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تسجيل مستخدم جديدLow Energy States of $^{81}_{31} Ga_{50}$ : Elements on the Doubly-Magic Nature of $^{78}$Ni
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David Verney
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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.
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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.
Nuclear magic numbers, which emerge from the strong nuclear force based on quantum chromodynamics, correspond to fully occupied energy shells of protons, or neutrons inside atomic nuclei. Doubly magic nuclei, with magic numbers for both protons and n
eutrons, are spherical and extremely rare across the nuclear landscape. While the sequence of magic numbers is well established for stable nuclei, evidence reveals modifications for nuclei with a large proton-to-neutron asymmetry. Here, we provide the first spectroscopic study of the doubly magic nucleus $^{78}$Ni, fourteen neutrons beyond the last stable nickel isotope. We provide direct evidence for its doubly magic nature, which is also predicted by ab initio calculations based on chiral effective field theory interactions and the quasi-particle random-phase approximation. However, our results also provide the first indication of the breakdown of the neutron magic number 50 and proton magic number 28 beyond this stronghold, caused by a competing deformed structure. State-of-the-art phenomenological shell-model calculations reproduce this shape coexistence, predicting further a rapid transition from spherical to deformed ground states with $^{78}$Ni as turning point.
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.
$beta$-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.
The nuclear magnetic moment of the ground state of $^{55}$Ni ($I^{pi}=3/2^{-}, T_{1/2}=204$ ms) has been deduced to be $|mu$^{55}Ni)$|=(0.976 pm 0.026)$ $mu_N$ using the $beta$-NMR technique. Results of a shell model calculation in the full textit{fp
} shell model space with the GXPF1 interaction reproduce the experimental value. Together with the known magnetic moment of the mirror partner $^{55}$Co, the isoscalar spin expectation value was extracted as $<sum sigma_z >=0.91 pm 0.07$. The $<sum sigma_z>$ shows a similar trend as that established in the textit{sd} shell. The present theoretical interpretations of both $mu(^{55}$Ni) and $<sum sigma_z>$ for the $T=1/2$, A=55 mirror partners support the softness of the $^{56}$Ni core.
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