Abstract
The Tz = -$\frac{3}{2}$ nucleus 21Mg has been studied by Coulomb excitation on 196Pt and 110</sup Pd targets. A 205.6(1)-keV γ-ray transition resulting from the Coulomb excitation of the $\frac{5}{2}$+ ground state to the first excited $\frac{1}{2}$+ state in 21Mg was observed for the first time. Coulomb excitation cross-section measurements with both targets and a measurement of the half-life of the $\frac{1}{2}$+ state yield an adopted value of B (E2; $\frac{5}{2}$+ → $\frac{1}{2}$+) = 13.3 (4) W.u. A new excited state at 1672(1) keV with tentative $\frac{9}{2}$+ assignment was also identified in 21Mg . This work demonstrates a large difference in the B (E2; $\frac{5}{2}$+ → $\frac{1}{2}$+) value between T = $\frac{3}{2}$, A = 21 mirror nuclei. Lastly, 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 s d shell.