NEUTRON SCATTERING AND INSTRUMENTATIONS

Neutron Training School 2014 seminar

27/05/14 Giorgia Albani – NTC 2014 seminar 1

Summary

1.  Physics of neutron scattering

2.  Neutron sources: ISIS and ILL

3.  A diffraction experiment

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Physics of neutron scattering

What is a scattering experiment?

�  In a scattering experiment, a beam of radiation is incident on a sample

�  The distribution of radiation scattered in a sample is measured

�  This is determined by the interaction potential of the radiation and the sample

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Scattering experiments �  Diffraction

�  Assumes no energy transferred to/from the sample

�  Measurements of crystal structure, atomic correlations in liquid/gases

�  Inelastic scattering (spectroscopy) �  Energy transferred to/from the sample

�  Measurements of lattice vibrations (phonons), atomic diffusion

�  Small angle scattering �  Diffraction at very small angle

�  Measuring large objects: proteins, nanoparticles, etc…

�  Reflectometry �  Diffraction from a source – specular or off-specular

�  Measures depth profile of thin films

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Neutron interactions

Nuclear interactions

�  electrons ignored

�  V ÷ b

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Atom: 10-10m

Nucleus: 10-15m

Magnetic interactions

�  Unpaired electrons

�  V ÷ μ. Β

Neutrons scatter via:

Neutrons are waves

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o  Neutron are located somewhere in Δx o  Heisenberg: must increase Δx to define λ better – or increase Δλ

to define x better o  The longer the Δx, the larger the spatial and temporal coherence of

the neutron – more monochromatic o  Neutrons are usually considered as infinite plane waves.

So why neutrons?

�  Neutrons have no electric charge:

Ø Very penetrating radiation

�  Strong scattering from light nuclei (e.g. H)

�  Strong magnetic interactions (S=1/2)

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λ=1,8 A

E=25 meV Thermal neutrons

λ~ interatomic spaces

E~ phonons excitations

INVESTIGATION OF MATTER

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Neutron sources

Neutron sources Fission reactor

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Spallation sources

Reactor sources (ILL)

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Institute Laue Langevin (ILL)

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Nuclear Fission

• Each fission of 235U releases 200 MeV!

• 2.35 neutrons produced!

• so 1.35 useful neutrons per fission!

• energies around 50-100 MeV per neutron

01n+ 92

235U→≈ 2.35n+ f +180MeV

•  1 n is needed to continue the reaction

•  The others n are moderated Ø  1,35 n useful per fission

•  Energies around 50-100 MeV per n

1.5x1015 neutrons per

second per cm2 are produced

Spallation Sources (ISIS)

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Spallation process

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Proton Energy: 800 MeV Frequency: 50 Hz Current: 200 uA # neutrons: 15-20 n/p Flux: 1.4 X 1016 n/s

ISIS

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Neutron moderators �  Both fission and spallation sources produce high

energy neutrons (E~1MeV, λ~3x10-4A°)

�  Neutron scattering requires “thermal” or “slow” neutrons (E~25meV, λ~1,8 A° at T=293K)

�  Neutrons can be slowed down by elastic collision with light atoms (e.g. H, D, C)

�  Moderators produce neutron beams with a near Maxwellian distributions of energies, at a characteristic temperature

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ILL and ISIS neutrons

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Diffraction experiments

Bragg’s law

�  λ= 2dsenϑ

�  K=2π/λ

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2ϑ

A Bragg peak is obtained if the reflected radiation off different planes interferes constructively. This condition is satisfied by the Bragg’s law:

Radiation with λcomparable with the interatomic spaces

DIFFRACTION PATTERN

Neutron Diffraction �  Neutron diffraction is used to measure the

differential cross section �  Crystalline solids �  Liquids and amorphous materials �  Large scale structures

�  Depending on the scattering angle, structure on different length scales, d, is measured:

d=λ/2sinϑ

�  For crystalline solids and liquids use wide angle diffraction. For large structures (proteins…) use small angle neutron scattering

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Neutron diffraction

�  Assumes that ki=kf (true for Bragg diffraction)

�  So only need to measure ki or kf

�  Performed on polycrystals (powder diffraction), or single crystals or disordered material

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Powder diffraction

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2ϑ

All possible crystal orientation presented to the beam

Reactor source method for fixed λwe measured d-spacing as function of 2ϑ

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λ= 2dsenϑ

source monochromator

detector

sample

2ϑ1

2ϑ2 2ϑ3

2ϑ4

Example: D1B (ILL)

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Time of flight method

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for fixed ϑ we measured d-spacing as function of λ(t)=h/mv λ= 2dsenϑ

source

sample

detector

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Time of flight method for fixed ϑ we measured d-spacing as function of λ(t)=h/mv λ= 2dsenϑ

Example: POLARIS

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Thank you for your attention!