neutrons: what for? and how? · 2013-08-12 · christian vettier 'the use of neutron...
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18 May 2011 1 Christian Vettier 'the use of neutron beams'
neutrons: what for? and how?
Christian Vettier
ESRF Grenoble, France
ESS AB, Lund Sweden
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experiments with neutron beams
18 May 2011 2 Christian Vettier 'the use of neutron beams'
• neutron properties
• neutron scattering applications
• neutron sources
• instruments and applications
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neutron properties
18 May 2011 3 Christian Vettier 'the use of neutron beams'
the discovery of the neutron James Chadwick 1932
spin 1/2 - magnetic moment 1.93 µN neutrons: no charge, mass close to proton – 1.675 10-23 kg
Cavendish Laboratory, Cambridge
free neutrons are unstable β-decay proton, electron, anti-neutrino life-time 886±1 sec
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neutrons as particles
18 May 2011 4 Christian Vettier 'the use of neutron beams'
neutron : an object to be studied fundamental interactions
confirmation of quantum effects in presence of gravity field
new developments
resonance spectrometry
V. Nesvizhevsky et al. Nature 415, 297 (2002)
T. Jenke et al. Nature Physics (2011)
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neutron properties
18 May 2011 5 Christian Vettier 'the use of neutron beams'
wave-particle duality
D.P Mitchell and N. Powers (1936)
diffraction of thermalised neutrons
€
E(meV) = 2.072 k (A0 −1
)⎡
⎣ ⎢
⎤
⎦ ⎥
2
≈9
λ (A0)
⎡
⎣
⎢ ⎢
⎤
⎦
⎥ ⎥
2
€
v (m /s) =3,956
λ (A0)
€
1(A0) ≈ 82 meV
1meV ≈ 8cm−1 ≈ 0.25THz ≈11.6K
kinetic energy
€
T =Lv
= 252.77 µsec ⋅ λ Ao⎡ ⎣ ⎢ ⎤ ⎦ ⎥ ⋅ L m[ ]
monochromatisation: diffraction or velocity selection
fortunately large value!
neutron dh = 1 m ≅ 0.1 µeV in gravity on Earth
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neutron properties
18 May 2011 6 Christian Vettier 'the use of neutron beams'
single nucleus process
what do neutrons ‘feel’ or ‘interact’ with?
like a billiards ball and a small compass
neutron-nuclei interaction Fermi length b ≈ 10-12 cm
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neutron properties
18 May 2011 7 Christian Vettier 'the use of neutron beams'
magnetism magnetisation densities
magnetic interaction spin 1/2 total magnetic moment M ≈ <L>+2<S>
order of magnitude gnr0M/2 ≈ 0.5 10-12 cm
neutron-nuclei interactions Fermi length b ≈ 10-12 cm
similar to x-ray Thomson scattering Zr0 ≈ 10-12 cm
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neutron properties
18 May 2011 8 Christian Vettier 'the use of neutron beams'
scattering form factors
brief summary
neutron-nuclei interactions isotopic sensitivity crystal structures atomic motions
neutron-magnetic moments interactions magnetic structures spin dynamics magnetic interactions “magnetic” electron densities
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neutron properties
18 May 2011 9 Christian Vettier 'the use of neutron beams'
different sites, no correlation
coherent and incoherent scattering
€
dσdΩ
= b j exp(iQ⋅ rj )j∑
2
€
dσdΩ
= b 2 exp[iQ⋅ (rj − rj ' )j , j '∑ ] + ( b2 − b 2)
j∑
€
b jb j' = b j b j' = b 2
€
b jb j' = b j2 = b2same site
isotopic dependence
nuclear spin dependence, I
€
ω + =I+12I+1
ω− =I
2I+1
€
σcoh = 4π b 2= 4πbcoh
2 σincoh = 4π b2 − b 2( )
€
σcoh = 4π ω+b+ +ω−b−( )2
σincoh = 4πω +ω− b+ −b−( )2
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neutron properties
18 May 2011 10 Christian Vettier 'the use of neutron beams'
what do we observe/measure with neutrons ?
isotope labelling (deuteration …..)
some b’s are < 0
… phase shift …
isotopic sensitivity
incoherent scattering
neutron-nuclei interactions Fermi length b ≈ 10-12 cm
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neutron properties
18 May 2011 11 Christian Vettier 'the use of neutron beams'
contrast variation
100 % H2O 100 % D2O
• null or full contrast : external shapes
• intermediate contrast : details
objects in solution different average cross-sections
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neutron properties
18 May 2011 12 Christian Vettier 'the use of neutron beams'
+ + =
sensitivity and selectivity isotopic substitution/contrast variation
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neutron properties
18 May 2011 13 Christian Vettier 'the use of neutron beams'
kinematics
scattering geometry
€
Q = ki − kf
€
Q2 = ki2 + kf
2 - 2kikf cos(2θ)
€
h2Q2
2mn
= Ei +Ef −2 Ei(Ei −hω ) cos(2θ )
velocities
momentum space
€
hω = Ei −Ef =h2
2mn
(ki2 −kf
2)
fixed Ei
direct geometry
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neutron sources
18 May 2011 14 Christian Vettier 'the use of neutron beams'
how to produce neutrons?
neutrons from fission process neutrons from spallation process neutrons from fusion process
free neutrons are created through ‘escape’ from nuclei
neutron stars
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neutron sources - nuclear reactors
18 May 2011 15 Christian Vettier 'the use of neutron beams'
a neutron source how does it work?
fission reactor slow neutrons splitting nuclei
ILL reactor
D2O moderator
hot source: graphite block 2400K
cold sources: liquid hydrogen 25K
thermal flux 1.5 1015 n/cm2/sec high power density (10 MW/litre) / achievable rate of cooling reactor core
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neutron sources - nuclear reactors
18 May 2011 16 Christian Vettier 'the use of neutron beams'
pulsed reactors time-of-flight methods on pulsed sources pulsing reactivity to become supercritical
• mechanically (fissile materials or reflector)
• electron pulses in the core - photo-fission process
• combination of the two above.
IBR-2 at Dubna, Russia
average power 2 MW
peak power 1,500 MW
rep rate 5 Hz, pulse width 215 µsec
peak flux 1.5 1016 n/cm2/sec
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neutrons from accelerators
18 May 2011 17 Christian Vettier 'the use of neutron beams'
pulsed sources
to overcome cooling problems
series of installations at Argonne Nat Lab, Rutherford, KEK
photo-fission
using high-energy electrons on heavy metal targets
deceleration leads to Bremsstrahlung
γ-rays produce fast neutrons
not efficient
3,000MeV per neutron
intense γ-ray beams
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neutrons from accelerators
18 May 2011 18 Christian Vettier 'the use of neutron beams'
spallation sources
using high-energy electrons on heavy metal targets
accelerator proton 2.0 GeV
high Z metal target
‘evaporation’ of 30 fast neutrons
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neutrons from accelerators
18 May 2011 19 Christian Vettier 'the use of neutron beams'
ISIS facility Rutherford Laboratory H- ion source
Linac 70 Mev
proton synchrotron 800 MeV
repetition rate 50 Hz
pulse width 100 ns
power 0.200 MW
2 target stations different moderators
ISIS/Diamond
neutron pulses width ~ 10-90 µs
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neutrons from accelerators
18 May 2011 20 Christian Vettier 'the use of neutron beams'
PSI continuous neutron source but pulsed accelerator
PSI SINQ
SNS
JPARC
1 MW
short pulses
0.8 MW 60Hz in 2012?
short pulses
protons pulses 0.3ns 50.6MHz
equivalent to 0.7MW
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neutron moderators
18 May 2011 21 Christian Vettier 'the use of neutron beams'
Maxwellian spectrum if thermal equilibrium
fission/spallation neutrons are in the MeV range moderation process thermal equilibrium with moderator
choice of moderator depends on applications hot neutrons <0.6 Å thermal neutrons 1- 4 Å cold neutrons 4-20 Å
€
λm =h
5mnkBT( )1/ 2 =19.483
T( )1/ 2 (Ao)
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neutron sources
18 May 2011 22 Christian Vettier 'the use of neutron beams'
Berkeley 37-inch cyclotron
350 mCi Ra-Be source
Chadwick
1930 1970 1980 1990 2000 2010 2020
105
1010
1015
1020
1
ISIS
Pulsed Sources
ZINP-P
ZINP-P/ KENS WNR
IPNS
ILL
X-10
CP-2
Steady State Sources
HFBR
HFIR NRU MTR NRX
CP-1
1940 1950 1960
Effe
ctiv
e th
erm
al n
eutro
n flu
x n/
cm2 -
s
(Updated from Neutron Scattering, K. Skold and D. L. Price, eds., Academic Press, 1986)
FRM-II SINQ
SNS
evolution of neutron sources characteristics ESS
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1-7 August 2009 23 C. Vettier ESSS 8th PSI Summer School - Zuoz
sources, now and in the future
landing the moon again by 2019!
a ‘dream’ neutron source?
A. Taylor et al. Science 315, 1092 (2007)
inertial fusion
D and T compressed by lasers
D and T combine : neutron plus α
propagating thermonuclear reaction
ESS
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neutron transport
18 May 2011 24 Christian Vettier 'the use of neutron beams'
neutron guide halls
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neutron optics
18 May 2011 25 Christian Vettier 'the use of neutron beams'
low brightness sources
compared with a 60 W (900 lm) light bulb
optimise neutron economy
• to collect (thermal/cold) neutrons
• to transport them to instruments
a few degrees!
~ 1014 n/cm2/steradian/sec
~ 1019 photons/cm2/steradian/sec
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neutron monochromators
18 May 2011 26 Christian Vettier 'the use of neutron beams'
crystal monochromators
selection of wavelength bandwidth (energy)
focusing
Heusler alloy: polarising monochromator
focusing mechanics
focusing supermirrors / guides
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neutron velocity selectors
18 May 2011 27 Christian Vettier 'the use of neutron beams'
principle for time-of-flight create pulse structure (even on reactors)
separate energies through velocity
select energies via time slicing disk chopper
opening of ~ 1 degree
rotational speed 300Hz
opening of about 80 µsec
disk diameter of 700 cm (speed about 600m/s)
Fermi chopper rotational speed 600Hz
opening of about 1 µsec
helical velocity selector typical bandwidth 10%
SANS experiments
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neutron velocity selection
18 May 2011 28 Christian Vettier 'the use of neutron beams'
high resolution method – neutron spin echo create neutron spin states
exploit Larmor precession of neutrons
vary time shift to probe time-dependent correlations
typical time scales: 0.01 - 500 ns
400 µeV – 8 neV
H. Hedgeland et al. Nature Phys. 5, 561 (2009) IN11 ILL
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neutron polarisation
18 May 2011 29 Christian Vettier 'the use of neutron beams'
production of polarised beams
Bragg reflection from magnetised crystal
total reflection from magnetised mirror
€
n' =1−Nλ2 b ±p( )
2π
θc ≈ λNπ
b ±p( )⎡
⎣ ⎢ ⎤
⎦ ⎥
1/2
€
dσdΩ
= FN2 Q( ) + 2 P⋅ µ( ) FN Q( ) FM Q( ) + FM
2 Q( )
B is guide field and polarising field
good matching for Cu2MnAl (111), Co0.92Fe0.08 (200), 57Fe (110)
€
dσdΩ
= FN Q( ) + FM Q( )2
€
dσdΩ
= FN Q( ) −FM Q( )2
up-state down-state
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neutron polarisation
18 May 2011 30 Christian Vettier 'the use of neutron beams'
production of polarised beams
transmission through polarising filter
€
σ↑↑ ≈ 0σ↑↓ ≈ 6000 barns
neutron absorption cross section of 3He is highly spin-dependent
anti-parallel neutron spins are absorbed
parallel neutron spins are transmitted
nuclear spin-polarised 3He gas
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neutron detection
18 May 2011 31 Christian Vettier 'the use of neutron beams'
the neutron is a weak probe! difficult to detect
thermal neutrons produce negligible ionisation
secondary ionisation due to neutron capture
emission of gamma rays or charged nuclei
most common absorbers: 3He, 10B and 6Li
image plates common in SRX community
need to convert neutron into photons (6Li or Gd)
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neutron detection
18 May 2011 32 Christian Vettier 'the use of neutron beams'
gas detectors 3He gas difficult to obtain nowadays
10B in BF3 toxic gas
position sensitive detectors detector
bank
micro-pattern gas counter
photolithography
multi-wire counter
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neutron instruments and applications
18 May 2011 33 Christian Vettier 'the use of neutron beams'
powder diffraction
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powder diffraction
18 May 2011 34 Christian Vettier 'the use of neutron beams'
molecular crystal CH3OH.H2O methanol monohydrate
negative linear compressibility negative thermal expansion
A D Fortes et al. Science 2011;331:742-746
applications to nano-switches but also planetary sciences …
parametric studies of lattice dimensions
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neutron instruments and applications
18 May 2011 35 Christian Vettier 'the use of neutron beams'
single crystal diffraction polarised neutrons
organic materials molecular magnets ‘high’ Tc magnetic couplings driven by radicals packing
S. Pillet et al. (2001) Nitrophenyl nitronyl nitroxide Tc=0.6K
D3 ILL
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neutron instruments and applications
18 May 2011 36 Christian Vettier 'the use of neutron beams'
diffraction - disordered materials
Sandals ISIS
D4 ILL
long range order observed in glasses (chemical and topological)
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neutron instruments and applications
18 May 2011 37 Christian Vettier 'the use of neutron beams'
diffraction and imaging engineering
residual stresses in large mechanical parts welds, ….
SALSA instrument ILL
neutron imaging of large objects FRM-2 PSI ILL
200 µsec exposures
fossil with leaves BMW engine
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neutron instruments and applications
18 May 2011 38 Christian Vettier 'the use of neutron beams'
diffraction and bio-molecules deuteration complementarity SRX neutrons
localisation and displacements of protons A.Y. Kovalevsky et a., Structure 18, 688 (2010)
LADI ILL
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neutron instruments and applications
18 May 2011 39 Christian Vettier 'the use of neutron beams'
instruments small angle neutron scattering
LOQ ISIS
D11 ILL
![Page 40: neutrons: what for? and how? · 2013-08-12 · Christian Vettier 'the use of neutron beams' 18 May 2011 13 kinematics scattering geometry € Q = k i −k f € Q2 =k i 2 +k f 2 -](https://reader030.vdocuments.us/reader030/viewer/2022013022/5fa3523bdf0a54212905cc15/html5/thumbnails/40.jpg)
neutron instruments and applications
18 May 2011 40 Christian Vettier 'the use of neutron beams'
‘soft matter’ systems
unified scheme of order and mutual orientation under ‘orientation’ fields: flow, shear, ….
S. Förster et al. Nature Materials 6, 888 (2007)
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neutron instruments and applications
18 May 2011 41 Christian Vettier 'the use of neutron beams'
instruments
neutron reflectometry
CRISP ISIS
Figaro ILL