Improved $\beta$ Decay Branching Ratios

The work we report here aims at increasing the precision possible in the measurement of branching ratios for superallowed $\beta ^+$decays. Such highly accurate values are essential in generating precise $ft$-values for $0^+\to 0^+$decays, which can then be used to test the Standard Model \textit{via} the unitarity of the Cabibbo-Kobayashi-Maskawa matrix [1]. The required precision is $\sim $0.1{\%} or better. While this limit was already achieved in the case of ${ }^{34}Ar$ [2], it would have been very difficult, if not impossible, to achieve it for other $\beta ^+$-decays without an upgrade to our acquisition and data-reduction systems. We have thus improved the controls over all the key elements in our experimental set-up: we now have direct control over the dead-time for the singles and coincidence channels and $<$0.1 mm control over the source-detector distance. In addition, we have extensively studied the efficiency of the $\beta $-detector with source-measurements tested against various Monte Carlo programs [3]. We have tested our new acquisition set-up on ${ }^{60}Co$ and ${ }^{22}Na$ ($\beta ^-$ and $\beta ^+$ emitters respectively) to validate our new methods. Preliminary results on the two sources are statistically consistent with the expected values. An ${ }^{34}Ar$ decay experiment using the new experimental configuration has already been performed and is currently analyzed. [1] J.C. Hardy and I.S. Towner, PRC \textbf{71}, 055501 (2005) [2] V. Iacob \textit{et al.}, BAPS \textbf{52}(3)B16; BAPS \textbf{52}(9)HF3 [3] V. Golovko \textit{et al.}, BAPS \textbf{52}(9)DH4; this BAPS