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 -

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

experiments with neutron beams


•  neutron properties

•  neutron scattering applications

•  neutron sources

•  instruments and applications

neutron properties


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

neutrons as particles


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)

neutron properties


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

neutron properties


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

neutron properties


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

neutron properties


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

neutron properties


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

neutron properties


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

neutron properties


contrast variation

100 % H2O 100 % D2O

•  null or full contrast : external shapes

•  intermediate contrast : details

objects in solution different average cross-sections

neutron properties


+ + =

sensitivity and selectivity isotopic substitution/contrast variation

neutron properties


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

neutron sources


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

neutron sources - nuclear reactors


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

neutron sources - nuclear reactors


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

neutrons from accelerators


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



spallation sources

using high-energy electrons on heavy metal targets

accelerator proton 2.0 GeV

high Z metal target

‘evaporation’ of 30 fast neutrons



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



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

neutron moderators


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)

neutron sources


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

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

neutron transport


neutron guide halls

neutron optics


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

neutron monochromators


crystal monochromators

selection of wavelength bandwidth (energy)

focusing

Heusler alloy: polarising monochromator

focusing mechanics

focusing supermirrors / guides

neutron velocity selectors


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

neutron velocity selection


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

neutron polarisation


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

neutron polarisation


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

neutron detection


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)

neutron detection


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

neutron instruments and applications


powder diffraction

powder diffraction


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



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



diffraction - disordered materials

Sandals ISIS

D4 ILL

long range order observed in glasses (chemical and topological)



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



diffraction and bio-molecules deuteration complementarity SRX neutrons

localisation and displacements of protons A.Y. Kovalevsky et a., Structure 18, 688 (2010)

LADI ILL



instruments small angle neutron scattering

LOQ ISIS

D11 ILL



‘soft matter’ systems

unified scheme of order and mutual orientation under ‘orientation’ fields: flow, shear, ….

S. Förster et al. Nature Materials 6, 888 (2007)



instruments

neutron reflectometry

CRISP ISIS

Figaro ILL

Documents

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 -