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Register a new userExistence of exotic torus configuration in high-spin excited states of $^{40}$Ca
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We investigate the possibility of the existence of the exotic torus configuration in the high-spin excited states of $^{40}$Ca. We here consider the spin alignments about the symmetry axis. To this end, we use a three-dimensional cranked Skyrme Hartree-Fock method and search for stable single-particle configurations. We find one stable state with the torus configuration at the total angular momentum $J=$ 60 $hbar$ and an excitation energy of about 170 MeV in all calculations using various Skyrme interactions. The total angular momentum J=60 $hbar$ consists of aligned 12 nucleons with the orbital angular momenta $Lambda=+4$, +5, and +6 for spin up-down neutrons and protons. The obtained results strongly suggest that a macroscopic amount of circulating current breaking the time-reversal symmetry emerges in the high-spin excited state of $^{40}$Ca.
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The superdeformed band, recently discovered in Ca-40 is analysed in an spherical shell model context. Two major oscillator shells, sd and pf are necessary to describe it. The yrast band of the fixed 8p-8h configuration fits extremely well with the experimental energies and transition rates of the superdeformed band. The 4p-4h configuration generates a normally deformed band plus a gamma-band pattern, both are also present in the experimental data.
The coexistence of various low-lying deformed states in $^{42}$Ca and $alpha$--$^{38}$Ar correlations in those deformed states have been investigated using deformed-basis antisymmetrized molecular dynamics. Wave functions of the low-lying states are obtained via parity and angular momentum projections and the generator coordinate method (GCM). Basis wave functions of the GCM calculation are obtained via energy variations with constraints on the quadrupole deformation parameter $beta$ and the distance between $alpha$ and $^{38}$Ar clusters. The rotational band built on the $J^pi = 0_2^+$ (1.84 MeV) state as well as the $J^pi = 0_3^+$ (3.30 MeV) state are both reproduced. The coexistence of two additional $K^pi = 0^+$ rotational bands is predicted; one band is shown to be built on the $J^pi = 0_3^+$ state. Members of the ground-state band and the rotational band built on the $J^pi = 0_3^+$ state contain $alpha$--$^{38}$Ar cluster structure components.
Recent experimental observation of magicity in $^{78}$Ni has infused the interest to examine the persistence of the magic character across the N$=$50 shell gap in extremely neutron rich exotic nucleus $^{78}$Ni in ground as well as excited states. A systematic study of Ni isotopes and N$=$50 isotones in ground state is performed within the microscopic framework of relativistic mean-field (RMF) and the triaxially deformed Nilson Strutinsky model (NSM). Ground state density distributions, charge form factors, radii, separation energies, pairing energies, single particle energies and the shell corrections show strong magicity in $^{78}$Ni. Excited nuclei are treated within the statistical theory of hot rotating nuclei where the variation of level density parameter and entropy shows significant magicity with a deep minima at N$=$50, which, persists up to the temperatures $approx$ 1.5$-$2 MeV and then slowly disappear with increasing temperature. Rotational states are evaluated and effect of rotation on N$=$50 (Z$=$20$-$30) isotones are studied. Our results agree very well with the available experimental data and few other theoretical calculations.
We investigate the precession motion of the exotic torus configuration in high-spin excited states of $^{40}$Ca. For this aim, we use the three-dimensional time-dependent Hartree-Fock (TDHF) method. Although the high-spin torus isomer is a unique quantum object characterized by the alignment of angular momenta of independent single-particle motions, we find that the obtained moment of inertia for rotations about an axis perpendicular to the symmetry axis is close to the rigid-body value. We also analyze the microscopic structure of the precession motion using the random-phase approximation (RPA) method for high-spin states. In the RPA calculation, the precession motion of the torus isomer is generated by coherent superposition of many one-particle-one-hole excitations across the sloping Fermi surface that strongly violates the time-reversal symmetry. By comparing results of the TDHF and the RPA calculations, we find that the precession motion obtained by the TDHF calculation is a pure collective motion well decoupled from other collective modes.
Multi-nucleon transfer reactions in $^{40}$Ca+$^{208}$Pb have been studied at several bombarding energies close to the Coulomb barrier. Light reaction products have been identified in mass and charge with a time-of-flight spectrometer. The energy spectra of the inclusive two neutron pick-up channel show a population in a narrow region of excitation energies which corresponds to the predicted energy of pairing vibration states in $^{42}$Ca.
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