Precision Tests of the Standard Model Using Nuclear β Decay at the Nuclear Science Laboratory

There are currently many searches looking for beyond the Standard Model physics including unitarity tests of the Cabbibo-Kobayashi-Masakawa matrix. This matrix has been considered consistent with unitarity for decades via determinations of the elements which comprise the top row. This evaluation is the result of many years of experimental and theoretical effort to determine ft-values for superallowed pure Fermi beta decays to extract a precise result for the V_ud matrix element as well as studying kaon decay to determine V_us. The reliability of the unitarity test for the top row largely depends on the precision and accuracy of its largest element, V_ud. Recent theoretical correction results in the last couple years have, however, led to a disagreement with unitarity at the 2.4σ confidence level. As the unitarity test now suggests inconsistencies with the Standard Model, it is important to test the accuracy of all results at the current precision. This can be accomplished by extracting V_ud from the ensemble of superallowed mixed beta decays to an improved precision. Experimentally, this involves a two pronged effort; there must be an increase in precision for the values used to calculate ft-values, and there must be a measurement of the Fermi to Gamow-Teller mixing ratio for these superallowed mixed decays. The work included here aligns with both of these routes.

The first aspect of this work includes two precise half-life measurements, of ²⁰F and ¹⁵O, with the Beta Counter at the University of Notre Dame’s Nuclear Science Laboratory. The ²⁰F measurement provided the grounds for an in-depth study of the impact of contamination on a half-life measurement with the system resulting in a measured value of 11.016 ± 0.016 seconds. The work resolved a decades long discrepancy in the half-life of ²⁰F, determining the world average to be 11.0062 ± 0.0080 seconds. Before our measurement, the half-life of ¹⁵O was the dominant source of uncertainty in the ft-value of that isotope. Therefore, this half-life was also measured, and determined to be 122.308 ± 0.049 seconds; the most precise measurement of this quantity to date. As a consequence of this measurement, the world average was updated from 122.24 ± 0.23 seconds to 122.27 ± 0.06 seconds, with a reduction of the uncertainty by a factor of 4, and the uncertainty on the ft-value is now dominated by the precision of the Q-value measurement.

The second aspect of this thesis includes the development of an experimental apparatus to measure the Fermi to Gamow-Teller mixing ratio of superallowed mixed decays, as this quantity is required to determine V_ud with this ensemble of nuclei. The instrument, named the St. Benedict ion trapping system, will be comprised of multiple beam guide components; a large-volume gas catcher, a radiofrequency carpet, a radiofrequency quadrupole, a radiofrequency quadrupole cooler-buncher, and a measurement Paul trap. An outline of each component is provided along with the commissioning of the radiofrequency quadrupole cooler-buncher which was determined to prepare ion bunches with a full-width-half-maximum of less than 80 nanoseconds, exceeding the requirements necessary for St. Benedict.