Nearly degenerate isomeric states of ${}^{75}\mathrm{Cu}$

The decays of two isomeric states in the neutron-rich nucleus ${}^{75}\mathrm{Cu}$ have been studied via delayed $\gamma$-ray spectroscopy. These states were populated by the fragmentation of a $60.4A\mathrm{MeV} {}^{86}\mathrm{Kr}$ primary beam, which impinged on a Be target at the LISE2k spectrometer of GANIL. Isomeric half-lives and branching ratios were measured, and a $\gamma -\gamma$ coincidence analysis performed. These have allowed a revised level scheme of ${}^{75}\mathrm{Cu}$ to be proposed. A comparison with large-scale shell-model calculations using different effective interactions and valence spaces shows the importance of proton excitations across the $Z=28$ shell gap for reproducing the energy spacing between the isomeric states. The coexisting collective and single-particle properties of the $(1/2^{-})$ and $(3/2^{-})$ states, respectively, are similar to those of the same states in the neighboring ${}^{69,71,73}\mathrm{Cu}$. The small energy spacings between the $(1/2^{-}), (3/2^{-})$, and $5/2^{-}$ states are responsible for the isomerism.

Figure 1

Schematic representation of experimental setup.

Figure 2

(a) Delayed $\gamma$ rays detected in coincidence with ${}^{75}\mathrm{Cu}$ ions. Contaminant peaks labeled with * correspond to delayed $\gamma$ decays from ${}^{71,72}\mathrm{Cu}$. Coincidence spectra obtained with gates set on (b) the 66.2- and (c) the 61.7-keV $\gamma$ rays. See text for more details.

Figure 3

Background subtracted time spectra for the two transitions observed in coincidence with ${}^{75}\mathrm{Cu}$ ions. In order to highlight the feeding between the two states the time distribution of the 61.7-keV transition (empty circles) was inverted with respect to the implantation time. The decay curve of the 66.2-keV $\gamma$-ray transition is marked with filled circles. A scale factor of 0.59 was applied to show that the decay time of 66.2 keV corresponds to the rise time of the 61.7 keV curve. The two proposed scenarios for the level scheme of ${}^{75}\mathrm{Cu}$ are shown in the inset.

Figure 4

Experimental energy (expressed in MeV) and reduced transition (given in W.u.) probabilities (black squares) of the $5/2^{-}$ states (top panels) and $1/2^{-}$ (bottom) in ${}^{69-75}\mathrm{Cu}$ compared with theoretical calculations using jj44b (blue triangles), JUN45 (red stars), and SMI interactions (green circles).

Figure 5

Partial experimental and calculated leveld schemes of ${}^{75}\mathrm{Cu}$.