On the difference between neutrons and X-rays ADPs: experiences and observations

Silvia C. Capelli

ISIS Neutron and Muon Source, Rutherford Appleton Laboratory, Harwell Science Campus,Didcot, OX11 0QX, United Kingdom.

e-mail: silvia.capelli@stfc.ac.uk

Although the theory behind single-crystal diffraction is exactly the same independently of using neutrons or x-rays, the two probes are sensitive to very different constituents of the same crystal: nuclei and electron clouds respectively. This simple observation brings about the question: when comparing results from these two probes, are we really comparing the same quantities? This is especially important when quantities like for example Anisotropic Displacement Parameters (ADPs) of hydrogen atoms are calculated by quantum mechanics and then compared to neutron diffraction derived ones, or when the calculated values are used together with x-ray data to derive additional physical properties. The last attempt at rationalizing the possible differences between neutrons and x-ray ADPs is due to Blessing in 1995 [1], but the last 30 years of improvements in equipment and technology for crystallography with both probes - from sources to detectors to sample sizes to experimental techniques - should be enough a reason to scrutinize the subject afresh. Some evident differences will persist: X-ray sources will always be way more powerful than neutron ones in terms of flux at the sample position and neutron samples will continue to be larger than those used with x-rays (after all, a nucleus will always be 4 to 5 orders of magnitude smaller than its electron cloud) and this will continue to show in effects like absorption and extinction. Other more subtle and sometimes overlooked differences, like for example a constant neutron scattering length vs a decaying x-ray form factor or the use or monochromatic vs polychromatic radiation and the use of time-of-flight in neutron diffraction, still need more careful investigation. Examples and observations will be presented to illustrate these points.

References:

[1] R.H. Blessing, Acta Cryst. (1995). B51, 816-823.