Laser polarized noble gases are made up of atoms such as helium, neon, and xenon, that are manipulated by lasers so that a large fraction of the nuclear spins point in a chosen direction. They have a remarkable number of applications ranging from magnetic resonance in medicine and biomedical research, to probes of elementary particle interactions among the atomsí constituents. For example, the NMR signal per atom from a sample of noble gases can be enhanced by factors of many thousand. This makes possible studies with many fewer atoms and lower concentrations than conventional bulk NMR studies, and allows for precision measurement of NMR frequencies.
Spin is an important variable to control in measurements of the elementary particle interactions of noble gas atoms and their constituents because the weakest of those interactions are characterized by correlations of the spin with other observables. Nuclear spins can also carry information encoded into the quantum states of a system of interacting spins. There have been three main interrelated thrusts to our current efforts: 1) precision measurement of atomic electric dipole moments of heavy atoms, specifically xenon and radon isotopes; 2) probes of the weak interaction in decay of polarized neutrons; 3) biomedical applications of NMR and MRI enhanced by laser polarized.
An atomic electric dipole moment is a separation of charge along the total angular momentum of an atom: = gd . The EDM, , is a polar vector that is even under time reversal and the total angular momentum is an axial vector that is odd under time reversal. Thus, the factor gd, arising due to elementary particle forces that induce the EDM, must be odd under both parity (P) and time reversal (T), indicating CP violating interactions. An EDM can be detected by measuring the Zeeman splitting (i.e. magnetic resonance frequency) of the atom in the presence of electric and magnetic fields that are reversed with respect to each other. For nuclear spin systems such as noble gases (the electron spins in the atom completely pair off), the state of the art requires measurement of a frequency shift of order 10-9 Hz. The stability required is characteristic of good atomic clocks, and we have developed a device we call a Nobel Gas Zeeman Maser for measurements of the 129Xe EDM. In such experiments, the magnetic moment of the system couples to any magnetic field, including that produced by leakage currents that flow due to the high voltage that produces the electric field. We have therefore undertaken a two species measurement that uses 3He, which is much less sensitive to the forces that induce an EDM. Though much more difficult, the two species measurement has the potential for much greater confidence in estimates of systematic effects because the magnetic moment interactions are more directly measured. The spin exhcange pumped maser is capable of precision better than 10-9 Hz, however the two species arrangement is more susceptible to frequency instability. It is therefore essential to stabilize many influences including temperature and ambient magnetic field, and we are progressing toward improving our current sensitivity of about 3 x 10-27 e-cm. Nuclear spin systems with special nuclear structure features can have greatly enhanced sensitivity to T-odd and P-odd elementary particle interactions. Isotopes of radon are among those expected to have enhancements of several hundred with respect to the current state of the art established by the 199Hg experiments at Seattle. Professor Chupp and Professor Carl Svensson of Guelph Univeristy are the spokesmen of an the Radon-EDM experiment, which has the goal of extending our work in laser polarized noble gases to rare-isotopes produced at the TRIUMF isotope production facility in Vancouver. Significant development is necessary on many fronts as we build on our experience, and we have established a strong collaboration to move forward with the full support of TRIUMF. We expect to measure with precision comparable to our 129Xe measurements by the end of 2014. This would likely extend sensitivity to T-odd and P-odd elementary particle interactions by more than an order of magnitude compared to 199Hg. This approach along with several other activities that combine precision measurement techniques with rare isotopes promises to benefit greatly from the planned Facility for Rare Isotopes Beams (currently FRIB, formerly RIA).
The neutron is a unique laboratory. It is the most fundamental unstable nuclear species, and itís decays can reveal the structure of elementary particle interactions. The most crucial measurements of neutron interactions are the neutron lifetime and measurements of the amount of parity violation in the decay of polarized neutrons. We are working to develop the key elements needed to measure the parity violating coefficients with a polarized pulsed cold neutron beam at the SNS - See tudies of polarized neutron beta decay. This will measure the asymmetry of proton emission with respect to the neutron spin. In our earlier work, we developed polarized 3He spin filters and neutron spin rotators (or spin flippers). These must both be refined so that the uncertainties on systematic errors are much less than 0.1%, a significant challenge. At NIST , we have also recently discovered a new, but not unexpected branch in neutron decay in which a photon is emitted along with the electron, antineutrino, and proton. A precision measurement has recently completed data taking and anaysis is underway.
NMR and MRI with laser polarized xenon has been a very exciting activity. We have enhanced polarization of 129Xe for use in a set of experiments with rats in vivo that show the potential for a 129Xe magnetic tracer of blood flow. We have shown that the 129Xe is carried from the heart to the brain and other organs and that xenon gas and xenon dissolved in the blood and different tissues can be separately imaged. In the brain, the xenon partitions into at least four separate chemical shifts plus the blood. We have been developing the technology for human imaging at 3 T, we have developed new techniques for analyzing tansient and steady state images to extract blood flow, and we have developed new microfluidic techniques for investigating these in-vitro. This effort exploits technology originally developed for some of the fundamental experiments listed above and is, in the true sense of the word, a SPINOFF.
Our work is supported by NSF and DOE.