arXiv subjects

B. Alex Brown

Publications and source records attributed to B. Alex Brown.

At least 37 records · Page 2Linked to original sources

arXiv2013

Constraints on Skyrme Equations of State from Properties of Doubly Magic Nuclei and Ab-Initio Calculations of Low-Density Neutron Matter

We use properties of doubly-magic nuclei and ab-initio calculations of low-density neutron matter to constrain Skyrme equations of state for neutron-rich conditions. All of these properties are consistent with a Skyrme functional form and a neutron-matter equation of state that depends on three parameters. With a reasonable range for the neutron-matter effective mass, the values of the two other Skyrme parameters are well constrained. This leads to predictions for other quantities. The neutron skins for $^{208}$Pb and $^{48}$Ca are predicted to be 0.182(10) fm and 0.173(5) fm, respectively. Other results including the dipole polarizability are discussed.

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arXiv2013

The Nuclear Pairing Gap -- How Low Can It Go?

The pairing gap for $^{53}$Ca obtained from new experimental data on the masses of $^{52-54}$Ca has the smallest value yet observed. This is explained in the framework of the nuclear shell model with schematic and realistic Hamiltonians as being due to shell gaps around the low-$ j $ orbital $ 1p_{1/2} $. Minima in the pairing gaps for all nuclei are shown and discussed

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arXiv2013

Constraints on the Skyrme Equations of State from Properties of Doubly Magic Nuclei

I use properties of doubly-magic nuclei to constrain nuclear matter and neutron matter equations of state. I conclude that the data determined the value of the neutron equation of state and the symmetry energy near a density of $ρ_{on}$ = 0.10 nucleons/fm$^{3}$. The slope at that point is constrained by the value of the neutron skin.

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arXiv2013

Investigating $β$-decay properties of spherical nuclei along the possible r-process path

The spherical QRPA method is used for the calculations of the $β$-decay properties of the neutron-rich nuclei in the region near the neutron magic numbers N=82 and N=126 which are important for determination of the r-process path. Our calculations differ from previous works by the use of realistic forces for the proton-neutron interaction. Both the allowed and first-forbidden $β$-decays are included. Detailed comparisons with the experimental measurements and the previous shell-model calculations are performed. The results for half-lives and beta-delayed neutron emission probabilities will serve as input for the r-process nucleosynthesis simulations.

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arXiv2013

Neutrino Pair Emission from Hot Nuclei During Stellar Collapse

We present shell-model calculations showing that residual interaction-induced configuration mixing enhances the rate of neutral current de-excitation of thermally excited nuclei into neutrino-antineutrino pairs. Though our calculations reinforce the conclusions of previous studies that this process is the dominant source of neutrino pairs near the onset of neutrino trapping during stellar collapse, our shell-model result has the effect of increasing the energy of these pairs, possibly altering their role in entropy transport in supernovae.

astro-ph.HE↗

arXiv2012

$β$-decay properties for neutron-rich Kr-Tc isotopes from deformed pn-QRPA calculations with realistic forces

In this work we studied $β$-decay properties for deformed neutron-rich nuclei in the region Z=36-43. We use the deformed pn-QRPA methods with the realistic CD-Bonn forces, and include both the Gamow-Teller and first-forbidden types of decays in the calculation. The obtained $β$-decay half-lives and neutron-emission probabilities of deformed isotopes are compared with experiment as well as with previous calculations. The advantages and disadvantages of the method are discussed.

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arXiv2012

Shape transitions in exotic Si and S isotopes and tensor-force-driven Jahn-Teller effect

We show how shape transitions in the neutron-rich exotic Si and S isotopes occur in terms of shell-model calculations with a newly constructed Hamiltonian based on V_MU interaction. We first compare the calculated spectroscopic-strength distributions for the proton 0d_5/2,3/2 and 1s_1/2 orbitals with results extracted from a 48Ca(e,e'p) experiment to show the importance of the tensor-force component of the Hamiltonian. Detailed calculations for the excitation energies, B(E2) and two-neutron separation energies for the Si and S isotopes show excellent agreement with experimental data. The potential energy surface exhibits rapid shape transitions along the isotopic chains towards N=28 that are different for Si and S. We explain the results in terms of an intuitive picture involving a Jahn-Teller-type effect that is sensitive to the tensor-force-driven shell evolution. The closed sub-shell nucleus 42Si is a particularly good example of how the tensor-force-driven Jahn-Teller mechanism leads to a strong oblate rather than spherical shape.

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arXiv2012

Tensor-force-driven Jahn-Teller effect and shape transitions in exotic Si isotopes

We show how the shape evolution of the neutron-rich exotic Si and S isotopes can be understood as a Jahn-Teller effect that comes in part from the tensor-driven evolution of single-particle energies. The detailed calculations we present are in excellent agreement with known experimental data, and we point out of new features that should be explored in new experiments. Potential energy surfaces are used to understand the shape evolutions. The sub-shell closed nucleus, $^{42}$Si, is shown to be a perfect example of a strongly oblate shape instead of a sphere through a robust Jahn-Teller mechanism. The distribution of spectroscopic factors measured by $^{48}$Ca(e,e'p) experiment is shown to be well described, providing a unique test on the tensor-driven shell evolution.

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arXiv2011

Effects of isospin mixing in the A=32 quintet

For the A=32 T=2 quintet we provide a unified theoretical description for three related aspects of isospin mixing: the necessity of more than three terms in the isobaric mass multiplet equation, isospin-forbidden proton decay, and a correction to the allowed Fermi beta decay. We demonstrate for the first time that all three effects observed in experiment can be traced to a common origin related to isospin mixing of the T=2 states with T=1 states.

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arXiv2011

Quenching of spectroscopic factors for proton removal in oxygen isotopes

We present microscopic coupled-cluster calculations of the spectroscopic factors for proton removal from the closed-shell oxygen isotopes $^{14,16,22,24,28}$O with the chiral nucleon-nucleon interaction at next-to-next-to-next-to-leading order. We include coupling-to-continuum degrees of freedom by using a Hartree-Fock basis built from a Woods-Saxon single-particle basis. This basis treats bound and continuum states on an equal footing. We find a significant quenching of spectroscopic factors in the neutron-rich oxygen isotopes, pointing to enhanced many-body correlations induced by strong coupling to the scattering continuum above the neutron emission thresholds.

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arXiv2010

Renormalized interactions with a realistic single particle basis

Neutron-rich isotopes in the sdpf space with Z < 15 require modifications to derived effective interactions to agree with experimental data away from stability. A quantitative justification is given for these modifications due to the weakly bound nature of model space orbits via a procedure using realistic radial wavefunctions and realistic NN interactions. The long tail of the radial wavefunction for loosely bound single particle orbits causes a reduction in the size of matrix elements involving those orbits, most notably for pairing matrix elements, resulting in a more condensed level spacing in shell model calculations. Example calculations are shown for 36Si and 38Si.

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arXiv2010

Configuration Interactions Constrained by Energy Density Functionals

A new method for constructing a Hamiltonian for configuration interaction calculations with constraints to energies of spherical configurations obtained with energy-density-functional (EDF) methods is presented. This results in a unified model that reproduced the EDF binding-energy in the limit of single-Slater determinants, but can also be used for obtaining energy spectra and correlation energies with renormalized nucleon-nucleon interactions. The three-body and/or density-dependent terms that are necessary for good nuclear saturation properties are contained in the EDF. Applications to binding energies and spectra of nuclei in the region above 208Pb are given.

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arXiv2009

Nuclei in the Cosmos

This white paper, directed to the Stars and Stellar Evolution panel, has three objectives: 1) to provide the Astro2010 Decadal Survey with a vista into the goals of the nuclear physics and nuclear astrophysics community; 2) to alert the astronomical community of joint opportunities for discoveries at the interface between nuclear physics and astronomy; and 3) to delineate efforts in nuclear physics and describe the observational and theoretical advances in astrophysics necessary to make progress towards answering the following questions in the Nuclear Science 2007 Long Range Plan: 1) What is the origin and distribution of the elements? 2) What are the nuclear reactions that power stars and stellar explosions? 3) What is the nature of dense matter? The scope of this white paper concerns the specific area of "low energy" nuclear astrophysics. We define this as the area of overlap between astrophysics and the study of nuclear structure and reactions. Of the questions listed above, two -- What is the origin of the elements? and What is the nature of dense matter? -- were specifically listed in the National Academies Study, "Connecting Quarks with the Cosmos".

astro-ph.SR↗

arXiv2004

New effective interaction for $pf$-shell nuclei and its implications for the stability of the $N$=$Z$=28 closed core

The effective interaction GXPF1 for shell-model calculations in the full $pf$ shell is tested in detail from various viewpoints such as binding energies, electro-magnetic moments and transitions, and excitation spectra. The semi-magic structure is successfully described for $N$ or Z=28 nuclei, $^{53}$Mn, $^{54}$Fe, $^{55}$Co and $^{56,57,58,59}$Ni, suggesting the existence of significant core-excitations in low-lying non-yrast states as well as in high-spin yrast states. The results of $N=Z$ odd-odd nuclei, $^{54}$Co and $^{58}$Cu, also confirm the reliability of GXPF1 interaction in the isospin dependent properties. Studies of shape coexistence suggest an advantage of Monte Carlo Shell Model over conventional calculations in cases where full-space calculations still remain too large to be practical.

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arXiv2001

Coherent and chaotic properties of nuclear pairing

The properties of the pairing interaction in the shell model framework are considered with the aid of the exact numerical solution utilizing the quasispin symmetry. We emphasize the features which are out of reach for the usual approximate techniques based on the BCS approach supplemented by the random phase approximation treatment of pair vibrations, especially in the region of weak pairing where the BCS+RPA theory fails. For the first time chaotic aspects of the mixing generated by the pairing interaction are studied. The level repulsion and large information entropy of the eigenstates coexist with the absence of thermalization of single-particle motion. The full spectrum of pair vibration in average displays the spin dependence similar to that for a rigid rotor.

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arXiv2001

Magic numbers in exotic nuclei and spin-isospin properties of {\it NN} interaction

The magic numbers in exotic nuclei are discussed, and their novel origin is shown to be the spin-isospin dependent part of the nucleon-nucleon interaction in nuclei. The importance and robustness of this mechanism is shown in terms of meson exchange, G-matrix and QCD theories. In neutron-rich exotic nuclei, magic numbers such as N = 8, 20, etc. can disappear, while N = 6, 16, etc. arise, affecting the structure of lightest exotic nuclei to nucleosynthesis of heavy elements.

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arXiv2001

Exact solution of the nuclear pairing problem

In many applications to finite Fermi-systems, the pairing problem has to be treated exactly. We suggest a numerical method of exact solution based on SU(2) quasispin algebras and demonstrate its simplicity and practicality. We show that the treatment of binding energies with the use of the exact pairing and uncorrelated monopole contribution of other residual interactions can serve as an effective alternative to the full shell-model diagonalization in spherical nuclei. A self-consistent combination of the exactly treated pairing and Hartree-Fock method is discussed. Results for Sn isotopes indicate a good agreement with experimental data.

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arXiv1997

Invariant Correlation Entropy and Complexity of Quantum States

We define correlational (von Neumann) entropy for an individual quantum state of a system whose time-independent hamiltonian contains random parameters and is treated as a member of a statistical ensemble. This entropy is representation independent and can be calculated as a trace functional of the density matrix which describes the system in its interaction with the noise source. We analyze perturbation theory in order to show the evolution from the pure state to the mixed one. Exactly solvable examples illustrate the use of correlational entropy as a measure of the degree of complexity in comparison with other available suggestions such as basis-dependent information entropy. It is shown in particular that a harmonic oscillator in a uniform field of random strength comes to a quasithermal equilibrium; we discuss the relation between effective temperature and canonical equilibrium temperature. The notion of correlational entropy is applied to a realistic numerical caculation in the framework of the nuclear shell model. In this system, which reveals generic signatures of quantum chaos, correlational entropy and information entropy calculated in the mean field basis display similar qualitative behavior.

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