Neutron Scattering Flashcards
What is neutron scattering most affected by?
Mass magnetic moment
When does neutron absorption become prohibitive?
When the scattering length of different elements and isotopes is close in absorption energies. This results in a complex scattering length.
When do the neutrons have the strongest resonant scattering?
When neutron energy is close to the transition to a stable state of a compound nucleus allowing neutron absorption
Why do X-rays have significant fall off in intensity with increasing angle
In a straight though direction all scattered waves due to electrons are in phase with each other. Destructive interference increases as the angle ($2\theta$) from the straight though direction increases.
Describe Compton scattering
Excitation of electrons by X-Rays to higher energy levels or ionisation, which results in inelastic scattering of the electrons
Can neutrons directly excite electrons? How?
No, as their energies are much lower than of electrons.
What can neutrons excite?
Vibrational transitions through inelastic scattering
3 disadvantages of x-ray tubes
- X-Ray energies from a given target are fixed
- Samples get hot
- X-Rays can not be tuned
3 advantages of synchrontron x-ray emission
- X-Rays emitted are coherent
- High radiation intensity
- Emission happens as pulses
What not to do with neutron calculations?
Do not convert neutron wavelengths into wavenumbers. Instead use a photon with the same energy
What does nuclear scattering arise from?
Atomic nuclei
What does magnetic scattering arise from?
Unpaired spins
Why is more material needed for neutron scattering compared to X-Ray?
Neutrons are less penetrating
How do nuclei behave in neutron scattering?
Point scatterers
Why do nuclei act as point scatterers?
Nuclei radius is much smaller than neutron wavelength
What helps to distinguish between neighbouring elements/isotopes?
Resonance scattering
What does eleastic coherent neutron scattering give?
Diffraction patterns
What does eleastic incoherent neutron scattering give?
Background noise
What are the two sources of nuclear incoherent scattering?
Isotopic & spin incoherence
What is isotopic incoherence?
When different isotopes have very different scattering lengths
What is spin incoherence?
When a given isotope can have two or more nuclear spin states that scatter neutrons differently.
Experimental neutron methods
2 ways of performing energy selection
- Choppers
- Crystal monochromator
Energy selection
Advantage and disadvantage of choppers
- Cheap
- Low resolution
Advantage of single crystal over powder diffraction
Incoherent background is not as bad
What will single crystal diffraction in combination with X-ray result help to determine?
Accurate determination of all atoms
What needs to happen to the sample before running powder diffraction?
It needs to be deutarated
Inelastic neutron scattering
What does intensity depend on?
Incoherent cross section and the amplitude of motion ^2
Advantage of inelastic neutron scattering over IR/Raman
- H vibration in conducting systems (IR/Raman opaque)
- M-H vibrations in organimetallic hydrides stand out (Raman: can be covered by CO)
Lattice vibrations
Define phonons
Normal modes of vibration
Lattice vibrations
Define acoustic
Mention wavelength/frequecy/energy
- With in-phase motion of neighbouring atoms
- Long wavelength & low frequency/energy
Lattice vibrations
Define optic
Mention wavelength/frequecy/energy
- Out of phase motion of neighbouring atoms
- Short wavelenth and high energy/frequency
Define asymmetric unit
Minimum number of atoms not related to each other by symmetry
Define angle-dispersive
Diffraction using radiation of fixed wavelength, while varying the diffraction angle.
Define wavelength-dispersive
Diffraction in which the detector angle is fixed and the wavelength is systematically varied
Exam Q 23 a
Explain why the intensity of elastic nuclear scattering of neutrons by an atom has
no fall-off with scattering angle 2x, whereas elastic scattering of X-rays does have
a significant fall-off with increasing 2x
2 marks / 4 points
- At greater angles neurtrons/X-rays need to go through more material
- Neutrons interact with small neucleus via short-range strong force
- X-rays interact with larger cloud of electrons via Compton scattering
- At greater angle there is a greater probability that an X-Ray has interracted with electrons leading to intensity fall off
Exam Q 23 b
What is the significance of both isotopes having a real scattering length?
- Intensity is increased allowing to distinguish between isotopes
- Overall scattering is not complex, therefore absorption is not prohibitive
