Isospin symmetry in B(E2) values: Coulomb excitation study of 21Mg

The T-z = -3/2 nucleus Mg-21 has been studied by Coulomb excitation on Pt-196 and Pd-110 targets. A 205.6(1)-keV gamma-ray transition resulting from the Coulomb excitation of the 5/2(+) ground state to the first excited 1/2(+) state in Mg-21 was observed for the first time. Coulomb excitation cross-section measurements with both targets and a measurement of the half-life of the 1/2(+) state yield an adopted value of B(E2; 5/2(+) -> 1/2(+)) = 13.3(4) W.u. A new excited state at 1672(1) keV with tentative 9/2(+) assignment was also identified in Mg-21. This work demonstrates a large difference in the B(E2; 5/2(+) -> 1/2(+)) value between T = 3/2, A = 21 mirror nuclei. The difference is investigated in the shell-model framework employing both isospin conserving and breaking USD interactions and using modern ab initio nuclear structure calculations, which have recently become applicable in the sd shell.