  The theoretical possibility to use an electron microscope as a spin polarizer
is studied. It turns out that a Bessel beam passing a standard magnetic
objective lens is intrinsically spin polarized. In the limit of infinitely
small detectors on axis, the spin polarisation tends to 100 % . Increasing the
detector size, the polarisation decreases rapidly, dropping below 10^-4 for
standard settings of medium voltage microscopes. For extremely low voltages,
the figure of merit increases by two orders of magnitude, approaching that of
existing Mott detectors. Our findings may lead to new desings of spin filters,
an attractive option in view of its inherent combination with the electron
microscope, especially at low voltage.
