A Study of the Superallowed $\beta^{+}$ Decay of $^{38}$Ca

To test the unitarity of the CKM matrix via precise measurements of superallowed $0^{+}\rightarrow 0^{+}$ nuclear $\beta^{+}$ decay, accurate calculations of small nuclear- structure-dependent corrections are essential. Currently, uncertainties in these calculations are comparable to the present level of experimental precision. However, these theoretical uncertainties can, in principle, be reduced by experiment. $^{38}$Ca is a particularly favorable case for this purpose because its nuclear-structure-dependent correction term is calculated to be one of the largest in the $sd$ shell [1]. We report our measured half-life to be 443.88(36) ms and the first preliminary results from a branching-ratio measurement. Combined with the well-known $Q_{EC}$ value [1,2], these two results will ultimately lead to a precise ${\cal F}t$ value for $^{38}$Ca, and a valuable test of the calculated nuclear- structure-dependent correction term.\\[4pt] [1] J.C. Hardy and I.S. Towner, Phys. Rev. C 79, 055502 (2009).\\[0pt] [2] T. Eronen $\textit{et al.}$, to be published.