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Register a new userMicroscopic shell-model description of the exotic nucleus ^{16}C
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The structure of the neutron-rich carbon nucleus ^{16}C is described by introducing a new microscopic shell model of no-core type. The model space is composed of the 0s, 0p, 1s0d, and 1p0f shells. The effective interaction is microscopically derived from the CD-Bonn potential and the Coulomb force through a unitary transformation theory. Calculated low-lying energy levels of ^{16}C agree well with the experiment. The B(E2;2_{1}^{+} to 0_{1}^{+}) value is calculated with the bare charges. The anomalously hindered B(E2) value for ^{16}C, measured recently, is well reproduced.
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The neutron rich exotic unbound 7He nucleus has been the subject of many experimental investigations. While the ground-state 3/2- resonance is well established, there is a controversy concerning the excited 1/2- resonance reported in some experiments as low-lying and narrow (E_R ~ 1 MeV, Gamma < 1 MeV) while in others as very broad and located at a higher energy. This issue cannot be addressed by ab initio theoretical calculations based on traditional bound-state methods. We introduce a new unified approach to nuclear bound and continuum states based on the coupling of the no-core shell model, a bound-state technique, with the no-core shell model/resonating group method, a nuclear scattering technique. Our calculations describe the ground-state resonance in agreement with experiment and, at the same time, predict a broad 1/2- resonance above 2 MeV.
We present ab initio calculations of resonances for $^7$He, a nucleus with no bound states, using the realistic nucleon-nucleon interaction Daejeon16. For this, we evaluate the $n{-}{^6rm He}$ elastic scattering phase shifts obtained within an $S$-matrix analysis of no-core shell model results for states in the continuum. We predict new broad resonances likely related to fragmentary experimental evidence.
The exotic $^9$He nucleus, which presents one of the most extreme neutron-to-proton ratios, belongs to the $N=7$ isotonic chain famous for the phenomenon of ground-state parity inversion with decreasing number of protons. Consequently, it would be expected to have an unnatural (positive) parity ground state similar to $^{11}$Be and $^{10}$Li. Despite many experimental and theoretical investigations, its structure remains uncertain. Apart from the fact that it is unbound, other properties including the spin and parity of its ground state and the very existence of additional low-lying resonances are still a matter of debate. In this work we study the properties of $^9$He by analyzing the $n+^8$He continuum in the context of the ab initio no-core shell model with continuum (NCSMC) formalism with chiral interactions as the only input. The NCSMC is a state-of-the-art approach for the ab initio description of light nuclei. With its capability to predict properties of bound states, resonances, and scattering states in a unified framework, the method is particularly well suited for the study of unbound nuclei such as $^9$He. Our analysis produces an unbound $^9$He nucleus. Two resonant states are found at the energies of ${sim}1$ and ${sim}3.5$ MeV, respectively, above the $n+^8$He breakup threshold. The first state has a spin-parity assignment of $J^{pi} = {1/2}^-$ and can be associated with the ground state of $^9$He, while the second, broader state has a spin-parity of ${3/2}^-$. No resonance is found in the ${1/2}^+$ channel, only a very weak attraction. We find that the $^9$He ground-state resonance has a negative parity and thus breaks the parity-inversion mechanism found in the $^{11}$Be and $^{10}$Li nuclei of the same $N=7$ isotonic chain.
The pseudo-SU(3) model is used to describe the low-energy spectra as well as $E2$ and $M1$ transition strengths in $^{158}$Gd. The Hamiltonian includes spherical single-particle energies, the quadrupole-quadrupole and proton and neutron pairing interactions, plus four rotor-like terms. The parameters of the Hamiltonian were fixed by systematics with the rotor-like terms determined through a least-squares analysis. The basis states are built as linear combinations of SU(3) states which are the direct product of SU(3) proton and neutron states with pseudo-spin zero. The calculated results compare favorably with the available experimental data, which demonstrates the ability of the model to describe such nuclei.
In order to test the $^{16}$C internal wave function, we perform microscopic coupled-channels (MCC) calculations of the $^{16}$C($0_1^+ to 2_1^+$) inelastic scattering by $^{208}$Pb target at $E/A$=52.7 MeV using the antisymmetrized molecular dynamics (AMD) wave functions of $^{16}$C, and compare the calculated differential cross sections with the measured ones. The MCC calculations with AMD wave functions reproduce the experimental data fairly well, although they slightly underestimate the magnitude of the cross sections. The absolute magnitude of calculated differential cross sections is found to be sensitive to the neutron excitation strength. We prove that the MCC method is a useful tool to connect the inelastic scattering data with the internal wave functions.
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