Introduction

Introduction

Neutrons are often pictured as tiny sub-atomic particles that make up approximately 50% of the mass of most atoms. However, due to the principle of wave-particle duality, neutrons also have a wavelength. The momentum, energy, spin, and lack of charge of the neutron makes it an ideal probe of the structure of matter. Neutrons were discovered many years after X-rays, and it was only after the development of nuclear reactors that intense beams of neutrons became viable for research use. The environmentally-friendly alternative to nuclear reactors for the production of intense beams of neutrons is the pulsed spallation source.

Crystal structure refinement from powder data was first developed for the case of neutron diffraction, resulting in the Rietveld method, which is still in use today. This section of the course explains why neutrons are used, together with powder neutron diffractometer design and function.

Neutron versus X-ray powder diffraction

Before discussing powder neutron diffraction in detail, a quick comparison with X-rays may be useful for those less familiar with the use of neutrons for powder diffraction studies. The figure below shows a typical laboratory data set of the mineral anglesite, PbSO₄, collected with Cu Kα₁ radiation in Bragg-Brentano geometry (to avoid absorption problems due to the lead atoms).

For comparison, a neutron diffraction pattern is shown of the same material collected on a constant wavelength medium-resolution powder diffractometer (D1A at the ILL, Grenoble) with a neutron wavelength equal to 1.909 Å.

There are several obvious differences:

Since the data sets were collected with different wavelengths, peaks due to identical d spacings will obviously appear at different angles (due to Bragg's law). (A more direct comparison can be made by plotting in either d or 1/d, but there are various arguments both for and against this that will not be discussed in detail here.)

The relative intensities of adjacent peaks are very different in the two data sets. Reflections that exhibit a high intensity in the X-ray data set may be relatively weak when measured by neutron diffraction, and vice-versa.

For the X-ray data set, the intensity of the reflections decays with increasing scattering angle due to the effect of the X-ray form factor, f, in stark constrast to that observed in the neutron data set, where the peaks are still intense at high scattering angle.

For the particular data sets shown, the peaks are a lot broader in the neutron data set than in the X-ray data set due to the different instrumental resolution functions. It is probably less obvious that the X-ray resolution function has its minimum at a relatively low scattering angle, while the minimum is observed at a relatively high scattering angle in the neutron case.

The neutron data set has a much higher background than the equivalent X-ray data set, thus giving a worse peak to background ratio.

The remainder of this section will attempt to explain why some of these differences occur and how powder neutron diffraction experiments exploit them.

Course Material Index

Section Index

Author(s): Jeremy Karl Cockcroft