Neutron scattering

The physics part• Neutrons are a wave.• In a scattering experiment, a beam of radiation is

incident on a sample.• The distribution of radiation scattered from the sample

is measured. • This is determined by the interaction potential and momentum transfer between the beam and the sample.

Neutron Scattering: A Primer, Roger Pynn, Los Alamos Science 19 (1990)www.fas.org/sgp/othergov/doe/lanl/pubs/number19.htm

Why use neutrons

• No charge • High energy• Weak interactions• Strong magnetic interactions• Scatter strongly from light nuclei

Neutrons vs X-raysNeutron X-ray

Highly penetrating due to their 0 charge. Therefore can probe the

internal structure of materialsNegative charge, and provides data

on the surface structure mainly

Interacts with nuclei, and therefore scatters strongly from light atoms,

as well as heavy atoms, and can differentiate easily between

isotopes

Interacts with the electron clouds, and therefore scatters strongly

from heavier elements with larger electron clouds.

Neutrons are spin- ½ particles, and carry a magnetic moment (good to

study magnetism over short ranges)

Can be used to study magnetism, through electromagnetic radiation,

but provide less information.

Expensive!!!! Less expensiveRequires the use of high energy

synchrotron ringsCan be done in house

Neutron sources in Europe

Types of experiment• Diffraction: - assumes “elastic scattering” - no energy transferred to/from sample

- measurement of crystal structures, atomic correlations in liquids/glasses

• Inelastic scattering (spectroscopy):- energy transferred to/from sample- measurement of lattice vibrations (phonons), atomic diffusion, molecular modes

• Small-angle scattering:- diffraction at very small angles- measuring large objects - proteins, colloids, aggregates, nanoparticles, etc

• Reflectometry:- diffraction from a surface - specular or off-specular- measures depth profile of thin films, membranes, etc.

• Imaging: - Images and maps of molecular arrangement on surfaces

SANS• Instruments:

• SANS2D, LOQ, LAMOUR, NIMROD, SANDALS (ISIS)• D11, D22 (ILL)• KWS-1, KWS-2, KWS-3, SANS-1 (MLZ)

How it works

Theory• Scattering of neutrons at small angles gives information about their

structure and size, giving a scattering pattern.

• This pattern is subjected to Fourier Transformation, and described in terms of a momentum transfer vector (Q), instead of a diffraction angle.

• Q is scattering in reciprocal space, rather than actual (d) space.• Small Q values look at long distances, large Q values look at short

distances.• Q values of 0.006 to 0.28 Å-1 probes distances of about 10 to 1000

Å ( 1 to 100 nm ).

Sample preparation

• All samples need to be made in D20.• Samples can be contrast matched, allowing for parts of the

sample to become invisible to the neutron beam.

• Contrast variation involves matching the density of part of the sample, with ratios of D2O and H2O.

• All samples need to be placed in clean quartz cuvettes, and placed in the instrument.

The data

• The data generated provides info on the shape and structure (but needs to be fitted).

• Changes in Q (the wiggles) define a change in scatering length density• From this, the data can be fitted to different models, which can provide

information on the size of the particle, interactions, radius of gyration, structure, shape etc…

Fitting• Many fitting programmes available:

• SASview• Fish• SASfit

• Fitting based on prior knowledge about system, and uses other experimentally determined values

SummaryPro’s Con’s

Powerful technique to characterise internal and external structural features

Requires previous knowledge on system from other techniques

Contrast matching can make parts of the system “invisible”

Requires knowledge on the density of different parts of the system

Very good at looking at probing size and structural effects in “smart” systems

Looking at samples in different conditions is time consuming. Only a limited amount of

beam time is availableGood in characterisation of simple systems

(e.g. sphecircal particles/polymers), up to very complex systems in complicated sample

environments (e.g. temperature responsive nanoparticles in a supercritical environment)

Data fitting gets more complicated as more parameters are added to a system, which

requires more previous knowledge

Looks very good in papers, and increases the impact of the article Need to apply for beam time

Can be used for many types of particle in many different environments

Specific fields requires more expertise in the area e.g. biological samples.

Good experience and chances to network Expensive and highly competitive

Reflectometry • Instruments:

• CRISP, SURF, INTER, OFFSPEC, POLREF (ISIS)• SuperADAM, D17, FIGARO (ILL)• MARIA, NREX, REFSANS (MLZ)

Uses of

How it works

Theory• A highly collimated beam is fired at a flat surface,

and the intensity of the beam reflected is measured as a function of wavelength.

• The reflection is usually described in terms of a momentum transfer vector (Q) i.e. the change in momentum before and after the neutron beam hits the surface.

Sample preparation

• All samples need to be made in D20.• Again, contrast matching can be used to mask

certain parts of the system.• Once made, samples need to be poured onto a

trough completely free of hydrophilic and hydrophobic molecules, as they can alter results.

• Samples are then placed in the instrument and experiment run.

Data/fitting

• Lots of programmes to choose from.• Depending on the type of experiment, fitting gives information

on thickness of layer/bilayer, composition, and so on.• Again, previous information from other techniques is required

e.g. ATIR, Brewster angle microscopy.

SummaryPro’s Con’s

Good in determining the thickness of interfaces, and thin films

Requires previous knowledge on system from other techniques including refractive index

Contrast matching can make parts of the system “invisible”

Requires knowledge on the density of different parts of the system, and deuterated samples

(very expensive)

Can be used to examine lots of things about an interface

Very time consuming, one run can take up to 2/3 hours. Lots of late nights!!

Can be used in many field e.g. bilayer composition and enzyme kinetics in biology

Data fitting gets more complicated as more parameters are added to a system, which

requires more previous knowledge

Looks very good in papers, and increases the impact of the article Need to apply for beam time

Very versatile Different instruments look at different parameters.

Good experience and chances to networkComplicated technique with lots of theory.

Fitting also needs certain values, which require use of equations

Applying for beam time• Case needs to be justified• Why are neutrons needed?• Xpress beam-time• Peer review• Experimental evidence needed• Things that help a case:

– Popular subjects– Contrast matching– “Quirky” systems– Magnetic moments– Strange sample environments– Interesting samples