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Physical > Neutron Scattering > Flashcards

Neutron Scattering Flashcards

1
Q

What is neutron scattering most affected by?

A

Mass magnetic moment

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2
Q

When does neutron absorption become prohibitive?

A

When the scattering length of different elements and isotopes is close in absorption energies. This results in a complex scattering length.

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3
Q

When do the neutrons have the strongest resonant scattering?

A

When neutron energy is close to the transition to a stable state of a compound nucleus allowing neutron absorption

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4
Q

Why do X-rays have significant fall off in intensity with increasing angle

A

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.

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5
Q

Describe Compton scattering

A

Excitation of electrons by X-Rays to higher energy levels or ionisation, which results in inelastic scattering of the electrons

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6
Q

Can neutrons directly excite electrons? How?

A

No, as their energies are much lower than of electrons.

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7
Q

What can neutrons excite?

A

Vibrational transitions through inelastic scattering

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8
Q

3 disadvantages of x-ray tubes

A
  1. X-Ray energies from a given target are fixed
  2. Samples get hot
  3. X-Rays can not be tuned
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9
Q

3 advantages of synchrontron x-ray emission

A
  1. X-Rays emitted are coherent
  2. High radiation intensity
  3. Emission happens as pulses
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10
Q

What not to do with neutron calculations?

A

Do not convert neutron wavelengths into wavenumbers. Instead use a photon with the same energy

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11
Q

What does nuclear scattering arise from?

A

Atomic nuclei

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12
Q

What does magnetic scattering arise from?

A

Unpaired spins

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13
Q

Why is more material needed for neutron scattering compared to X-Ray?

A

Neutrons are less penetrating

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14
Q

How do nuclei behave in neutron scattering?

A

Point scatterers

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15
Q

Why do nuclei act as point scatterers?

A

Nuclei radius is much smaller than neutron wavelength

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16
Q

What helps to distinguish between neighbouring elements/isotopes?

A

Resonance scattering

17
Q

What does eleastic coherent neutron scattering give?

A

Diffraction patterns

18
Q

What does eleastic incoherent neutron scattering give?

A

Background noise

19
Q

What are the two sources of nuclear incoherent scattering?

A

Isotopic & spin incoherence

20
Q

What is isotopic incoherence?

A

When different isotopes have very different scattering lengths

21
Q

What is spin incoherence?

A

When a given isotope can have two or more nuclear spin states that scatter neutrons differently.

22
Q

Experimental neutron methods

2 ways of performing energy selection

A
  1. Choppers
  2. Crystal monochromator
23
Q

Energy selection

Advantage and disadvantage of choppers

A
  1. Cheap
  2. Low resolution
24
Q

Advantage of single crystal over powder diffraction

A

Incoherent background is not as bad

25
Q

What will single crystal diffraction in combination with X-ray result help to determine?

A

Accurate determination of all atoms

26
Q

What needs to happen to the sample before running powder diffraction?

A

It needs to be deutarated

27
Q

Inelastic neutron scattering

What does intensity depend on?

A

Incoherent cross section and the amplitude of motion ^2

28
Q

Advantage of inelastic neutron scattering over IR/Raman

A
  • H vibration in conducting systems (IR/Raman opaque)
  • M-H vibrations in organimetallic hydrides stand out (Raman: can be covered by CO)
29
Q

Lattice vibrations

Define phonons

A

Normal modes of vibration

30
Q

Lattice vibrations

Define acoustic

Mention wavelength/frequecy/energy

A
  • With in-phase motion of neighbouring atoms
  • Long wavelength & low frequency/energy
31
Q

Lattice vibrations

Define optic

Mention wavelength/frequecy/energy

A
  • Out of phase motion of neighbouring atoms
  • Short wavelenth and high energy/frequency
32
Q

Define asymmetric unit

A

Minimum number of atoms not related to each other by symmetry

33
Q

Define angle-dispersive

A

Diffraction using radiation of fixed wavelength, while varying the diffraction angle.

34
Q

Define wavelength-dispersive

A

Diffraction in which the detector angle is fixed and the wavelength is systematically varied

35
Q

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

A
  • 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
36
Q

Exam Q 23 b

What is the significance of both isotopes having a real scattering length?

A
  • Intensity is increased allowing to distinguish between isotopes
  • Overall scattering is not complex, therefore absorption is not prohibitive

Physical (4 decks)

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