TESTS OF ISOSPIN MIXING CORRECTIONS IN SUPERALLOWED 0-]0+ BETA-DECAYS()

Measurements of superallowed 0+ --> 0+ nuclear beta decays, corrected for radiative and charge-dependent effects, yield fundamental properti es of the weak interaction. Model predictions of the necessary correct ions exist, but they have never been tested directly by experiment. Th e present work reports, for the first time, observations of (or limits on) nonanalog 0+ --> 0+ branches in K-38m, V-46, Mn-50, and Co-54, an d tests of the model predictions for isospin-mixing corrections. The r esulting unitarity test of the top row of the Kobayashi-Maskawa matrix is 0.9983 +/- 0.0015.