Projected shell model description of nuclear level density: Collective, pair-breaking, and multiquasiparticle regimes in even-even nuclei

Abstract

There is overwhelmingly experimental evidence indicating that excited nuclear states are dominated by quasiparticle (qp) excitations, which form many-body configurations with broken nucleon-pairs from different orbitals. By using these multi-qp states as building blocks for a shell-model basis, we propose a novel shell-model method to calculate the nuclear level density (NLD) in deformed nuclei. The shell-model diagonalization with two-body residual interactions yields a large ensemble of eigenstates of angular momentum and parity. We demonstrate that NLD as a statistical quantity depends sensitively on the structure of deformed single-particle states. As the first example to introduce this method, we take a well-deformed rare-earth nucleus, ${}^{164}$Dy, for which NLD has been studied extensively by the Oslo method. By a quantitative comparison with discrete levels from spectroscopic measurements, we show that while the pronounced stepwise structure in the low-energy NLD curve can be understood as the collective excitation and nucleon-pair breaking, the exponential growth of levels in the higher-energy NLD can be described by the combination of the broken-pair states, subject to the Pauli principle. According to the nature of NLD with increasing excitation, we divide the entire NLD curve into (1) collective regime, (2) pair-breaking regime, and (3) multi-qp regime. We discuss the formation mechanism and characteristic features of NLD for the three regimes. In addition, the parity dependence and angular-momentum dependence in NLD are investigated with a strong emphasis on the structure effect.

References in corpus (32)

• Nuclear ground-state masses and deformations: FRDM(2012)
• Random Matrices and Chaos in Nuclear Physics
• Current Status of r-Process Nucleosynthesis
• Reference Database for Photon Strength Functions
• Analysis of possible systematic errors in the Oslo method
• Novel technique for constraining r-process (n,$\gamma$) reaction rates
• The sign of the overlap of HFB wave functions
• Spin projection in the shell model Monte Carlo method and the spin distribution of nuclear level densities
• Nuclear level statistics: extending the shell model theory to higher temperatures
• Novel Techniques for Constraining Neutron-Capture Rates Relevant for r-Process Heavy-Element Nucleosynthesis
• Verification of detailed balance for $\gamma$ absorption and emission in Dy isotopes
• Heavy deformed nuclei in the shell model Monte Carlo method
• Nonaxial-octupole effect in superheavy nuclei
• Atlas of Nuclear Isomers -- Second Edition
• Crossover from vibrational to rotational collectivity in heavy nuclei in the shell-model Monte Carlo approach
• Experimental level densities of atomic nuclei
• Test of nuclear level density inputs for Hauser-Feshbach model calculations
• Multi-phonon $\gamma$-vibrational bands in odd-mass nuclei studied by triaxial projected shell model approach
• A High-Performance Fortran Code to Calculate Spin- and Parity-Dependent Nuclear Level Densities
• Independent Normalization for $\gamma$-ray Strength Functions: The Shape Method
• Competition in rotation-alignment between high-j neutrons and protons in transfermium nuclei
• Toward extremes of angular momentum: Application of the Pfaffian algorithm in realistic calculations
• Matrix elements of one-body and two-body operators between arbitrary HFB multi-quasiparticle states
• Stochastic Estimation of Nuclear Level Density in the Nuclear Shell Model: An Application to Parity-Dependent Level Density in ${}^{58}$Ni
• Superdeformed Band in ^{36}Ar Described by Projected Shell Model
• Entropy of thermal quasiparticles in nuclei
• gamma-vibrational states in superheavy nuclei
• New Bardeen-Cooper-Schrieffer-type theory at finite temperature with particle-number conservation
• Microscopic calculations of nuclear level densities with the Lanczos method
• Multi-Shell Shell Model for Heavy Nuclei
• A new software implementation of the Oslo method with rigorous statistical uncertainty propagation
• Parity-Dependence in the Nuclear Level Density

Cited by in corpus (2)

• Inference of Parameters for Back-shifted Fermi Gas Model using Feedback Neural Network
• Nuclear level density studied in odd-mass nuclei in the framework of the projected shell model