MRI: Development of a Rubidium Spin Filter as a Source of Polarized Electrons

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A team of University of Nebraska-Lincoln physicists will develop a novel, "turn-key" polarized electron beam source. The use of polarized electrons is widespread in physics, from probing the spin-structure of nucleons and nuclei to studies of magnetic domain structure and the spin-dependence of atomic collisions. State-of-the-art polarized electron sources are based on GaAs photocathode technology. While this source has significant advantages over other available methods, it is very difficult to operate. In university settings, the learning curve for reliable operation by a graduate student of these sources is routinely measured in months or years, not weeks. At accelerator facilities such as CEBAF or MAMI, dedicated staffs of technicians and Ph.D.s maintain and operate these sources. The polarized electron source to be developed is based on new technology: the optically-pumped rubidium spin filter. In this scheme, unpolarized electrons diffuse under the action of an electric field through a target of optically-pumped Rb and a buffer gas. As the electrons collide with the spin-polarized Rb, they become polarized via exchange collisions. They are then extracted and formed into a beam. A proof-of-principle experiment has already demonstrated this technique. Such a source is expected make many current experiments easier, and it would also extend the range and quality of the data they produce. More importantly, it may lead groups with no current expertise in polarized electron technology to consider new types of experiments. As an example, spin-polarized low-energy microscopy (SPLEEM), a powerful technology for the study of magnetic surfaces, is currently used in only a few labs around the world due to the operational complexity of the GaAs sources it employs. A spin filter would make such technology much more accessible to the general condensed matter community. Students and postdocs working on the development of the Rb spin filter will receive training in state-of-the-art optical instrumentation and technology. The key element of this source is optical pumping, which is an enabling technology for other fields, such as nuclear physics and medicine. The faculty involved with this project have been very effective at integrating undergraduate students into their research programs. They have made a particularly strong effort to hire and mentor women and underrepresented minorities early in the academic "pipeline." The results of this instrument development effort will be broadly disseminated through refereed scientific publications, and the potential for commercialization will be evaluated.