Attainable polarization level

The level of polarization attainable with hyperpolarized Helium-3 spin-exchange optical pumping depends not only on the spin-exchange rate, discussed above, but also the spin relaxation rate of the Helium-3 in the polarizer. Because the time constants are so long, on the order of several hours, any impurity or imperfection can affect the final polarization level. Even unintended inconsistencies in fabrication can drastically affect the resulting performance.

The polarization of Helium-3 will be affected by collisions. Collisions with alkali vapor will drive the helium polarization towards the polarization level of that vapor. Collisions against the surface of the cell, however, will cause the Helium-3 polarization to relax back to zero. The rate of that relaxation can be measured by cooling the cell and measuring the time constant.

When the cell is hot, however, there appears to be an additional mechanism for Helium-3 relaxation that has not been fully understood. This so-called X-factor is the primary limitation of the attainable polarization. The X-factor has now been measured in scores of cells and reported in the literature. Values vary between 0.2 and 0.6. For the largest cells the values are typically 0.2.

Xemed is actively exploring a candidate hypothesis for identifying and eliminating the X-factor in order to achieve higher Helium-3 polarizations from spin exchange optical pumping. We believe this program will allow Xemed to produce polarizers with consistently low X-factors. Alternatively, Xemed can continue with the standard practice in the polarization research community: fabricate several polarization cells, measure their properties, and select only the best ones for installation.