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β -decay is one of the key factors for understanding the r-process and evolution of massive stars. The Gamow–Teller (GT) transitions drive the β -decay process. We employ the proton–neutron quasiparticle random phase approximation (pn-QRPA) model to calculate terrestrial and stellar β -decay rates for 50 top-ranked nuclei possessing astrophysical significance according to a recent survey. The model parameters of the pn-QRPA model affect the predicted results of β -decay. The current study investigates the effect of nucleon–nucleon pairing gaps on charge-changing transitions and the associated β decay rates. Three different values of pairing gaps, namely TF, 3TF, and 5TF, were used in our investigation. It was concluded that both GT strength distributions and half-lives are sensitive to pairing gap values. The 3TF pairing gap scheme, in our chosen nuclear model, resulted in the best prediction with around 80% of the calculated half-lives within a factor 10 of the measured ones. The 3TF pairing scheme also led to the calculation of the biggest β -decay rates in stellar matter. [ABSTRACT FROM AUTHOR]

We re-examine the nuclear structure properties of waiting point nuclei around A ∼ 70 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 (β 2). The shape co-existence 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 β -decay properties (Gamow-Teller strength distributions, β -decay half-lives and branching ratios) of selected nuclei as a function of β 2. The results revealed a significant variation in calculated half-lives and Gamow-Teller strength distributions as the shape parameter was changed. The β 2 computed via DD-ME2 functional resulted in half-lives in best agreement with the measured data. [ABSTRACT FROM AUTHOR]

Recent studies (Boillos and Sarriguren, Phys. Rev. C 3(91):034311, 2015; Moreno et al., Phys. Rev. C, 73(5):054302, 2006) predicted the sensitivity of the Gamow-Teller (GT) strength distributions on nuclear deformation in neutron-deficient Hg isotopes. Motivated by this work we investigate nuclear ground state properties and GT strength distributions for neutron-deficient Hg isotopes ( 177 -- 193 Hg). The nuclear deformation ( β 2 ) values were calculated using the Relativistic Mean Field (RMF) model. The RMF approach with different density-dependent interactions, namely, DD-ME2 (meson-exchange) and DD-PC1 (point-coupling), was used to compute nuclear shape parameters. The computed deformation values were later used within the framework of deformed proton-neutron quasi-random phase approximations (pn-QRPA) model, with a separable interaction, to calculate the allowed GT strength distributions for these Hg isotopes. Our calculations validate the findings of Boillos and Sarriguren (Phys. Rev. C 3(91):034311, 2015) and confirmed the effect of deformation on GT strength distributions. Our study may further provide a complementary signature for nuclear shape isomers. Noticeable differences are highlighted between the current and previous calculations. The calculation of Boillos and Sarriguren (Phys. Rev. C 3(91):034311, 2015) shows that 174 -- 182 Hg posses prolate and 184 -- 196 Hg have oblate shapes. Our calculation, on the other hand, predicts prolate shape for 177 -- 185 Hg and oblate shape for 186 -- 193 Hg isotopes. We finally analyze the effect of deformation on the calculated T 1 / 2 for the selected Hg isotopes. Our half-life calculations do not validate the findings of Moreno et al. (Phys. Rev. C 73(5):054302, 2006) that T 1 / 2 values are not good enough observables to study deformation effects. [ABSTRACT FROM AUTHOR]

The nuclear isotopic structure can be understood easily via the intermediate-energy charge exchange reactions of (p, n) and (3 He, t) type. In the current contribution, we present some results for charge exchange reactions induced by 3He on targets lying in mass region 1 8 ≤ A ≤ 2 0 8 within the theoretical framework of plane wave impulse approximation (PWIA) and distorted wave impulse approximation (DWIA). Here, the recoil effects in PWIA have also been considered. Particularly, the angular distributions and the unit cross-sections have been calculated and compared with the available data. Further, the importance of inclusion of the exchange contribution in these reactions is also considered, which eventually enhance the matching with data. [ABSTRACT FROM AUTHOR]

In this work, we explore the role of deformation parameter (β) on the calculated Gamow-Teller (GT) strength distributions and electron capture cross-sections (ECC) for 46,48,50Cr isotopes within the formalism of the proton neutron-quasi-particle random phase approximation (pn-QRPA). Three different β parameters were used in the present study. Two of them were calculated by using the interacting boson model (IBM) and the macroscopic-microscopic (Mac-mic) models. The third one is the experimental β values obtained by employing its relation with the experimental B(E2)↑ values. The GT strength distributions were widely dispersed among all the daughter states of the given isotopes. They were found to have inverse relation with β parameter i.e decreasing with increasing the β value. The ECC were computed as a function of β parameter and the results suggest that the calculated ECC decreased with decreasing value of the β for the selected cases. [ABSTRACT FROM AUTHOR]

Electron capture and β − decay are the dominant decay processes during late phases of evolution of heavy stars. Previous simulation results show that weak rates on isotopes of Molybdenum (Mo) have a meaningful contribution during the development of phases of stars before they go supernova. The relative abundance coupled with the stellar weak rates on Mo isotopes may change the lepton-to-baryon content of the core material. Here we report on the calculation of nuclear structure properties of 82−138Mo isotopes employing the RMF model. Later we calculate the weak decay rates of these isotopes. We use the pn-QRPA model to compute these rates. In the first step, the ground-state nuclear properties of Mo isotopes such as binding energy per nucleon, neutron and proton separation energies, charge radii, total electric quadrupole moments and deformation parameter of electric quadrupole moments have been calculated using density dependent version of RMF model with DD-PC1 and DD-ME2 functionals. The calculated electric quadrupole deformation parameters have been used in a deformed pn-QRPA calculation in the second phase of this work to calculate half-lives and weak decay rates for these Mo isotopes in stellar matter. We calculate the electron capture and β -decay rates over an extensive range of temperature (0.01 × 10 9 K to 30 × 10 9 K) and density (10 to 10 11 ) g/cm3. Our study can prove useful for simulation of presupernova evolution processes of stars. [ABSTRACT FROM AUTHOR]

The electron capture plays significant role in the presupernova 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 ( 4 6 , 4 8 , 5 0 Cr). 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 f p -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. [ABSTRACT FROM AUTHOR]

Charge-changing transitions play a significant role in stellar weak-decay processes. The fate of the massive stars is decided by these weak-decay rates including lepton (positron and electron) captures rates, which play a consequential role in the dynamics of core collapse. As per previous simulation results, weak interaction rates on nickel (Ni) isotopes have significant influence on the stellar core vis-à-vis controlling the lepton content of stellar matter throughout the silicon shell burning phases of high mass stars up to the presupernova stages. In this paper, we perform a microscopic calculation of Gamow-Teller (GT) charge-changing transitions, in the -decay and electron capture (EC) directions, for neutron-rich Ni isotopes (Ni). We further compute the associated weak-decay rates for these selected Ni isotopes in stellar environment. The computations are accomplished by employing the deformed proton-neutron quasiparticle random phase approximation (pn-QRPA) model. A recent study showed that the deformed pn-QRPA theory is well suited for the estimation of GT transitions. The astral weak-decay rates are determined over densities in the range of 10-10g/cm3 and temperatures in the range of 0.01-30K. The calculated lepton capture rates are compared with the previous calculation of Pruet and Fuller (PF). The overall comparison demonstrates that, at low stellar densities and high temperatures, our EC rates are bigger by as much as two orders of magnitude. Our results show that, at higher temperatures, the lepton capture rates are the dominant mode for the stellar weak rates and the corresponding lepton emission rates may be neglected. [ABSTRACT FROM AUTHOR]

The deformed proton–neutron quasiparticle random phase approximation (QRPA) has been developed and applied to evaluate Gamow–Teller (GT) transition strength distributions, including high-lying excited states. The data of high-lying excited states are recently available beyond one or two nucleon threshold by charge exchange reactions using hundreds of MeV projectiles. Our calculations started with single-particle states calculated using a deformed, axially symmetric Woods–Saxon potential. The neutron–neutron and proton–proton pairing correlations are explicitly taken into account at the deformed Bardeen–Cooper–Schriffer theory. Additionally, the ground state correlations and two-particle and two-hole mixing states were included in the deformed QRPA. In this work, we used a realistic two-body interaction, given by the Brueckner G -matrix based on the CD Bonn potential to reduce the ambiguity on the nucleon–nucleon interactions inside nuclei. We applied our formalism to the GT transition strengths for 76 Ge, 76,82 Se, and 90,92 Zr, and compared the results with the available experimental data. The GT strength distributions were sensitive to the deformation parameter as well as its sign, i.e. , oblate or prolate. The Ikeda sum rule, which is usually thought to be satisfied under the one-body current approximation, regardless of nucleon models, was used to test our numerical calculations and shown to be satisfied without introducing the quenching factor, if high-lying GT excited states were properly taken into account. Most of the GT strength distributions of the nuclei considered in this work have the high-lying GT excited states beyond one-nucleon threshold, which are shown to be consistent with the available experimental data. [ABSTRACT FROM AUTHOR]

Abstract: A 71Ga(3He, t)71Ge charge-exchange experiment was performed to extract with high precision the Gamow–Teller (GT) transition strengths to the three lowest-lying states in 71Ge, i.e., the ground state , the 175 keV and the 500 keV excited states. These are the relevant states, which are populated via a charged-current reaction induced by neutrinos from reactor-produced 51Cr and 37Ar sources. A precise measurement of the GT transition strengths is an important input into the calibration of the SAGE and GALLEX solar neutrino detectors and addresses a long-standing discrepancy between the measured and evaluated capture rates from the 51Cr and 37Ar neutrino calibration sources, which has recently spawned new ideas about unconventional neutrino properties. [Copyright &y& Elsevier]

The β-decay studies of neutron-rich N isotopes have been performed using β-n, β-γ, and β-n-γ coincidence methods. The N ions were produced by the fragmentation of the Ne and Mg beams, respectively, on a thick beryllium target. The time of flight of the emitted neutrons following the β-decay of N was measured by a neutron detector system with wide energy detection range and low-energy detection threshold. In addition, several clover germanium detectors were used to detect the β-delayed γ-rays. The half-lives of the β-decays of N and N were determined to be (619±2) ms and (82.9±7.5) ms, respectively. Several new β-delayed neutron groups were observed with a total branching ratio of (6.98±1.46)% and (90.5±4.2)% for N and N, respectively. The level schemes of O and O were deduced. The experimental Gamow-Teller β-decay strengths of N and N to these levels were compared with the shell model calculations. [ABSTRACT FROM AUTHOR]

Abstract: Charge-exchange reactions of () and () types at intermediate energies are introduced as a tool for the study of nuclear matrix element in  decay. Here, the () type reactions are realized through , where 2He refers to two protons in a singlet state and where both of these are momentum analyzed and detected by the same spectrometer and detector. These reactions have been developed and performed exclusively at KVI, Groningen (NL), using an incident deuteron energy of 183 MeV. The  reaction is of () type and was developed at the RCNP facility in Osaka (JP) at incident energies of 420 MeV. Using both reaction types one can extract the Gamow–Teller transition strengths  and , which define the two “legs” of the decay matrix elements for the  decay. The high resolution available in both reactions allows a detailed insight into the excitations of the intermediate odd–odd nuclei and, as will be shown, some unexpected features are being unveiled. [Copyright &y& Elsevier]

Gamow–Teller states, possibly with narrow widths, have been searched for by the 4He(3He, t) reaction with an energy resolution of 100 keV. The search is motivated by theoretical discussions on the role of pions in nuclear structure and simulation of supernovae explosion. In the preliminary analysis of the spectrum at around 0°, no significant narrow structure is observed. [ABSTRACT FROM AUTHOR]

Abstract: Double differential cross sections between 0°–12° were measured for the 90Zr(n, p) reaction at 293 MeV over a wide excitation energy range of 0–70 MeV. A multipole decomposition technique was applied to the present data as well as the previously obtained 90Zr(p, n) data to extract the Gamow–Teller (GT) component from the continuum. The GT quenching factor Q was derived by using the obtained total GT strengths. The result is , not including an overall normalization uncertainty in the GT unit cross section of 16%. [Copyright &y& Elsevier]

Abstract: The Gamow–Teller strength (GT) distributions and electron capture rates on 55Co and 56Ni have been calculated using the proton–neutron quasiparticle random phase approximation theory. We calculate these weak interaction mediated rates over a wide temperature (–) and density () domain. Electron capture process is one of the essential ingredients involved in the complex dynamics of supernova explosion. Our calculations of electron capture rates show differences with the reported shell model diagonalization approach calculations and are comparatively enhanced at presupernova temperatures. We note that the GT strength is fragmented over many final states. [Copyright &y& Elsevier]

In two complementary measurements, an array of high-resolution germanium detectors and a total-absorption γ-spectrometer were used to investigate the β decay of 102In, a one proton–hole, three-neutron particle nucleus with respect to the 100Sn core. A total of 509 β-delayed γ rays (494 new) were observed for 102In, and a scheme comprising 234 levels (223 new) was constructed for the decay daughter 102Cd. The QEC value of the decay and the half-life were determined, the latter one being 23.3(1) s. The summed Gamow–Teller strength deduced from the high-resolution experiment is as low as 0.60±0.07±0.11 compared to a value of 4.70±0.05±0.20±1.0 obtained from the total absorption measurement, and to a shell-model prediction of 13.3. [Copyright &y& Elsevier]

We study Gamow–Teller strength distributions in some selected nuclei of particular astrophysical interest within the iron mass region. The theoretical framework is based on a proton–neutron quasiparticle random phase approximation built on a deformed selfconsistent mean field basis obtained from two-body density-dependent Skyrme forces. We compare our results to available experimental information obtained from (n,p) and (p,n) charge exchange reactions. [Copyright &y& Elsevier]

Beta-delayed proton decay of 57Zn has been investigated at the GSI on-line isotope separator. The studied 57Zn nuclei were produced in fusion evaporation reactions by using a 150 MeV 32S beam on a 28Si target. Beta-delayed protons were measured by a charged-particle telescope detector. The observed decay pattern was used to construct the level scheme of 57Cu and to extract the beta feeding distribution. The experimental results are compared with shell-model calculations. [ABSTRACT FROM AUTHOR]

Abstract: The influence of the nuclear shape on Gamow–Teller strength distributions is considered here for the case of neutron-deficient lead isotopes with even mass number from 184 to 194. Our theoretical framework is based on a deformed Hartree–Fock (HF) mean field with pairing interactions within Bardeen–Cooper–Schrieffer approximation (BCS). Residual spin–isospin interactions in the particle–hole and particle–particle channels are also included within quasiparticle random phase approximation (QRPA). After considering the sensitivity of the Gamow–Teller (GT) strength distributions to the various ingredients in the formalism, we conclude that the -decay patterns of these isotopes show specific signatures for each nuclear shape. [Copyright &y& Elsevier]

Electron capture on nuclei plays an essential role in the dynamics of several astrophysical objects, including core-collapse and thermonuclear supernovae, the crust of accreting neutron stars in binary systems and the final core evolution of intermediate-mass stars. In these astrophysical objects, the capture occurs at finite temperatures and densities, at which the electrons form a degenerate relativistic electron gas. The capture rates can be derived from perturbation theory, where allowed nuclear transitions [Gamow-Teller (GT) transitions] dominate, except at the higher temperatures achieved in core-collapse supernovae, where forbidden transitions also contribute significantly to the capture rates. There has been decisive progress in recent years in measuring GT strength distributions using novel experimental techniques based on charge-exchange reactions. These measurements not only provide data for the GT distributions of ground states for many relevant nuclei, but also serve as valuable constraints for nuclear models which are needed to derive the capture rates for the many nuclei for which no data yet exist. In particular, models are needed to evaluate stellar capture rates at finite temperatures, where capture can also occur on nuclei in thermally excited states. There has also been significant progress in recent years in the modeling of stellar capture rates. This has been made possible by advances in nuclear many-body models as well as in computer soft- and hardware. Specifically, to derive reliable capture rates for core-collapse supernovae, a dedicated strategy has been developed based on a hierarchy of nuclear models specifically adapted to the abundant nuclei and astrophysical conditions present under various collapse conditions. In particular, for the challenging conditions where the electron chemical potential and the nuclear Q values are of the same order, large-scale shell-model diagonalization calculations have proved to be an appropriate tool to derive stellar capture rates, often validated by experimental data. Such situations are relevant in the early stage of the core collapse of massive stars, for the nucleosynthesis of thermonuclear supernovae, and for the final evolution of the cores of intermediate-mass stars involving nuclei in the mass range A ∼ 20-65. This manuscript reviews the experimental and theoretical progress recently achieved in deriving stellar electron capture rates. It also discusses the impact these improved rates have on our understanding of the various astrophysical objects., (© 2021 IOP Publishing Ltd.)