## Doctoral Dissertations

Spring 1998

Dissertation

Physics

#### Degree Name

Doctor of Philosophy

L C Balling

#### Abstract

The past several years have witnessed an increase in applications of polarized noble gases. In particular, the development of high power lasers has enabled the realization of dense, highly polarized samples of $\sp3$He. We report here the development of a system to produce high density, highly polarized samples of $\sp3$He gas. We developed a liquid helium system to cryogenically increase the density of $\sp3$He gas to pressures exceeding 10 atmospheres. We developed a system based on adiabatic fast passage nuclear magnetic resonance to measure and monitor the $\sp3$He nuclear polarization. Polarizations as high as 64% were measured and cells with polarization decay lifetimes as long as 180 hours were produced.

We present the first known two dimensional images acquired with polarized noble gases at 21 gauss. Under typical magnetic resonance imaging conditions the polarization of protons is ${\sim}5.2\times10\sp{-4}$ %. Using spin exchange optical pumping, we are able to produce $\sp3$He polarizations of tens of percent with no field dependence. We acquired images at 21 gauss with a custom imager and at 47,000 gauss using a commercial imager with equivalent resolution and signal strength. We acquired images of a rat lung at 21 gauss with a measured $\rm T\sb1\approx40$ s and $\rm T\sbsp{2}{\*}>100$ ms. To demonstrate the applicability of low field imaging in and around conductive materials, we acquired images of a polarized $\sp3$He sample inside a 25.4 $\mu$m brass box at 21 gauss. Additionally, compared to the large distortions observed at high fields, we imaged a $\sp3$He sample near known paramagnetic materials with minimal distortions.

We fabricated polarized $\sp3$He based neutron spin-filters and used them in two experiments at the Los Alamos Neutron Science Center. Spin-filters were produced at three and six atmospheres with flat entrance and exit windows for a constant thickness of $\sp3$He. The first experiment utilized one of these spin-filters to measure the neutron polarization for neutron energies ranging from 25 meV to 10 eV. The second experiment required a spin-filter to polarize neutrons and another one to analyze the spin of the neutrons having passed through a $\sp{139}$La target.

COinS