from the air we breathe
Spin-Exchange Optical Pumping of MagniXene™
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 conceived of the counter-flow method and invented the counter-flow xenon polarizer. Using funding from the NIH and later from internal sources at UNH, Hersman’s team achieved a technological breakthrough in 2004. Magnetization output, the product of polarization times production rate, exceeded the output of the commercial Amersham polarizer by a factor of fifty, and exceeded that of the world’s best polarizer, a scaled-up version of the Amersham polarizer developed at the National High Magnetic Field Laboratory, by a factor of ten to twenty.
The counter-flow polarizer embodies a main operating principle and two auxiliary supporting ideas, enabling a new regime of operation. The main principle is to flow the mixed gases along a long reaction chamber towards a laser beam emanating from the opposite direction. The gases are initially illuminated with the attenuated beam achieving some degree of polarization, but near the end of the process when gases are highly polarized they are subjected to the most intense laser light to assure 100% rubidium polarization and raise the xenon polarization still further. This “counter-current” principle is used in efficient heat exchangers (see figure).
The first auxiliary idea is to saturate the flowing gases with rubidium in a preparatory chamber outside the laser polarization region. This assures homogeneous saturation of the gases to a well-defined rubidium density (dependent on the thermal bath temperature) essentially independent of flow velocity and laser power. The Princeton (and NHMFL) design had the rubidium droplets in the polarization chamber. Higher flow rates resulted in incomplete saturation of the flowing gases with rubidium. Higher laser intensity resulted in a non-linear runaway condition, in which laser power absorption heated the gas vaporizing additional rubidium, which in turn increased laser absorption. Higher laser intensity in our system actually reduces rubidium density (higher temperature at constant partial pressure) maintaining stability.
The second auxiliary idea is to extract the rubidium in the presence of the polarizing light. By bringing the mixed flowing gases into contact with a cold surface in the presence of the laser light, the rubidium remains polarized, continuing to transfer polarization to the xenon, until it is extracted. The Princeton system exhausted mixed gases from the chamber still saturated with rubidium. If we employed that strategy, our rubidium would quickly lose its polarization and begin to depolarize the xenon.
The new operating regime enabled by these advances is: high gas flow velocity, low gas pressure, and very low xenon partial pressure. The key benefit to low xenon partial pressure is the resulting ability to maintain high rubidium polarization with the laser. High xenon partial pressure leads to rapid rubidium depolarization. Of course low xenon partial pressure then requires higher flow rates to polarize the same quantity of xenon.
The key benefit to low total gas pressure is the increase in polarization transfer from rubidium to xenon. Rubidium transfers polarization to xenon by two distinct processes: instantaneous binary collisions and formation of van der Waals molecules. Spin exchange by molecule formation has a much higher probability if the molecule is not broken up by a subsequent collision, and therefore dominates at low pressures. We are able to achieve almost an order of magnitude increase in spin exchange rate and flow at much higher velocities.
Low pressure however can reduce the efficiency of extracting the laser light by the rubidium. Most polarizers require higher pressure to pressure broaden the rubidium absorption spectral line. We achieved efficient laser absorption by increasing the length of the cell by a factor of ten, from a previous maximum length of 20cm to almost two meters. (More recently we have succeeded in narrowing the laser spectral output, aiding laser absorption on low pressure gas.)
