Polarized 3 He, its Effect on the EPR Spectrum of Potassium and its Use for Neutron Structure Studies

Author: ORCID icon orcid.org/0000-0001-9030-1808
Katugampola, Sumudu, Physics - Graduate School of Arts and Sciences, University of Virginia
Cates, Gordon, AS-Physics, University of Virginia

This thesis describes studies involving nuclear polarized 3He, and its application as a target for electron-scattering experiments that probe the structure of the neutron. Central to this work is a precision measurement of the fundamental atomic parameter κ0 for a K-3He system. This dimensionless quantity parameterizes the frequency shift that occurs in the electron paramagnetic resonance (EPR) spectrum of an alkali-metal atom when it is in the presence of polarized 3He. Our determination of κ0 will enable a large improvement in polarimetry when working with polarized 3He. Also described in this thesis are the production and characterization of a new generation of polarized 3He target cells. These targets were used in a recently completed experiment at Jefferson Laboratory (JLab) in Newport News to measure the spin-asymmetry A1n, and represent the first application of polarized 3He target cells in which the gas flow is controlled using convection.

To measure κ0, we used samples of 3He, contained in nominally spherical glass cells, that were polarized using spin-exchange optical pumping. The basic technique was to measure the 3He polarization using the NMR technique of adiabatic fast passage (AFP) while also measuring the frequency shifts of the potassium EPR lines. Crucial to this technique was calibrating the absolute polarization of our AFP signals using thermally polarized water samples that were nominally the same size and shape as the 3He cells; also the calibration needed to be carried out at around 36 G where the water signals were tiny. To accommodate the small water signals, a custom NMR system was designed and built that incorporated electromagnetic shielding, isolation from mechanical vibrations and very-low-noise electronics, including the RF system, which for AFP measurements was on during scans. Our system was nearly Johnson-noise limited, and provided roughly 30:1 SNR for a single measurement of water with no averaging. When comparing AFP signals from 3He and water to calibrate the system at the sub-percent level, it was important to account for the dissimilarities in the shape of the glass containers holding the two samples. A method to capture the shapes of the glass containers (water and 3He) using imaging techniques and to extract a correction factor to account for the geometric differences was developed. Using the above mentioned techniques, three values for κ0 at 225C, 235C and 245C have been extracted and a final more precise value at 235C has also been reported with an accuracy of roughly 1%.

Historically, polarized 3He has been used as an effective neutron target in electron scattering experiments to study the spin structure of the neutron. Another important part of the work presented here includes the bench testing of some 18 polarized 3He target cells that were constructed for the recently completed A1n experiment (E12-06-110) that was conducted at JLab. The target cells were tested using a modified version of the same apparatus that we used to measure κ0. The results of these bench tests were critical both for guiding our efforts in constructing the targets, as well as in selecting the best target cells for use at JLab.

This thesis also discusses 3He spin relaxation on different glass and metal surfaces. With the anticipated increased electron beam current in the upcoming GEn (E12-09-016) experiment at JLab, thin metal “end windows” (through which the electron beam enters and exits) on our target cells would be quite desirable to achieve higher tolerance for radiation and temperature. After extensively studying the spin relaxation properties of different metal surfaces and inner coatings, gold-coated (electro-plated) OFHC (Oxygen Free High Conductivity) copper substrates have been shown to have favorable spin-relaxation properties. Further, a reproducible methodology for incorporating metal into all-glass target cells has been developed.

PHD (Doctor of Philosophy)
optical pumping, EPR, spin-exchange
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