A cylindrically symmetric "micro-Mott" electron polarimeter.

A small, novel, cylindrically symmetric Mott electron polarimeter is described. The effective Sherman function, Seff, or analyzing power, for 20 kV Au target bias with a 1.3 keV energy loss window is 0.16 ± 0.01, where uncertainty in the measurement is due primarily to uncertainty in the incident electron polarization. For an energy loss window of 0.5 keV, Seff reaches its maximum value of 0.24 ± 0.02. The device's maximum efficiency, I/Io, defined as the detected count rate divided by the incident particle rate, is 3.7 ± 0.2 × 10(-4) at 20 keV. The figure-of-merit of the device, η, is defined as Seff (2)IIo and equals 9.0 ± 1.6 × 10(-6). Potential sources of false asymmetries due to detector electronic asymmetry and beam misalignment have been investigated. The new polarimeter's performance is compared to published results for similar compact retarding-field Mott polarimeters, and it is concluded that this device has a relatively large Seff and low efficiency. SIMION(®) electron trajectory simulations and Sherman function calculations are presented to explain the differences in performance between this device and previous designs. This design has an Seff that is insensitive to spatial beam fluctuations and, for an energy loss window >0.5 keV, negligible background due to spurious ion and X-ray production at the target.

New technique for the reduction of helicity-correlated instrumental asymmetries in photoemitted beams of spin-polarized electrons.

We present a new optical system that significantly reduces helicity-dependent instrumental intensity asymmetries. It is an extension of a previous scheme [Appl. Opt.47, 2465 (2008)], where one laser beam is split using a polarizing beam splitter into two with orthogonal linear polarizations. The beams are sent through a chopper, allowing only one to pass at a time. The two temporally separated beams are then spatially recombined using a second beam splitter. A liquid crystal retarder preceding the first beam splitter controls the relative intensity of the two oppositely polarized beams, allowing reduction of instrumental asymmetries. This system has been modified to include a spatial filter and a Pockels cell placed after the second beam splitter to act as a second active polarization element. Using this method, we can control instrumental asymmetries to ∼5×10(-7) in 1 h of data taking, which is comparable to the precision achieved in "second-generation" high energy electron-nuclear scattering parity violation experiments.