Page 66 - Neutrons for Sciences and Society

Neutrons for Science
3.2.1 Diffraction
Neutrons can fulfil the same role as X-rays, however they have two benefits in certain applications which largely compensate for the low intensity of available beams. I have already mentioned
the neutron’s magnetic moment which makes them unique for determining magnetic structures. This was the first field of success for neutrons, worth the Nobel Prize received by Cliff Shull in 1994.
The other advantage arising from impact of neutrons with an atom is that they interact with the nucleus, whereas X-rays are scattered by the electrons orbiting the nucleus. As a consequence X-rays have an interaction 92 times greater with a uranium atom than with a hydrogen atom. The latter will contribute very little to a diffraction image from X-rays. On the contrary, with neutrons the contribution to this image from an atom of hydrogen and
a much heavier atom are of the same order of magnitude. It is thus easier to see hydrogen atoms with neutrons than X-rays. There have been numerous applications of this. Very early, in 1969 Benno Schoenborn52 using the Brookhaven HFBR studied the protein myoglobin, completing the X-ray results of John Kendrew for which the latter was awarded a Nobel Prize in 1962. An additional advantage arises from the fact that hydrogen has
a negative scattering amplitude while deuterium has a positive amplitude. The applications in biology which make use of this difference have been described by Heinrich Stuhrmann53. While facilitating the location of hydrogen atoms the use of neutrons in
52 Schoenborn B.P., Nature, (1969), 224, p143-146, DOI 10.1038/224143a0
53 H.B. Stuhrmann, Rep. Prog. Phys. (2004), 67, p1073-1115, DOI 10.1088/0034-4885/67/7/R02
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