Targets of Møller polarimeters always consist of thin ferromagnetic foils. Foils allow to have a large density of electrons, ferromagnets are easily polarized.
The classical target set-up consists of a foil magnetized in its plane using two
Helmholtz coils. The target is oriented to have a small angle (10-20 ) with
respect to the beam to get longitudinal polarization. Magnetization of the target is
measured with the help of pick-up coils which sense the change of flux upon
reversal of the B-field. Precision of this method is limited by systematic errors of
this measurement process. This leads to typical uncertainties of target polarization
between 2 and 5 %.
The target planned for Hall C will be of a novel design. The target will be
magnetized to saturation in a field of 4T to the foil plane
using a superconducting split coil. The target magnetization is monitored with help
of a laser beam. This design is more costly but has decisive advantages:
The target will consist of a pure iron foil, as iron is the ferromagnet with the
best known properties. Maximum target polarization at saturation is 2.216 / 26 =
8.52 %. This value must be temperature corrected as ferromagnetism becomes weaker
with increasing temperature. For iron the magnetization vanishes at T =
770
C. The relation between magnetization and polarization is is given by:
The value of M (the magnetization per unit volume) can either be measured in a
separate experiment, or taken with very high precision from the literature. The
factor g (the contribution of the orbital moment to the magnetization)
contains the largest uncertainty and is the only value which cannot be measured by
ourselves. Its uncertainty is 0.1%.
The magnetization is monitored with help of the magneto-optical Kerr effect. A
polarized laser beam is reflected on the target. If the target is magnetized the
reflected beam has an altered polarization plane. The rotation of the polarization
plane linearly depends on the magnetization. The laser probes the magnetization in
the first 50 nm only, but it has been shown that surface magnetization is always
than its bulk value. Thus saturation of the foil in the surface
proves that magnetic saturation exists throughout the material. The set-up is shown
in fig.8.
Figure 8: Setup of a Kerr apparatus: P = polarizer(Glan-Thompson), PEM = photoelastic
modulator, A = analyzer(Glan-Thompson), LIA = lock-in amplifier