cern, 12/5/09
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Neutron Optics. Optics reflection refraction diffraction polarization Neutron Instruments source transport focusing, divergence wavelength encoding polarization encoding. CERN, 12/5/09. Ken Andersen. Neutrons vs Light. Neutron scattering. 10 barns. 1 barn. - PowerPoint PPT PresentationTRANSCRIPT
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INSTITUT MAX VON LAUE - PAUL LANGEVINCERN, 12/5/09 Ken Andersen
Neutron Optics
• Optics– reflection– refraction– diffraction– polarization
• Neutron Instruments– source– transport– focusing, divergence– wavelength encoding– polarization encoding
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light neutrons
λ < μm < nm
E > eV > meV
n 1→4 0.9997→1.0001
θc 90° 1°
Φ/ΔΩ 1019 p/cm2/ster/s
(60W lightbulb)
1014 n/cm2/ster/s
(60MW reactor)
P left-right up-down
spin 1 ½
interaction electromagnetic strong force, magnetic
charge 0 0
Neutrons vs Light
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Neutron scattering
24 b
10 barns
1 barn
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• About 10 neutron facilities worldwide
• Fission (continuous)• Spallation (pulsed)• User Facilities• ILL:
– 40 instruments– 700 experiments/year– mainly solid-state physics,
but also fundamental physics, chemistry, biology
ILL
ISIS
Neutron Sources
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ILL & ESRF in Grenoble
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2.5m
cold thermal hot
moderator liquid D2, H2, CH4
liquid H2O
graphite
moderator temperature
15–25K 300K 2000K
neutron wavelength
3→20Å 1→3Å 0.3→1Å
sample lengthscale
1Å→100nm
0.3→5Å 0.1→2Å
sample timescale
1kHz→1THz
0.1→10 THz
1→100 THz
Neutron Moderators
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1011
1012
1013
1014
0 1 2 3 4 5 6 7 8
Hot source
Thermal source
Cold source
wavelength (A)
Neutron Moderators
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20cm
5cm
30cm
200cm
reflective internal surfaces Max angle ≈ 1°
Source Optics
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Reflecting Surfaces
n=1
n’<1
incident
refracted
reflected
θ critical angle of total reflection θc
Nb/πλθ
2θ1cosθ2π
bNλ1n'
n'nn'cosθ
c
2cc
2c
for natural Ni,
θc = λ[Å]0.1
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Increasing the Critical Angle
d1}
c
d2} d3
} d4
}
Interface reflection:
θc(Ni) = λ[Å]0.1
Equivalent Bragg diffraction:
λ = 2dsinθ
d = λ/2θ = 200 Å
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An Fe/Si multilayer
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Neutron Supermirrors
• Multilayers with up to several 1000s of layers feasible by magnetron sputtering
• 4×θNi is commercially available
• Layer thicknesses > 20 Å• Interlayer roughness < 3 Å
– limited by roughness of substrate (1-8 Å)
• < 0.5 m2 deposition in one batch• Technology transfer from neutron labs to industry• Neutron guides up to >100m
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Instrument Example: Powder Diffraction
λ = 2dsinθ d = λ/2sinθ
Structure Determination
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Monochromating by time-of-flight
300 Hz
~μs burst time
distance
time
Choppers
Δλ/λ ≈ 1%
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Monochromating by time-of-flight
Velocity selector
Δλ/λ ≈ 10%
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Single-crystal Monochromators
fwhm < 10-4°
→Perfect crystal
<hkl>
cotB Mosaic crystal
fwhm > 0.1 °
Bragg’s law:
λ = 2dsinθ
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Single-crystal Monochromators
d-spacingGermanium 333
1.089 Å
Copper 111 2.087 ÅSilicon 111 3.135 ÅGraphite 002 3.355 Åstage 1 K-intercalated graphite 002
5.35 Å
stage 2 K-intercalated graphite 002
8.74 Å
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Focusing
guide ~ 100 cm2 samples < 1 cm2
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Focusing Devices
Crystal monochromators
Copper 200Graphite 002
Supermirror optics
Kirkpatrick-Baez mirrors
Focusing guides
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θB
A
B
Monochromator Focusing
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θA
Monochromator Focusing
θB
A
B
/sin4Q
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Limitations of focusing
• Liouville’s theorem: phase-space density is constant– Increase in spatial density implies a reduction in
angular density– worse resolution
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Limitations of focusing
• Liouville’s theorem: phase-space density is constant– Increase in spatial density implies a reduction in
angular density– worse resolution
• Source brightness ~ 1014 n/cm2/ster/s– ΔΩ ≈ 10-3 ster– Δλ/λ ≈ 1%
• Flux impinging on sample < 108 n/s– strongly limited by Poisson statistics
• Sometimes S/N can be more important
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Polarization Optics
• Magnetism– neutron magnetic moment interacts with
that of unpaired electrons– magnetic scattering depends strongly on
relative orientation of neutron spin, electron spin and momentum transfer
– unambiguous separation of magnetic and nuclear scattering
• Precession techniques– polarization vector precesses around field
direction– frequency ~ B– phase measurement gives time spent in
field neutron speed
21ns+½ħ
-½ħ
“Spin-up” (+)
“Spin-down” (-)
21ns
B
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Polarizing Supermirrors
B
FeSi
Si
0
5
10
15
Si Fe Si Fe Si Fe Si substrate
N.b
(1
0-6 A
-2)
0
5
10
15
Si Fe Si Fe Si Fe Si substrate
N.b
(1
0-6 A
-2)
• with
B
0
5
10
15
Si Fe Si Fe Si Fe Si substrate
N.b
(1
0-6 A
-2)
B
B=0
0
0.2
0.4
0.6
0.8
1
1.2
0 0.5 1 1.5 2 2.5 3 3.5
0 1 2 3 4
R+R-
Omega
m=/ c(Ni)
FeSi m=3.8
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Cu2MnAl (Heusler) crystal
Polarizing Crystals
ILL is the only producer
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MEOP
Metastability Exchange Optical Pumping
B0
OPC
3He bottle Purifier
Capillary
Optical pumping Cells
Yb fiber laser
OPC
Buffer 2,5l
5.2 liter compressor
Hydraulic piston
Polarised 3He Cells
Optical polarimeter
Mirrors
Discharge
Polarized-3He Spin-filters
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0
0.2
0.4
0.6
0.8
1
0 10 20 30 40 50 60
Transmission TBeam Polarization P
Figure of Merit P2T
Opacity (bar.cm.A)
Polarized-3He Spin-filters
Hen PΟΟT )(cosh)(exp)(
Hen PΟP )(tanh)(
where O(λ) = 7.28×10-2×P[bar] ×t[cm] ×λ[Å]
0
0.1
0.2
0.3
0.4
0.5
0 0.2 0.4 0.6 0.8 13He Polarization
Flipping ratio = 40P
n = 0.95
2002
2008
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Polarized-3He Spin-filters
• 3He Polarization < 80%• World leaders
– Supplying 3He filling stations, spin-filter cells & magnetic-field environments to neutron labs in UK, Australia, Germany, Taiwan, USA
• Applications:– magnetic structures in single
crystals– magnetic domain structures in thin
films– disorder in frustrated magnetic
systems– magnetic excitations in high-Tc
superconductors• Medical applications
– functional lung imaging (MRI)
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• User facilities– experiments performed mainly by outside groups– ILL: 200 days/yr, 40 instruments, 700 experiments/year, 1200 users/year– mainly solid-state physics, but also fundamental physics, chemistry,
biology• Sources
– thermalized Maxwellian spectrum– low brightness, large sources– beam distribution by guides
• Monochromatization– time-of-flight: velocity selectors, pulsing choppers– crystal monochromators: Bragg formula, perfect crystals, mosaic crystals
• Focusing– sample size vs source/guide size, resolution degradation– crystal monochromators– guides
• Polarization– crystal monochromators– supermirrors– polarized 3He
Summary