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 ³⁸Ca and ⁴⁶V has been studied in this dissertation. For ³⁸Ca, 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 ³⁸Ca were produced and the decay positrons detected in a high-efficiency 4[pi] proportional gas counter. Since the beta⁺ decay of ³⁸Ca feeds ³⁸K^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 ³⁸Ca, with a precision of 0.08%, is a factor of five improvement on the best previous result. The branching-ratio of ³⁸Ca 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 ³⁸Ca transition into the desired range of 0.1%. With our half-life and superallowed branching ratio results for ³⁸Ca, we obtain the Ft to be 3072(17) s, in good agreement with the conserved vector current expectation. The half-life of ⁴⁶V 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 $\overline{\mathcal{F}t}$ value of 3072.08(79) s established from the 13 best-known superallowed transitions. This demonstrates that previously accepted half-lives of ⁴⁶V were correct in their contribution to a precision test of the conserved vector current hypothesis.

Precision measurements of superallowed Fermi β-decay allow for stringent tests of the magnitude of isospin-symmetry-breaking effects in nuclei, the validity of the conserved vector current hypothesis, and the unitarity of the Cabibbo-Kobayashi-Maskawa (CKM) quark-mixing matrix. A high-precision measurement of the branching ratio for the superallowed β+ decay of 74Rb has been performed at the Isotope Separator and ACcelerator (ISAC) facility at TRIUMF. The 8π spectrometer, an array of 20 HPGe detectors, was used to detect γ-rays emitted following the Gamow-Teller and non-analogue Fermi β-decays of 74Rb. PACES, an array of 5 Si(Li) detectors, was used to detect emitted conversion electrons, while half of SCEPTAR, a close-packed array of 10 plastic scintillators, was used to detect emitted β particles. In this experiment, 23 excited states were identified in 74Kr which were populated following the β-decay of 74Rb. A total of 58 γ-rays were identified following the 8.241(4)x108 detected β-particles. An observed non-superallowed branching ratio of 0.396(7)% was determined from the intensity of the identified γ-rays, while the unobserved non-superallowed intensity was calculated to be 0.05(5)%, leading to a superallowed branching ratio of 99.55(5)%. The superallowed branching ratio is now the most precise experimental quantity in the determination of the superallowed ft-value of 74Rb. Combining the half-life and Q-value with the superallowed branching ratio measured in this work leads to a superallowed ft-value of 3082.6(66) s. Finally, comparisons between the superallowed ft-value, the world average Ft-value, and the non-analogue Fermi branching ratio are made to provide future guidance in the refinement of the theoretical models required to describe the crucial isospin-symmetry-breaking term in superallowed β-decay.