Coulomb Excitation of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mmultiscripts><mml:mi>Cu</mml:mi><mml:mprescripts/><mml:none/><mml:mrow><mml:mn>68</mml:mn><mml:mo>,</mml:mo><mml:mn>70</mml:mn></mml:mrow></mml:mmultiscripts></mml:math>: First Use of Postaccelerated Isomeric Beams

We report on the first low-energy Coulomb excitation measurements with radioactive ${I}^{\ensuremath{\pi}}={6}^{\ensuremath{-}}$ beams of odd-odd nuclei $^{68,70}\mathrm{Cu}$. The beams were produced at ISOLDE, CERN and were post-accelerated by REX-ISOLDE to $2.83\text{ }\text{ }\mathrm{MeV}/\mathrm{\text{nucleon}}$. $\ensuremath{\gamma}$ rays were detected with the MINIBALL spectrometer. The ${6}^{\ensuremath{-}}$ beam was used to study the multiplet of states (${3}^{\ensuremath{-}}$, ${4}^{\ensuremath{-}}$, ${5}^{\ensuremath{-}}$, ${6}^{\ensuremath{-}}$) arising from the $\ensuremath{\pi}2{p}_{3/2}\ensuremath{\nu}1{g}_{9/2}$ configuration. The ${4}^{\ensuremath{-}}$ state of the multiplet was populated via Coulomb excitation and the $B(E2;{6}^{\ensuremath{-}}\ensuremath{\rightarrow}{4}^{\ensuremath{-}})$ value was determined in both nuclei. The results obtained illustrate the fragile stability of the $Z=28$ shell and $N=40$ subshell closures. A comparison with large-scale shell-model calculations using the $^{56}\mathrm{Ni}$ core shows the importance of the proton excitations across the $Z=28$ shell gap to the understanding of the nuclear structure in the neutron-rich nuclei with $N\ensuremath{\approx}40$.

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