[Show abstract][Hide abstract] ABSTRACT: The properties of loosely bound proton-rich nuclei around A = 20 are
investigated within the framework of nuclear shell model. In these nuclei, the
strength of the effective interactions involving the loosely bound proton s1=2
orbit are significantly reduced in comparison with those in their mirror
nuclei. We evaluate the reduction of the effective interaction by calculating
the monopole-baseduniversal interaction (VMU) in the Woods-Saxon basis. The
shell-model Hamiltonian in the sd shell, such as USD, can thus be modified to
reproduce the binding energies and energy levels of the weakly bound
proton-rich nuclei around A = 20. The effect of the reduction of the effective
interaction on the structure and decay properties of these nuclei is also
[Show abstract][Hide abstract] ABSTRACT: Remarkable improvements in the evaluation of neutrino-nucleus reaction cross sections are obtained based on new shell-model Hamiltonians with proper tensor components. New ν-induced reaction cross sections on 12C, 13C, 56Fe, 56Ni and 40Ar are presented, and predictions for nucleosynthesis in supernova explosions, ν-oscillation effects and low-energy reactor and solar neutrino detection are discussed based on these new cross sections.
[Show abstract][Hide abstract] ABSTRACT: We calculate β-transition rates for URCA nuclear pairs using the sd-shell-model framework of Wildenthal [ B. H. Widenthal Prog. Part. Nucl. Phys. 11 5 (1984)] with the modified Hamiltonian. We consider many β transitions including excited states necessary for electron capture and β decay with high Fermi energy of electrons at high electron density ρYe and temperature T relevant for the massive star and super-asymptotic-giant-branch star evolution. We provide β-transition rates for URCA nuclear pairs (A=23, 25, and 27) in the range of density 8.0 <log10ρYe< 9.2 in steps of 0.02 (Ye is the electron mole number and ρ is the nucleon density in units of g cm−3) and temperature 8.0 <log10T< 9.2 in steps of 0.05 (T is the temperature in units of K). This fine mesh is able to provide clearly the URCA density at log10ρYe=8.92 for A=23 and at log10ρYe=8.81 for A=25, while the URCA density is not clear for A=27. For the evolution of 8–10M⊙ stars, the use of the fine mesh is found to increase the cooling effect and affects the final fate of these stars.
Physical Review C 07/2013; 88(1). · 3.72 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Neutrino–nucleus reaction processes play important roles in the synthesis of 7Li, 11B, 138La, 180Ta and other elements in explosive environments realized in supernovae. We have constructed new shell-model Hamiltonians based on recent progress in the physics of exotic nuclei. The shell evolutions toward drip-lines and the spin responses of nuclei are found to be remarkably improved by the new Hamiltonians. Proper tensor components in the Hamiltonians are shown to be important for the improvements. We then applied these Hamiltonians to calculate the neutrino–nucleus reaction cross sections, and obtained more precise theoretical estimates of supernova nucleosynthesis of 7Li, 11B and 55Mn etc, including the neutrino processes. Finally, we propose a new method for determining the neutrino oscillation parameter, the mixing angle θ13, and the mass hierarchy, by making use of the strong dependence of the yields of 7Li and 11B on these parameters.
Journal of Physics G Nuclear and Particle Physics 06/2013; 40(8):083101. · 5.33 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The stellar mass range 8<M/Mo<12 corresponds to the most massive AGB stars
and the most numerous massive stars. It is host to a variety of supernova
progenitors and is therefore very important for galactic chemical evolution and
stellar population studies. In this paper, we study the transition from
super-AGB star to massive star and find that a propagating neon-oxygen burning
shell is common to both the most massive electron capture supernova (EC-SN)
progenitors and the lowest mass iron-core collapse supernova (FeCCSN)
progenitors. Of the models that ignite neon burning off-center, the 9.5Mo model
would evolve to an FeCCSN after the neon-burning shell propagates to the
center, as in previous studies. The neon-burning shell in the 8.8Mo model,
however, fails to reach the center as the URCA process and an extended (0.6 Mo)
region of low Ye (0.48) in the outer part of the core begin to dominate the
late evolution; the model evolves to an EC-SN. This is the first study to
follow the most massive EC-SN progenitors to collapse, representing an
evolutionary path to EC-SN in addition to that from SAGB stars undergoing
thermal pulses. We also present models of an 8.75Mo super-AGB star through its
entire thermal pulse phase until electron captures on 20Ne begin at its center
and of a 12Mo star up to the iron core collapse. We discuss key uncertainties
and how the different pathways to collapse affect the pre-supernova structure.
Finally, we compare our results to the observed neutron star mass distribution.
The Astrophysical Journal 06/2013; 772(2). · 6.73 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Gamow-Teller (GT) strength in 40Ar is studied by shell-model calculations with monopole-based universal interaction, which has tensor components of π+ρ-meson exchanges. Calculated GT strength is found to be consistent with the experimental data obtained by recent (p,n) reactions. Neutrino capture cross sections on 40Ar for solar neutrinos from 8B are found to be enhanced compared with previous calculations. The reaction cross sections for multipoles other than 0+ and 1+ are obtained by random-phase approximation (RPA). Their contributions become important for neutrino energies larger than 50 MeV.
Physical Review C 01/2013; 87(1). · 3.72 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Beta decays of N=126 isotones are studied by shell model calculations by
including both the Gamow-Teller (GT) and first-forbidden (FF)
transitions. The half-lives of the isotones are found to be reduced
considerably by the the contributions from the FF transitions, and short
compared to the standard half-lives usually used for the nucleosynthesis
network calculations. Possible implications of the short half-lives of
the waiting point nuclei on the r-process nucleosynthesis during the
supernova explosions are discussed.
[Show abstract][Hide abstract] ABSTRACT: Spin-dependent modes in nuclei are studied and important roles of tensor
and three-body forces on nuclear structure are discussed. New shell
model Hamiltonians, which have proper tensor components, are shown to
explain spin properties of both stable and exotic nuclei. Gamow-Teller
(GT) strengths in Ni isotopes, especially in 56Ni, are found
to be well described by pf-shell Hamiltonian GXPF1J, which leads to a
remarkable improvement in the evaluation of electron capture rates in
stellar environmnets. GT strength in 40Ar obtained with VMU
(monopole-based universal interaction) is found to be consistent with
the experimental strength, and neutrino capture reaction cross sections
for solar neutrinos from 8B are found to be enhanced compared
with previous calculations. The repulsive monopole corrections to the
microscopic two-body interactions in isospin T=1 channel are important
for the proper shell evolutions in neutron-rich isotopes. The three-body
force, in particular the Fujita-Miyazawa force induced by δ
excitations, is pointed out to be responsible for the repulsive
corrections among the valence neutrons. The important roles of the
three-body force on the energies of exotic calcium isotopes as well as
on the closed-shell nature of 48Ca and M1 transition in
48Ca are demonstrated.
[Show abstract][Hide abstract] ABSTRACT: Nuclear weak processes are investigated based on new shell model
Hamiltonians, which give successful description of spin responses in
nuclei, and applied to astrophysical problems. Neutrino-induced
reactions on 12C and synthesis of light elements by supernova
neutrinos, and effects of contamination of 13C, whose natural
isotopic abundance is 1.1%, on inclusive ν-12C reactions
are discussed. Spin-dipole transitions and ν-induced reactions on
16O are studied by using a new Hamiltonian with proper tensor
components, and compared with conventional calculations and previous
CRPA results. Gamow-Teller transition strength in 40Ar and
ν-induced reactions on 40Ar by solar neutrinos are studied
based on monopole-based-universal interaction (VMU). We finally discuss
electron capture reactions on Ni isotopes in stellar environments.
Journal of Physics Conference Series 09/2012; 381(1):2118-.
[Show abstract][Hide abstract] ABSTRACT: We study boron, carbon, nitrogen, and oxygen isotopes with a newly constructed shell-model Hamiltonian developed from a monopole-based universal interaction (VMU). The present Hamiltonian can reproduce well the ground-state energies, energy levels, electric quadrupole properties, and spin properties of these nuclei in full psd model space including (0−3)ℏω excitations. Especially, it correctly describes the drip lines of carbon and oxygen isotopes and the spins of the ground states of 10B and 18N while some former interactions such as WBP and WBT fail. We point out that the inclusion of 2ℏω excitations is important in reproducing some of these properties. In the present (0+2)ℏω calculations small but constant E2 effective charges appear to work quite well. As the inclusion of the 2ℏω model space makes a rather minor change, this seems to be related to the smallness of the 4He core. Similarly, the spin g factors are very close to free values. The applicability of tensor and spin-orbit forces in free space, which are taken in the present Hamiltonian, is examined in shell-model calculations.
Physical Review C 06/2012; 85(6). · 3.72 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We investigate the effects of newly measured beta-decay half-lives on
r-process nucleosynthesis. These new rates were determined by recent
experiments at the radioactive isotope beam factory facility in the RIKEN
Nishina Center. We adopt an r-process nucleosynthesis environment based on a
magnetohydrodynamic supernova explosion model that includes strong magnetic
fields and rapid rotation of the progenitor. A number of the new beta-decay
rates are for nuclei on or near the r-process path, and hence they affect the
nucleosynthesis yields and time scale of the r-process. The main effect of the
newly measured beta-decay half-lives is an enhancement in the calculated
abundance of isotopes with mass number A = 110 -- 120 relative to calculated
abundances based upon beta-decay rates estimated with the finite-range droplet
mass model. This effect slightly alleviates, but does not fully explain, the
tendency of r-process models to underproduce isotopes with A = 110 -- 120
compared to the solar-system r-process abundances.
Physical Review C 03/2012; 85(4). · 3.72 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Neutrino‐nucleus reactions at MeV region are evaluated by using new shell model Hamiltonians, which are found to improve the description of spin degrees of freedom in nuclei as well as shell evolutions. The light element synthesis by neutrino processes in supernova explosions is discussed with the use of the new reaction cross sections for 12C and 4He. Possible constraints on the mixing angle θ13 and the neutrino mass hierarchy are pointed out to be imposed by the production yields of 7Li and 11B. Spin‐dipole transition strengths in 16O and neutrino‐induced reactions on 16O are investigated with the new improved Hamiltonian. The Gamow‐Teller transition strength in 56Ni for the new Hamiltonian, which is more fragmented compared to previous calculations, is found to lead to the enhancement of the production yield of 55Mn in population III stars.
[Show abstract][Hide abstract] ABSTRACT: Beta decays of the isotones with N=126 are studied by shell model
calculations taking into account both the Gamow-Teller (GT) and first-forbidden
(FF) transitions. The FF transitions are found to be important to reduce the
half-lives, by nearly twice to several times, from those by the GT
contributions only. Possible implications of the short half-lives of the
waiting point nuclei on the r-process nucleosynthesis during the supernova
explosions are discussed. A slight shift of the third peak of the element
abundances in the r-process toward higher mass region is found.
[Show abstract][Hide abstract] ABSTRACT: Spin modes in stable and unstable exotic nuclei are studied and important roles of tensor and three-body forces on nuclear structure are discussed. New shell model Hamiltonians, which have proper tensor components, are shown to explain shell evolutions toward drip-lines and spin properties of both stable and exotic nuclei, for example, Gamow-Teller transitions in 12C and 14C and an anomalous M1 transition in 17C. The importance and the necessity of the repulsive monopole corrections in isospin T = 1 channel to the microscopic two-body interactions are pointed out. The corrections are shown to lead to the proper shell evolutions in neutron-rich isotopes. The three-body force, in particular the Fujita-Miyazawa force induced by Δ excitations, is pointed out to be responsible for the repulsive corrections among the valence neutrons. The important roles of the three-body force on the energies and transitions in exotic oxygen and calcium isotopes are demonstrated.
[Show abstract][Hide abstract] ABSTRACT: Electron capture rates in Ni isotopes are studied in stellar environments, that is, at high densities and high temperatures during the core-collapse and postbounce explosive nucleosynthesis in supernovae. Reaction rates in 58Ni and 60Ni, as well as in 56Ni, 62Ni, and 64Ni, are evaluated by shell-model calculations with the use of a new shell-model Hamiltonian in the fp shell, GXPF1J. While the previous shell-model calculations failed to reproduce the measured peaks of Gamow-Teller strength in 58Ni and 60Ni, the present new Hamiltonian is found to reproduce them very well, as well as the capture rates obtained from the observed strengths. Strengths and energies of the Gamow-Teller transitions in 56Ni, 62Ni, and 64Ni are also found to be consistent with the observations.
Physical Review C 04/2011; 83(4). · 3.72 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Spin dependent nuclear weak processes and nucleosynthesis in stars are investigated based on recent advances in shell model studies of stable and unstable exotic nuclei. Three topics on (1) neutrino–nucleus reactions in supernova explosions and nucleosynthesis of light elements as well as Mn, (2) electron capture reaction rates on Ni and Co isotopes at high densities and temperatures in the core-collapse process, and (3) new β-decay half-lives of N=126 isotones obtained by including both the Gamow–Teller and the first-forbidden transitions, and the effects on the element abundance in the r-process at the third peak region (A∼195), are studied with the use of new shell model Hamiltonians.
Progress in Particle and Nuclear Physics 01/2011; 66(2):385-389. · 2.26 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Gamow-Teller (GT) and first-forbidden (FF) transitions in nuclei and their possible consequences on astrophysical processes in stars are discussed. GT transition strengths in fp-shell nuclei are studied by shell model calculations with the use of new shell model Hamiltonians, GXPF1. Neutrino-nucleus reaction cross sections are re-evaluated and compared with previous investigations. Proton emission cross sections on 56Ni are pointed out to be enhanced and lead to the enhancement of the production yields of 55Mn and 59Co in population III supernovae. Next, electron capture reactions on Ni and Fe isotopes in steller environments are studied. The capture rates depend sensitively on the distributions of the GT strengths. Capture rates on 58Ni and 60Ni obtained by GXPF1 are found to reproduce rather well those obtained from experimental GT strengths. Finally, beta decays of the N = 126 isotones are studied by shell model calculations taking into account both the GT and FF transitions. The FF transitions are found to be important to reduce the half-lives by twice to several times of those by the GT contributions only. Possible implications on the short half-lives of the waiting point nuclei on the r-process nucleosynthesis are discussed for certain astrophysical conditions.
Journal of Physics Conference Series 01/2011; 312(4).
[Show abstract][Hide abstract] ABSTRACT: Understanding and predicting the formation of shell structure from nuclear
forces is a central challenge for nuclear physics. While the magic numbers
N=2,8,20 are generally well understood, N=28 is the first standard magic number
that is not reproduced in microscopic theories with two-nucleon forces. In this
Letter, we show that three-nucleon forces give rise to repulsive interactions
between two valence neutrons that are key to explain 48Ca as a magic nucleus,
with a high 2+ excitation energy and a concentrated magnetic dipole transition
strength. The repulsive three-nucleon mechanism improves the agreement with
experimental binding energies.
Journal of Physics G Nuclear and Particle Physics 09/2010; 39(8). · 5.33 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Beta decays of N = 126 isotones are studied by shell model calculations. Both the Gamow-Teller (GT) and first-forbidden (FF) transitions are taken into account to evaluate the half-lives of the isotones (Z = 64-72) with the use of shell model interactions based on G-matrix. The FF transitions are found to be important to reduce the half-lives by twice to several times of those obtained by the GT contributions only. Possible implications of the short half-lives of the waiting point nuclei on the r-process nucleosynthesis during the supernova explosions are discussed.