Coulomb Excitation ofCu68,70: First Use of Postaccelerated Isomeric Beams

Abstract

We report on the first low-energy Coulomb excitation measurements with radioactive I 6 ÿ beams of odd-odd nuclei 68;70 Cu. The beams were produced at ISOLDE, CERN and were post-accelerated by REX-ISOLDE to 2:83 MeV=nucleon. rays were detected with the MINIBALL spectrometer. The 6 ÿ beam was used to study the multiplet of states (3 ÿ , 4 ÿ , 5 ÿ , 6 ÿ ) arising from the 2p 3=2 1g 9=2 configuration. The 4 ÿ state of the multiplet was populated via Coulomb excitation and the BE2; 6 ÿ ! 4 ÿ 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 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 40. Radioactive beams provide great opportunities for investigating the nuclear structure away from the stable nuclei. One of the regions of the nuclear chart that has attracted a considerable interest in the past years is the one close to 68 Ni [1][2][3][4][5][6][7][8]. Coulomb excitation experiments with radioactive beams of even-even isotopes showed that the coupling of a few extra particles to the 68 Ni core induces large polarization effects [2,3]. These effects were associated with a weakening of the Z 28 and N 40 gaps when neutrons start filling the 1g 9=2 orbital [2]. Beyond N 40, results of -decay measurements in the neutronrich [69][70][71][72][73] Cu isotopes revealed a dramatic and sudden lowering of the 1f 5=2 orbital with the increased occupancy of the 1g 9=2 orbital [4]. Referred to as monopole migration, this energy shift was interpreted as originating from the residual proton-neutron interaction and it is expected to have profound implications on the structure of the doubly magic nucleus 78 Ni [4,5].Shell-model calculations using different effective nucleon-nucleon interactions were used in order to understand the observed properties in the nuclei around 68 Ni and predict the evolution of the shell structure towards 78 Ni [5][6][7][8]. The calculations indicated that the values of the Z 28 and N 40, 50 energy gaps strongly depend on the effective interaction used. A consistent understanding of the evolution of the nuclear structure in these regions requires also experimental information such as excitation energies and transition rates in the odd-A and odd-odd nuclei.

Coulomb excitation of the 3−isomer in70Cu

“…The isomeric beams were produced in a similar way as in Refs. [2][3][4]15], where narrow-band laser scans provided the optimum values of the laser frequency that maximize the ionization of the different isomers [17]. The 6 − and 3 − beams of 70 Cu, post-accelerated by the radioactive beam experiment (REX) at ISOLDE [18] up to 2.8 MeV/nucleon, were used to bombard a 120 Sn 2.3 mg/cm 2 target inducing Coulomb excitation.…”

“…The experimental setup used in the present work is identical to the one described in Refs. [4,16]. γ rays following the deexcitation of the levels populated by Coulomb excitation were detected by the MINIBALL HPGe array [19], while the scattered projectile and recoiling target nuclei were detected in an annular Compact Disk (CD)-shaped double-sided silicon strip detector [20].…”

“…been subject to several experimental, e.g., Refs. [1][2][3][4][5][6][7][8], and theoretical papers, e.g., Refs. [9][10][11].…”

“…These long-lived isomeric states were also investigated using laser spectroscopy [6,15], whereby the spin and parity were firmly fixed to 1 + , 3 − , and 6 − in the case of 70 Cu and 1 + and 6 − in case of 68 Cu. Thanks to recent developments in resonant-laser ionization and post-acceleration, energetic isomeric beams have been produced; and soon after, this opportunity was used to perform pioneering Coulomb excitation (Coulex) studies of the 6 − state in 68,70 Cu [4]. Coulex investigations are an important probe for nuclear-structure studies, as they provide information on electromagnetic (E2) transition rates between nuclear states, on energies, and, based on selection rules, on spin and parities of the excited levels.…”

“…The set of data reported in Ref. [4] have been therefore reanalyzed on the basis of the new analysis procedure, and more reliable values have been extracted. This also ensures that the data acquired in the two 70 Cu experiments are treated coherently.…”