Test of the Standard Electroweak Model in superallowed 0+ -> 0+ nuclear beta decays

Precise measurements of superallowed 0+ -> 0+ nuclear b decays provide demanding tests
of the fundamental symmetries and properties of the electroweak interaction. Collectively,
and once corrected, the comparative half-lives (the so-called ft-values) of these transitions
allow a sensitive (~ 3´10-4) probe of the CVC hypothesis. Moreover, by providing the most
accurate determination of Vud, the up-down quark-mixing element, the superallowed data
serve as a stringent test of the unitarity of the CKM matrix, a mandatory ingredient of the
three-generation Standard Model. However, prior to new results for Vus, the sum of squares of
the top-row CKM elements failed to meet unity by more than 2 s. As a possible explanation
for the long standing unitarity default, uncertainties in the theoretical Coulomb corrections
applied to the experimental ft-values were pointed out. It became therefore essential to extend
the study to heavy odd-odd (A ³ 62, TZ = 0) and medium mass (18 £ A £ 42, TZ = -1) nuclei
where these corrections are predicted to be much larger. In this context, the b-g decays of
62Ga and 38Ca have been investigated at the IGISOL (Finland) and ISOLDE (CERN)
facilities. The experimental results obtained will be presented. Prospects for the continuation
of this work will be also discussed.