High-precision half-life measurements for superallowed Fermi beta decays

High-precision measurements of the ft values for superallowed Fermi beta decays between 0+ isobaric analogue states have, for decades, provided demanding tests of the Standard Model description of electroweak interactions. In order to significantly contribute to these tests experimentally, beta decay half-lives and branching ratios must be determined to overall precisions of +/- 0.05% or better, and beta decay Q values must be deduced to at least +/- 0.01%. For beta decay half-lives in particular, this demanding requirement is generally accomplished using direct beta counting techniques. This method was employed as part of this thesis in order to deduce the half-life of the superallowed beta + emitter 62Ga using mass-separated radioactive ion beams provided by the Isotope Seperator and Accelerator (ISAC) facility at TRIUMF. The result of this analysis, T1/2( 62Ga)beta 116.100 +/- 0.025 ms, is now the single most precise superallowed half-life ever reported.; In cases where there are large amounts of contaminant or daughter activities, one must instead rely on a measurement of the half-life using the gamma-ray activity. Half-life measurements using the technique of gamma-ray photopeak counting have, however, been previously limited by a systematic bias associated with detector pulse pile-up effects. While detector pulse pile-up has been qualitatively understood for decades, there has not been a quantitative description of its effects on half-life measurements to the level of precision required (+/- 0.05%) for superallowed Fermi beta decay studies. Using the 8pi gamma-ray spectrometer, a spherical array of 20 HPGe detectors at ISAC, a new method was developed that, for the first time, provides the necessary quantitative description of detector pulse pile-up to the required level of precision. This novel technique has been verified through both a detailed Monte-Carlo simulation and experimentally using radioactive beams of 26Na. Following a correction of nearly 30 statistical standard deviations for pulse pile-up, the half-life of 26Na deduced in this work, T1/2(26Na) gamma 1.07167 + 0.00055 s, is precise to the level of 0.05% and is in excellent agreement with the corresponding value, T 1/2(26Na)beta = 1.07128 +/- 0.00025 s, deduced from direct beta counting. This study has demonstrated the feasibility of using the gamma-ray counting technique to deduce beta decay half-lives to the necessary level of +/- 0.05% precision. As an extension to this work the half-life of the superallowed beta+ emitter 18Ne was determined to be, T1/2( 18Ne)gamma = 1.6656 +/- 0.0019 s, a result that is a factor of four times more precise than the previous world average.