| Noble-gas nuclei can achieve large nonequilibrium nuclear polarizations through a process known as spin-exchange optical pumping (SEOP). These polarizations are of order 10% and are independent of the external field strength. This makes these hyperpolarized (HP) gases useful in several applications that require large polarizations and long relaxation times. In order to maximize the lifetime and achieve the highest polarizations of hyperpolarized gas, particularly 3He, the relaxation interactions need to be understood and controlled. One main relaxation mechanism deals with interactions of the hyperpolarized 3He with the walls of the glass container or cell, known commonly as wall relaxation.; By using nuclear magnetic resonance (NMR) to periodically sample a HP 3He free induction decay (FID) signal one can obtain information about wall relaxation mechanisms. One particular wall relaxation mechanism, known as T1 hysteresis, is a dependence of T1 on the history of exposure of a spin-exchange cell to large magnetic fields (>100 G). This type of relaxation is believed to be due to multidomained ferromagnetic sites on the surface of the glass container correlated with the presence of Rb used in the SEOP process. We demonstrate that a similar mechanism is also present in spin-exchange cells after exposure to small fields (30 G), making this particular mechanism important to all other researchers using HP 3He. We also demonstrate that the onset of T1 hysteresis is not only correlated with the presence of Rb, but also occurs after rinsing the cells with a hydrofluoric or nitric acid solution.; Along with an NMR method used to detect these ferromagnetic sites this dissertation addresses detection of the sites using a superconducting quantum interference device (SQUID). We demonstrate the existence of ferromagnetic sites in small crushed glass samples both in the presence and absence of Rb using this SQUID method.; Impurities in the Rb, the glass and in the acids are examined carefully as possible explanations for these sites, but are found to be an unlikely source. Very preliminary evidence of formation of a ferromagnetic chemical compound is also presented, along with data supporting the extreme robustness of the T1 hysteresis effect once it is onset. |
