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Polarization diagnostics for rubidium-80 in a spin-polarized beta decay experiment Roberge, Daniel Grant


The TRINAT group uses optical pumping to polarize atoms for precision spin-polarized beta decay experiments. This thesis describes two methods for determining the degree of polarization achieved in an experiment using ⁸⁰Rb: one method using atomic observables and one using nuclear observables. The atomic diagnostic uses a measurement of the excited state population over the duration of the optical pumping process to infer the polarization, while the nuclear diagnostic measures the beta decay asymmetry of the polarized atoms for a more direct polarization measurement. The atomic diagnostic uses a simulation of the optical pumping process to calculate the excited state population over time as the atoms are optically pumped, along with the resulting polarization. The excited state population is in turn measured using a pulsed 355nm laser, with sufficient energy per photon to photoionize the excited state but not the ground state. Through variation of a ’polarization-spoiling’ parameter the simulation can be fit to the data and a polarization value derived. The simulation is also applied to determine the laser power from a measurement of hyperfine pumping. The atomic diagnostic gives poIarization vaIues of -0.887 [superscript= +0.031 +0.022] [subsrcipt= -0.039 -0.021] and 0.868 [superscript = +0.020 +0.0.026] [subscript= -0.010 -0.030·]. The nuclear diagnostic depends on a pair of back-to-back beta detectors placed at a 30 degree angle from the polarization axis. These detectors are used to measure the frontback asymmetry of the beta particles, which due to parity violation is proportional to the polarization of the decaying atoms. Due to systematic differences between the two detectors and their energy dependent corrections, the front-back asymmetry did not produce a useful polarization. Instead, the asymmetry in each detector between positive and negative polarization was used, which is also proportional to the polarization if the difference between the two polarizations is ignored. From this, one detector gives a polarization of (0.44 ± 0.01) and the other gives a polarization of (0.51 ± 0.01). These values are inconsistent both with each other and with the atomic result, but several differences between the sensitivity of the two methods may explain this discrepancy.

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