Neutron Scattering

Some polarizers using Bragg reflections, for instance the (200) reflection from Co o.92Fe o .o8
alloy crystals, exhibit a very high degree of polarization (~ 0 .99 for the whole set-up :
initial polarization - spin-flipper - final polarization) . However, only a narrow band of
wavelengths can be used and also the accepted divergence is small, which could be demanding
in terms of scattering intensities . An experimental set-up for polarization analysis
using the total reflection of long super-mirrors or shorter benders (a stack of such
super-mirrors) perform also reasonably well in terms of polarization (typically ~ 0 .95
or better), and in addition a comparatively wide band in wavelength or energy of cold
neutrons is accepted (particularly useful for time-of-flight spectrometers) . For thermal
neutrons 'He-filtern seem to be a very appropriate choice. The device does not interfere
with the divergence, which han been set otherwise in the experiment . The beam transmission
and degree of polarization can be optimized by varying the gas pressure and can
be matched to the spectrum of neutron energies . The most efficient performance is, however,
a compromise between intensity and a modest degree of neutron polarization (say
about 50%) . Of course, this requires to perform corrections due to the finite degree of
polarization . However, such corrections can easily be performed and the final result for
the scattering intensities depend just on the accuracy with which ones knows the degree
of polarization .
4 .3 Polarization and scattering processes
4 .3 .1 Coherent nuclear scattering
Within the firnt Born approximation the scattering cross-section is determined by
du Mn ) 2 ~kS~
d9 (2z1i
(4 .3)
First, we calculate the matrix element of the interaction potential V between the
initial and final states for pure nuclear scattering at a nucleus wich spin I = 0, i . e . the
scattering amplitude A(Q) (nee chapter 3) . With b(Q) _ ~-, z btie i Q.r s the matrix element is
A(Q) - (Sz b(Q) Sz) - b(Q)(S,' l Sz)
NSF
SF
(4 .4)
4-6