INSTITUT MAX VON LAUE - PAUL LANGEVIN CERN, 12/5/09 Ken Andersen Neutron Optics Optics –reflection –refraction –diffraction –polarization Neutron Instruments

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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

INSTITUT MAX VON LAUE - PAUL LANGEVIN

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


Neutron scattering

24 b

10 barns

1 barn


• 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


ILL & ESRF in Grenoble


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


1011

1012

1013

1014

0 1 2 3 4 5 6 7 8

Hot source

Thermal source

Cold source

wavelength (A)

Neutron Moderators



20cm

5cm

30cm

200cm

reflective internal surfaces Max angle ≈ 1°

Source Optics


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


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


An Fe/Si multilayer


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



Instrument Example: Powder Diffraction

λ = 2dsinθ d = λ/2sinθ

Structure Determination


Monochromating by time-of-flight

300 Hz

~μs burst time

distance

time

Choppers

Δλ/λ ≈ 1%


Monochromating by time-of-flight

Velocity selector

Δλ/λ ≈ 10%


Single-crystal Monochromators

fwhm < 10-4°

→Perfect crystal

<hkl>

cotB Mosaic crystal

fwhm > 0.1 °

Bragg’s law:

λ = 2dsinθ


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


Focusing

guide ~ 100 cm2 samples < 1 cm2


Focusing Devices

Crystal monochromators

Copper 200Graphite 002

Supermirror optics

Kirkpatrick-Baez mirrors

Focusing guides


θB

A

B

Monochromator Focusing


θA

Monochromator Focusing

θB

A

B

/sin4Q


Limitations of focusing

• Liouville’s theorem: phase-space density is constant– Increase in spatial density implies a reduction in

angular density– worse resolution


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


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


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


N.b

(1

0-6 A

-2)

• with

B

0

5

10

15


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


Cu2MnAl (Heusler) crystal

Polarizing Crystals

ILL is the only producer


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


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)


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



• 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)


• 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

Documents

INSTITUT MAX VON LAUE - PAUL LANGEVIN CERN, 12/5/09 Ken Andersen Neutron Optics Optics –reflection –refraction –diffraction –polarization Neutron Instruments