Continuous Spin-Exchange Optical Pumping in High Flux Neutron Beams

We collaborated on tests at the Insitut Laue Langevin (ILL) to study the effects of high flux neutron beams on spin-exchange optical pumping (SEOP).  The key result of this published work was the finding that the alkali spin relaxation induced by the beam, which increases the laser power demand, scales as the square root of the neutron flux.  

However, this work opened new questions as the data suggested a possible dependence on the nitrogen pressure in the cells. Nitrogen is required to suppress radiative emission of alkali atoms in SEOP cells. In addition, both fast and slow (~300 s) components of the beam-induced relaxation were observed. In a second set of experiments at the ILL, we collaborated on further investigations. We found that the slow component appears to be associated with nitrogen. However, interpreting the results has proven to be remarkably complicated for several reasons, such as the shortening of stopping lengths for charged particles for higher nitrogen densities.  The experiments provided new data, but again opened new questions for further study.

In parallel, we are developing and testing "double cells," which can potentially bypass this issue by separating the optical pumping volume from the spin filter volume. We tested a small double cell in the high flux ILL beam and the data indicate that it was insensitive to the neutron beam. Such cells have other advantages for continuous pumping, but also present greater demands on the laser power required for maximum polarization.> To test a large double cell, we employed an electrically heated oven in a solenoid for SEOP.Although a 150 h relaxation time was observed for this cell, the polarization was lower than expectations. This discrepancy will be investigated further.

Neutron beam effects on SEOP will be understood and addressed, allowing the advantages of ³He spin filters to be exploited for high flux experiments at the NCNR and other US and overseas laboratories.  New cell approaches that will be useful for the general goal of continuous optical pumping will be developed.


Image Description: On the top is shown a small double cell that was tested in the high neutron flux beam at the ILL. As shown in the plot on the bottom, the rise in polarization as the cell was polarized by SEOP was unaffected when the cell was exposed to the neutron beam.