Low-energy Coulomb excitation of neutron-rich zinc isotopes

Abstract

At the radioactive ion beam facility REX-ISOLDE, neutron-rich zinc isotopes were investigated using lowenergy Coulomb excitation. These experiments have resulted in B(E2, 2 74,76 Zn and the determination of the energy of the first excited 2 + 1 states in 78,80 Zn. The zinc isotopes were produced by high-energy proton-(A = 74, 76, 80) and neutron-(A = 78) induced fission of 238 U, combined with selective laser ionization and mass separation. The isobaric beam was postaccelerated by the REX linear accelerator and Coulomb excitation was induced on a thin secondary target, which was surrounded by the MINIBALL germanium detector array. In this work, it is shown how the selective laser ionization can be used to deal with the considerable isobaric beam contamination and how a reliable normalization of the experiment can be achieved. The results for zinc isotopes and the N = 50 isotones are compared to collective model predictions and state-of-the-art large-scale shell-model calculations, including a recent empirical residual interaction constructed to describe the present experimental data up to 2004 in this region of the nuclear chart.

Coulomb excitation of the 3−isomer in70Cu

“…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].…”

“…target nucleus (in this case 120 Sn), the beam composition is a crucial parameter in the normalization to the target excitation [16].…”

“…[4,16] by comparing measurements with laser ON and laser OFF. Values of 30(3)% Ga contamination were found in the 70 Cu beam when the lasers were tuned to the I π = 6 − , while 50(3)% Ga contamination was obtained when the lasers were tuned to the I π = 3 − beam.…”

“…In the former case, the experiment with the 6 − enhanced beam, the excitation induced by the gallium was determined as described in Ref. [16] by considering the ratio of scattered particles detected in laser ON and laser OFF separated runs.…”

“…[2]. The characteristic γ rays from the β decay of the three long-lived states that were detected in the MINIBALL germanium array during the off-beam periods [16], allowed us to determine the isomeric content of the beam. The analysis showed that when the laser was tuned to the maximum production of the 6 − beam, 85(5)% of the total 70 Cu ion yield was produced in this spin state, while the (3 − ,1 + ) isomers were found to contribute almost equal amounts (≈7%) to the total Cu beam.…”