arXiv subjects

M. M. Sharma

Publications and source records attributed to M. M. Sharma.

68 records · Page 4Linked to original sources

arXiv1999

Compressibility of Nuclear Matter from Shell Effects in Nuclei

The compressibility of nuclear matter has received significant attention in the last decade and a variety of approaches have been employed to extract this fundamental property of matter. Recently, significant differences have emerged between the results of relativistic and non-relativistic calculations of breathing mode giant monopole resonance (GMR). This is due to a lack of understanding of the dynamics of GMR and of its exact relationship to the compression modulus of the infinite nuclear matter. Here, I present an alternative approach based upon nuclear shell effects. The shell effects are known to manifest experimentally in terms of particle-separation energies with an exceedingly high precision. Within the framework of the non-relativistic density-dependent Skyrme theory, it is shown that the compressibility of nuclear matter has a significant influence on shell effects in nuclei. It is shown that 2-neutron separation energies and hence the empirical shell effects can be used to constrain the compressibility of nuclear matter.

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arXiv1999

Carbon Isotopes Near Drip Lines in the Relativistic Mean-Field Theory

We have investigated the ground-state properties of carbon isotopes in the framework of the relativistic mean-field (RMF) theory. RMF calculations have been performed with the non-linear scalar self-coupling of the $σ$ meson using an axially symmetric deformed configuration. We have also introduced the vector self-coupling of the $ω$ meson for the deformed mean-field calculations. The results show that the RMF predictions on radii and deformations are in good agreement with the available experimental data. It is shown that several carbon isotopes possess a highly deformed shape akin to a superdeformation. The single-particle structure of nuclei away from the stability line has been discussed with a view to understand the properties near the neutron drip line. Predictions of properties of carbon isotopes away from the stability line are made.

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arXiv1998

Dependence of direct neutron capture on nuclear-structure models

The prediction of cross sections for nuclei far off stability is crucial in the field of nuclear astrophysics. We calculate direct neutron capture on the even-even isotopes $^{124-145}$Sn and $^{208-238}$Pb with energy levels, masses, and nuclear density distributions taken from different nuclear-structure models. The utilized structure models are a Hartree-Fock-Bogoliubov model, a relativistic mean field theory, and a macroscopic-microscopic model based on the finite-range droplet model and a folded-Yukawa single-particle potential. Due to the differences in the resulting neutron separation and level energies, the investigated models yield capture cross sections sometimes differing by orders of magnitude. This may also lead to differences in the predicted astrophysical r-process paths. Astrophysical implications are discussed.

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arXiv1997

Light Nuclei near Neutron and Proton Drip Lines in the Relativistic Mean-Field Theory

We have made a detailed study of the ground-state properties of nuclei in the light mass region with atomic numbers Z=10-22 in the framework of the relativistic mean-field (RMF) theory. The nonlinear $σω$ model with scalar self-interaction has been employed. The RMF calculations have been performed in an axially deformed configuration using the force NL-SH. We have considered nuclei about the stability line as well as those close to proton and neutron drip lines. It is shown that the RMF results provide a good agreement with the available empirical data. The RMF predictions also show a reasonably good agreement with those of the mass models. It is observed that nuclei in this mass region are found to possess strong deformations and exhibit shape changes all along the isotopic chains. The phenomenon of the shape coexistence is found to persist near the stability line as well as near the drip lines. It is shown that the magic number N=28 is quenched strongly, thus enabling the corresponding nuclei to assume strong deformations. Nuclei near the neutron and proton drip lines in this region are also shown to be strongly deformed.

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arXiv1996

Superheavy Nuclei in the Relativistic Mean Field Theory

We have carried out a study of superheavy nuclei in the framework of the Relativistic Mean-Field theory. Relativistic Hartree-Bogoliubov (RHB) calculations have been performed for nuclei with large proton and neutron numbers. A finite-range pairing force of Gogny type has been used in the RHB calculations. The ground-state properties of very heavy nuclei with atomic numbers Z=100-114 and neutron numbers N=154-190 have been obtained. The results show that in addition to N=184 the neutron numbers N=160 and N=166 exhibit an extra stability as compared to their neighbors. For the case of protons the atomic number Z=106 is shown to demonstrate a closed-shell behavior in the region of well deformed nuclei about N=160. The proton number Z=114 also indicates a shell closure. Indications for a doubly magic character at Z=106 and N=160 are observed. Implications of shell closures on a possible synthesis of superheavy nuclei are discussed.

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arXiv1995

Rare-Earth Nuclei: Radii, Isotope-Shifts and Deformation Properties in the Relativistic Mean Field Theory

A systematic study of the ground-state properties of even-even rare earth nuclei has been performed in the framework of the Relativistic Mean-Field (RMF) theory using the parameter set NL-SH. Nuclear radii, isotope shifts and deformation properties of the heavier rare-earth nuclei have been obtained, which encompass atomic numbers ranging from Z=60 to Z=70 and include a large range of isospin. It is shown that RMF theory is able to provide a good and comprehensive description of the empirical binding energies of the isotopic chains. At the same time the quadrupole deformations $β_{2}$ obtained in the RMF theory are found to be in good agreement with the available empirical values. The theory predicts a shape transition from prolate to oblate for nuclei at neutron number N=78 in all the chains. A further addition of neutrons up to the magic number 82 brings about the spherical shape. For nuclei above N=82, the RMF theory predicts the well-known onset of prolate deformation at about N=88, which saturates at about N=102. The deformation properties display an identical behaviour for all the nuclear chains. A good description of the above deformation transitions in the RMF theory in all the isotopic chains leads to a successful reproduction of the anomalous behaviour of the empirical isotopic shifts of the rare-earth nuclei. The RMF theory exhibits a remarkable success in providing a unified and microscopic description of various empirical data.

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arXiv1995

Ground state properties of exotic nuclei near Z=40 in the relativistic mean-field theory,

Study of the ground-state properties of Kr, Sr and Zr isotopes has been performed in the framework of the relativistic mean field (RMF) theory using the recently proposed relativistic parameter set NL-SH. It is shown that the RMF theory provides an unified and excellent description of the binding energies, isotope shifts and deformation properties of nuclei over a large range of isospin in the Z=40 region. It is observed that the RMF theory with the force NL-SH is able to describe the anomalous kinks in isotope shifts in Kr and Sr nuclei, the problem which has hitherto remained unresolved. This is in contrast with the density-dependent Skyrme Hartree-Fock approach which does not reproduce the behaviour of the isotope shifts about shell closure. On the Zr chain we predict that the isotope shifts exhibit a trend similar to that of the Kr and Sr nuclei. The RMF theory also predicts shape coexistence in heavy Sr isotopes. Several dramatic shape transitions in the isotopic chains are shown to be a general feature of nuclei in this region. A comparison of the properties with the available mass models shows that the results of the RMF theory are generally in accord with the predictions of the finite-range droplet model.

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arXiv1994

Nuclear Breathing Mode in the Relativistic Mean Field Theory

The breathing-mode giant monopole resonance is studied within the framework of the relativistic mean-field (RMF) theory. Using a broad range of parameter sets, an analysis of constrained incompressibility and excitation energy of isoscalar monopole states in finite nuclei is performed. It is shown that the non-linear scalar self-interaction and the resulting surface properties influence the breathing-mode considerably. It is observed that dynamical surface properties respond differently in the RMF theory than in the Skyrme approach. A comparison is made with the incompressibility derived from the semi-infinite nuclear matter and with constrained nonrelativistic Skyrme Hartree-Fock calculaions.

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arXiv1994

Isospin Dependence of the Spin-Orbit Force and Effective Nuclear Potentials

The isospin dependence of the spin-orbit potential is investigated for an effective Skyrme like energy-functional suitable for density-dependent Hartree-Fock calculations. The magnitude of the isospin dependence is obtained from a fit to experimental data on finite spherical nuclei. It is found to be close to that of relativistic Hartree models. Consequently the anomalous kink in the isotope shifts of Pb nuclei is well reproduced.

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arXiv1994

Anomalous Isotope Shifts in Pb Nuclei in RMF Theory

We have studied the anomalous behaviour of isotopic shifts of Pb nuclei in the relativistic mean field theory. It has been shown that the relativistic mean field provides an excellent description of the anomalous kink in the isotopic shifts about $^{208}$Pb. This is in contrast from density-dependent Skyrme forces which do not reproduce the observed trend in the empirical data on the charge radii. We discuss some differences in the description of isotope shifts in the RMF theory and the Skyrme mean field.

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arXiv1994

Direct Capture at Low Energies

The importance of direct capture for (n,$γ$)--reactions on intermediate-- and heavy--mass target nuclei occuring in the s-- and r--process is investigated. It is shown that the direct mechanism is non--negligible for magic and neutron rich target nuclei. For some double magic and neutron rich nuclei in the r--process direct capture is even the dominant reaction mechanism.

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arXiv1994

Nuclear Breathing Mode in the Relativistic Mean-Field Theory

The breathing-mode giant monopole resonance is studied within the framework of the relativistic mean-field (RMF) theory. Using a broad range of parameter sets, a systematic analysis of constrained incompressibility and excitation energy of isoscalar monopole states in finite nuclei is performed. A comparison is made with the incompressibility derived from the semi-infinite nuclear matter and with constrained nonrelativistic Skyrme Hartree-Fock calculations. Investigating the dependence of the breathing-mode energy on the nuclear matter incompressibility, it is shown that dynamical properties of surface respond differently in the RMF theory than in the Skyrme approach.

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arXiv1993

Shell Effects in Nuclei Near Neutron Drip

Shell effects in nuclei close to the neutron-drip lines have been investigated. It has been demonstrated in the relativistic mean-field theory that nuclei very far from stability manifest the shell effects strongly. This behaviour is in accord with the predictions of nuclear masses in the finite-range droplet model including shell corrections. As a consequence we find a large neutron halo in nuclei near the neutron-drip line. The shell effects predicted in the existing Skyrme mean-field theory in comparison are significantly weaker than those of the other approaches.

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arXiv1993

Anomaly in the charge radii of Pb isotopes

The anomalous behaviour of the charge radii of the isotopic chain of Pb nuclei has been studied in the relativistic mean field theory. It has been shown that the relativistic mean field provides an excellent description of the anomalous kink in the isotopic shifts about $^{208}$Pb. This contrasts strongly from the Skyrme mean field, where almost all the known and realistic forces fail to reproduce the observed trend in the empirical data on the charge radii. The results have been discussed in the perspective of differences in the ansätze of the relativistic and the Skyrme mean-field theories.

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