Ph.D. Defense: Final Oral Examination for the Degree of Doctor of Philosophy of Michelle Dunlop

Event Details

  • Date:
  • Time: 10:00am
  • Location: MacNaughton 222

Thesis Title:

High-Precision Half-Life Measurements for the Superallowed Fermi β+ Emitters 10C and 22Mg


High-precision measurements of the Ft values for superallowed Fermi β transitions between T = 1, Jπ = 0 + isobaric analogue states allow for demanding tests of the electroweak interaction described by the Standard Model. These transitions provide an experimental probe of the conserved vector current hypothesis, they provide the most precise determination of the up-down element (Vud) of the Cabibbo-Kobayashi-Maskawa quark-mixing matrix, and are used to set stringent limits on the existence of scalar currents in the weak interaction.

In order to use the superallowed Ft values to sensitively test fundamental properties of the Standard Model, both precise experimental determinations of the ft values, via measurements of the half-life, superallowed branching ratio, and QEC value, as well as theoretical corrections are required. This thesis focuses on the high-precision half-life measurements of the superallowed Fermi β + emitters, 10C and 22Mg, which were performed at the Isotope Separator and Accelerator Facility at TRIUMF. For both cases, the half-life measurements were motivated by discrepancies between previous half-life measurements found in literature.

For 10C, two independent half-life measurements were performed. The first measurement was performed via γ-ray photopeak counting using an array of 20 Compton suppressed high-purity germanium detectors with the resulting half-life of T1/2 = 19.2969± 0.0072 s. The second measurement was performed via direct β counting using a 4π continuous-flow gas proportional β counter which yielded a half-life measurement of T1/2 = 19.3009 ± 0.0017 s. The latter is the most precise superallowed half-life measurement reported to date and the first ever to achieve a relative precision below 10−4 . The impact that the improvement in the 10C half-life measurement has on setting limits on scalar currents in the weak interaction will be discussed.

The 22Mg half-life measurement was performed via direct β counting using a 4π continuous-flow gas proportional counter. The result of T1/2 = 3.87400 ± 0.00079 s is a factor of 3 more precise than the previously adopted world average and resolves a discrepancy between the two previously published 22Mg half-life measurements. The implications for testing the isospin symmetry breaking theoretical corrections in superallowed Fermi β decays will be discussed.

Examination Committee:

  • Dr. Robert Wickham, Chair
  • Dr. Carl Svensson, Advisor
  • Dr. Paul Garrett
  • Dr. Joanne O'Meara
  • Dr. Maxime Brodeur, External Examiner (University of Notre Dame)



M. R. Dunlop et al., Phys. Rev. Lett., 116, 172501 (2016).

M. R. Dunlop et al., Phys. Rev. C, 96, 045502 (2017).