arXiv subjects

Chirashree Lahiri

Publications and source records attributed to Chirashree Lahiri.

10 recordsLinked to original sources

arXiv2023

Allowed $β^-$ decay of bare atoms with A$\sim$ 60-80 in stellar environments

We have calculated $β^-$ decay rates to the continuum and bound states of some fully ionized atoms in the stellar s-process environment having free electron density and temperature in the range $n_e = 10^{26} $ cm$^{-3} - 10^{27} $ cm$^{-3}$ and $T = 10^8$ K - $5 \times 10^{8}$ K, respectively. The presence of bare atoms in these particular situations has been confirmed by solving Saha ionization equation taking into account the ionization potential depression (IPD). At these temperatures, low lying excited energy levels of parent nuclei may have thermal equilibrium population and those excited levels may also decay via $β^-$ emission. The Nuclear Matrix Element (NME) of all the transitions of the set of 15 nuclei is calculated using nuclear shell-model. These NME are then used to calculate the comparative half-life ($ft_{1/2}$) of the transitions. Calculated terrestrial half-lives of the $β^-$ decays are in good agreement with the experimental results in most of the cases. Decay to bound and continuum states of bare atoms from ground/isomeric levels and excited nuclear levels have been calculated separately. The ratio of bound state to continuum state decay rates as a function of IPD modified $Q$-value reveals that bound state $β^-$ decay rate may compete and even dominate for $Q$-value $<$ 100 keV. The importance of the bound state $β^-$ decay in stellar situations has been shown explicitly. We have calculated total $β^-$ decay rates (bound state plus continuum state) taking into account IPD corrected neutral atom $Q$-value as a function of density and temperature. We have also presented results for the stellar $β^-$ half-lives and compared the ratio of neutral atom to bare atom half-lives for different density and temperature combinations. These results may be useful for s-process nucleosynthesis calculations.

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arXiv2019

Bound and continuum state $β^-$ decay of bare atoms: enhancement of decay rate and changes in $β^{-}$ decay branching

We have calculated rates of $β^{-}$ decay to both continuum and bound states separately for some fully ionized (bare) atoms in the mass range A $\approx$ 60-240. One of the motivations of this work is that the previous theoretical calculations were very old and/or informatically incomplete. Probably no theoretical study on this subject has been done in the last three decades. For the calculation, we have derived a framework from the usual $β^{-}$ decay theory used by previous authors. Dependence of the calculated rates on the nuclear radius and neutral atom Q-value have been examined. We have used the latest experimental data for nuclear and atomic observables, such as $β^{-}$ decay Q-value, ionization energy, neutral atom $β^{-}$ decay branchings, neutral atom half-lives etc. Results of $β^{-}$ decay rates for decay to continuum and bound states and the enhancement factor due to the bound state decay for a number of nuclei have been tabulated and compared with the previously calculated values, if available. The effective rate or half-life calculated for bare atom might be helpful to set a limit for the maximum enhancement due to bound state decay. Finally, $β^{-}$ decay branching for bare atom has been calculated. The changes in branching in bare atom compared to that in the neutral atom and for the first time branching flip for a few cases have been obtained. Reason for this branching change has been understood in terms of Q-values of the transitions in the neutral and bare atoms. Verification of this branching change / flip phenomenon in bare atom decay might be of interest for future experiments.

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arXiv2019

Importance of $α$-induced reactions and the inverse $(γ,α)$ on $p$-nuclei

We have calculated astrophysical reaction cross-sections for $(γ,α)$ reactions of some nuclei important for the calculation of $p$-process reaction-decay network. Reaction rates for $α$-induced reactions are calculated with the semi-microscopic optical potential constructed using double folding method, where nuclear density distributions for finite nuclei along with the effective nucleon-nucleon interaction are the important components of the folded potential. For this purpose density distributions of target nuclei are obtained from relativistic mean field approach. Astrophysical reaction cross section for elastic scattering of $α$-particle from $^{92}$Mo target is compared with the existing experimental results to constrain the newly formed potential. Further, to check the credibility of the present theoretical framework, the astrophysical S-factor for ($α$,$γ$) reactions are compared with the experimental observation, wherever available. Finally, an estimate of dominant photodisintegration channels at various astrophysical temperature is discussed for $p$-nuclei $^{74}$Se and $^{96}$Ru.

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arXiv2013

Importance of Q-values in astrophysical rp-proces

The importance of measuring Q-values in rapid proton capture process has been investigated. The microscopic optical model, derived using a nucleon nucleon interaction and densities from relativistic mean field calculations, has been utilized to calculate the reaction rates. It has been observed that the Q-values involved in the reactions at waiting points at A = 60 and 64 are very important in determining the final abundance of the process. Some other Q-values also play a crucial role in the final abundance of nuclei near the end point of the process.

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arXiv2012

Low energy proton reactions of astrophysical interest in A$\sim90-100$ region

Semimicroscopic optical potentials for low energy proton reactions in mass 90-100 region have been obtained by folding the density dependent M3Y interaction with relativistic mean field densities. Certain parameters in the potential have been deduced by comparing calculated results with the data for elastic scattering. Low energy proton reactions in this mass region have been studied in the formalism with success. Rates of important astrophysical reaction in the mass region have been calculated.

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arXiv2012

Neutron rich nuclei in a new binding energy formula and the astrophysical $r-$process

Neutron rich nuclei has been studied with a new phenomenological mass formula. Predictions of different mass formulas for the location of the neutron dripline are compared with those from the present calculation. The implications of the new mass formula for r-process nucleosynthesis are discussed. It is found that though the neutron drip line obtained from this formula differs substantially from other formulas, the r-process abundance upto mass 200 are unlikely to be significantly different. The errors inherent in the mass formula are found to play an insignificant role beyond mass A = 80.

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arXiv2012

Endpoint of $rp$ process using relativistic mean field approach and a new mass formula

Densities from relativistic mean field calculations are applied to construct the optical potential and, hence calculate the endpoint of the rapid proton capture ($rp$) process. Mass values are taken from a new phenomenological mass formula. Endpoints are calculated for different temperature- density profiles of various X-ray bursters. We find that the $rp$ process can produce significant quantities of nuclei upto around mass 95. Our results differ from existing works to some extent.

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arXiv2011

Relativistic Mean Field in $A\approx$80 nuclei and low energy proton reactions

Relativistic Mean Field calculations have been performed for a number of nuclei in mass $A\approx$80 region. Ground state binding energy, charge radius and charge density values have been compared with experiment. Optical potential have been generated folding the nuclear density with the microscopic nuclear interaction DDM3Y. S-factors for low energy ($p,γ$) and ($p,n$) reactions have been calculated and compared with experiment.

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arXiv2011

Proton dripline in a new formula for nuclear binding energy

The location of the proton dripline in a new phenomenological mass formula is calculated. Predictions of different mass formulas for the dripline are compared. The implications of the new mass formula for rapid proton nucleosynthesis beyond $^{56}$Ni are discussed. It is seen that the new formula indicates that masses up to A=80 are easily synthesized in a typical X-ray burst.

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arXiv2011

Microscopic calculation of proton capture reactions in mass 60-80 region and its astrophysical implications

Microscopic optical potentials obtained by folding the DDM3Y interaction with the densities from Relativistic Mean Field approach have been utilized to evaluate S-factors of low-energy $(p,γ)$ reactions in mass 60-80 region and to compare with experiments. The Lagrangian density FSU Gold has been employed. Astrophysical rates for important proton capture reactions have been calculated to study the behaviour of rapid proton nucleosynthesis for waiting point nuclei with mass less than A=80.

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