The beta decay of 38Ca: Sensitive test of isospin symmetry-breaking   corrections from mirror superallowed 0+-to-0+ transitions
  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. 
H.I. Park, J.C. Hardy, V.E. Iacob, M. Bencomo, L. Chen, V Horvat, N.   Nica, B.T. Roeder, E. Simmons, R.E. Tribble and I.S. ...
nucl-ex hep-ph nucl-th