Polarized 3He Circulating Technologies for Neutron Analyzers
The United States and several other countries are investing in new neutron scattering facilities as well as upgrading existing ones due to their promise for fundamental and applied research that can catalyze advances in energy, telecommunications, manufacturing, plastics, transportation, biotechnology, and health. The performance of one new type of instrument, the wide-angle polarized neutron spectrometer, is severely constrained by the volume of polarized3 He gas that can be produced and the level of polarization that it can maintain. Currently 3He polarizers produced in Europe based on Metastability Exchange Optical Pumping dominate the international market, with existing installations in Germany and France and two multimillion dollar contracts recently closed for ISIS in England and ANSTO in Australia. Even those systems have limitations in capacity and reliability, leading developers of new wide-angle spectrometers, such as the TOPAS group at Jlich, to seek an alternative with greater capacity, higher polarization, and better reliability. In an ongoing parallel project, our team developed a large-scale 3He polarizer based on Hybrid-alkali Spin Exchange Optical Pumping. By illuminating a large 8.5 liter glass polarizer cell inside a pressure vessel with a 1.2 kW laser we demonstrated spin-up rates of 20% per hour for 50 STP liters, a world record. We recently upgraded that polarizer with a custom-built 2.5 kW spectrally-narrowed laser and a fully integrated support infrastructure. Testing of new monolithic glass cells in this upgraded polarizer is imminent. In Phase 1 of this project, we demonstrated feasibility for utilizing this large-scale 3He polarizer to allow automated filling and emptying of a large analyzer with minimal polarization losses. In particular, we proposed and accomplished four objectives: we designed and had built a non-ferrous vacuum pump capable of evacuating a 40 liter analyzer volume in 15 minutes with minimal polarization loss. We operated the compressor, demonstrating it can transfer helium from 100 torr to 8000 torr, simulating the required evacuation of the analyzer volume back into the polarizer. We developed a sapphire orifice throttle and installed it and the compressor in a rig for testing their polarization preserving properties. We began characterizing polarization losses using an integrated NMR system. These substantial accomplishments within a nine- month Phase 1 project give confidence that the technical challenges of assembling a high- polarization, low-loss, long-running prototype will be accomplished within our Phase 2 plan. Our overall plan seeks routine operation of a 40 liter neutron analyzer with average polarization exceeding 75%, a challenging goal demanded by the science. Since our exotic pump arrived just as Phase I ended, our first aim for Phase II is a thorough study of the existing setup. We will also extend our archival measurements of surface relaxivity to various metal, ceramic, and plastic materials to guide our choices for use in the final pumping system, transfer lines, valves, and orifices. Our second aim is to improve our polarizer. We will develop materials processing techniques that will allow our cells to be blown from GE180, and retrofit our laser with dedicated components that achieve 2.5kW with 0.15nm line width. Our third aim is to incorporate new subsystems and implement process automation that will be essential for routine operation of a large-scale 3He polarizer and wide-angle neutron analyzer. Commercial Applications and Other Benefits: We estimate a market size of a dozen large scale 3He polarizer-analyzer systems for fundamental and applied physics research over the next decade, with synergistic benefits in pediatric lung imaging.
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