Applications of ³He and ¹²⁹Xe polarization in nuclear physics

Polarized neutron target

The technology for producing samples of nuclear polarized noble gases was originally motivated by fundamental questions in nuclear physics. Since ³He is composed of two protons paired off to spin zero, the single neutron is principally responsible for the magnetism of the nucleus. Magnetically aligned ³He is provides a sample of magnetically aligned neutrons. Under some conditions when beams of high energy particles strike this neutron, it behaves approximately as if the other protons weren’t even there. A polarized gas of ³He is a good substitute for a bottle of polarized neutrons.

Our system for polarizing ³He could usefully serve measurements of this type. Presently these experiments rely on a two-cell system with one cell illuminated by the laser and diffusively coupled to the other cell that the beam passes through. Cells are optimized to meet the requirements for both spin exchange optical pumping and those of the beam. By performing the spin-exchange optical pumping on the side of the beam line and flowing the gas periodically to the target cell it may be possible to operate at twice the polarization, twice the pressure, and four times the beam current. Since the figure-of-merit increases like polarization-squared, this could provide an improvement factor as high as 32 times.

Neutron spin-filter

Polarized ³He can also play a role in preparing a beam of polarized neutrons. When a beam of unpolarized neutrons impinges on a vessel of polarized ³He, the gas acts as a neutron spin filter. An unpolarized beam of neutrons can be considered as a mixture in which half the neutrons are spin-up and half spin-down. If the polarized ³He is spin-up, then it will preferentially absorb the part of the incoming neutron beam that is spin-down and allow only spin-up neutrons to pass through.

The NPDGamma experiment, recently completed at Los Alamos, utilized a polarized ³He spin-filter for the first time in a multi-month experiment. The experiment was successful in maintaining efficient operation of the spin-filter over this time period. Two observations are relevant: First, the cell containing the ³He and the rubidium suffered nuclear and chemical reactions leaving milky-cloudy residue inside, increasingly blocking the laser illumination. Second, the cell polarization always dropped immediately when the beam was turned on. This depolarization was later traced to depolarization of the rubidium by highly ionized tritons, protons, and electrons resulting from the neutron being absorbed into the helium nucleus. These two mechanisms will become even more significant if ³He spin-filters are utilized at higher-flux facilities, like the Spallation Neutron Source in Oak Ridge, Tennessee.

Our system for polarizing ³He could usefully serve measurements of this type. The polarization we are able to achieve off-line should be more than double that which was typical at NPDGamma. Because the system polarizing the ³He is not placed in the beam, the ionization will not affect the polarization of the rubidium. Also any impurities generated in the target cell could be filtered out before returning the gases to the polarizing system and reacting with alkali metal. Consequently a system such as ours could allow operation at facilities with higher neutron flux like the SNS with lower likelihood of long-term degradation of performance.

Parity-violation and time-reversal invariance

Although parity violation is most evident in weak interactions involving leptonic and semi-leptonic processes, parity violation is also observed as a manifestation of the weak interaction in purely hadronic processes, reactions involving only nucleons and mesons. Measurements of neutrons interacting with heavy nuclei performed at Los Alamos have identified a number of resonances that are strongly parity violating, including one such resonance in xenon. The isotope responsible for this resonance is believed to be ¹³¹Xe. This is significant because a parity violating resonance in a polarizable nucleus could serve as a test-bed for a measurement of time-reversal symmetry.

The XeBox-E10 polarizer is capable of polarizing nearly a mole of xenon atoms every four hours. Although we typically utilize the long-lived signals from ¹²⁹Xe, we have confirmed that ¹³¹Xe also becomes polarized in the process. The lifetime of the polarization for ¹³¹Xe, however, is known to be much shorter than that of ¹²⁹Xe. Whether a viable time-reversal experiment could be performed with XeBox-E10 is still an open question. It is clear, nevertheless, that the high production rate of XeBox-E10 brings the nuclear physics community a factor of several hundred closer to that goal.