Electron capture in stellar collapse

Electron-capture rates are calculated for the high-density regime of pre-supernova stellar collapse. The Gamow-Teller contribution arises from thermal unblocking and proceeds mainly through captures beneath the T = 0 energy threshold. Parity-forbidden (Δl = 1) processes also contribute, particularly the 2⁻ unique forbidden transition. Both are quenched by the effective renormalization of the vector and axial vector charges by ground-state correlations and isobars. Special attention is paid to the temperature-induced unblocking of the GT's and the thermal redistribution of the forbidden-transition strength. A statistical treatment averages the contributions of nuclear species present in the mass range 60 < A < 100. Dynamical collapse calculations indicate that the electron fraction at neutrino-trapping densities is not much reduced from its initial value. In contrast to previous work, the entropy is shown to decrease due to the below threshold GT captures as well as those on free protons. The small amount of electron capture and the entropy production are both useful in producing successful supernova explosions.     

Electron capture rates during collapse of the supernova core

Electron capture rates in the core of a collapsing star are calculated. It is shown that allowed Gamow-Teller transitions are quickly suppressed and that the neutronization rates are dominated by first forbidden transitions. These rates are evaluated using a simple model of the nucleus. In the calculation of the total neutronization rates, the contribution of free protons is also included. Comparison shows that early investigators overestimated the rates by a factor of ～20.     

Electron capture rates for iron group nuclei at the surface of magnetar

Effects of an ultra-strong magnetic field on electron capture rates for 55Co are analyzed in the nuclear shell model and under the Landau energy levels quantized approximation in the ultra-strong magnetic field, and the electron capture rates on 10 abundant iron group nuclei at the surface of a magnetar are given. The results show that electron capture rates on 55Co are increased greatly in the ultra-strong magnetic field, by about 3 orders of magnitude generally. These conclusions play an important role in future study of the evolution of magnetars.     

Fine-grid calculations for stellar electron and positron capture rates on Fe isotopes

The acquisition of precise and reliable nuclear data is a prerequisite to success for stellar evolution and nucleosynthesis studies. Core-collapse simulators find it challenging to generate an explosion from the collapse of the core of massive stars. It is believed that a better understanding of the microphysics of core-collapse can lead to successful results. The weak interaction processes are able to trigger the collapse and control the lepton-to-baryon ratio (Y _e ) of the corematerial. It is suggested that the temporal variation of Y _e within the core of a massive star has a pivotal role to play in the stellar evolution and a fine-tuning of this parameter at various stages of presupernova evolution is the key to generate an explosion. During the presupernova evolution of massive stars, isotopes of iron, mainly ^54–56Fe, are considered to be key players in controlling Y _e ratio via electron capture on these nuclides. Recently an improved microscopic calculation of weak-interaction-mediated rates for iron isotopes was introduced using the proton-neutron quasiparticle random-phase-approximation (pn-QRPA) theory. The pn-QRPA theory allows a microscopic state-by-state calculation of stellar capture rates which greatly increases the reliability of calculated rates. The results were suggestive of some fine-tuning of the Y _e ratio during various phases of stellar evolution. Here we present for the first time the fine-grid calculation of the electron and positron capture rates on ^54–56Fe. The sensitivity of the pn-QRPA calculated capture rates to the deformation parameter is also studied in this work. Core-collapse simulators may find this calculation suitable for interpolation purposes and for necessary incorporation in the stellar evolution codes.     

Neutrino-antineutrino pair emission from thermally excited nuclei in stellar collapse

We examine the rate of neutrino-antineutrino pair emission by hot nuclei in collapsing stellar cores. The rates are calculated assuming that only allowed charge-neutral Gamow-Teller (GT₀) transitions contribute to the decay of thermally excited nuclear states. To obtain the GT₀ transition matrix elements, we employ the quasiparticle random phase approximation extended to finite temperatures within the thermo field dynamics formalism. The decay rates and the energy emission rates are calculated for the sample nuclei ⁵⁶Fe and ⁸²Ge at temperatures relevant to core collapse supernovae.     

恒星能量下核子俘获反应率的晕核效应

恒星能量下俘获截面很难直接测量。¹⁰Be（n，γ）¹¹Be俘获反应涉及到非均匀宇宙大爆炸核合成，无直接测量实验截面数据。利用转移反应¹⁰Be（d，p）¹¹Be的渐近归一化系数（ANC）方法，计算了¹⁰Be（n，γ）¹¹Be俘获反应截面和反应率。¹¹Be是中子晕核。研究表明，在恒星能量下俘获到晕态的截面和反应率显著增大。     

Beta decay rates of sd-shell nuclei in stellar interiors

Gamow-Teller matrix elements of sd-shell nuclei relevant for stellar evolution of massive stars are calculated by using the sd-shell model wave functions of Wildenthal. Emphasis is placed on the calculations of GT transitions between excited states, which are not obtainable by experiment. Our results are compared with the previous work by Fuller et al. and are found notably different in many cases. The beta decay rates, as calculated with and without the contributions of excited states, are demonstrated to be quite different under conditions of high density and high temperature.     

Low-entropy adiabats for stellar collapse

The equation of state of hot, dense matter is computed along two adiabats (s =1.0 k and s = 1.5 k) using the nuclear Thomas-Fermi model.     

Consequences of nuclear electron capture in core collapse supernovae

The most important weak nuclear interaction to the dynamics of stellar core collapse is electron capture, primarily on nuclei with masses larger than 60. In prior simulations of core collapse, electron capture on these nuclei has been treated in a highly parametrized fashion, if not ignored. With realistic treatment of electron capture on heavy nuclei come significant changes in the hydrodynamics of core collapse and bounce. We discuss these as well as the ramifications for the postbounce evolution in core collapse supernovae.     

Exchange current corrections for partial capture rates of muons in nuclei

A calculation is made of exchange current corrections for partial capture rates of muons in¹²C,¹⁶O,³²S and⁴⁰Ca nuclei. One-pion exchange is taken into account. Adler-Dothan's results are used to obtain the weak pion production amplitude. The nuclear states are described in the Tamm-Dancoff approximation. It is found that the corrections are very erratic and state dependent. A large effect is found for the 0⁺→0^? transitions due to the size of the time part of the axial exchange current.     

Electron capture during gravitational collapse in type II supernov AE

We consider different mechanisms of unblocking the blocked allowed electron capture in stellar collapse, finding the mode through first-forbidden operators of negligible importance. A new mechanism of unblocking due to deformation of the nuclei sets the blocking in at a higher value of A. We also allow for neutrino phase-space blocking using an improved neutrino diffusion equation.     

Radiative muon capture on nuclei

Radiative muon capture rates of stopped meons, spectra, circular polarisation, and asymmetries of emitted -quanta in¹⁶O and⁴⁰Ca are analysed using the shell-model nuclear wave functions. Calculated characteristics show considerable sensitivity to the value of induced pseudoscalar coupling constantg _p. Where possible we compare the theoretical results with the preliminary experimental data.Dedicated to the 25th anniversary of the Joint Institute for Nuclear Research.On leave of absence fromthe Institute of Nuclear Physics, Czechosl. Acad. Sci., 250 68 e, Czechoslovakia.     

Muon capture on neutron-rich nuclei

Future facilities will allow the exploration of extremely neutron-rich nuclei far from the valley of stability. It is discussed that the strong-neutron excess results in changes in the collective excitations of such nuclei compared to conventional stable nuclei. We propose muon capture as an experimental tool to explore such changes. We will quantify our discussion by the calculation of the total and differential muon capture rates on selected calcium isotopes between ⁴⁰Ca and ⁶⁰Ca. Our calculations are based on the random phase approximation and agree nicely with the measured rates for ⁴⁰Ca and ⁴⁴Ca. Received: 6 April 2001 / Accepted: 14 May 2001     

Microscopic calculations of weak interaction rates of nuclei in stellar environment for A = 18 to 100

We report here the microscopic calculation of weak interaction rates in stellar matter for 709 nuclei with A = 18 to 100 using a generalized form of proton-neutron quasiparticle RPA model with separable Gamow-Teller forces. This is the first ever extensive microscopic calculation of weak rates calculated over a wide temperature-density grid which includes 10⁷≤ T(K) ≤ 30 × 10⁹ and 10 ≤ρ Y_e (gcm⁻³) ≤ 10¹¹, and over a larger mass range. Particle emission processes from excited states, previously ignored, are taken into account, and are found to significantly affect some β decay rates. The calculated capture and decay rates take into consideration the latest experimental energy levels and ft value compilations. Our calculation of electron capture and β-decay rates, in the fp-shell, show considerable differences with a recently reported shell model diagonalization approach calculation. Received: 16 April 1999     

Spontaneous lepton-number violation and de-leptonization in stellar collapse

We consider core collapse in weak interaction models where global lepton-number invariance is spontaneously broken. We find that the initial neutrino number prevents finite temperature restoration of lepton-number conservation. Assuming the lepton-number violating reactions occur rapidly, we show that the neutrino number is driven to a sufficiently small value to allow electron capture reactions to de-leptonize the core, and lead to a high-entropy collapse.     

Thermal effects on electron capture for neutron-rich nuclei

Thermal effects on allowed and first-forbidden proton-neutron transitions in neutron-rich nuclei are studied with the formalism of the thermofield dynamics. The p → n transition strength distributions as well as an electron capture cross section for the parent nucleus ⁷⁶Ge are calculated at different temperatures with the Hamiltonian of the quasiparticle-phonon nuclear model. Original Russian Text ? A.A. Dzhioev, A.I. Vdovin, V.Yu. Ponomarev, J. Wambach, 2009, published in Izvestiya Rossiiskoi Akademii Nauk. Seriya Fizicheskaya, 2009, Vol. 73, No. 2, pp. 236–240.     

Electron scattering for exotic nuclei

A brand-new electron scattering facility, the SCRIT Electron Scattering Facility, will soon start its operation at RIKEN RI Beam Factory, Japan. This is the world’s first electron scattering facility dedicated to the structure studies of short-lived nuclei. The goal of this facility is to determine the charge density distributions of short-lived exotic nuclei by elastic electron scattering. The first collision between electrons and exotic nuclei will be observed in the year 2014.     

SU(4) breaking,allowed transitions and total muon capture rates in nuclei

We study muon capture rates in light nuclei and show that one can evaluate them in a model-independent way by correcting the Primakoff closure approximation to the first order in the neutrino energy dependence. In this framework the interplay between the existence of allowed transitions and SU(4) breaking is analyzed. In the correction to the first order in the neutrino energy dependence by a sum rule, Majorana and SU(4) breaking potentials are taken into account. The method is also applied to single multipolar transitions, in particular for the dipole transitions. General considerations are illustrated for ⁶Li and ¹²C by explicit calculations. The stability of the results varying the mean neutrino energy allows the use of capture rates as a very sensitive tool to investigate the configuration mixing of the target because we connect the corrective term to spectroscopic information using directly the matrix elements of the effective potentials.     

Low-energy resonance strengths for proton capture on Mg and Al nuclei

Absolute strengths of selected low-energy resonances for proton captures on the nuclei ²⁴Mg, ²⁵Mg, ²⁶Mg and ²⁷Al have been determined. The experiments were carried out by measuring the number of resonant γ-rays, integrated over the yield curve, simultaneously with the number of Rutherford scattered protons. The method applied in the present work is independent of target stoichiometries and uniformities, stopping powers, beam straggling and current integration. The ratio of charged-particle and γ-ray detection efficiencies was measured by using the ¹⁹F(p,α₂γ)¹⁶O reaction at E_R=340 keV. A new set of reliable resonance strength standards at low bombarding energies is presented and our results are compared with previous work.