arXiv subjects

P. C. Srivastava

Publications and source records attributed to P. C. Srivastava.

51 records · Page 3Linked to original sources

arXiv2013

High-spin structures of $^{136}_{54}$Xe, $^{137}_{55}$Cs, $^{138}_{56}$Ba, $^{139}_{57}$La, and $^{140}_{58}$Ce : A shell model description

In the present work recently available experimental data [A. Astier et al, Phys. Rev. C 85, 064316 (2012)] for high-spin states of five N=82 isotones, $^{136}_{54}$Xe, $^{137}_{55}$Cs, $^{138}_{56}$Ba, $^{139}_{57}$La, and $^{140}_{58}$Ce have been interpreted with state-of-the-art shell model calculations. The calculations have been performed in the 50-82 valence shell composed of $1g_{7/2}$, $2d_{5/2}$, $1h_{11/2}$, $3s_{1/2}$, and $2d_{3/2}$ orbitals. We have compared our results with the available experimental data for excitation energies including high-spin states, occupancy numbers and transition probabilities. As expected the structure of these isotones are due to proton excitations across Z=50 shell. The structure of the positive-parity states are mainly from $(πg_{7/2}πd_{5/2})^n$ and $(πg_{7/2}πd_{5/2})^{n-2}(πh_{11/2})^2$ configurations, while the negative-parity states have $(πg_{7/2}πd_{5/2})^{n}(πh_{11/2})^1$ configuration. Additionally, for the $^{136}_{54}$Xe, $^{137}_{55}$Cs and $^{138}_{56}$Ba isotones the excitation of the neutrons across N=82 gap is important.

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arXiv2012

Structure of odd Ge isotopes with $40 < N < 50$

We have interpreted recently measured experimental data of $^{77}$Ge, and also for $^{73,75,79,81}$Ge isotopes in terms of state-of-the-art shell model calculations. Excitation energies, B(2) values, quadrupole moments and magnetic moments are compared with experimental data when available. The calculations have been performed with the recently derived interactions, namely with JUN45 and jj44b for ${f_{5/2}pg_{9/2}}$ space. We have also performed calculation for ${fpg_{9/2}}$ valence space using an ${fpg}$ effective interaction with $^{48}$Ca core and imposing a truncation to study the importance of the proton excitations across the Z=28 shell in this region. The predicted results of jj44b interaction are in good agreement with experimental data.

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arXiv2012

Shell model description of Ge isotopes

A shell model study of the low energy region of the spectra in Ge isotopes for $38\leq N\leq 50$ is presented, analyzing the excitation energies, quadrupole moments, $B(E2)$ values and occupation numbers. The theoretical results have been compared with the available experimental data. The shell model calculations have been performed employing three different effective interactions and valence spaces.We have used two effective shell model interactions, JUN45 and jj44b, for the valence space $f_{5/2} \, p \,g_{9/2}$ without truncation. To include the proton subshell $f_{7/2}$ in valence space we have employed the $fpg$ effective interaction due to Sorlin {\it et al.}, with $^{48}$Ca as a core and a truncation in the number of excited particles.

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arXiv2012

Shell Model Description of $^{102-108}$Sn Isotopes

We have performed shell model calculations for neutron deficient even $^{102-108}$Sn and odd $^{103-107}$Sn isotopes in $sdg_{7/2}h_{11/2}$ model space using two different interactions. The first set of interaction is due to Brown {\it et al.} and second is due to Hoska {\it et al}. The calculations have been performed using doubly magic $^{100}$Sn as core and valence neutrons are distributed over the single particle orbits 1$g_{7/2}$, 2$d_{5/2}$, 2$d_{3/2}$, 3$s_{1/2}$ and 1$h_{11/2}$. In more recent experimental work for $^{101}$Sn [Phys. Rev. Lett. {\bf 105} (2010) 162502], the g.s. is predicted as 5/2$^+$ with excited 7/2$^+$ at 172 keV. We have also performed another two set of calculations by taking difference in single particle energies of 2$d_{5/2}$ and 1$g_{7/2}$ orbitals by 172 keV. The present state-of-the-art shell model calculations predicts fair agreements with the experimental data. These calculations serve as a test of nuclear shell model in the region far from stability for unstable Sn isotopes near the doubly magic $^{100}$Sn core.

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arXiv2012

Nuclear structure study with core excitations in Ni region: for $fpg_{9/2}$ space

Shell model calculations for Ni, Cu and Zn isotopes by modifying $fpg$ interaction due to Sorlin {\it et. al.,} [Phys. Rev. Lett. 88, 092501 (2002)] have been reported. In the present work 28 two body matrix elements of the earlier interaction have been modified. Present interaction is able to explain new experimental results for this region.

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arXiv2011

Nuclear structure study around Z=28

Yrast levels of Ni, Cu and Zn isotopes for $40 \leq N \leq50$ have been described by state-of-the-art shell model calculations with three recently available interactions using $^{56}$Ni as a core in the $f_{5/2}pg_{9/2}$ model space. The results are unsatisfactory viz. large $E(2^+)$ for very neutron rich nuclei, small B(E2) values in comparison to experimental values. These results indicate an importance of inclusion of $πf_{7/2}$ and $νd_{5/2}$ orbitals in the model space to reproduce collectivity in this region.

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arXiv2011

Structure of $^{71-78}$Ga isotopes in $f_{5/2}pg_{9/2}$ and $fpg_{9/2}$ spaces

We have performed comprehensive set of shell model calculations for Ga isotopes including high-spin states with three different effective interactions. This work will add more information in the earlier work by Cheal {\it et al.} for odd-even Ga isotopes [ Phys. Rev. Lett. 104, 252502 (2010)] and Mané {\it et al.} for odd-odd Ga isotopes [ Phys. Rev. C 84, 024303 (2011)], where only few excited states are studied in $f_{5/2}pg_{9/2}$ space. For lighter isotopes $fpg$ interaction is better and for heavier isotopes jj44b is quantitatively better than JUN45. These results show that limitation of existing interactions and calling for further improvements to predict nuclear structure properties of Ga isotopes.

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arXiv2011

Identification of the slow E3 transition 136mCs -> 136Cs with conversion electrons

We performed at ISOLDE the spectroscopy of the decay of the 8- isomer in 136Cs by and conversion-electron detection. For the first time the excitation energy of the isomer and the multipolarity of its decay have been measured. The half-life of the isomeric state was remeasured to T1/2 = 17.5(2) s. This isomer decays via a very slow 518 keV E3 transition to the ground state. In addition to this, a much weaker decay branch via a 413 keV M4 and a subsequent 105 keV E2 transition has been found. Thus we have found a new level at 105 keV with spin 4+ between the isomeric and the ground state. The results are discussed in comparison to shell model calculations.

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arXiv2011

Shell-model results in $fp$ and ${fpg_{9/2}}$ spaces for $^{61,63,65}$Co isotopes

Low-lying spectra and several high-spin states of odd--even $^{61,63,65}$Co isotopes are calculated in two different shell-model spaces. First set of calculations have been carried out in ${fp}$ shell valence space (full $fp$ space for $^{63,65}$Co and a truncated one for $^{61}$Co) using two recently derived ${fp}$ shell interactions, namely GXPF1A and KB3G, with $^{40}$Ca as core. Similarly, the second set of calculations have been performed in ${fpg_{9/2}}$ valence space using an ${fpg}$ effective interaction due to Sorlin {\it et al}., with $^{48}$Ca as core and imposing a truncation. It is seen that the results of GXPF1A and KB3G are reasonable for $^{61,63}$Co. For $^{65}$Co, shell-model results show that the ${fpg}$ interaction adopted in the study is inadequate and also point out that it is necessary to include orbitals higher than $1g_{9/2}$ for neutron-rich Co isotopes.

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arXiv2011

One plus two-body random matrix ensembles with parity: Density of states and parity ratios

One plus two-body embedded Gaussian orthogonal ensemble of random matrices with parity [EGOE(1+2)-$π$] generated by a random two-body interaction (modeled by GOE in two particle spaces) in the presence of a mean-field, for spinless identical fermion systems, is defined, generalizing the two-body ensemble with parity analyzed by Papenbrock and Weidenmüller [Phys. Rev. C {\bf 78}, 054305 (2008)], in terms of two mixing parameters and a gap between the positive $(π=+)$ and negative $(π=-)$ parity single particle (sp) states. Numerical calculations are used to demonstrate, using realistic values of the mixing parameters appropriate for some nuclei, that the EGOE(1+2)-$π$ ensemble generates Gaussian form (with corrections) for fixed parity eigenvalue densities (i.e. state densities). The random matrix model also generates many features in parity ratios of state densities that are similar to those predicted by a method based on the Fermi-gas model for nuclei. We have also obtained, by applying the formulation due to Chang et al [Ann. Phys. (N.Y.) {\bf 66}, 137 (1971)], a simple formula for the spectral variances defined over fixed-$(m_1,m_2)$ spaces, where $m_1$ is the number of fermions in the +ve parity sp states and $m_2$ is the number of fermions in the -ve parity sp states. Similarly, using the binary correlation approximation, in the dilute limit, we have derived expressions for the lowest two shape parameters. The smoothed densities generated by the sum of fixed-$(m_1,m_2)$ Gaussians with lowest two shape corrections describe the numerical results in many situations. The model also generates preponderance of +ve parity ground states for small values of the mixing parameters and this is a feature seen in nuclear shell model results.

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arXiv2010

Monopole Shift in Odd Neutron-Rich F Isotopes: A Shell Model Description

Monopole shift in the proton states in odd neutron rich F isotopes has been studied with neutron number changing from N = 10 to 20 for three different interactions. The variation of monopole shift point out towards appearance of shell closure at N = 14 and N = 16 and its disappearance at N = 20. Low lying states of odd F isotopes have also been calculated in the limited configuration space of {\it sd} shells for these interactions.

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arXiv2010

Large scale shell model calculations for odd-odd $^{58-62}$Mn isotopes

Large scale shell model calculations have been carried out for odd-odd $^{58-62}$Mn isotopes in two different model spaces. First set of calculations have been carried out in full $\it{fp}$ shell valence space with two recently derived $\it{fp}$ shell interactions namely GXPF1A and KB3G treating $^{40}$Ca as core. The second set of calculations have been performed in ${fpg_{9/2}}$ valence space with the $fpg$ interaction treating $^{48}$Ca as core and imposing a truncation by allowing up to a total of six particle excitations from the 0f$_{7/2}$ orbital to the upper $\it{fp}$ orbitals for protons and from the upper $\it{fp}$ orbitals to the 0g$_{9/2}$ orbital for neutron. For low-lying states in $^{58}$Mn, the KB3G and GXPF1A both predicts good results and for $^{60}$Mn, KB3G is much better than GXPF1A. For negative parity and high-spin positive parity states in both isotopes $fpg$ interaction is required. Experimental data on $^{62}$Mn is sparse and therefore it is not possible to make any definite conclusions. More experimental data on negative parity states is needed to ascertain the importance of 0g$_{9/2}$ and higher orbitals in neutron rich Mn isotopes.

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arXiv2009

Large scale shell model calculations for even-even $^{62-66}$Fe isotopes

The recently measured experimental data of Legnaro National Laboratories on neutron rich even isotopes of $^{62-66}$Fe with A=62,64,66 have been interpreted in the framework of large scale shell model. Calculations have been performed with a newly derived effective interaction GXPF1A in full $\it{fp}$ space without truncation. The experimental data is very well explained for $^{62}$Fe, satisfactorily reproduced for $^{64}$Fe and poorly fitted for $^{66}$Fe. The increasing collectivity reflected in experimental data when approaching N=40 is not reproduced in calculated values. This indicates that whereas the considered valence space is adequate for $^{62}$Fe, inclusion of higher orbits from $\it{sdg}$ shell is required for describing $^{66}$Fe.

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arXiv2007

Shell model study of neutron rich oxygen isotopes

Shell model calculations for low lying energy states of neutron rich oxygen isotopes 19O, 20O, 21O, 22O, 23O, 24O, 25O, 26O have been performed using OXBASH code. The configuration space consists of 0d5/2, 1s1/2, 0d3/2 orbital for neutrons outside the 16O core. Different interactions namely, Wildenthal, Preedom-Wildenthal, Wildenthal-Mcgrory and renormalized Kuo and Brown, which are either empirical or realistic in nature have been used in the calculation, The calculated energy spectrum are in good agreement with the experimental data wherever available for the empirical interactions and the correct ordering of levels is reproduced. The levels obtained from realistic interactions, though have a small rms deviation, do not reproduce the correct ordering in some of the cases. In the case of 21O, realistic interactions predict a much too compressed 1/2+ state at energy 0.157 MeV compared to the experimental value of 1.218 MeV. For even isotopes the variation of the energy of the first 2+exited state has been studied as a function of neutron number N. A sharp rise in the value is observed at N=16 for both empirical and realistic interactions and only at N=14 for empirical interactions. Significantly higher energy of first 2+ exited state compared to the value in the neighboring even-even nuclei is considered as a signature for magic nuclei at N=14 and N=16.

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arXiv2006

Parity Inversion And Different Properties of Be11 halo Nuclei

The recent developments of radioactive nuclear beams has opened up the possibility of exploring a wide variety of nuclei far from the valley of beta-stability. Of late several theoretical studies have come up in the past for describing nuclei in these exotic region. Extension of traditional shell model techniques to explain the exotic feature in the structure of these nuclei such as, formation of neutron halo large r.m.s radii, soft dipole resonances etc have confirmed the necessity to modify the single particle shell model potential when dealing with nuclei far from stability. The ground state nuclear structure properties of 4Be11 have been calculated in the framework of shell model using analytically soluble mean field potential given by Ginocchio. The potential is highly versatile in nature and depends on four parameters, which define its depth, range and shape. Potential parameters, which generate highly diffuse shape, account for the small binding and halo structure of the valence neutron in 4Be11. The problem of ground state parity inversion is also addressed.

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