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.
SCIENTIFIC RESULTS This NSF MRI Award provided the University of Nebraska with funds to develop a new scientific tool: a source of "polarized" electrons that is easy for scientists to use. In addition to their electric charge, electrons can be thought of as tiny bar magnets. Electrons that are polarized have their magnetic poles (or at least some fraction of them) lined up in the same direction. Polarized electrons can be used as probes of the magnetic properties of matter such as molecules or crystalline solids. Experiments that use polarized electrons can provide detailed information about physical interactions or the structure of materials that is unobtainable when unpolarized electrons are used. The current state-of-the-art for producing polarized electrons is technologically challenging. Thus, there are very few labs in the world equipped to do experiments of this type. In the work supported by this award, a source of polarized electrons was developed based on the spin-exchange of unpolarized electrons with a vapor of the alkali metal rubidium. The rubidium atoms have an active electron which is polarized by "optical pumping," in which laser light with angular momentum transfers its angular momentum to the active electrons of the rubidium, thereby aligning the active electron's magnetic axes. Unpolarized electrons that collide with the rubidium switch places with the polarized rubidium electrons, thus producing a beam of oriented electrons. The so-called "rubidium spin filter" that does this is shown in Figure 1 and operates as follows. Unpolarized electrons are produced in a standard thermionic gun (3) and are guided with magnetic fields produced by solenoidal coils (2) through a rubidium spin-exchange cell (4). The rubidium in this cell is polarized by a pump laser (10) counter-propagating with the electron beam. After passage through the exchange cell, the electron beam is polarized; its degree of polarization can be measured using a newly-developed electron-optical polarimeter (7),(9). This source is much simpler to operate than the current standard source of polarized electrons: the GaAs source. It is also more reliable. It can produce electron beam currents of 4 microamperes, with a polarization of 24%, that is to say, with a beam with 24% of its electrons being completely polarized and 76% being unpolarized. This performance is comparable to the GaAs sources used in the few university laboratories that have them. In the course of this work, a new, compact, and highly efficient optical electron polarimeter was developed to analyze the polarization of the electron beams produced by the rubidium spin filter. In this device, the electron beam to be analyzed excites a target of helium gas, which in turn fluoresces. If the gas has been excited by electrons that are polarized, the fluorescence will in turn be circularly polarized. The polarimeter measures the polarization of the fluorescent light, from which the electron polarization can be determined very accurately. In the course of this work, extensive studies were made of how to best polarize alkali metals such as rubidium through the use of optical pumping. Highly-polarized alkalis are used extensively in fields ranging from high-energy nuclear physics to medical imaging of human lungs. This work has broadened our knowledge base about how to produce the highest polarization in such systems. BROADER IMPACTS Scientifically, the rubidium spin filter has the potential to greatly expand the number of laboratories that can use polarized electrons to study problems in areas as diverse as material science, astrobiology, and nuclear physics. The electron optical polarimeter is being considered for use at the large nuclear accelerator facility at Thomas Jefferson National Laboratory to better analyze the polarization of the electron beams they produce. The results of this work were disseminated both in the literature and through talks given at international scientific conferences. Of these talks, five were invited, and three of them were given by undergraduate physics majors. The work funded by this award has had two overarching goals: the development of novel technologies in the service of science, and the development of new teachers and researchers in physics. The latter goal is achieved primarily through our training of graduate and undergraduate students. In this regard, we have emphasized the recruiting and mentoring of women undergraduates and high school students in order to get them into the research 'pipeline' as early as possible, and to keep them there. This award has provided training, in total, to 6 graduate students, 10 undergraduates, and 1 high school student. Three of these students were women. Two graduate students and 4 undergraduates were given direct financial support by this award to work on the Rb spin filter development effort. The others benefited from working on projects that used the lasers and other laboratory optical infrastructure purchased in the course of this work.
