High Precision Half-Life Measurement of $^{38}$Ca

The measured $ft$ values for superallowed 0$^{+}$ $\rightarrow$ 0$^{+}$ nuclear $\beta$ decay can be used to test the Conserved Vector Current (CVC) hypothesis and the unitarity of the Cabbibo-Kobayashi-Maskawa (CKM) matrix. One of the essential elements of this test is the calculated radiative and isospin-symmetry breaking corrections that must be applied to experimental data [1]. Some of these corrections depend on nuclear structure and their uncertainties can, in principle, be reduced by improving the precision of the experimental $ft$ values. The case of $^{38}$Ca is particularly interesting since its structure-dependent correction is calculated to be one of the largest in the $sd$ shell. The $Q_{EC}$ value of the $^{38}$Ca decay is already well measured [2] and we have now measured its half-life to better than 0.1\% precision. Preliminary results will be presented.\\[4pt] [1] I.S. Towner and J.C. Hardy, Phys. Rev. C {\bf 77}, 025501 (2008).\\[0pt] [2] R. Ringle {\it et al.}, Phys. Rev. C {\bf 75}, 055503 (2007).