91 research outputs found
Neutrinoless decay nuclear matrix elements in an isotopic chain
We analyze nuclear matrix elements (NME) of neutrinoless double beta decay
calculated for the Cadmium isotopes. Energy density functional methods
including beyond mean field effects such as symmetry restoration and shape
mixing are used. Strong shell effects are found associated to the underlying
nuclear structure of the initial and final nuclei. Furthermore, we show that
NME for two-neutrino double beta decay evaluated in the closure approximation,
, display a constant proportionality with respect to
the Gamow-Teller part of the neutrinoless NME, . This
opens the possibility of determining the matrix
elements from Gamow-Teller strength functions. Finally, the
interconnected role of deformation, pairing, configuration mixing and shell
effects in the NMEs is discussed
Systematic study of infrared energy corrections in truncated oscillator spaces
We study the convergence properties of nuclear binding energies and
two-neutron separation energies obtained with self-consistent mean-field
calculations based on the Hartree-Fock-Bogolyubov (HFB) method with Gogny-type
effective interactions. Owing to lack of convergence in a truncated working
basis, we employ and benchmark one of the recently proposed infrared energy
correction techniques to extrapolate our results to the limit of an infinite
model space. We also discuss its applicability to global calculations of
nuclear masses.Comment: 12 pages, 12 figure
Sensitivity study of explosive nucleosynthesis in type Ia supernovae: Modification of individual thermonuclear reaction rates
Background: Type Ia supernovae contribute significantly to the nucleosynthesis of many Fe-group and
intermediate-mass elements. However, the robustness of nucleosynthesis obtained via models of this class of
explosions has not been studied in depth until now.
Purpose: We explore the sensitivity of the nucleosynthesis resulting from thermonuclear explosions of massive
white dwarfs with respect to uncertainties in nuclear reaction rates. We put particular emphasis on indentifying
the individual reactions rates that most strongly affect the isotopic products of these supernovae.
Method: We have adopted a standard one-dimensional delayed detonation model of the explosion of a
Chandrasekhar-mass white dwarf and have postprocessed the thermodynamic trajectories of every mass shellwith
a nucleosynthetic code to obtain the chemical composition of the ejected matter. We have considered increases
(decreases) by a factor of 10 on the rates of 1196 nuclear reactions (simultaneously with their inverse reactions),
repeating the nucleosynthesis calculations after modification of each reaction rate pair. We have computed as
well hydrodynamic models for different rates of the fusion reactions of 12C and of 16O. From the calculations we
have selected the reactions that have the largest impact on the supernova yields, and we have computed again
the nucleosynthesis using two or three alternative prescriptions for their rates, taken from the JINA REACLIB
database. For the three reactions with the largest sensitivity we have analyzed as well the temperature ranges
where a modification of their rates has the strongest effect on nucleosynthesis.
Results: The nucleosynthesis resulting from the type Ia supernova models is quite robust with respect to variations
of nuclear reaction rates,with the exception of the reaction of fusion of two 12C nuclei. The energy of the explosion
changes by less than ∼4% when the rates of the reactions 12C + 12C or 16O + 16O are multiplied by a factor of
×10 or ×0.1. The changes in the nucleosynthesis owing to the modification of the rates of these fusion reactions
are also quite modest; for instance, no species with a mass fraction larger than 0.02 experiences a variation of
its yield larger than a factor of 2. We provide the sensitivity of the yields of the most abundant species with
respect to the rates of the most intense reactions with protons, neutrons, and α. In general, the yields of Fe-group
nuclei are more robust than the yields of intermediate-mass elements. Among the species with yields larger than
10−8M , 35S has the largest sensitivity to the nuclear reaction rates. It is remarkable that the reactions involving
elements with Z > 22 have a tiny influence on the supernova nucleosynthesis. Among the charged-particle
reactions, the most influential on supernova nucleosynthesis are 30Si + p 31P + γ , 20Ne + α 24Mg + γ ,
and 24Mg + α 27Al + p. The temperatures at which a modification of their rate has a larger impact are in the
range 2 T 4 GK.Postprint (published version
Energy density functional study of nuclear matrix elements for neutrinoless decay
We present an extensive study of nuclear matrix elements (NME) for the
neutrinoless double beta decay of the nuclei Ca, Ge, Se,
Zr, Mo, Cd, Sn, Te, Te,
Xe, and Nd based on state-of-the-art energy density functional
methods using the Gogny D1S functional. Beyond mean-field effects are included
within the generating coordinate method with particle number and angular
momentum projection for both initial and final ground states. We obtain a
rather constant value for the NME's around 4.7 with the exception of Ca
and Nd, where smaller values are found. We analyze the role of
deformation and pairing in the evaluation of the NME and present detailed
results for the decay of Nd.Comment: accepted in Phys. Rev. Let
Shell Model Applications in Nuclear Astrophysics
In recent years, shell model studies have significantly contributed in improving the nuclear input, required in simulations of the dynamics of astrophysical objects and their associated nucleosynthesis. This review highlights a few examples such as electron capture rates and neutrino-nucleus cross sections, important for the evolution and nucleosynthesis of supernovae. For simulations of rapid neutron-capture (r-process) nucleosynthesis, shell model studies have contributed to an improved understanding of half lives of neutron-rich nuclei with magic neutron numbers and of the nuclear level densities and γ-strength functions that are both relevant for neutron capture rates
On the nuclear robustness of the r process in neutron-star mergers
We have performed r-process calculations for matter ejected dynamically in
neutron star mergers based on a complete set of trajectories from a
three-dimensional relativistic smoothed particle hydrodynamic simulation. Our
calculations consider an extended nuclear network, including spontaneous,
- and neutron-induced fission and adopting fission yield distributions
from the ABLA code. We have studied the sensitivity of the r-process abundances
to nuclear masses by using different models. Most of the trajectories,
corresponding to 90% of the ejected mass, follow a relatively slow expansion
allowing for all neutrons to be captured. The resulting abundances are very
similar to each other and reproduce the general features of the observed
r-process abundance (the second and third peaks, the rare-earth peak and the
lead peak) for all mass models as they are mainly determined by the fission
yields. We find distinct differences in the abundance yields at and just above
the third peak, which can be traced back to different predictions of neutron
separation energies for r-process nuclei around neutron number . The
remaining trajectories, which contribute 10% by mass to the total integrated
abundances, follow such a fast expansion that the r process does not use all
the neutrons. This also leads to a larger variation of abundances among
trajectories as fission does not dominate the r-process dynamics. The total
integrated abundances are dominated by contributions from the slow abundances
and hence reproduce the general features of the observed r-process abundances.
We find that at timescales of weeks relevant for kilonova light curve
calculations, the abundance of actinides is larger than the one of lanthanides.
Hence actinides can be even more important than lanthanides to determine the
photon opacities under kilonova conditions. (Abridged)Comment: 17 pages, 7 figures, resubmitted to PRC addressing referee comment
Testing the importance of collective correlations in neutrinoless ββ decay
We investigate the extent to which theories of collective motion can capture the physics that determines the nuclear matrix elements governing neutrinoless double-β decay. To that end we calculate the matrix elements for a series of isotopes in the full pf shell, omitting no spin-orbit partners. With the inclusion of isoscalar pairing, a separable collective Hamiltonian that is derived from the shell model effective interaction reproduces the full shell-model matrix elements with good accuracy. A version of the generator coordinate method that includes the isoscalar pairing amplitude as a coordinate also reproduces the shell model results well, an encouraging result for theories of collective motion, which can include more single-particle orbitals than the shell model. We briefly examine heavier nuclei relevant for experimental double-β decay searches, in which shell-model calculations with all spin-orbit partners are not feasible; our estimates suggest that isoscalar pairing also plays a significant role in these nuclei, though one we are less able to quantify precisely.This work was supported in part by an International Research Fellowship from the Japan Society for the Progress of Science (JSPS), and JSPS KAKENHI Grant No. 26·04323, by the Deutsche Forschungsgemeinschaft through Contract No. SFB 634, by the Helmholtz
Association through the Helmholtz Alliance Program, Contract No. HA216/EMMI “Extremes of Density and Temperature: Cosmic Matter in the Laboratory”, by the European Research Council under Grant No. 307986 STRONGINT, by the U.S. Department of Energy through Contract No. DE-FG02-97ER41019, and by the Spanish
MINECO under Programa Ramón y Cajal 11420 and FIS-2014-53434-
Impact of pions on binary neutron star mergers
We study the impact of pions in simulations of neutron star mergers and
explore the impact on gravitational-wave observables. We model charged and
neutral pions as a non-interacting Boson gas with a chosen, constant effective
mass. We add the contributions of pions, which can occur as a condensate or as
a thermal population, to existing temperature and composition dependent
equations of state. Compared to the models without pions, the presence of a
pion condensate decreases the characteristic properties of cold, non-rotating
neutron stars such as the maximum mass, the radius and the tidal deformability.
We conduct relativistic hydrodynamical simulations of neutron star mergers for
these modified equations of state models and compare to the original models,
which ignore pions. Generally, the inclusion of pions leads to a softening of
the equation of state, which is more pronounced for smaller effective pion
masses. We find a shift of the dominant postmerger gravitational-wave frequency
by up to 150~Hz to higher frequencies and a reduction of the threshold binary
mass for prompt black-hole formation by up to 0.07~. We evaluate
empirical relations between the threshold mass or the dominant postmerger
gravitational-wave frequency and stellar parameters of nonrotating neutron
stars. These relations are constructed to extract these stellar properties from
merger observations and are built based on large sets of equation of state
models which do not include pions. Comparing to our calculations with pions, we
find that these empirical relations remain valid to good accuracy, which
justifies their use although they neglect a possible impact of pions. We also
address the mass ejection by neutron star mergers and observe a moderate
enhancement of the ejecta mass by a few ten per cent. (abridged)Comment: 27 pages, 24 figures, accepted for publication in Phys. Rev.
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