Spin-Exchange Optical Pumping of MagniLium™

In 1991 at the University of New Hampshire, Professor Bill Hersman began a technology development program to produce polarized helium cells to serve as particle beam targets for nuclear physics experiments. In 1997 after learning of the need for better methods to polarize xenon, he redirected his attention to that problem, inventing the counter-flow method of xenon polarization. Over the decade that followed, his team grew increasingly capable at designing and implementing polarization technologies of commercial quality on an industrial scale, capabilities that are now the core competencies of Xemed. In response to urgings from the polarized Helium-3 community, in 2006 Xemed initiated a project to optimize polarized Heium-3 production by spin-exchange optical pumping (SEOP).

Like xenon, optical pumping spin-exchange of helium requires an alkali vapor as an intermediary, and rubidium is the most practical choice. Collisions between optically polarized rubidium atoms and helium atoms are characterized by two fundamental rates: the spin-exchange rate proportional to the probability that the rubidium atom’s polarization is transferred to the helium nucleus, and the alkali spin destruction rate proportional to the probability that the collision destroys the rubidium atom’s polarization without any benefit. A preferred choice of alkali would have a high probability of transferring its polarization (which is still very small) and a low probability of losing its own polarization (which is still quite high). If the alkali atom loses its polarization, it will extract another photon from the laser beam and try again.

Atomic physicists have measured these rates for all the alkali metals. Potassium has a similar rate for transferring its polarization to helium, but a much lower rate than rubidium for losing its polarization during a collision. Consequently potassium would seem to be a better choice of alkali for polarizing helium. Practical considerations favor rubidium, including the availability of high power lasers, and the large separation of rubidum’s D1 absorption line from its D2 line.

Xemed employs a hybrid strategy using both potassium and rubidium. The rubidium absorbs the laser light and contributes to polarizing helium. The potassium vapor is readily polarized by the rubidium, and contributes its part to polarizing helium. Because the potassium is much more efficient at polarizing helium we use an alkali mixture with much more potassium than rubidium. Operating the polarizer at higher temperature, around 250°C, allows us to make full use of these higher efficiencies.

Xemed MagniLium™ production technology also benefits from Xemed’s experience working with polarization components on a scale that is very large by traditional measures. Our polarizer cell encloses a volume of over eight liters and operates at pressures up to nine atmospheres. We illuminate the apparatus with up to 2.5 kW of laser light in a beam with divergence as low as a couple milliradians. The combination of all these factors provide spin-up times as short as four hours for polarization volumes holding fifty liters of Helium-3 at a time.