Ph.D. Defense: Final Oral Examination for the Degree of Doctor of Philosophy of Alex Laffoley

Event Details

  • Date:
  • Time: 9:30 AM
  • Location: MacN 222

 Thesis Title:

"High-Precision Half-Life Measurements for the Superallowed Fermi β Emitters 14O and 18Ne"


The study of superallowed Fermi β transitions between nuclear isobaric analogue states of spin 0+ provides demanding, and fundamental, tests of the properties of the electroweak interaction. These studies, which include some 200+ individual measurements, have been used to test the conserved vector current hypothesis, set limits on weak scalar currents, test the unitarity of the Cabibbo-Kobayashi-Maskawa quark mixing matrix, and set limits on physics beyond the Standard Model.

A series of high-precision half-life measurements for the superallowed Fermi β emitters 14O and 18Ne were performed at the Isotope Separator and Accelerator Facility at TRIUMF. For each nucleus, 2 independent high-precision half-life measurements were made resulting in systematic self-consistent zests of the analysis methods.

The half-life of 14O was measured simultaneously through both direct β and γ counting techniques using a thin plastic scintillator to detect the β particles and an array of high-purity germanium detectors to detect the characteristic 2312.6 keV γ-ray emitted in >99% of 14O β decays. The results of this analysis yielded T1/2(14O)β = 70.610 ± 0.030 s and T1/2(14O)γ = 70.610 ± 0.030 s, resulting in a new world-average half-life for 14O of T1/2(14O) = 70.619 ± 0.011 s with a χ2/ν = 0.89.

The half-life of 18Ne was measured via direct β counting techniques using a 4π gas proportional counter during two separate experiments with different beam properties and different beam energies. The results of these measurements were T1/2 = 1.66424+0.00072 -0.00064 s and T1/2 = 1.66368+0.00084 -0.00050 s. These two results are consistent with each other and their average yields T1/2(18Ne) = 1.66400+0.00057 -0.00048 s, an improvement in precision by a factor of 2 over the previous world half-life data set for 18Ne.

In addition, the half-life of 23Ne was measured as part of an investigation of possible systematic uncertainties associated with the diffusion of neon sample implanted in aluminum tape systems.  The new world average 23Ne half-life is T1/2(23Ne) = 37.148 ± 0.032 s, which represents an improvement in precision by a factor of 1.7 when compared to the world average prior to the measurements performed in this work.

The precise half-life measurements for 14O and 18Ne reported here can be combined with the respective branching ratio and Q value measurements for these isotopes in order to determine the ft values for these superallowed Fermi β transitions.

Examination Committee:

Dr. Robert Wickham, Chair

Dr. Carl Svensson, Advisor

Dr. Paul Garrett

Dr. Alexandros Gezerlis

Dr. Dan Melconia, External Examiner (Texas A & M University)