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




Dunlop, Michelle

Journal Title

Journal ISSN

Volume Title


University of Guelph


High precision measurements of the ft values for superallowed Fermi β transitions be- tween Jπ = 0+, T = 1 isobaric analogue states allow for stringent tests of the electroweak interaction described by the Standard Model. These transitions provide an experimental probe of the Conserved-Vector-Current hypothesis, the most precise determination of Vud, the up- down element of the Cabibbo-Kobayashi-Maskawa quark-mixing matrix, and set stringent limits on the existence of scalar currents in the weak interaction. This thesis focuses on high-precision half-life measurements of two superallowed Fermi β+ emitters, 10C and 22Mg. These half-life measurements were performed at TRIUMF’s Isotope Separator and Accelerator facility. For both cases, measurements of their half-lives were motivated by the discrepancies between previous measurements found in literature that resulted in inflations in the uncertainties to account for the inconsistent measurements. Two independent measurements of the 10C half-life were performed. The first measurement was performed via γ-ray photopeak counting using the 8π γ-ray spectrometer, an array of 20 Compton-suppressed high-purity germanium detectors, by measuring the time profile of the characteristic 718-keV γ-ray. This analysis yielded T1/2 = 19.2969 ± 0.0072 s. A second measurement was performed via direct β counting, using a 4π continuous-flow gas proportional β counter. The results from this analysis yields T1/2 = 19.3009 ± 0.0017 s, and is the most precise superallowed half-life measurement reported to date and the first to ever achieve a relative precision below 10−4. A half-life measurement of the superallowed β+ emitter 22Mg was performed via direct β counting using the 4π continuous-flow gas proportional counter. This analysis yielded a half-life of T1/2 = 3.87400 ± 0.0079 s. This measurement resolved a discrepancy between the previous world-average dataset, which was composed of only two measurements. These two half-life measurements showed a disagreement characterized by a χ2/ν = 4.0, however, upon the inclusion of the new measurement presented in this thesis, now has a χ2/ν = 1.1. The high-precision half-life measurements reported in this thesis, once included with the other high-precision superallowed data, can be used to test fundamental properties of the Standard Model. In particular, the improved uncertainty of the 10C half-life can be used to provide a more stringent limit on the contribution of scalar currents in the Standard Model. The improvement in the uncertainty of the 22Mg half-life provides further tests of the isospin symmetry breaking corrections in superallowed Fermi β decays.



Nuclear Physics, Superallowed Beta Decay, Half-Life, Beta Decay