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Register a new userStudy of the N=50 major shell effect close to $^{78}$Ni : First evidence of a weak coupling structure in $^{83}_{32}$Ge$_{51}$ and three-proton configuration states in $^{81}_{31}$Ga$_{50}$
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Authors
D. Verney
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New levels were attributed to $^{81}_{31}$Ga$_{50}$ and $^{83}_{32}$Ge$_{51}$ which were fed by the $beta$-decay of their respective mother nuclei $^{81}_{30}$Zn$_{51}$ and $^{83}_{31}$Ga$_{52}$ produced by fission at the PARRNe ISOL set-up installed at the Tandem accelerator of the Institut de Physique Nucleaire, Orsay. We show that the low energy structure of $^{81}_{31}$Ga$_{50}$ and $^{83}_{32}$Ge$_{51}$ can easily be explained within the natural hypothesis of a strong energy gap at N=50 and a doubly-magic character for $^{78}$Ni.
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Excited levels were attributed to $^{81}_{31}$Ga$_{50}$ for the first time which were fed in the $beta$-decay of its mother nucleus $^{81}$Zn produced in the fission of $^{nat}$U using the ISOL technique. We show that the structure of this nucleus is consistent with that of the less exotic proton-deficient N=50 isotones within the assumption of strong proton Z=28 and neutron N=50 effective shell effects.
Atomic masses of the neutron-rich isotopes $^{76-80}$Zn, $^{78-83}$Ga, $^{80-85}Ge, $^{81-87}$As and $^{84-89}$Se have been measured with high precision using the Penning trap mass spectrometer JYFLTRAP at the IGISOL facility. The masses of $^{82,83}$Ga, $^{83-85}$Ge, $^{84-87}$As and $^{89}$Se were measured for the first time. These new data represent a major improvement in the knowledge of the masses in this neutron-rich region. Two-neutron separation energies provide evidence for the reduction of the N=50 shell gap energy towards germanium Z=32 and a subsequent increase at gallium (Z=31). The data are compared with a number of theoretical models. An indication of the persistent rigidity of the shell gap towards nickel (Z=28) is obtained.
Low-lying excited states in the $N=32$ isotope $^{50}$Ar were investigated by in-beam $gamma$-ray spectroscopy following proton- and neutron-knockout, multi-nucleon removal, and proton inelastic scattering at the RIKEN Radioactive Isotope Beam Factory. The energies of the two previously reported transitions have been confirmed, and five additional states are presented for the first time, including a candidate for a 3$^-$ state. The level scheme built using $gammagamma$ coincidences was compared to shell-model calculations in the $sd-pf$ model space, and to ab initio predictions based on chiral two- and three-nucleon interactions. Theoretical proton- and neutron-knockout cross sections suggest that two of the new transitions correspond to $2^+$ states, while the previously proposed $4^+$ state could also correspond to a $2^+$ state.
A measurement of the $^{50}$Ti($d$,$p$)$^{51}$Ti reaction at 16 MeV was performed using a Super Enge Split Pole Spectrograph to measure the magnitude of the $N=32$ subshell gap in Ti. Seven states were observed that had not been observed in previous ($d$,$p$) measurements, and the textit{L} transfer values for six previously measured states were either changed or measured for the first time. The results were used to determine single neutron energies for the $p_{3/2}$, $p_{1/2}$ and $f_{5/2}$ orbitals. The resulting single neutron energies in $^{51}$Ti confirm the existence of the $N=32$ gap in Ti. These single neutron energies and those from previous measurements in $^{49}$Ca, $^{53}$Cr and $^{55}$Fe are compared to values from a covariant density functional theory calculation.
$beta$-decay rates play a decisive role in understanding the nucleosynthesis of heavy elements and are governed by microscopic nuclear-structure information. A sudden shortening of the half-lives of Ni isotopes beyond $N=50$ was observed at the RIKEN-RIBF. This is considered due to the persistence of the neutron magic number $N=50$ in the very neutron-rich Ni isotopes. By systematically studying the $beta$-decay rates and strength distributions in the neutron-rich Ni isotopes around $N=50$, I try to understand the microscopic mechanism for the observed sudden shortening of the half-lives. The $beta$-strength distributions in the neutron-rich nuclei are described in the framework of nuclear density-functional theory. I employ the Skyrme energy-density functionals (EDF) in the Hartree-Fock-Bogoliubov calculation for the ground states and in the proton-neutron Quasiparticle Random-Phase Approximation (pnQRPA) for the transitions. Not only the allowed but the first-forbidden (FF) transitions are considered. The experimentally observed sudden shortening of the half-lives beyond $N=50$ is reproduced well by the calculations employing the Skyrme SkM* and SLy4 functionals. The sudden shortening of the half-lives is due to the shell gap at $N=50$ and cooperatively with the high-energy transitions to the low-lying $0^-$ and $1^-$ states in the daughter nuclei. The onset of FF transitions pointed out around $N=82$ and 126 is preserved in the lower-mass nuclei around $N=50$. This study suggests that needed is a microscopic calculation where the shell structure in neutron-rich nuclei and its associated effects on the FF transitions are selfconsistenly taken into account for predicting $beta$-decay rates of exotic nuclei in unknown region.
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