  We report the first branching-ratio measurement of the superallowed 0+-to-0+
beta transition from 38Ca. The result, 0.7728(16), leads to an ft value of
3062.3(68)s with a relative precision of +/-0.2%. This makes possible a
high-precision comparison of the ft values for the mirror superallowed
transitions, 38Ca-to-38mK and 38mK-to-38Ar, which sensitively tests the isospin
symmetry-breaking corrections required to extract Vud, the up-down quark-mixing
element of the Cabibbo-Kobayashi-Maskawa (CKM) matrix, from superallowed beta
decay. The result supports the corrections currently used, and points the way
to even tighter constraints on CKM unitarity.
