A compact optical electron polarimeter using a helium target is described. It offers a maximum fluorescence detection efficiency of ~20 Hz/nA, which is an order of magnitude higher than that of earlier designs. With an argon target, this device is expected to have a polarimetric figure-of-merit of 270 Hz/nA. By relying on a magnetic field to guide a longitudinally spin-polarized electron beam, the present instrument employs fewer electrodes. It also uses a commercially available integrated photon counting module. These features allow it to occupy a smaller volume and make it easier to operate.

We describe the operation of a prototype polarized-electron source. Rubidium vapor, contained in a cell, is optically pumped in the presence of a buffer gas. Unpolarized electrons from a tungsten filament are injected into the cell and extracted after undergoing spin exchange with the Rb atoms. We compare the performance of the source when different buffer gases are used. We measure a decrease in electron polarization as their injection energy increases, but find an unexpected regime at higher injection energies yielding increased electron polarization accompanied by a 40-fold increase in current, suggesting the production of slow secondary electrons in the target cell. With ethylene, we have measured electron currents of 4 μA simultaneously with electron polarizations of 24%. This work offers the promise of a simple, benchtop, "turnkey" source of polarized electrons.The use of polarized electrons is widespread in physics, from probing of the spin structure of nucleons and nuclei [1] to studying magnetic domain structure [2] and the spin dependence of atomic collisions [3]. The additional information provided by studies with incident polarized-electron beams comes at a price: The technological demands of polarized-electron sources are severe. The current state-ofthe-art source technology is based on photoemission from negative electron affinity (NEA) strained GaAs photocathodes [4,5]. The modern GaAs source can produce high-current (ß1 mA), high-brightness beams with ࣙ80% polarization. Also, importantly, it is optically reversible; the electron-spin direction can be flipped by reversing the helicity of the light producing photoemission from the GaAs. This enables a relatively straightforward diagnosis of systematic experimental error due to instrumental asymmetries. The biggest drawback of the GaAs source is that it is difficult to operate, particularly with regard to production of the NEA conditions that allow reasonable photocurrents to be extracted. In university labs, the learning curve for reliable graduate student operation of such sources is measured in months (or years); at accelerators, e.g., CEBAF, a dedicated scientific staff maintains and runs these sources. Several attempts to develop the GaAs source into a "blackbox" commercial technology have failed [6].We are currently investigating the interaction of longitudinally polarized electrons, which are chiral, with chiral molecular targets. The observation of handedness-specific scattering provides information about novel molecular collision dynamics [3], as well as possible clues to the mystery of why all naturally occurring DNA has the same handedness [7]. In tabletop experiments with vapor targets, such as ours, destruction of the photocathode's NEA surface conditions by organic and other vacuum contaminants can make GaAs sources unusable or, at best, highly problematic. If a different, user-friendly source of polarized electrons, such as the one we describe here, were widely available, it could enable a broad range of experiments, including those in...

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