Pulse-Nmr Studies of Spin Relaxation Relevant to Laser-Polarized Noble Gases.

The past few years have witnessed a tremendous increase in applications of noble gases polarized by spin -exchange with a laser-optically pumped alkali-metal vapor. The applications have benefited from the prior and ongoing study of more fundamental questions, both directly and indirectly related to spin exchange. This thesis addresses several such questions and also presents a new application of laser-polarized gases--biological magnetic resonance imaging. We present the first known measurements of nuclear relaxation in four of the alkali-metal hydrides: NaH, KH, RbH, and CsH. At room temperature and in a 14 kG magnetic field, the proton relaxation is non-exponential in these polycrystalline samples, with characteristic times (T _1) ranging from 15 min in NaH to 150 min in CsH. We also measured T_1=13.1s for ^{23}Na in NaH and T_1=330s for ^{133 }Cs in CsH. We discuss possible mechanisms for the proton relaxation and suggest that it might proceed through cross relaxation at the crystallite surfaces with the shorter-lived alkali-metal nuclei. The long proton -relaxation times make the alkali-hydrides a good candidate for cross-polarization with laser-polarized noble gases. We have measured the Rb-^{129}Xe binary spin-exchange rates at high (~1 atm) xenon pressures and found them to agree reasonably with theoretical estimates. We have explored energy pooling in Rb vapor, whereby intense (~1W/cm ^2) cw laser excitation of a resonant atomic transition produces non-resonant fluorescence. We discuss the effect energy pooling might have on optical pumping, particularly in light of increasing available laser power. In collaboration with Duke University's Center for In-Vivo Microscopy, we have produced the first ever mr images using laser-polarized ^3He. In a related experiment, we made precise measurements of the longitudinal relaxation rate for ^3He in the presence of oxygen, an important paramagnetic relaxing agent in the lung environment. We find 1/T_1=0.49 s^{-1} per atmosphere of oxygen at room temperature. Much of the above work was carried out with a superheterodyned quadrature-detection pulse-nmr spectrometer, partially designed and entirely constructed by the author. Before discussing the above work, we document the new instrument and explain its operation.