Speaker
Paul Finlay
(Katholieke Universiteit Leuven (BE))
Description
High-precision half-life and branching-ratio measurements for the superallowed $\beta^+$ emitter $^{26}\mathrm{Al}^m$ were performed at the TRIUMF-ISAC radioactive ion beam facility located in Vancouver, Canada. The branching ratio measurement was performed with the 8$\pi$ Spectrometer, an array of 20 high-purity germanium detectors, in conjunction with SCEPTAR, a plastic scintillator array used to detect the emitted beta particles. An upper limit of $\le$ 12 ppm at 90$\%$ confidence level was found for the second forbidden $\beta^+$ decay of $^{26}\mathrm{Al}^m$ to the state at 1809 keV in $^{26}\mathrm{Mg}$. An inclusive upper limit of $\le$ 15 ppm at 90$\%$ confidence level was found when considering all possible non-analogue $\beta^+$/EC decay branches of $^{26}\mathrm{Al}^m$, resulting in a superallowed branching ratio of $100.0000^{+0}_{-0.0015}\%$.
The half-life measurement was performed using a 4$\pi$ continuous-flow gas proportional counter and fast tape transport system. The resulting value for the $^{26}\mathrm{Al}^m$ half-life, T$_{1/2} = 6.34654(76)$ s, is consistent with, but 2.5 times more precise than, the previous world average, and represents the single most precisely measured half-life of any superallowed emitting nucleus to date.
Combining these results with world-average $Q$-value measurements yields a superallowed $\beta$-decay ${ft}$ value of 3037.58(60) s, the most precisely determined ${ft}$ value for any superallowed emitting nucleus to date. Combined with the small, and precisely quoted, theoretical isospin-symmetry-breaking corrections for this nucleus, the corrected ${\mathcal{F}t}$ value for $^{26}\mathrm{Al}^m$ of 3073.1(12) s is also the most precisely determined for any superallowed emitter by nearly a factor of two and now rivals the precision of all the other 12 precisely measured superallowed $\beta$ decays combined. The high-precision experimental ${ft}$ value for $^{26}\mathrm{Al}^m$ superallowed decay reported here provides a new benchmark to refine theoretical models of isospin-symmetry-breaking effects in superallowed Fermi $\beta$ decays.
Primary author
Paul Finlay
(Katholieke Universiteit Leuven (BE))
