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The Tz=-32 nucleus Mg21 has been studied by Coulomb excitation on Pt196 and Pd110 targets. A 205.6(1)-keV γ-ray transition resulting from the Coulomb excitation of the 52+ ground state to the first excited 12+ state in Mg21 was observed for the first time. Coulomb excitation cross-section measurements with both targets and a measurement of the half-life of the 12+ state yield an adopted value of B(E2;52+→12+)=13.3(4) W.u. A new excited state at 1672(1) keV with tentative 92+ assignment was also identified in Mg21. This work demonstrates a large difference in the B(E2;52+→12+) value between T=32, 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.

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Physical Review C