Electron Capture Rates Research Articles

Production of Lithium and Heavy Elements in AGB Stars Experiencing PIEs

Asymptotic giant branch (AGB) stars can experience proton ingestion events (PIEs), leading to a rich nucleosynthesis. During a PIE, the intermediate neutron capture process (i-process) develops, leading to the production of trans-iron elements. It is also suggested that lithium is produced during these events. We investigate the production of lithium and trans-iron elements in AGB stars experiencing a PIE with 1<Mini/M⊙<3 and −3<[Fe/H]<0. We find that lithium is produced in all PIE models with surface abundances 3< A(Li) <5. The surface enrichment and overall AGB lithium yield increases with decreasing stellar mass. The lithium enrichment is accompanied by a production of 13C with 3<12C/13C <9 at the surface just after the PIE. AGB stars experiencing PIE may be related to J-type carbon stars whose main features are excesses of lithium and13C. In addition to Li and 13C, heavy elements (e.g., Sr, Ba, Eu, Pb) are significantly produced in low-metallicity stars up to [Fe/H] ≃−1. The yields of our models are publicly available. Additionally, of interest to the Li nucleosynthesis, we provide an updated fitting formula for the 7Be(e−,νe)7Li electron capture rate.

Investigation of nuclear structure and [formula omitted]-decay properties of As isotopes

The nuclear ground state properties of 67−80As nuclei have been investigated within the framework of relativistic mean field (RMF) approach. The RMF model with density-dependent (DD-ME2) interaction is utilized for the calculation of potential energy curves and the nuclear ground-state deformation parameters (β2) of selected As isotopes. Later, the β-decay properties of As isotopes were studied using the proton–neutron quasi particle random phase approximation (pn-QRPA) model. These include Gamow Tellar (GT) strength distributions, log ft values, β-decay half-lives, stellar β± decays and stellar electron/positron capture rates. The β2 values computed from RMF model were employed in the pn-QRPA model as an input parameter for the calculations of β-decay properties for 67−80As. The calculated log ft values were in decent agreement with the measured data. The predicted β-decay half-lives matched the experimental values within a factor of 10. The stellar rates were compared with the shell model results. Only at high temperature and density values, the sum of β+ and electron capture rates had a finite contribution. On the other hand, the sum of β− and positron capture rates were sizeable only at low density and high temperature values. For all such cases, the pn-QRPA rates were found to be bigger than the shell model rates up to a factor of 33 or more. The findings reported in the current investigation could prove valuable for simulating the late-stage stellar evolution of massive stars.

Probing nuclear physics with supernova gravitational waves and machine learning

ABSTRACT Core-collapse supernovae (CCSNe) are sources of powerful gravitational waves (GWs). We assess the possibility of extracting information about the equation of state (EOS) of high density matter from the GW signal. We use the bounce and early post-bounce signals of rapidly rotating supernovae. A large set of GW signals is generated using general relativistic hydrodynamics simulations for various EOS models. The uncertainty in the electron capture rate is parametrized by generating signals for six different models. To classify EOSs based on the GW data, we train a convolutional neural network (CNN) model. Even with the uncertainty in the electron capture rates, we find that the CNN models can classify the EOSs with an average accuracy of about 87 per cent for a set of four distinct EOS models.

Impact of the Brink-Axel hypothesis on unique first-forbidden β-transitions for r-process nuclei* *J.-U. Nabi is supported by the Higher Education Commission Pakistan through Project (0-15394/NRPU/R&D/HEC/2021)

Key nuclear inputs for the astrophysical r-process simulations are the weak interaction rates. Consequently, the accuracy of these inputs directly affects the reliability of nucleosynthesis modeling. The majority of the stellar rates, used in simulation studies are calculated by invoking the Brink-Axel (BA) hypothesis. The BA hypothesis assumes that the strength functions of all parent excited states are the same as for the ground state, only shifted in energies. However, the BA hypothesis has to be tested against microscopically calculated state-by-state rates. In this project, we study the impact of the BA hypothesis on calculated stellar -decay and electron capture rates. Our investigation include both unique first forbidden (U1F) and allowed transitions for 106 neutron-rich trans-iron nuclei ([27, 77] ≤ [Z, A] ≤ [82, 208]). The calculations were performed using the deformed proton-neutron quasiparticle random-phase approximation (pn-QRPA) model with a simple plus quadrupole separable and schematic interaction. Waiting-point and several key r-process nuclei lie within the considered mass region of the nuclear chart. We computed electron capture and -decay rates using two different prescriptions for strength functions. One was based on invoking the BA hypothesis and the other was the state-by-state calculation of strength functions, under stellar density and temperature conditions ([10, 1] ≤ [ ( ), T( )] ≤ [1011, 30]). Our results show that the BA hypothesis invoked U1F rates are overestimated by 4–5 orders of magnitude as compared to microscopic rates. For capture rates, more than two orders of magnitude differences were noted when applying the BA hypothesis. It was concluded that the BA hypothesis is not a reliable approximation, especially for -decay forbidden transitions.

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

This work presents the microscopic calculation of energy rates (γ-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 γ-ray heating rates due to electron capture rates rise with the temperature and density values of the stellar core. On the other hand, the γ-ray heating due to β-decay increases with the core temperature values but decreases by orders of magnitude when the stellar core stiffens. The pn-QRPA computed γ 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.

Photovoltaic Response of Heterojunction Photodiode based on Ba0.8Sr0.2TiO3 Nanorod Film

The BST/Si photodiode has been effectively synthesized by growth BST on the p-type Si (100) substrate utilizing the hydrothermal process. Screen printing was used to prepare TiO2 film deposited on Si substrate, then immersing TiO2 film in Ba(OH)2 and Sr(OH)2 solution to fabricate BST/Si photodiode using a hydrothermal process. The BST film was studied using XRD, FESEM, and reflection spectra. The band gap was calculated for a BT film using the reflection methods. Hall measurement confirmed the n-type conductivity of BST film. Curie temperature of the BST film was observed at 87 °C according to DC measurement. The dark and illuminated (J-V) characteristics of the BST/Si photodiode have been measured under simulated AM1 conditions using a Xenon lamp. The Shockley-Read- Hall recombination caused unequal electron and hole capture rates that dominated the I-V characteristics resulting in an absence of classic superposition phenomena. The open circuit voltage (Voc) measurement is carried out to know the recombination mechanism in an open circuit. The BST/Si film showed more conductivity after increasing illuminating power density, which qualifies it for photovoltaic applications.

Target Density Effects on Charge Transfer of Laser-Accelerated Carbon Ions in Dense Plasma.

We report on charge state measurements of laser-accelerated carbon ions in the energy range of several MeV penetrating a dense partially ionized plasma. The plasma was generated by irradiation of a foam target with laser-induced hohlraum radiation in the soft x-ray regime. We use the tricellulose acetate (C_{9}H_{16}O_{8}) foam of 2 mg/cm^{3} density and 1mm interaction length as target material. This kind of plasma is advantageous for high-precision measurements, due to good uniformity and long lifetime compared to the ion pulse length and the interaction duration. We diagnose the plasma parameters to be T_{e}=17 eV and n_{e}=4×10^{20} cm^{-3}. We observe the average charge states passing through the plasma to be higher than those predicted by the commonly used semiempirical formula. Through solving the rate equations, we attribute the enhancement to the target density effects, which will increase the ionization rates on one hand and reduce the electron capture rates on the other hand. The underlying physics is actually the balancing of the lifetime of excited states versus the collisional frequency. In previous measurement with partially ionized plasma from gas discharge and z pinch to laser direct irradiation, no target density effects were ever demonstrated. For the first time, we are able to experimentally prove that target density effects start to play a significant role in plasma near the critical density of Nd-glass laser radiation. The finding is important for heavy ion beam driven high-energy-density physics and fast ignitions. The method provides a new approach to precisely address the beam-plasma interaction issues with high-intensity short-pulse lasers in dense plasma regimes.

Introducing an n-type electron deceleration layer to enhance the luminous efficiency of AlGaN-based DUV-LEDs

The internal quantum efficiency (IQE) of conventional AlGaN-based deep-ultraviolet (DUV) light-emitting diodes (LEDs) is seriously limited by the poor and inhomogeneous carrier injection. The typical solution is to optimize the structure parameters of p-type region and active region. In this work, however, we try to address this issue by introducing an n-type electron deceleration layer (EDL) underneath multiple quantum wells (MQWs). On one hand, the electron deceleration layer helps to decrease the electron velocity and thus increase the electron capture rate. On the other hand, it can also reduce barrier heights in the band valence and thus enhance the hole transport in the multiple quantum wells. As a consequence, the concentrations of electrons and holes in the multiple quantum wells were significantly increased, resulting in the enhancement of radiative recombination. Compared to the conventional structure, the DUV-LED structure with an electron deceleration layer achieves a higher internal quantum efficiency, leading to a 39% improvement in the light output power. It is believed that performing energy-band engineering in n-type region has great application prospects for high-performance DUV-LEDs.

Clarifying the atomic origin of electron killers in β-Ga2O3 from the first-principles study of electron capture rates

The emerging wide bandgap semiconductor -Ga2O3 has attracted great interest due to its promising applications for high-power electronic devices and solar-blind ultraviolet photodetectors. Deep-level defects in -Ga2O3 have been intensively studied towards improving device performance. Deep-level signatures E 1, E 2, and E 3 with energy positions of 0.55–0.63, 0.74–0.81, and 1.01–1.10 eV below the conduction band minimum have frequently been observed and extensively investigated, but their atomic origins are still under debate. In this work, we attempt to clarify these deep-level signatures from the comparison of theoretically predicted electron capture cross-sections of suggested candidates, Ti and Fe substituting Ga on a tetrahedral site (TiGaI and FeGaI) and an octahedral site (TiGaII and FeGaII), to experimentally measured results. The first-principles approach predicted electron capture cross-sections of TiGaI and TiGaII defects are 8.56 × 10–14 and 2.97 × 10–13 cm2, in good agreement with the experimental values of E 1 and E 3 centers, respectively. We, therefore, confirmed that E 1 and E 3 centers are indeed associated with TiGaI and TiGaII defects, respectively. Whereas the predicted electron capture cross-sections of FeGa defect are two orders of magnitude larger than the experimental value of the E 2, indicating E 2 may have other origins like CGa and Gai, rather than common believed FeGa.

Comparison of Electron Capture Rates in the N = 50 Region using 1D Simulations of Core-collapse Supernovae

Recent studies have highlighted the sensitivity of core-collapse supernovae (CCSNe) models to electron-capture (EC) rates on neutron-rich nuclei near the N = 50 closed-shell region. In this work, we perform a large suite of one-dimensional CCSN simulations for 200 stellar progenitors using recently updated EC rates in this region. For comparison, we repeat the simulations using two previous implementations of EC rates: a microphysical library with parametrized N = 50 rates (LMP), and an older independent-particle approximation (IPA). We follow the simulations through shock revival up to several seconds post-bounce, and show that the EC rates produce a consistent imprint on CCSN properties, often surpassing the role of the progenitor itself. Notable impacts include the timescale of core collapse, the electron fraction and mass of the inner core at bounce, the accretion rate through the shock, the success or failure of revival, and the properties of the central compact remnant. We also compare the observable neutrino signal of the neutronization burst in a DUNE-like detector, and find consistent impacts on the counts and mean energies. Overall, the updated rates result in properties that are intermediate between LMP and IPA, and yet slightly more favorable to explosion than both.

Beta decay and electron capture rates of manganese isotopes in astrophysical environments

The isotopes of manganese in the mass range of A=53−63 are abundant in the core materials of high-mass stars and they are considered to be of prime importance in the progression of the pre-collapse phases. During these late evolutionary phases, nuclear processes associated with weak interactions, including β−-decay and electron capture (EC) of these isotopes, significantly alter the lepton to baryon ratio (Ye) in the core’s composition. The temporal change in this parameter is one of the basic elements used to simulate a successful explosion. Hence, the β−-decay and EC rates of manganese (Mn) nuclides may serve as important inputs in the simulation codes for core-collapse supernova. In this study, we investigated the weak decay characteristics of 55−63Mn nuclides. The strength distributions of Gamow–Teller transitions in the directions of β−-decay and EC were calculated for these isotopes using the proton–neutron quasi-particle random-phase approximation (pn-QRPA) model. The β-decay half-life values of Mn isotopes under terrestrial conditions were computed and compared with measured data and previous calculations. The pn-QRPA estimated half-lives were in good agreement with the experimental values. The β− and EC rates were then calculated over a large range of stellar temperatures (0.01−30)×109K and densities (10−1011) g/cm3. The computed rates were compared with previous calculations performed using the large-scale shell model (LSSM) and independent particle model (IPM). For most of the isotopes, in high temperature and density regimes, the pn-QRPA calculated β−-decay rates were smaller than the LSSM and IPM rates by up to an order of magnitude. By contrast, the EC rates were larger by up to an order of magnitude (factor of 3−10) at high temperature compared with the LSSM (IPM) results.

Electron capture and β-decay rates for nuclei with A = 65–80

Recently a list of top 50 most important electron capture (ec) and β-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 = 65–80, 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 the previous pn-QRPA and independent particle model (IPM) results is presented for the first time. The reported ec rates are almost the same when compared with the previous pn-QRPA calculation. On the other hand, the reported bd rates are generally smaller up to an order of magnitude. Comparison with IPM results show that our calculated rates are bigger, by two orders of magnitude. The current calculation may contribute to a more realistic simulation of late phases of stellar evolution and modeling of x-ray bursts.

Shell Model Applications in Nuclear Astrophysics

In recent years, shell model studies have significantly contributed in improving the nuclear input, required in simulations of the dynamics of astrophysical objects and their associated nucleosynthesis. This review highlights a few examples such as electron capture rates and neutrino-nucleus cross sections, important for the evolution and nucleosynthesis of supernovae. For simulations of rapid neutron-capture (r-process) nucleosynthesis, shell model studies have contributed to an improved understanding of half lives of neutron-rich nuclei with magic neutron numbers and of the nuclear level densities and γ-strength functions that are both relevant for neutron capture rates.

Finite-temperature electron-capture rates for neutron-rich nuclei near N=50 and effects on core-collapse supernova simulations

Electron capture on neutron-rich nuclei near $N=50$ plays an important role during the gravitational collapse of massive stars prior to a supernova explosion, as neutrinos emitted in the electron-capture process can freely leave the stellar core. At the high temperatures in the stellar core the electron-capture rates are determined from thermally excited states whose properties differ from those of the ground state. The authors perform finite-temperature calculations with different theoretical approaches and use the resulting electron-capture rates as input in core-collapse supernova simulations. This work shows that the various sets of electron-capture rates lead to very small differences in the simulations suggesting that the rates are well-constrained.

How effective is the Brink–Axel hypothesis for astrophysical weak rates?

Abstract We explore the effectiveness of the Brink–Axel hypothesis (BAH) for the computation of stellar electron capture (EC) and β-decay (BD) rates, namely that the transition strength function depends only upon the transition energy and not upon the details of the initial state. For this purpose, we calculated Gamow–Teller (GT) strength distributions for a selection of sd-shell nuclides, using two different microscopic models, namely the proton–neutron quasiparticle random phase approximation and the full configuration-interaction shell model, taking into account the first 100 states of both the initial and final nuclides. The GT transition strengths among these levels evolve with initial state energy. These transition strength functions we folded into weak-interaction mediated rates in stellar matter, specifically EC and BD rates, for a range of densities 10 g cm−3 ⩽ ρ ⩽ 1011 g cm−3 and range of temperatures 1 GK ⩽ T ⩽ 30 GK. When transitions from excited states were approximated using the BAH, augmented by so-called ‘back-resonance’ transitions, the rates were affected by up to three orders of magnitude or more at high temperatures and densities. Thus the BAH is not a reliable approximation for the calculation of stellar rates, especially in high temperature–density environments.

Competition of Electron Capture and Beta Decay Rates in Explosive Astrophysical Scenario of Type II Supernova

Stellar weak interaction processes play a significant role during the supernova explosion condition after collapse leading to the formation of neutron star. In dynamic events like core-collapse supernovae the high entropy wind scenario arises from considerations of the newly born proto-neutron star. Here, the late neutrinos interact with matter of the outermost neutron star layers leading to moderately neutron rich ejecta. We study the electron capture and beta decay rates of Co and Cd isotopes at various temperature and density conditions in an astrophysical environment and found that the beta decay rates are much higher than the corresponding electron capture rates at all the conditions.

Impacts of Strongly Magnetized Degenerate Plasma on the Electron-capture Rates

The strongly magnetized degenerate astrophysical plasma is investigated.Arelativistic Hartree self-consistent field method is applied to calculate the screening potential. A profile froma15M⊙ core collapsing supernova (SN) progenitor is applied to evaluate the electron-capture rates of 54Fe. It is found that the screening potential at high field is enhanced compared with the previous study. If the field is high enough and only the lowest Landau level is allowed, two orders of magnitude reduction of the capture rates are found in the high-density region. Such deviations of the electron capture rates are essential since the rates determine the neutron richness of the progenitor model as well as the iron core mass, which are crucial for MHD-Jet SNe explosion calculation.

Description of weak-interaction rates within the relativistic energy density functional theory

A new theoretical framework has been established and applied in the calculation of electron capture (EC) and β-decay rates in stellar environment, characterized by high density and temperature. For the description of the nuclear properties, the finite-temperature Hartree Bardeen-Cooper-Schrie_er (FTHBCS) theory based on the relativistic derivative-coupling D3C* interaction is employed. In order to describe the charge-exchange transitions, the finitetemperature proton-neutron quasi-particle random-phase approximation is developed (FT-PNRQRPA) which includes both temperature and pairing correlations. In the FT-HBCS calculations, only the isovector pairing is included, while in the residual interaction of the FT-PNRQRPA both the isovector and isoscalar pairing contribute. In this work, results for EC and β-decay rates are presented in the temperature interval T = 0–1.5 MeV and stellar density ρYe = 107 and 109 g/cm3. Both allowed 0+, 1+ and first-forbidden transitions 0−, 1− and 2− are included in the calculations. It is shown that interplay between pairing correlations and finite-temperature effects can lead to significant changes in rates. It is also important to include de-excitations, i.e. transitions with negative Q-value, that become increasingly significant at higher temperatures especially for p f -shell nuclei.

Calculations of electron capture rates of <sup>66</sup>Fe in astrophysical enviroment

The calculation of weak interaction rates plays a very important role in studying nuclear physics and nuclear astrophysics. In this work, we calculate the electron capture rate of <sup>66</sup>Fe in the framework of shell model. We mainly focus on the contribution of allowed transition and forbidden transition to the total rate. It is found that in some astrophysical environments the forbidden transition is very important in contribution to the electron capture rate, in which the non-unique forbidden transition plays a major role. This is very important for nuclear structures and astrophysics.

Screening Effects on Electron-Capture Rates and Type Ia Supernova Nucleosynthesis

Nucleosynthesis of iron-group elements in Type Ia supernovae is studied for single-degenerate models with the use of electron-capture rates updated with the new shell-model Hamiltonian in pf -shell. An over-production problem of neutron-rich iron-group isotopes compared with the solar abundances is now found to be suppressed within a factor of about twice for the updated weak rates. Effects of screening on nucleosynthesis are investigated for explosion models of fast deflagration and slow deflagration with delayed detonation. The e-capture rates are reduced by the screening, especially by the screening effects on the ions. The production yields of most neutron-rich isotopes such as 50Ti, 54Cr and 58Fe are found to be suppressed most by the screening. The inclusion of the screening is desirable for precise evaluation of abundances of neutron-rich nuclides.