Investigation of the collective behaviour of nuclei around mass 70 towards the proton drip-line

Abstract

In even-even nuclei, deformation of the nucleus is related to reduced transition probability, B(E2) values. By measuring the 2 + states and corresponding B(E2) values, collectivity can be studied in a unique way. In order to obtain these values, intermediate energy exotic beams have been used as a spectroscopic tool through Coulomb excitations and nuclear inelastic scatterings. The inelastic scattering of 72 Kr, 70 Kr, 70 Br and 68 Se isotopes on 9 Be and 197 Au targets has been performed. Production of these very exotic nuclei, at the proton drip-line, was achieved at the Radioactive Isotope Beam Factory (RIBF), Japan. A78 Kr primary beam with an energy of 345 MeV/u was impinged on a 9 Be target to produce the ions of interest as a secondary beam. The BigRIPS fragment separator was used in order to deliver the secondary beam isotopes with an energy of 175 MeV/u to the secondary target. The reaction products were identified in the ZeroDegree Spectrometer (ZDS) employing the Bρ-ΔE-ToF method. The γ-rays emitted due to the excitation were measured by an array of γ-ray detectors, DALI2, that was placed around the secondary target.The experiments were performed using two different targets in order to increase the probability of Coulomb or nuclear interaction. While the exotic beam at intermediate energies scatters through the electromagnetic field of an Au target, Coulomb and nuclear forces interfere. The fraction of these two kinds of excitation was identified by scattering the same beam off a Be target and increase the relative strength of nuclear scattering. For each isotope, the experimental conditions were simulated to obtain the response functions of the transitions. Experimental results were then fitted to these response functions from the simulations in order to determine the number of emitted γ-rays. Then, the excitation cross-sections were deduced for both cases and used in order to determine the deformation lengths δn and δc , for nuclear and Coulomb excitation, respectively. These deformation lengths were obtained using ECIS-97 code. Finally, using the relation between electromagnetic interaction deformation length and the deformation parameter, B(E2) values were determined.