arXiv subjects

Asim Ullah

Publications and source records attributed to Asim Ullah.

16 recordsLinked to original sources

arXiv2025

Nuclear Structure Properties of even-even Chromium Isotopes and the E ect of Deformation on Calculated Electron Capture Cross Sections

In this study, we investigate the role of nuclear deformation on the calculated electron capture cross section (ECC) of even-even chromium (Cr) isotopes. We first determined the nuclear structure properties of these nuclei within the interacting boson model-1 (IBM-1). The energy spectra and E2 transition probabilities were calculated by fitting the parameters in the model formalism. The analysis of the potential energy surface was also performed to predict the geometric shape of the Cr nuclei by plotting their contour plot in the plane of (beta, gamma) deformation parameters. Later, we calculated the ECC within the proton-neutron quasiparticle random phase approximation (pn-QRPA) model. In particular, we studied how the calculated ECC changed with different values of the nuclear deformation parameter. The calculated Gamow-Teller (GT) strength distributions were widely spread among the daughter states. The total GT strength decreased with increasing value of the beta parameter. The computed ECC values, however, increased with increasing beta values of the Cr isotopes.

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arXiv2025

$\beta$-decay of N=126 isotones for the r-process nucleosynthesis

The $\beta$-decay properties of nuclei with neutron number $N = 126$ is investigated in this paper. Two different versions of the proton-neutron quasi particle random phase (pn-QRPA) model were employed to compute $\beta$-decay rates and half-lives for the N = 126 isotones. The first set of calculation solves the pn-QRPA equations using the schematic model (SM) approach. The Woods-Saxon potential was employed as a mean-field basis. A spherical shape assigned for each waiting point nuclei throughout all simulations. Both allowed Gamow-Teller (GT) and first-forbidden (FF) transitions were considered in the particle-hole (ph) channel. The second set uses the pn-QRPA model in deformed Nilsson basis to calculate $\beta$-decay rates for allowed GT and unique first-forbidden (U1F) transitions under terrestrial and stellar conditions. Our results are in agreement with shell model findings that first-forbidden transitions lead to a considerable decrement in the calculated half-lives of the isotones. Inclusion of the first-forbidden contribution led to a decent agreement of our computed terrestrial $\beta$-decay half-lives with measured ones, much better than the previous calculations. The possible implication of the waiting point nuclei on r-process nucleosynthesis is discussed briefly.

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arXiv2025

Investigation of pairing correlations on computed Gamow-Teller strength distributions and associated $\beta$-decay half-lives

We investigate the effect of pairing correlations on the computed Gamow-Teller (GT) strength distributions and corresponding $\beta$-decay half-lives. The calculations are performed for a total of 47 sd-shell nuclei, for $20 < A < 30$, employing the pn-QRPA model. Our calculations use three different values of pairing gaps computed using three different empirical formulae. The GT strength distribution and centroid values change considerably with a change in the pairing gap values. This, in turn, leads to differences in computed half-lives. The pairing gaps computed using the mass-dependent formula result in the calculated half-lives being in better agreement with the measured data.

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arXiv2025

Investigation of effects of pairing correlations on calculated $\beta$-decay half-lives of fp-shell nuclei

Pairing of nucleons plays a key role in solving various nuclear physics problems. We investigate the probable effects of pairing correlations on the calculated Gamow-Teller (GT) strength distributions and the associated $\beta$-decay half-lives. Computations are performed for a total of 35 fp-shell nuclei using the proton-neutron quasiparticle random phase approximation (pn-QRPA) model. The nuclei were selected because of their importance in various astrophysical environments. Pairing gaps are one of the key parameters in the pn-QRPA model to compute GT transitions. We employed three different values of the pairing gaps obtained from three different empirical formulae in our calculation. The GT strength distributions changed significantly as the pairing gap values changed. This in turn resulted in contrasting centroid and total strength values of the calculated GT distributions and led to differences in calculated half-lives using the three schemes. The half-life values computed via the three-term pairing formula, based on separation energies of nucleons, were in best agreement with the measured data. We conclude that the traditional choice of pairing gap values, $\Delta_p = \Delta_n = 12/\sqrt{A}$, may not lead to half-life values in good agreement with measured data. The findings of this study are interesting but warrant further investigation.

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arXiv2025

Electron capture and \b{eta}-decay rates for nuclei with A=65-80

Recently, a list of the top 50 most important electron capture (EC) and $\beta$-decay (BD) nuclei, averaged throughout the stellar trajectory for $0.500 > Y_e > 0.400$, was published. The current study presents the calculation of EC and BD rates, from the published list with $A = 65\text{--}80$, on a detailed temperature-density grid. The EC and BD rates were calculated using the proton-neutron quasiparticle random phase approximation model (pn-QRPA). Our calculation did not employ the Brink-Axel hypothesis. A systematic comparison of the current calculation with previous pn-QRPA and independent particle model (IPM) results is presented for the first time. The reported EC rates are nearly the same when compared with the previous pn-QRPA calculation. On the other hand, the reported BD rates are generally smaller by up to an order of magnitude. Comparison with IPM results shows that our calculated rates are larger by two orders of magnitude. The current calculation may contribute to a more realistic simulation of the late phases of stellar evolution and the modeling of X-ray bursts.

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arXiv2025

Investigation of $\beta$-decay properties of neutron-rich Cerium isotopes

Reliable and precise knowledge of the $\beta$-decay properties of neutron-rich nuclei is important for a better understanding of the $r$-process. We report the computation of $\beta$-decay properties of neutron-rich Cerium isotopes calculated within the proton-neutron quasiparticle random phase approximation (pn-QRPA) approach. A total of 34 isotopes of Ce in the mass range $120 \leq A \leq 157$ were considered in our calculation. Pairing gaps are recognized amongst the key parameters in the pn-QRPA model to compute Gamow-Teller (GT) transitions. We employed two different values of the pairing gaps obtained from two different empirical formulae in our computation. The GT strength distributions changed considerably with changes in the pairing gap values. This, in turn, resulted in contrasting centroid and total strength values of the GT distributions and led to differences in calculated half-lives using the two schemes. The traditional pairing gaps resulted in significant fragmentation of GT strength. However, the pairing gaps, calculated employing the formula based on separation energies of neutron and proton, led to computed half-lives in better agreement with the measured data.

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arXiv2025

Energy rates due to Fe isotopes during presupernova evolution of massive stars

This work presents the microscopic calculation of energy rates ({\gamma} ray heating and (anti)neutrino cooling rates) due to weak decay of selected Fe isotopes. The isotopes have astrophysical significance during the presupernova evolution of massive stars. The energy rates are calculated using the pn QRPA model and compared with the independent particle model (IPM), large scale shell model (LSSM) and recent shell model calculation (GXPF1J). The reported (anti)neutrino cooling rates are smaller by up to two orders of magnitude at low core temperature values than the IPM rates. The two calculations compare well at T = 30 GK. The comparison of cooling rates with the LSSM is interesting. The pn QRPA cooling rates due to even even Fe isotopes are smaller (up to 2 orders of magnitude). For the odd A isotopes, the reported rates are bigger up to an order of magnitude. The pn QRPA computed cooling rates are, up to 2 orders of magnitude, bigger when compared with the GXPF1J calculation. The {\gamma} ray heating rates due to electron capture rates rise with the temperature and density values of the stellar core. On the other hand, the {\gamma} ray heating due to \b{eta} decay increases with the core temperature values but decreases by orders of magnitude when the stellar core stiffens. The pn QRPA computed {\gamma} heating rates are bigger (up to 3 orders of magnitude) at high temperatures and densities (for the case of 55 56Fe) when compared with the recent shell model results. Owing to the importance of energy rates, this study may contribute to a realistic simulation of presupernova evolution of massive stars.

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arXiv2025

Effect of Nuclear Deformation on Electron Capture Cross-section on Chromium Isotopes

The electron capture plays significant role in the pre supernova and supernova evolutions of massive stars which in turn are of great importance in synthesizing heavy elements beyond iron. In this paper we study the effect of nuclear deformation on the computed electron capture cross section on selected even even chromium isotopes (464850Cr). The nuclear deformation parameters were computed using two different theoretical models: Interacting Boson Model (IBM 1) and Macroscopic (Yukawa plus exponential) microscopic (Folded Yukawa) model (Mac mic model). A third value of deformation parameter was adopted from experimental data. We chose the pn QRPA model to perform our calculations. The predictive power of the chosen model was first tested by calculating Gamow Teller (GT) strength distributions of selected fp shell nuclei where measured GT data was available. The calculated GT strength distributions were well fragmented over the energy range 0 12 MeV and were noted to be in decent agreement with experimental data. The total GT strength was found to increase (decrease) with decrease (increase) in the value of deformation parameter for the three chromium isotopes. The computed GT strength distributions satisfied the model independent Ikeda sum rule. The ECC were calculated as a function of the deformation parameter at core temperature 1.0 MeV. Our results show that the calculated ECC increased with increasing value of nuclear deformation.

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arXiv2025

Investigation of Gamow-Teller strength of 186Hg within deformed pn-QRPA

Recently the the total absorption gamma spectroscopy technique was used to determine the Gamow Teller (GT) distribution of \b{eta} decay of 186Hg. It was concluded that the best description of the measured data was obtained with dominantly prolate components for both parent 186Hg and daughter 186Au. Motivated by the recent findings, we investigate the effect of nuclear deformation on the energy distribution of the GT strength of the decay of 186Hg into 186Au within the framework of pn QRPA based on the deformed Nilsson potential. To do the needful, we first calculate the energy levels and shape prediction of 186Hg within the interacting boson model. The computed GT strength distribution satisfied the model independent Ikeda sum rule 100 % (99.98 %) for the prolate (oblate) case. Based on the strength distributions, the deformed pn QRPA model with separable interaction prefers a prolate shape for the ground state of 186Hg and supports the shape coexistence for this nucleus.

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arXiv2025

Impact of Coulomb Correction Factor on Rate of Change of Lepton Fraction during Presupernova Evolution

We reexamine the weak interaction nuclei having largest contribution to the lepton to baryon fraction Ye by coupling the stellar weak rates and mass abundances for post silicon burning conditions during the presupernova evolution of massive stars. The stellar weak rates were recently calculated by Nabi et al. 2021 employing the fully microscopic pnQRPA model without invoking the Brink Axel hypothesis. We compute the mass abundances for a total of 728 nuclei, with A equal 1to 100, using Sahas equation and assuming nuclear statistical equilibrium with the incorporation of Coulomb correction factor to the chemical potential. We compile a list of top 50 electron capture ec and \b{eta} decay bd nuclei on the basis of largest contribution to Ye forpost silicon burning conditions where 11 percent ec and 6percent bd nuclei debuted dueto Coulomb corrections. The calculated mass abundances and corresponding Ye values are enhanced up to 3 orders of magnitude for heavier nuclei once Coulomb corrections were incorporated. This enhancement led to anincrement in total Y bde and Yece values, at Ye equal to 0.425 (\r{ho} equal 2.20 multiply 109 g/cm3 of 80percent and 91percent respectively. After incorporating the Coulomb corrections we propose a revised interval of Ye equal 0.423 0.455 where bd rates surpass thecompeting ec rates and is 3.2 percent bigger than the one suggested by Nabi et al. (2021).

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arXiv2025

Stellar Weak Rates and Mass Fractions of 20 Most Important $fp$-shell Nuclei with $A < 65$

This work presents stellar weak rates and mass fractions of 20 most important electron capture (ec) and beta decay (bd) nuclei with $A < 65$ according to a recent study during the presupernova evolution of massive stars. The mass fractions of these nuclei were calculated using the Sahas equation which assumes nuclear statistical equilibrium for a set of initial conditions ($T_9$, $\rho$ and $Y_e$) that represents the trajectory which a massive stars central region takes after its silicon core burns. Our computed mass fractions were found in decent comparison in most cases, and up to a factor 4 difference was noted when compared with the Independent Particle Model results. The weak interaction (ec and bd) rates were calculated in a totally microscopic fashion using the proton neutron quasiparticle random phase ap proximation model and without assuming the Brink Axel hypothesis. The rates were computed for a wide range of density ($10$-$10^{11}$) g/cm$^3$ and temperature (0.01-30) GK. In comparison with large scale shell model, our computed rates were found bigger at high values of core temperature. The current study may contribute in a more realistic simu lation of stellar evolution processes and modeling of core collapse supernovae.

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arXiv2024

Reexamination of nuclear structure properties and shape coexistence of nuclei around A70

We reexamine the nuclear structure properties of waiting point nuclei around A70 using the interacting boson model 1 (IBM 1) and the relativistic mean field (RMF) model. Effective density dependent meson exchange functional (DD ME2) and density dependent point coupling functional (DD PC1) were used for the RMF calculations. We calculated the energy levels, the geometric shapes, binding and separation energies of nucleons and quadrupole deformation parameters (\b{eta}2). The shape coexistence phenomena in A 70 nuclei (68Se, 70Se, 70Br, 70Kr, 72Kr, 74Kr, 74Rb, and 74Sr) was later investigated. Spherical and deformed shapes of the selected waiting point nuclei were computed using the IBM 1 and RMF models, respectively. The proton neutron quasiparticle random phase approximation (pn QRPA) model was used to calculate \b{eta} decay properties (Gamow Teller strength distributions, \b{eta} decay half lives, and branching ratios) of selected nuclei as a function of \b{eta}2. The results revealed a significant variation in calculated half lives and Gamow Teller strength distributions as the shape parameter was changed. The \b{eta}2 computed via DD ME2 functional resulted in half lives in best agreement with the measured data.

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arXiv2024

Re examination of \b{eta} decay in Hg, Pb and Po Isotopes

This study re examines the effect of nuclear deformation on the calculated Gamow Teller (GT) strength distributions of neutron deficient (178 192Hg, 185 194Pb and 196 206Po) nuclei. The nuclear ground state properties and shape parameters were calculated using the Relativistic Mean Field model. Three different density dependent interactions were used in the calculation. Estimated shape parameters were later used within the framework of deformed proton-neutron quasi random phase approximations model, with a separable interaction, to calculate the GT strength distributions, half lives and branching ratios for these neutron deficient isotopes. It was concluded that half lives and GT strength distributions vary considerably with change in shape parameter.

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arXiv2024

Nuclear structure properties and weak interaction rates of even-even Fe isotopes

Nuclear structure properties and weak interaction rates of neutron rich even even iron (Fe) isotopes (A = 50 70) are investigated using the Interacting Boson Model1 (IBM1) and the proton neutron Quasiparticle Random Phase Approximation (pnQRPA) model. The IBM1 is used for the calculation of energy levels and the B(E2) values of neutron rich Fe isotopes. Later their geometry was predicted within the potential energy formalism of the IBM1 model. Weak interaction rates on neutron rich nuclei are needed for the modeling and simulation of presupernova evolution of massive stars. In the current study, we investigate the possible effect of nuclear deformation on stellar rates of even even Fe isotopes. The pnQRPA model is applied to calculate the weak interaction rates of selected Fe isotopes using three different values of deformation parameter. It is noted that, in general, bigger deformation values led to smaller total strength and larger centroid values of the resulting Gamow Teller distributions. This later translated to smaller computed weak interaction rates. The current finding warrants further investigation before it may be generalized.The reported stellar rates are up to 4 orders of magnitude smaller than previous calculations and may bear astrophysical significance.

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arXiv2024

Allowed and forbidden beta-decay log ft values of neutron-rich Pb and Bi isotopes

The beta-decay log ft values for 210 215Pb 210 215Bi and 210 215Bi 210 215Po transitions in the north east region of 208Pb nuclei are estimated using the proton neutron quasiparticle random phase approximation model. The pn-QRPA equations were solved using the schematic model approach. The Woods Saxon (WS) potential was inserted as a mean field basis and nuclei were treated as spherical. Allowed Gamow Teller (GT) and first forbidden (FF) transitions were investigated in the particle hole (ph) channel. The calculated log ft values of the allowed GT and FF transitions using the pn-QRPA(WS) were found closer to the experimental values. Later we performed calculation of beta-decay rates in stellar environment. Here we solved the RPA equations in deformed Nilsson basis, both in the particle particle (pp) and particle hole (ph) channels. Allowed beta decay and unique first-forbidden (U1F) rates were calculated in stellar matter. For certain cases, the calculated U1F contribution was much more than the allowed beta decay rates under prevailing stellar conditions in line with previous findings. Increasing temperature of the stellar core affected the allowed GT rates more than the U1F rates.

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arXiv2021

Investigation of important weak interaction nuclei in presupernova evolution

The project aims to investigate most important weak interaction nuclei in the presupernova evolution of massive stars. To achieve the goal, an ensemble containing 728 nuclei in the mass range A = 1--100 was considered. We computed the mass fractions of these nuclei using Saha's equation for predetermined values of \textit{$T$}, \textit{$\rho$} and \textit{$Y_e$} and assuming nuclear statistical equilibrium. The nuclear partition functions were obtained using a newly introduced recipe where excited states, up to 10 $MeV$, were treated as discrete. The weak interaction rates (electron capture (\textit{ec}) and $\beta$-decay (\textit{bd})) were calculated in a \textit{totally} microscopic fashion using the proton-neutron quasiparticle random phase approximation (pn-QRPA) model and without assuming the Brink-Axel hypothesis. The calculated rates were coupled with the computed mass fractions to investigate the time rate of change of lepton to baryon fraction of the stellar matter. We compare our results with the previous calculations reported in the literature. Noticeable differences up to orders of magnitude are reported with previous calculations. These differences may influence the evolution of the star in the later stages of presupernova. We present a list of top 50 \textit{ec} and \textit{bd} nuclei which have the largest effect on $Y_e$ for conditions after silicon core burning. The competition between the \textit{ec} and \textit{bd} rates in stellar core was investigated and it was found that $Y_e$ = 0.424--0.455 is the interval where the \textit{bd} results are bigger than the \textit{ec} rates.

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