POLARIZED NEUTRON-SCATTERING FROM POLARIZED NUCLEI NEAR PARAMAGNETIC CENTERS

Polarized neutron scattering from dynamically polarized targets has be en used for the study of hydrogenous materials in several laboratories . A new variant, which is less dependent on specific deuteration, is p roposed. This is based on the observation that a radio frequency (RF) held swept over a selected part of an NMR line depolarizes the spins d epending on their spatial distribution in and around paramagnetic mole cules. An RF field swept over a narrow frequency interval has little e ffect on polarized nuclear spins in the paramagnetic centers, whereas remote nuclear spins can be depolarized. When applying the sweep over a much larger frequency range, all spins are depolarized. This was obs erved both by NMR and by polarized neutron scattering after selective depolarization of the proton spins in a solid solution of protonated E HBA-Cr-V, Na(C12H20O7CrV). D2O, in a mixture of deuterated glycerol an d water. The decay time of proton spin polarization in domains associa ted with the paramagnetic centers varies between 5 h in a deuterated s olvent and less than 1 min in a protonated solvent. The size of such a domain is barely larger than 1 nm. The same observation is also made when the nuclear polarization is reversed by adiabatic fast passage. I f the RF sweep is limited to frequencies close to the central peak of the proton NMR line, the proton spins far away from the paramagnetic c enters are reversed, whereas those of EHBA-Cr-V and close to the cente rs maintain their original polarization. This method may become a usef ul tool in neutron scattering once the paramagnetic centers can be fix ed as spin labels to well defined sites in a macromolecule, preferably in a crystal lattice.