X-ray Diffraction- Neutron Powder Diffraction

Question 1

Properties of neutrons

Answer 1

Zero charge
Rest mass 1.675x10^-27kg
Spin 1/2

Question 2

Formula for wavelength of a neutron

Answer 2

λ=h/rt(2mE)
Where E is kinetic energy
m is mass

Question 3

Energy of neutrons from different sources

Answer 3

Neutrons arising from a reactor or pulsed spallation source that have been thermally equilibrated have energy E about kT.

Question 4

Wavelength of thermal neutrons

Answer 4

T is 300-400K so λ is 1-2Å, same order as x-ray wavelengths

Question 5

Diffraction of neutrons

Answer 5

Scattered in accordance with Bragg law. Scattered by atomic nucleus radius about 10^-4Å. Scattering independent of sinθ/λ. Neutron scattering length is property of the nucleus and no regular variation with Z. Scattering much weaker than x-rays so need large samples roughly 5g. Neutrons have a magnetic moment and are scattered by atomic magnetic moments. Scattered by periodic magnetic and nuclear structures

Question 6

Is diffraction of neutrons different for different isotopes?

Answer 6

Yes unlike for XRD

Question 7

Reactor source of neutrons

Answer 7

Core designed to maximise neutron flux and minimise heat. U-235 absorbs neutron to make Kr, Ba and more neutrons. Fuel rods enriched to about 93% U-235. D2O is moderator and coolant. Flux in core about 10^15 cm^-2s^-1. For angle dispersive experiment fixed λ and variable θ.

Question 8

Pulsed spallation source of neutrons

Answer 8

Synchrotron accelerates protons to relativistic speeds. Pulses of protons strike heavy metal U-Ta target causing ejection or spallation of neutrons over wide range of velocities. Done in pulses so not continuous heating which would lead to melting. Liquid methane or Gd(NO3)3 poisoned H2O moderator. Energy dispersive experiment has variable λ and fixed θ. Neutron wavelength determined from time of flight. Time averaged flux about 2% of reactor flux

Question 9

Applications: location of H atoms

Answer 9

Needs deuterated samples because of incoherent scattering by H. Neutron scattering by deuterium and heavier elements is similar. Accurately locate deuterium atoms in drug molecules, inorganic hydrides etc

Question 10

Applications: atomic distributions

Answer 10

Cannot differentiate between elements with similar Z by x-rays as similar scattering factors. But neutron scattering lengths may vary considerably. Can determine site occupancy in transition metal spinels

Question 11

Applications: magnetic scattering

Answer 11

Neutrons scattered by interaction of neutron magnetic moment and those of atoms/ions. So neutrons diffracted by periodic arrays of magnetic moments. Diffraction pattern contain both nuclear and magnetic scattering reflections. Analysis of magnetic reflections allows determination of magnetic structure