Neutron Scattering

40-
E¢. 0. or P
travel
-03
E4- é0- Ki- direction
E4 --(»
-0,
l
1 -1
^4~ P
Fig .5a Guide field for longitudinal Fig .5b Guide f eld far longitudinal
polarisation, P Il H , the polarisation, _P antiparallel _N
small arrow is the spin of the
neutrons
Fig .Sc Guide fietd for transversal
polarisation, P parallel N
Eig .5d Guide field for transversal
polarisation,- P antiparallel N
Figure 4 .1 : Neutrons in a guide field H with spins parallel or anti-parallel[6] .
sheet, for example, and polarized neutrons parallel to the firnt guide field will be
kept in an anti-parallel orientation to the second field when passing the sheet . This
case w » WL is also used in ends operating as spin flippers .
In neutron scattering experiments with polarized neutrons, the principle of slow field
changes is used, for instance, to align the polarization along a particular direction ai the
Sample position . As will be Seen in the examples below, the polarizations perpendicular
and parallel to the scattering vector and the magnetization of the Sample are of particular
interest . Usually, we assume that the field at the Sample position is sufficiently weck so
Chat the magnetization of the Samples remains undisturbed . Using three (orthogonal)
pairs of ends the polarization can be turned arbitrarily into any direction at the Sample
position to probe the orientation the magnetization of a Sample . While turning the
neutron polarization ai the Sample position by an additional field H and Chus keeping the
neutrons moments in guide fields one can only distinguish scattering processes in which
the spin direction is preserved (non-spin flip) or reversed (spin-flip) for a given direction .
In general, there may be an arbitrary angle of rotation of the neutron polarization, and
it can be measured only if the Sample is in a field-free space . Super-conducting sheets
are ideal for shielding the Sample environment from external magnetic fields (sudden
4-3