Target and polarization measurement

  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:

• In principle there is no need to measure the target magnetization as long as magnetic saturation can be assumed. This is certainly true for low beam currents as there depolarization due to target heating can be neglected.
• With the laser monitoring foil magnetization can be observed on-line. Problems with target depolarization can be noticed immediately.
• During experiments only the relative target polarization needs to be measured (due to the saturation). The absolute calibration is known from the literature.
• The measurement is insensitive to errors in geometry as the target is perpendicular to the beam.

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