In this study, we have reported key nuclear properties of weak β-decay processes on Yttrium isotopes for the mass number range A = 101 − 108. This mass region has importance while dealing with astrophysical r-process abundances. Our study might be helpful in r-process simulations. We have computed charge-changing strength distributions, β-decay half-lives, β-delayed neutron emission probabilities and β − (EC) weak rates under stellar conditions. We have performed microscopic calculations based on deformed proton-neutron quasi-particle random phase approximation (p-n-QRPA) over a wide temperature (10 7 − 3 × 10 10 ) K and density (10 − 10 11 ) g/cm 3 domain. Unique first-forbidden (U1F) transitions have been included in the calculations in addition to the allowed transitions. A significant decrease in calculated half-lives in ceratin cases, e.g., in 107 Y ( 108 Y) by about 67% (42%), has been observed because of the contribution from U1F transitions. We have compared present results with measured and theoretical works. A good agreement of our half-lives with experimental data is observed.

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The knowledge of beta-decay transitional probabilities and Gamow-Teller (GT) strength functions from highly excited states of nuclides is of particular importance for applications to astrophysical network calculations of nucleosynthesis in explosive stellar events. These quantities are challenging to achieve from the experimental measurements or computations using various nuclear models. Due to unavailability of feasible alternatives, many theoretical studies often rely on the Brink-Axel (BA) hypothesis, that is, the response of strength functions depends merely on the transition energy of the parent nuclear ground state and is independent of the underlying details of the parent state, for the calculation of stellar rates. BA hypothesis has been used in many applications from nuclear structure determination to nucleosynthesis yield in the astrophysical matter. We explore here the the validity of BA hypothesis in the calculation of stellar beta-decay (BD) and electron-capture (EC) weak rates of fp- and fpg-shell nuclides for GT transitions. Strength functions have been computed in a microscopic way by employing fully microscopic proton-neutron QRPA (quasi-particle random-phase approximation) within a broad density, ρYe = (10 - 1011) [g cm-3], and temperature, T = (1−30) [GK], grids relevant to the pre-collapse astrophysical environment. Our work provides evidence that the use of the approximation based on the BA hypothesis does not lead to reliable calculations of excited states strength functions under extreme temperature-density conditions characteristic of presupernova and supernova evolution of massive stars. Weak rates obtained by incorporating the BA hypothesis in the calculation of strength functions substantially deviate from the rates based on the state-by-state microscopically calculated strength functions. Deviation in the two calculations becomes significant as early as neon burning phases of massive stars. The deviation in the calculation of BD rates is even more pronounced, reaching up to three orders of magnitude.

β-decay is amongst the key properties of nuclei required for the modeling of r-process nucleosynthesis. It also governs the flow of abundances among neighboring isotopic chains of high-mass elements. In the present work, a simple proton-neutron quasi particle random phase approximation (pn-QRPA) model has been used for the calculation of β-decay half-lives of Rb, Sr, Y and Zr neutron-rich isotopes. For 97−103Rb, 98−107Sr, 101−109Y and 104−112Zr, where the experimental data were available, the half-life values are reproduced with reasonable accuracy. The same set of model parameters are later used to predict half-lives for few neutron-rich nuclei (104−112Rb, 108−113Sr, 110−114Y and 113−115Zr) where measured data is not available. The pn-QRPA results (including only allowed transitions) are compared with previous calculations (allowed plus forbidden) and exhibit agreement within a factor of 2.0 when compared with the recent available experimental data.

Rates for (anti-)neutrino energy loss on nickel isotopes, due to interactions involving weak decays (β±-decay and lepton captures) are regarded as having fundamental importance during late evolutionary stages of massive stars. These rates substantially affect the leptonic ratio (Ye) of stellar interior. For the densities less than 1011 g/cm3, weak processes produce (anti-)neutrinos which cause reduction in the stellar core’s entropy. In this paper, rates for neutrino and anti-neutrino energy loss on nickel neutron-rich isotopes (66-71Ni) have been presented. Rates for energy loss have been determined by applying the deformed pn-QRPA model. The ranges for temperature and density, have been used to determine the rates, are from 0.01 to 30 (109 K) and 101 to 1011 (g/cm3), respectively. Our computed rates for energy loss, at higher temperature regions, are enhanced in comparison with previously reported rates of Pruet and Fuller (PF).