High-Precision Measurements of the Superallowed Beta+ Decays of 38Ca and 46V



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As a part of our program to test the unitarity of the Cabibbo-Kobayashi-Maskawa matrix, the decay of the superallowed 0? --> 0? beta emitters ??Ca and ??V has been studied in this dissertation. For ??Ca, the half-life, 443.88(36) ms, and superallowed branching ratio, 0.7738(41), have been measured. In our half-life experiment, pure sources of ??Ca were produced and the decay positrons detected in a high-efficiency 4[pi] proportional gas counter. Since the beta? decay of ??Ca feeds ??K^m, which is itself a superallowed beta? emitter, the data were analyzed as a linked parent-daughter decay. Our result for the half-life of ??Ca, with a precision of 0.08%, is a factor of five improvement on the best previous result. The branching-ratio of ??Ca depended on beta-delayed gamma-ray intensities being measured with a high-purity germanium detector calibrated for absolute efficiency to 0.2% precision. This branching-ratio result represents our first step in bringing the ft value for the superallowed ??Ca transition into the desired range of 0.1%. With our half-life and superallowed branching ratio results for ??Ca, we obtain the Ft to be 3072(17) s, in good agreement with the conserved vector current expectation. The half-life of ??V has been measured to be 422.66(6) ms, a factor of two more precise than the best previous measurement. Our present result determines the corresponding Ft value to be 3074.5(26) s, which is consistent with the average Ft value of 3072.08(79) s established from the 13 best-known superallowed transitions. This demonstrates that previously accepted half-lives of ??V were correct in their contribution to a precision test of the conserved vector current hypothesis.