Incoherent spin-flip

Measurements of the self-correlation function with neutrons are normally performed on protonated materials since incoherent scattering is particularly strong there. This is a consequence of the spin-dependent scattering lengths of hydrogen. Due to spin-flip scattering, which leads to a loss of polarization, this... 

The spin-incoherent scattering (prominent for protons) involves a change of the spin-state of the scattered neutrons (spin-flip scattering) with a probability of%. 

From (3.39) we see that nuclear coherent scattering is always non spin-flip (-(- -(-,--), as is nuclear incoherent scattering which is due to the random iso-... 

For nuclear incoherent scattering from a non-magnetic system with randomly oriented nuclear spins, the cross-sections per atom are independent of the neutron polarization direction and the spin-flip scattering cross section is twice that for non spin-flip scattering ... 

Spin-flip transitions, between the manifolds, are controlled by the incoherent cross section of hydrogen, 80.3 bam, and are enabled by spin exchange with the neutron, / = 1/2. Thus, for the J(l -0) transition... 

Coherent and isotopic-incoherent scattering involve no spin-flip, whereas spin incoherent scattering (i.e., for hydrogenated molecules) inverts the neutron spin with a probability of 2/3. Since spin-polarized neutrons are used in the neutron spin-echo technique, the polarization of the neutron beam, after spin-incoherent scattering woifld be reversed and three times less intense. 

The consequences of spin-incoherent scattering, which can have a significantly deleterious effect on the NSE signal, are shown in Figure 4. Spin-incoherent scattering causes, with a nucleus-dependent probability, a spin flip of both the inelastically and elastically scattered neutrons, e.g., 2/3 of the neutrons scattered from H undergo a spin flip, whereas deuterium, which has no nuclear spin, has no influence on the neutron spin. 

Quite often, spin dephasing times are orders of magnitude shorter than the decay times measured in the (incoherent) population relaxation experiments. For a few transition metal chelates in the excited triplet state we will argue later that the homogeneous spin dephasing is determined by hyperfine interactions of the triplet electron spin with randomly flipping ligand nuclear spins. 

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