The Instrument at the Cavendish Laboratory

A Mott polarimeter has been available at the Cavendish Laboratory for some time. The main purpose of the polarimetry project to date has been to increase its reliability as a measuring device to the point where it can be used to determine the polarization of a reflected, diffracted or secondary electron beam from an ultra-thin magnetic film structure. The latter has now been achieved using a specularly reflected electron beam from a film structure that previous research has suggested is of interest (chapter 2.8.) This has required the finding of technical solutions to a variety of instrumental difficulties [8] (sections 5.4, 5.5.)

The polarimeter now in use at the Cavendish Laboratory is of the compact, retarding-potential type (figure 3.1) [71,34]. The detector position is chosen because there is a broad maximum in the Sherman function $S$, defined above, for metal foils at a scattering angle of around $120^{\circ}$ [34]. Electrons are focused onto the thorium target by the electrostatic grids and lenses at the front of the device (figure 3.1,) and are accelerated to energies between $20\,\mathrm{keV}$ and $25\,\mathrm{keV}$, suitable for the required low impact parameter scattering, by an electrostatic potential applied at the thorium foil [34]. An electrostatic potential applied at the retarding grids (figure 3.1) turns back electrons which have lost more than some specified amount of energy, chosen by setting the retarding grid potential, during the scattering process [34]. This eliminates a large proportion of multiply scattered electrons, for which the Sherman function $S$ is reduced, without the reduction in the total cross-section $\sigma _0$, which is brought about by using a thin film target to reduce multiple scattering [72].

Figure 3.1: The Compact Retarding Potential Mott Polarimeter. $G$ Represents the Rate at which Electrons Arrive at the Front of the Polarimeter, $a$ the Fraction of Those Electrons that Are Accepted into the Polarimeter, and $H_i$ the Rate at which Electrons Are Scattered towards Channeltron $i$.

The use of a retarding potential also means that the external electrodes can all be kept grounded, with the obviation of the need for bulky shielding on the detector as well as the direct safety benefits [34]. This, coupled with the relatively small target potential of $\sim{}22\,\mathrm{kV}$, which restricts the inter-electrode distances needed to avoid electrical breakdown, keeps the instrument small (the distance between the two channeltrons is $\sim{}10\,\mathrm{cm}$) [34], which is convenient for its attachment to a vacuum chamber (chapter 4) for experiments and for development and maintenance. In addition, the closeness of the channeltrons to the target, and the large solid angle which they therefore subtend, allows for a high total scattering cross-section $\sigma _0$ (equation 3.2) [34]. The relatively low energy of electrons incident on the thorium foil (figure 3.1) also increases $\sigma _0$.