Workshop on Polarized Electron Sources and Polarimeters

with a fast sensor and for the electron beam with an ultra-fast coaxial Faraday cup.
For the capturing process of the beam into the superconducting accelerating structures
the electron-bunch length need to be of the order of 5 ps. Therefore a new chopper and
prebuncher system has been developed, based on a similar setup at MAMI [9].
Polarization manipulation and measurement
The degree of polarization of 100 keV electrons for the teststand is measured with a
Mott polarimeter detecting count-rate asymmetries in electron scattering on nuclei due
to the spin-orbit interaction. The asymmetry occurs only when the electron spin direction
is not parallel to the scattering plane. Thus the initially longitudinal oriented spins must
be rotated by 90 ◦ into the transverse direction. This is done by a Wien filter [10]. This
device is also necessary tool for adjusting the spin orientation at different experimental
areas of the S-DALINAC.
The ideal Wien filter acts as a drift space in the vertical plane, but its horizontal
electron-optical properties are similar to those of a quadrupole: It focusses the electron
beam. This effect was minimized: the focus has been set inside the filter with help of
newly designed compact quadrupole triplet in front of the Wien filter. An additional
quadrupole doublet installed after the device compensates its influence on the beam.
The installed spin rotator can provide 100 degree spin rotation.
The 100 keV Mott polarimeter [11] houses 4 silicon-surface-barrier detectors which
are oriented in two perpendicular scattering planes to measure both transverse polarization
components simultaneously. As targets self-supporting gold foils of the thickness
in the range from 42.5 nm to 500 nm are utilized. The polarimeter contains also a BeO
viewer to control the beam position and one empty target for background measurements.
The scattering angle of (120 ± 1.5) ◦ is determined by an aluminum collimator with 2
mm diameter. This angle matches the broad maximum of the analyzing power or socalled
Sherman function at this angle. The typical energy resolution of the detector is
Counts x 10 4
2
1.5
1
0.5
= 60 nm
0
10 20 30 40
Channel
50 60
10 20 30 40 50 60 70
Channel
FIGURE 2. Measurement of Mott spectra with the two detectors of a scattering plane for reversing
electron helicities. The helicity can be switched by reversing the polarization of the laser light.
about 20 keV and the peak-to-background ratio is found to be 14 : 1 for the 122 nm gold
foil. The polarization of the electrons can be switched from parallel to antiparallel to the
electron momentum by changing the incident light helicity.