Search

The neutron-rich nucleus \nucleus{200}{Pt} is investigated via in-beam \gamma-ray spectroscopy in order to study the shape evolution in the neutron-rich platinum isotopes towards the N = 126 shell closure. The two-neutron transfer reaction \nucleus{198}{Pt}(\nucleus{82}{Se}, \nucleus{80}{Se})\nucleus{200}{Pt} is used to populate excited states of \nucleus{200}{Pt}. The Advanced Gamma Ray Tracking Array (AGATA) demonstrator coupled with the PRISMA spectrometer detects \gamma rays coincident with the \nucleus{80}{Se} recoils, the binary partner of \nucleus{200}{Pt}. The binary partner method is applied to extract the \gamma-ray transitions and build the level scheme of \nucleus{200}{Pt}. The level at 1884\,keV reported by Yates et. al [Phys. Rev. C 37, 1889] was confirmed to be at 1882.1\,keV and assigned as the (6^+_1) state. An additional \gamma ray was found and it presumably de-excites the (8^+_1) state. The results are compared with state-of-the-art beyond mean-field calculations, performed for the even-even \nucleus{190-204}{Pt} isotopes, revealing that \nucleus{200}{Pt} marks the transition from the \gamma-unstable behaviour of lighter Pt nuclei towards a more spherical one when approaching the N=126 shell closure.

No data on the fusion excitation function of 48Ti + 58Fe in the energy region near the Coulomb barrier existed prior to the present work, while fusion of 58Ni + 54Fe was investigated in detail some years ago, down to very low energies and clear evidence of fusion hindrance was noticed at relatively high cross sections. 48Ti and 58Fe are soft and have a low-lying quadrupole excitation lying at ~800-900 keV only. Instead, 58Ni and 54Fe have a closed shell (protons and neutrons, respectively) and are rather rigid. Investigating: 1) the possible in influence of the different structure of the involved nuclei on the fusion excitation functions far below the barrier ; 2) in particular, whether hindrance is observed in 48Ti + 58Fe, and to compare the results with current CC models. Methods: 48Ti beams from the XTU Tandem accelerator of INFN-Laboratori Nazionali di Legnaro were used. The experimental set-up was based on an electrostatic beam separator, and fusion-evaporation residues (ER) were detected at very forward angles. Angular distributions of ER were measured. Fusion cross sections of 48Ti + 58Fe have been obtained in a range of nearly 6 orders of magnitude around the Coulomb barrier, down to2 microb. The sub-barrier cross sections of 48Ti + 58Fe are much larger than those of 58Ni + 54Fe. Significant differences are also observed in the logarithmic derivatives and astrophysical S-factors. No evidence of hindrance is observed, because coupled-channels calculations using a standard Woods-Saxon potential are able to reproduce the data in the whole measured energy range. Analogous calculations for 58Ni + 54Fe predict clearly too large cross sections at low energies. The two fusion barrier distributions are wide and display a complex structure that is only qualitatively fit by calculations. It is pointed out that all these different trends originate from the dissimilar low-energy nuclear structure of the involved nuclei. In particular, the strong quadrupole excitations in 48Ti and 58Fe produce the relative cross section enhancement and make the barrier distribution ~2 MeV wider, thus probably pushing the threshold for hindrance below the measured limit.