SPECS 2D-CCD, User Manual

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Manual V2.docx | Status 10.12.2015 | Version 1
installed. They serve to shape the beam for high pass energies in the analyzer. Electrons
scattered at these apertures are stopped from reaching the 90° deflector by a retarding
potential applied to the next lens L5A. The triplet L4A, L5A, L4B thus serves as a repeller for
scattered electrons.
When the electrons leave the acceleration lens and enter the 90° deflector, they have a
kinetic energy of 3000 eV, the pass energy of the 90° deflector.
5.1.3 Rotator lens
A magnetic lens is fitted for rotating the transversal component of the spin polarization
vector.
1
The spin polarization component parallel to the direction of flight of the electrons is
not affected.
For the transversal spin rotation angle around the optical axis, η, in a coil lens
2
, where it is assumed that the optical axis is along the z-axis. B
z
is the z-component of the
magnetic field and g the gyromagnetic ratio, g = 2.00232.
v
is the relativistic velocity of the
electrons,
e
the electron charge and
m
the electron mass. The kinetic energy of the electrons
in the rotator lens is 1800 eV.
Since electrons will follow a spiral shaped path through a magnetic coil lens, the lens will
rotate an electron optical image an angle
θ
around the optical axis. In rotationally symmetric
magnetic fields, it holds
2
Combining these equations gives the result that the spin rotation angle is close to two times
the image rotation angle.
2
This gives a convenient way to check the operation of the rotator lens. By switching off the
90° deflector, the electron beam is directed to a phosphor screen in front of a camera, see
Figure 13. The angle of rotation of the beam image when the polarity of the current through
the rotator lens is changed is half the spin rotation angle.
1
Engwall, D. A., B. M. Dunham, et al. (1993). "A spin manipulator for electron accelerators."
Nucl. Instr. Meth. A324: 409-420.