International
Tables for Crystallography Volume C Mathematical, physical and chemical tables Edited by E. Prince © International Union of Crystallography 2006 |
International Tables for Crystallography (2006). Vol. C. ch. 2.6, p. 108
Section 2.6.2.3.1. Spin-contrast variation
R. Mayb
|
For a long time, the magnetic properties of the neutron have been neglected as far as `nonmagnetic' matter is concerned. Spin-contrast variation, proposed by Stuhrmann (Stuhrmann et al., 1986; Knop et al., 1986), takes advantage of the different scattering lengths of the hydrogen atoms in its spin-up and spin-down states. Normally, these two states are mixed, and the cross section of unpolarized neutrons with the undirected spins gives rise to the usual value of the scattering amplitude of hydrogen. If, however, one is able to orient the spins of a given atom, and especially hydrogen, then the interaction of polarized neutrons with the two different oriented states offers an important contribution to the scattering amplitude: where b is the isotropic nuclear scattering amplitude, B is the spin-dependent scattering amplitude, s is the neutron spin, and I the nuclear spin. For hydrogen, b = −0.374 × 10−12 cm, B = 2.9 × 10−12 cm.
The sample protons are polarized at very low temperatures (order of mK) and high magnetic fields (several tesla) by dynamic nuclear polarization, i.e. by spin–spin coupling with the electron spins of a paramagnetic metallo-organic compound present in the sample, which are polarized by a resonant microwave frequency. It is clear that the principles mentioned above also apply to other than biological and chemical material.
References
Knop, W., Nierhaus, K. H., Nowotny, V., Niinikoski, T. O., Krumpolc, M., Rieubland, J. M., Rijlart, A., Schärpf, O., Schink, H.-J., Stuhrmann, H. B. & Wagner, R. (1986). Polarised neutron scattering from dynamic polarised targets of biological origin. Helv. Phys. Acta, 59, 741–746.Google ScholarStuhrmann, H. B., Schärpf, O., Krumpolc, M., Niinikoski, T. O., Rieubland, M. & Rijllart, A. (1986). Dynamic nuclear polarisation of nuclear matter. Eur. Biophys. J. 14, 1–6.Google Scholar