spallation sources

Research reactors
A kind of nuclear reactors but simpler
than power reactors
 Mechanism: The “chain reaction”


n U  U
235
92
236
92
U  Ba Kr  3n  177MeV
236
92
144
56
89
36
Spallation sources
Spallation is a process in which
fragments of materials (spall) are
ejected from a body due to impact or
stress
 The bullet: high energy species, such as
proton(1 to 2 GeV)
 The target: heavy metal, such as
Mercury and Tantulum
 20 to 30 neutrons are generated per
impact

ORNL neutron facilities
HFIR(High Flux Isotope Reactor)
 SNS(Spallation Neutron Source)

HFIR
The highest flux reactor-based source of
neutrons for condensed matter research
in the United States
 Fuel: Uranium-235
 Reflector: Beryllium
 Moderator: Water

HFIR beam tubes and experiment locations
SNS
Negatively
charged
hydrogen
Linear
accelerator
Heavy metal
target
(Mercury)
Proton pulses
High energy
neutron
pulses
Moderator
(Water)
foil
P
strip off electrons
Accumulating
ring
Cold and
thermal
neutrons
Summary
Neutron is a powerful probe to study
complex materials
 We can get information of both the
lattice structure and magnetic structure
 Two general neutron source: reactor and
spallation source

References
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[1] J. Chadwick, Nature (London) 129 312 (1932)
[2] W. M. Elsasser, C. R. Acad. Sci. Paris 202 1029 (1936)
[3] H. Halban and P. Preiswerk, C. R. Acad. Sci. Paris 203 73 (1936)
[4] D. P. Mitchell and P. N. Powers, Phys. Rev. 50 486 (1936)
[5] F. Bloch, Phys. Rev. 50 259 (1936)
[6] B. N. Brockhouse, Nobel Lecture, December 8, 1994
[7] URL http://en.wikipedia.org/wiki/Neutron_temperature
[8] URL http://en.wikipedia.org/wiki/Neutron_diffraction
[9] Tapan Chatterji, Neutron Scattering from Magnetic Materials
URL http:// www.sciencedirect.com/science/book/9780444510501
[10]URL http://neutrons.ornl.gov
[11]URL http://www.khwarzimic.org/takveen/seaborg.pdf
[12] J. R. Alonso “The spallation neutron source project”
Proceeding of the 1999 particle accelerator conference, New York, 1999
[13]URL http://irfu.cea.fr/en/Phocea/Vie_des_labos/Ast/ast_visu.php?id_ast=2215
[14] V.F. Sears, Methods of Experimental Physics, vol. 23, eds. K. Sköld and D.L. Price, Part A,
Academic Press, London (1986)
[15] D.L. Price and K. Sköld, in: Methods of Experimental Physics, vol. 23, Part A, p.1,
Academic Press, London (1987)
[16] R. Mittal, L. Pintschovius, D. Lamago, R. Heid, K-P. Bohnen, D. Reznik, S. L. Chaplot, Y. Su, N. Kumar,
S. K. Dhar, A. Thamizhavel and Th. Brueckel
Phys. Rev. Lett. 102, 217001(2009)
[17] P. Dai, J. A. Fernandez-Baca, E. W. Plummer, Y. Tomioka and Y. Tokura
Phys. Rev. B64, 224429 (2001)