Magnetically activated and guided isotope separation




Mazur, Thomas Rolf

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This dissertation describes a proof-of-principle experiment demonstrating a technique for stable isotope enrichment called Magnetically Activated and Guided Isotope Separation (MAGIS) (1). Over the past century a large number of enriched isotopes have become available, thanks largely to electromagnetic separators called calutrons that were developed during World War II. These isotopes have found applications across an array of fields including medicine, basic science, and energy. Due to substantial maintenance and operating costs, the United States decommissioned the last of its calutrons in 1998, leading to demand for alternative methods of isotope separation. Our experiment suggests the promise for MAGIS as a viable alternative for replenishing stockpiles previously provided by calutrons. Our apparatus combines optical pumping with a scalable magnetic field gradient to enrich lithium-7 (Li-7) by suppressing lithium-6 (Li-6) throughput in a lithium atomic beam. We first evaporate lithium metal in a crucible in order to generate thermal, high flux beam. We then perform optical pumping on Li-6 atoms, magnetically polarizing a substantial fraction of Li-6 atoms into the entirely high-field seeking 2²S₁/₂, F = 1/2 ground state. The resultant beam then samples a magnetic field gradient produced by a 1.5 m long array of rare-earth permanent magnets bent over its length by 20 mrad. This geometry prevents high-field seeking lithium atoms from reaching the plane beyond the magnets, while efficiently deflecting low-field seeking atoms. We measured Li-6 suppression – using independent techniques – along the plane after the magnets beyond a factor of 200, corresponding to Li-7 enrichment to better than 99.95%. As apparatus-specific hindrances appeared to limit this suppression, we believe that we should achieve better enrichment on a commercial apparatus. We also measured both the absolute flux beyond the single, 1.5 in tall magnet array and the efficiency for guiding feedstock material to the collection plane. Given the planar configuration for the field gradient, the flux that we measured should scale linearly with both magnet height and the number of arrays surrounding the source. Our measurements therefore indicate that – at source temperatures that we actually investigated – a commercial apparatus fitting within a volume of just several cubic meters should yield hundreds of grams of enriched (to beyond 99.95%) Li-7 per year. In addition, we observed a competitive ratio between collected material and feedstock with greater than 20% of lithium incident upon the magnet array reaching beyond the magnets. Benchmarking our work against the calutron, we demonstrated comparable enrichment in a manner that should scale to the production of similar quantities. In contrast, however, MAGIS should require vastly less energy input. While calutrons required massive currents for maintaining a static magnetic field over a substantial area, the only non-shared energy expense for MAGIS is the cost for running the low power lasers for optical pumping. Via additional analysis, we have supplemented this proof-of-principle experiment with schemes for applying MAGIS to over half of the stable isotopes in the periodic table. Due to the success of this demonstration and the broad applicability of the principles, we believe that MAGIS will play an important role in the future of stable isotope enrichment.




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