  We investigate isospin-symmetry breaking effects in the sd-shell region with
large-scale shell-model calculations, aiming to understand the recent anomalies
observed in superallowed Fermi beta-decay. We begin with calculations of
Coulomb displacement energies (CDE's) and triplet displacement energies (TDE's)
by adding the T=1,J=0 isospin nonconserving (INC) interaction into the usual
isospin-invariant Hamiltonian. It is found that CDE's and TDE's can be
systematically described with high accuracy. A total number of 122 one- and
two-proton separation energies are predicted accordingly, and locations of the
proton drip-line and candidates for proton-emitters are thereby suggested.
However, attempt to explain the anomalies in the superallowed Fermi beta-decay
fails because these well-fitted T=1,J=0 INC interactions are found no effects
on the nuclear matrix elements. It is demonstrated that the observed large
isospin-breaking correction in the 32Cl beta-decay, the large isospin-mixing in
the 31Cl beta-decay, and the small isospin-mixing in the 23Al beta-decay can be
consistently understood by introducing additional T=1,J=2 INC interactions
related to the s1/2 orbit.
