Neutron and X-ray Scattering Flashcards
What is the key difference between X-rays and neutrons in terms of what they are sensitive to?
X-rays are primarily sensitive to charge (electrons), while neutrons are sensitive to nuclear mass and magnetic moments.
What are the three main types of interactions that can cause radiation to scatter off atoms?
1) Charge (electrons)
2) Mass (nuclei)
3) Magnetic moments
What is the scattering length?
A measure of the strength of the interaction between a particle, such as a neutron or an X-ray photon, and a target atom or molecule.
What is coherent scattering?
When the outgoing scattered wave is in phase with the incident wave.
What is the total scattered X-ray wave from an atom composed of?
It is the superposition of all the Thomson scattered waves from the individual electrons in the atom.
What is the atomic scattering factor f?
The atomic scattering factor f is a function of sin(θ)/λ and is the Fourier transform of the electron density distribution in the atom.
What additional contribution arises when X-rays/neutrons scatter resonantly close to an absorption edge?
An additional real and imaginary component to the scattering factor, e.g. f = f0 + f’ + if” for X-rays.
What type of scattering can probe the spin arrangements in magnetically ordered materials?
Magnetic neutron scattering can measure diffraction peaks from ordered ferro/antiferromagnetic structures.
For a centrosymmetric crystal structure, what can be said about the structure factor Fhkl?
Fhkl is always real (has no imaginary component) due to cancellation of sine terms.
What two effects occur due to resonant scattering?
1) Strong absorption by the resonant atoms
2) Change in magnitude and phase of the scattered wave
Give an example of when resonant scattering is useful for X-rays?
It is useful for determining absolute configurations of enantiomeric crystals, phasing in structure solution, and distinguishing elements in disordered solids.
For X-rays, when does resonant scattering occur?
Near the ionisation edges (K edge = 1s electrons, L edges = 2s, 2p electrons, etc.)
Neutrons can also show resonant scattering effects. Near what energies does this occur?
Resonant effects for neutrons occur when the neutron energy is close to the energy required to form a stable compound nucleus (original nucleus + neutron).
How does magnetic scattering differ from the other types of scattering?
Magnetic moment is a vector quantity, so the scattering is dependent on the moment direction as well as magnitude.
What are the two sources of nuclear incoherent scattering?
Isotopic incoherence
Spin incoherence
