Neutron Scattering

Question 1

What is neutron scattering most affected by?

Answer 1

Mass magnetic moment

Question 2

When does neutron absorption become prohibitive?

Answer 2

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

Question 3

When do the neutrons have the strongest resonant scattering?

Answer 3

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

Question 4

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

Answer 4

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.

Question 5

Describe Compton scattering

Answer 5

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

Question 6

Can neutrons directly excite electrons? How?

Answer 6

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

Question 7

What can neutrons excite?

Answer 7

Vibrational transitions through inelastic scattering

Question 8

3 disadvantages of x-ray tubes

Answer 8

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

Question 9

3 advantages of synchrontron x-ray emission

Answer 9

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

Question 10

What not to do with neutron calculations?

Answer 10

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

Question 11

What does nuclear scattering arise from?

Answer 11

Atomic nuclei

Question 12

What does magnetic scattering arise from?

Answer 12

Unpaired spins

Question 13

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

Answer 13

Neutrons are less penetrating

Question 14

How do nuclei behave in neutron scattering?

Answer 14

Point scatterers

Question 15

Why do nuclei act as point scatterers?

Answer 15

Nuclei radius is much smaller than neutron wavelength

Question 16

What helps to distinguish between neighbouring elements/isotopes?

Answer 16

Resonance scattering

Question 17

What does eleastic coherent neutron scattering give?

Answer 17

Diffraction patterns

Question 18

What does eleastic incoherent neutron scattering give?

Answer 18

Background noise

Question 19

What are the two sources of nuclear incoherent scattering?

Answer 19

Isotopic & spin incoherence

Question 20

What is isotopic incoherence?

Answer 20

When different isotopes have very different scattering lengths

Question 21

What is spin incoherence?

Answer 21

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

Question 22

Experimental neutron methods

2 ways of performing energy selection

Answer 22

1. Choppers
2. Crystal monochromator

Question 23

Energy selection

Advantage and disadvantage of choppers

Answer 23

1. Cheap
2. Low resolution

Question 24

Advantage of single crystal over powder diffraction

Answer 24

Incoherent background is not as bad

Question 25

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

Answer 25

Accurate determination of all atoms

Question 26

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

Answer 26

It needs to be deutarated

Question 27

# Inelastic neutron scattering What does intensity depend on?

Answer 27

Incoherent cross section and the amplitude of motion ^2

Question 28

Advantage of inelastic neutron scattering over IR/Raman

Answer 28

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

Question 29

# Lattice vibrations Define **phonons**

Answer 29

Normal modes of vibration

Question 30

# Lattice vibrations Define **acoustic** ## Footnote Mention wavelength/frequecy/energy

Answer 30

* With in-phase motion of neighbouring atoms * **Long** wavelength & **low** frequency/energy

Question 31

# Lattice vibrations Define **optic** ## Footnote Mention wavelength/frequecy/energy

Answer 31

* Out of phase motion of neighbouring atoms * **Short** wavelenth and **high** energy/frequency

Question 32

Define **asymmetric unit**

Answer 32

Minimum number of atoms not related to each other by symmetry

Question 33

Define **angle-dispersive**

Answer 33

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

Question 34

Define **wavelength-dispersive**

Answer 34

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

Question 35

# 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 ## Footnote 2 marks / 4 points

Answer 35

* 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

Question 36

# Exam Q 23 b What is the significance of both isotopes having a real scattering length?

Answer 36

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