arXiv subjects

B. K. Agrawal

Publications and source records attributed to B. K. Agrawal.

At least 55 records · Page 3Linked to original sources

arXiv2017

Interdependence of different symmetry energy elements

Relations between the nuclear symmetry energy coefficient and its density derivatives are derived. The relations hold for a class of interactions with quadratic momentum dependence and a power-law density dependence. The structural connection between the different symmetry energy elements as obtained seems to be followed by almost all reasonable nuclear energy density functionals, both relativistic and non-relativistic, suggesting a universality in the correlation structure. This, coupled with known values of some well-accepted constants related to nuclear matter, helps in constraining values of different density derivatives of the nuclear symmetry energy shedding light on the isovector part of the nuclear interaction.

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arXiv2017

Nuclear symmetry energy with mesonic cross-couplings in the effective chiral model

The effective chiral model is extended by introducing the contributions from the cross-couplings between isovector and isoscalar mesons. These cross-couplings are found to be instrumental in improving the density content of the nuclear symmetry energy. The nuclear symmetry energy as well as its slope and curvature parameters at the saturation density are in harmony with those deduced from a diverse set of experimental data. The equation of state for pure neutron matter at sub-saturation densities is also in accordance with the ones obtained from different microscopic models. The maximum mass of neutron star is consistent with the measurement and the radius at the canonical mass of the neutron star is within the empirical bounds.

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arXiv2017

New parameterization of the effective field theory motivated relativistic mean field model

A new parameter set is generated for finite and infinite nuclear system within the effective field theory motivated relativistic mean field (ERMF) formalism. The isovector part of the ERMF model employed in the present study includes the coupling of nucleons to the δ and \r{ho} mesons and the cross-coupling of \r{ho} mesons to the σ and ω mesons. The results for the finite and infinite nuclear systems obtained using our parameter set are in harmony with the available experimental data. We find the maximum mass of the neutron star to be 2.03M\odot? and yet a relatively smaller radius at the canonical mass, 12.69 km, as required by the available data.

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arXiv2017

Warm unstable asymmetric nuclear matter: critical properties and the density dependence of the symmetry energy

The spinodal instabilities in hot asymmetric nuclear matter and some important critical parameters derived thereof are studied using six different families of relativistic mean-field (RMF) models. The slopes of the symmetry energy coefficient vary over a wide range within each family. The critical densities and proton fractions are more sensitive to the symmetry energy slope parameter at temperatures much below its critical value ($T_c\sim$14-16 MeV). The spread in the critical proton fraction at a given symmetry energy slope parameter is noticeably larger near $T_c$, indicating that the warm equation of state of asymmetric nuclear matter at sub-saturation densities is not sufficiently constrained. The distillation effects are sensitive to the density dependence of the symmetry energy at low temperatures which tend to wash out with increasing temperature.

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arXiv2017

Limiting symmetry energy elements from empirical evidence

In the framework of an equation of state (EoS) constructed from a momentum and density-dependent finite-range two-body effective interaction, the quantitative magnitudes of the different symmetry elements of infinite nuclear matter are explored. The parameters of this interaction are determined from well-accepted characteristic constants associated with homogeneous nuclear matter. The symmetry energy coefficient $a_2$, its density slope $L_0$, the symmetry incompressibility $K_δ$ as well as the density dependent incompressibility $K(ρ)$ evaluated with this EoS are seen to be in good harmony with those obtained from other diverse perspectives. The higher order symmetry energy coefficients $a_4,~a_6$ etc are seen to be not very significant in the domain of densities relevant to finite nuclei, but gradually build up at supra-normal densities. The analysis carried with a Skyrme-inspired energy density functional obtained with the same input values for the empirical bulk data associated with nuclear matter yields nearly the same results.

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arXiv2017

Structure effects on fission yields

The structure effects of the fission fragments on their yields are studied within the statical theory with the inputs, like, excitation energies and level density parameters for the fission fragments at a given temperature calculated using the temperature dependent relativistic mean field formalism (TRMF). For the comparison, the results are also obtained using the finite range droplet model. At temperatures $T =1-2$ MeV, the structural effects of the fission fragments influence their yields. It is also seen that at $T = $ 3 MeV, the fragments become spherical and the fragments distribution peaks at a close shell or near close shell nucleus.

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arXiv2016

Ternary fission within the temperature dependent relativistic mean field approach

For the first time, we apply the temperature dependent relativistic mean field (TRMF) model to study the ternary fission of heavy nucleus using level density approach. The probability of yields of a particular fragment is obtained by evaluating the convolution integrals which employ the excitation energy and the level density parameter for a given temperature calculated within the TRMF formalism. To illustrate, we have considered the ternary fissions in 252Cf, 242Pu and 236U with fixed third fragment A3 = 48Ca, 20O and 16O respectively. The relative yields are studied for the temperatures T = 1, 2 and 3 MeV. For the comparison, the relative yields are also calculated from the single particle energies of the finite range droplet model (FRDM). In general, the larger phase space for the ternary fragmentation is observed indicating that such fragmentations are most probable ones. For T = 2 and 3 MeV, the Sn + Ni + Ca is the most probable combination for the nucleus 252Cf. However, for the nuclei 242Pu and 236U, the maximum fragmentation yields at T = 2 MeV differ from those at T = 3 MeV. For T = 3 MeV, the closed shell (Z = 8) light mass fragments with its corresponding partners has larger yield values. But, at T = 2 MeV Si/P/S are favorable fragments with the corresponding partners. It is noticed that the symmetric binary fragmentation along with the fixed third fragment for 242Pu and 236U are also favored at T = 1 MeV. The temperature dependence of the nuclear shape and the single particle energies are also discussed.

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arXiv2016

Strong correlations of neutron star radii with the slopes of nuclear matter incompressibility and symmetry energy at saturation

We examine the correlations of neutron star radii with the nuclear matter incompressibility, symmetry energy, and their slopes, which are the key parameters of the equation of state (EoS) of asymmetric nuclear matter. The neutron star radii and the EoS parameters are evaluated using a representative set of 24 Skyrme-type effective forces and 18 relativistic mean field models, and two microscopic calculations, all describing 2$M_\odot$ neutron stars. Unified EoSs for the inner-crust-core region have been built for all the phenomenological models, both relativistic and non-relativistic. Our investigation shows the existence of a strong correlation of the neutron star radii with the linear combination of the slopes of the nuclear matter incompressibility and the symmetry energy coefficients at the saturation density. Such correlations are found to be almost independent of the neutron star mass in the range $0.6\text{-}1.8M_{\odot}$. This correlation can be linked to the empirical relation existing between the star radius and the pressure at a nucleonic density between one and two times saturation density, and the dependence of the pressure on the nuclear matter incompressibility, its slope and the symmetry energy slope. The slopes of the nuclear matter incompressibility and the symmetry energy coefficients as estimated from the finite nuclei data yield the radius of a $1.4M_{\odot}$ neutron star in the range $11.09\text{-}12.86$ km.

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arXiv2016

Model dependence of the neutron-skin thickness on the symmetry energy

The model dependence in the correlations of the neutron-skin thickness in heavy nuclei with various symmetry energy parameters is analyzed by using several families of systematically varied microscopic mean field models. Such correlations show a varying degree of model dependence once the results for all the different families are combined. Some mean field models associated with similar values of the symmetry energy slope parameter at saturation density $L$, and pertaining to different families, yield a greater-than-expected spread in the neutron-skin thickness of the $^{208}$Pb nucleus. The effective value of the symmetry energy slope parameter $L_{\rm eff}$, determined by using the nucleon density profiles of the finite nucleus and the density derivative $S^\prime(ρ)$ of the symmetry energy starting from about saturation density up to low densities typical of the surface of nuclei, seems to account for the spread in the neutron-skin thickness for the models with similar $L$. The differences in the values of $L_{\rm eff}$ are mainly due to the small differences in the nucleon density distributions of heavy nuclei in the surface region and the behavior of the symmetry energy at subsaturation densities.

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arXiv2016

Sensitivity of elements of the symmetry energy of nuclear matter to the properties of neutron-rich systems

The sensitivity of nuclear symmetry energy elements at the saturation density to the binding energies of ultra neutron-rich nuclei (neutron to proton ratio $\sim$ 2) and the maximum mass of neutron star is explored within a relativistic mean field model. Values of the interaction parameters governing the isovector strengths and the symmetry elements are determined in tighter bounds. Assessments based on the sensitivity matrix reveal that the properties of extreme neutron-rich systems play a predominant role in narrowing down the uncertainties in the various symmetry energy parameters. The calculations are extended over a wide range of nuclear matter density and the results are discussed.

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arXiv2016

Correlation of the neutron star crust-core properties with the slope of the symmetry energy and the lead skin thickness

The correlations of the crust-core transition density and pressure in neutron stars with the slope of the symmetry energy and the neutron skin thickness are investigated, using different families of relativistic mean field parametrizations with constant couplings and non-linear terms mixing the $σ$, $ω$ and $ρ$-meson fields. It is shown that the modification of the density dependence of the symmetry energy, involving the $σ$ or the $ω$ meson, gives rise to different behaviors: the effect of the $ω$-meson may also be reproduced within non-relativistic phenomenological models, while the effect of the $σ$-meson is essentially relativistic. Depending on the parametrization with $σ-ρ$ or $ω-ρ$ mixing terms, different values of the slope of the symmetry energy at saturation must be considered in order to obtain a neutron matter equation of state compatible with results from chiral effective field theory. This difference leads to different pressures at the crust-core transition density. A linear correlation between the transition density and the symmetry energy slope or the neutron skin thickness of the $^{208}$Pb nucleus is obtained, only when the $ω$-meson is used to describe the density dependence of the symmetry energy. A comparison is made between the crust-core transition properties of neutron stars obtained by three different methods, the Relativistic Random Phase Approximation (RRPA), the Vlasov equation and Thermodynamical method. It is shown that the RRPA and the Vlasov methods predict similar transition densities for $pne$ $β$-equilibrium stellar matter.

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arXiv2015

The neutron skin thickness from the measured electric dipole polarizability in $^{68}$Ni, $^{120}$Sn, and $^{208}$Pb

The information on the symmetry energy and its density dependence is deduced by comparing the available data on the electric dipole polarizability $α_D$ of ${}^{68}$Ni, ${}^{120}$Sn, and ${}^{208}$Pb with the predictions of the Random Phase Approximation, using a representative set of nuclear energy density functionals. The calculated values of $α_D$ are used to validate different correlations involving $α_D$, the symmetry energy at the saturation density $J$, the corresponding slope parameter $L$ and the neutron skin thickness $Δr_{\!np}$, as suggested by the Droplet Model. A subset of models that reproduce simultaneously the measured polarizabilities in ${}^{68}$Ni, ${}^{120}$Sn, and ${}^{208}$Pb are employed to predict the values of the symmetry energy parameters at saturation density and $Δr_{\!np}$. The resulting intervals are: $J\!=\!30 \text{-}35$ MeV, $L\!=\!20 \text{-} 66$ MeV; and the values for $Δr_{\!np}$ in ${}^{68}$Ni, ${}^{120}$Sn, and ${}^{208}$Pb are in the ranges: 0.15\text{-}0.19 fm, 0.12\text{-}0.16 fm, and 0.13\text{-}0.19 fm, respectively. The strong correlation between the electric dipole polarizabilities of two nuclei is instrumental to predict the values of electric dipole polarizabilities in other nuclei.

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arXiv2015

Pairing reentrance in warm rotating $^{104}$Pd nucleus

Pairing reentrance phenomenon in the warm rotating $^{104}$Pd nucleus is studied within the Bardeen-Cooper-Schrieffer (BCS)-based approach (the FTBCS1). The theory takes into account the effect of quasiparticle number fluctuations on the pairing field at finite temperature and angular momentum within the pairing model plus noncollective rotation along the symmetry axis. The numerical calculations for the pairing gaps and nuclear level densities (NLD), of which an anomalous enhancement has been experimentally observed at low excitation energy $E^*$ and high angular momentum $J$, show that the pairing reentrance is seen in the behavior of pairing gap obtained within the FTBCS1 at low $E$ and high $J$. This leads to the enhancement of the FTBCS1 level densities, in good agreement with the experimental observation. This agreement indicates that the observed enhancement of the NLD might be the first experimental detection of the pairing reentrance in a finite nucleus.

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arXiv2015

Constraining the symmetry energy content of nuclear matter from nuclear masses: a covariance analysis

Elements of nuclear symmetry energy evaluated from different energy density functionals parametrized by fitting selective bulk properties of few representative nuclei are seen to vary widely. Those obtained from experimental data on nuclear masses across the periodic table, however, show that they are better constrained. A possible direction in reconciling this paradox may be gleaned from comparison of results obtained from use of the binding energies in the fitting protocol within a microscopic model with two sets of nuclei, one a representative standard set and another where very highly asymmetric nuclei are additionally included. A covariance analysis reveals that the additional fitting protocol reduces the uncertainties in the nuclear symmetry energy coefficient, its slope parameter as well as the neutron-skin thickness in $^{208}$Pb nucleus by $\sim 50\%$. The central values of these entities are also seen to be slightly reduced.

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arXiv2015

Diversities in the properties of neutron stars at a fixed neutron-skin thickness in $^{208}$Pb nucleus

We study the diversities in the properties of the neutron stars arising due to the different choices for the cross-coupling between various mesons which governs the density dependence of the nuclear symmetry energy in the extended relativistic mean-field(RMF) model. For this purpose, we obtain two different families of the extended RMF model corresponding to different non-linear cross-coupling term in the isovector part of the effective Lagrangian density. The lowest order contributions for the $δ$ mesons are also included. The different models within the same family are so obtained that they yield wide variation in the value of neutron-skin thickness in the $^{208}$Pb nucleus. These models are employed to compute the neutron star properties such as, core-crust transition density, radius and red shift at canonical mass ($1.4M_{\odot}$), tidal polarizability parameter, and threshold mass required for the enhanced cooling through direct Urca process. Most of the neutron star properties considered are significantly different(10\%-40\%) for the different families of models at a smaller neutron-skin thickness ($\sim 0.15$ fm) in the $^{208}$Pb nucleus.

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arXiv2015

Reassessing nuclear matter incompressibility and its density dependence

Experimental giant monopole resonance energies are now known to constrain nuclear incompressibility of symmetric nuclear matter $K$ and its density slope $M$ at a particular value of sub-saturation density, the crossing density $ρ_c$. Consistent with these constraints, we propose a reasonable way to construct a plausible equation of state of symmetric nuclear matter in a broad density region around the saturation density $ρ_0$. Help of two additional empirical inputs, the value of $ρ_0$ and that of the energy per nucleon $e(ρ_0)$ are needed. The value of $K(ρ_0)$ comes out to be $211.9\pm 24.5$ MeV.

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arXiv2014

Equation of state of nuclear matter from empirical constraints

From empirically determined values of some of the characteristic constants associated with homogeneous nuclear matter at saturation and sub-saturation densities, within the framework of a Skyrme-inspired energy density functional, we construct an equation of state (EoS) of nuclear matter.This EoS is then used to predict values of density slope parameters of symmetry energy $L(ρ)$, isoscalar incompressibility $K(ρ)$ and a few related quantities. The close consonance of our predicted values with the currently available ones for the density dependence of symmetry energy and incompressibility gleaned from diverse approaches offers the possibility that our method may help in settling their values in tighter bounds. Extrapolation of our EoS at supranormal densities shows that it is in good harmony with the one extracted from experimental data.

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arXiv2014

Core-crust transition properties of neutron stars within systematically varied extended relativistic mean-field model

The model dependence and the symmetry energy dependence of the core-crust transition properties for the neutron stars are studied using three different families of systematically varied extended relativistic mean field model. Several forces within each of the families are so considered that they yield wide variations in the values of the nuclear symmetry energy $a_{\rm sym}$ and its slope parameter $L$ at the saturation density. The core-crust transition density is calculated using a method based on random-phase-approximation. The core-crust transition density is strongly correlated, in a model independent manner, with the symmetry energy slope parameter evaluated at the saturation density. The pressure at the transition point dose not show any meaningful correlations with the symmetry energy parameters at the saturation density. At best, pressure at the transition point is correlated with the symmetry energy parameters and their linear combination evaluated at the some sub-saturation density. Yet, such correlations might not be model independent. The correlations of core-crust transition properties with the symmetry energy parameter are also studied by varying the symmetry energy within a single model. The pressure at the transition point is correlated once again with the symmetry energy parameter at the sub-saturation density.

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