Coulomb Excitation of ${}^{68,70}\mathrm{Cu}$: First Use of Postaccelerated Isomeric Beams

We report on the first low-energy Coulomb excitation measurements with radioactive $I^{\pi }=6^{-}$ beams of odd-odd nuclei ${}^{68,70}\mathrm{Cu}$. The beams were produced at ISOLDE, CERN and were post-accelerated by REX-ISOLDE to $2.83\mathrm{MeV}/\mathrm{\text{nucleon}}$. $\gamma$ rays were detected with the MINIBALL spectrometer. The $6^{-}$ beam was used to study the multiplet of states ($3^{-}$, $4^{-}$, $5^{-}$, $6^{-}$) arising from the $\pi 2p_{3/2}\nu 1g_{9/2}$ configuration. The $4^{-}$ state of the multiplet was populated via Coulomb excitation and the $B(E2;6^{-}\to 4^{-})$ 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 $N\approx 40$.

Figure 1

Top: particle–$\gamma$-ray coincidence spectrum acquired with the $6^{-}$ beam of ${}^{68}\mathrm{Cu}$. The partial level scheme and deexcitation $\gamma$ rays shown in the upper right corner are based on Refs. 15, 21 and this work. Energies are given in keV. Levels drawn with thick lines represent the $\beta$-decaying states. Bottom: particle–$\gamma$-ray coincidence spectrum acquired with the $1^{+}$ beam. No Doppler correction was applied to these spectra.

Figure 2

Particle–$\gamma$-ray coincidence spectrum obtained with $6^{-}$ beam of ${}^{70}\mathrm{Cu}$. The spectrum is Doppler corrected for mass $A=70$. Upper right corner: partial level scheme of ${}^{70}\mathrm{Cu}$ taken from Ref. 12.