Projects

Optical Electron Polarimetry

An important step in realizing an Accurate Electron Spin Optical Polarimeter (AESOP) with an accuracy of 0.4% is improving our knowledge of the analyzing power’s energy dependence. The fundamental formula relating the electron spin polarization Pe and the circular polarization of light P3 is given by [1]

Circular polarization equation

where P1 is the amount of linear polarization and a and b are coefficients obtained by simple angular momentum coupling algebra. The analyzing power A of an electron polarimeter is defined as Pe = AP3, where A depends on the linear polarization from the previous formula which in turn may have some dependence on the energy of the incident polarized electrons.

Thus, to enhance our electron polarization measurements, we want to carefully quantify the linearly polarized light produced by electrons as their incident energy varies over the excitation resonance in noble gases required for optical electron polarimeters [2]. We realize this desire in the experimental apparatus shown in Figure 1. It consists of a vacuum chamber containing our electron beam apparatus, where the electrons excite the noble gas resonances, and an optical analysis section where we can precisely measure the polarization of the light emitted.

Experimental apparatus for near-threshold experiment

Figure 1: Experimental apparatus for near-threshold experiment.

Electron beam apparatus

Figure 2: Electron beam apparatus which resides horizontally inside the vacuum chamber of Fig. 1. Molybdenum electrostatic optics accelerate and select electrons for exciting the noble gas atomic beam.

Inside of the vacuum chamber is the electron beam apparatus consisting of a tungsten filament for thermionically producing electrons that are then guided by electro static optics through a trochoidal monochromator. This greatly increases the energy resolution of the electrons and can achieve energy full-width-half-maxima of 0.02 eV [3]. The electrons then enter the collision chamber where cross them with an atomic beam of neon or other noble gas. The electron beam continues to the collector plate where we measure the energy distribution, and the light from the collision process exits the aperture on the side of the collision cell toward the light analysis section and PMT.



References

  • [1] T J Gay 1983 J. Phys. B: At. Mol. Phys.16 L553
  • [2] T. J. Gay, J. E. Furst, K. W. Trantham, and W. M. K. P. Wijayaratna 1996 Phys. Rev. A, Vol 53, No 3
  • [3] A. Stamatovic and G. J. Schulz 1968 Rev. Sci. Inst., Vol 39, No 11

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