Electron Capture Rates Research Articles

Laser Irradiated Enhancement of the Atomic Electron Capture Rate in Search of New Physics

Electron capture processes are important in the search for new physics. In this context, a high capture rate is desired. We investigate the possibility of enhancing the electron capture rate by irradiating laser beam to ''atom''. The possibility of such enhancement can be understood as a consequence of an enhancement of the electron wave function at the origin, $\Psi (0)$, through an increased effective mass of the electron. We find that an order of magnitude enhancement can be realized by using a laser with intensity on the order of $10^{10}$ W/mm$^2$ and a photon energy on the order of $10^{-3}$ eV.

Gravitational waves from accreting neutron stars

The observational evidence for gravitational wave emission from accreting millisecond pulsars, specifically the spin frequency distribution of the population, is reviewed. These continuous wave sources are promising candidates for detection by LIGO. Four theoretical mechanisms are discussed for producing the mass quadrupole moments inferred from the spin frequency data: thermal mountains (formed by gradients in the electron capture rate), r-modes in the core, magnetic mountains (formed by polar magnetic burial), and superfluid circulation in the core. For magnetic mountains, it is shown that the gravitational wave strain is inversely proportional to the magnetic moment, and that the gravitational wave spectrum displays distinctive sidebands as the magnetically confined mountain wobbles around. (The compression of the magnetic field towards the equator also modifies the physics of thermonuclear type I X-ray bursts in these objects, e.g., by thermally insulating the two hemispheres and leading to burst pairs.) For superfluid circulation in the core, it is shown that the high-Reynolds-number flow is nonaxisymmetric and emits a distinctive gravitational wave spectrum with two broad peaks.

Kinetics of an exciton-trion system in shallow GaAs/AlGaAs quantum wells

The formation of three-particle charged exciton complexes (trions) in shallow GaAs/AlGaAs quantum wells in the temperature range 1.7–15 K has been investigated by luminescence spectroscopy and resonant light scattering. The effect of the photon energy and the intensity of additional above-barrier illumination on the trion formation kinetics has been analyzed. It is established that, upon intrawell excitation, illumination leads to the formation of trions when the light photon energy corresponds to the regions of effective formation of trions in the photoluminescence excitation spectra. It is shown that, with an increase in the illumination level, the trion concentration first increases and then reaches a plateau since the quantum well acquires an electric charge whose field equalizes the electron and hole capture rates.

Electron-Capture Decay Rate of 7Be Encapsulated in C60 Cages

Electron-Capture Decay Rate of 7Be Encapsulated in C60 Cages

Measurement of weak rates for stellar evolution via the ( t, 3He) reaction

The (t,3He) charge-exchange reaction has been developed as a tool to extract Gamow-Teller strengths on nuclei of importance for stellar evolution. A secondary triton beam of 115 MeV/nucleon is used, either produced from a primary α beam, or, since recently, from a primary 16O beam. Here, the (t,3He) reaction is used to study the Gamow-Teller strength distribution in 58Co via the 58Ni(t,3He) reaction. The experimental results are compared with calculations in large-scale shell models using the kb3g and gxpf1 interactions, as well as existing data from 58Ni(n, p) and 58Ni(d,2He) experiments. The differences between the data and theoretical models are studied in terms of electron-capture rates in the pre-collapse stages of core-collapse supernovae.

Effects of energy distribution of interface traps on recombination dc current-voltage line shape

The effects of energy distributions of Si∕SiO2 interface traps in the energy gap of oxidized silicon on the current versus voltage line shape of the electron-hole recombination current are analyzed using the steady-state Shockley-Read-Hall kinetics. Slater’s [Insulators, Semiconductors and Metals; Quantum Theory of Molecules and Solids (McGraw-Hill, New York, 1967)] localized bulk perturbation theory applied by us to the interface anticipates U-shaped energy distributions of the density of neutral electron and hole interface traps from random variations of the Si:Si and Si:O bond angles and lengths. Conservation in dissipative transition energy anticipates the rate of electron capture into neutral electron trap to be faster for electron trap energy levels nearer the conduction band edge, and similarly, the rate of hole capture into neutral hole trap to be faster for hole trap energy levels nearer the valence band edge. Line shape broadening is analyzed for discrete and U-shaped energy distributions of interface trap energy levels. The broadened line shapes observed in past experiments, previously attributed to spatial variations of surface dopant impurity concentrations, could also arise from energy distributions of interface trap energy levels.

Screening effect on electron capture in presupernova stars

Aims. With the new electron capture results from the improved GT resonance distribution, we analyzed the electron screening effect on electron capture at the presupernova stage. Methods. The relativistically degenerate electron liquid screening potential derived from the linear response theory are adopted. The calculation of the electron capture rates is based on the shell-model. Both the change of electron energy and the shift in the threshold for electron capture in screening are taken into account. Results. The results indicate that the influence of electron screening on the electron capture is prominent in high-density matter. As the improved electron capture rate is considered, screening effect on the electron capture rate increases by about 1 percent at ρYe > 10 9 g/cm 3 . Conclusions. The influence of screening on the electron capture reaction is about 10 percent for 56 Co at ρYe ≈ 10 9 g/cm 3 . Screening may affect the structure and evolution of presupernova and should be included in any precise simulation.

Shell model method for Gamow-Teller transitions in heavy, deformed nuclei

A method for calculation of Gamow-Teller transition rates is developed by using the concept of the Projected Shell Model (PSM). The shell model basis is constructed by superimposing angular-momentum-projected multiquasiparticle configurations, and nuclear wave functions are obtained by diagonalizing the two-body interactions in these projected states. Calculation of transition matrix elements in the PSM framework is discussed in detail, and the effects caused by the Gamow-Teller residual forces and by configuration-mixing are studied. With this method, it may become possible to perform a state-by-state calculation for beta-decay and electron-capture rates in heavy, deformed nuclei at finite temperatures. Our first example indicates that, while experimentally known Gamow-Teller transition rates from the ground state of the parent nucleus are reproduced, stronger transitions from some low-lying excited states are predicted to occur, which may considerably enhance the total decay rates once these nuclei are exposed to hot stellar environments.

Explosions of O-Ne-Mg cores, the Crab supernova, and subluminous type II-P supernovae

We present results of simulations of stellar collapse and explosions in spherical symmetry for progenitor stars in the 8-10 solar mass range with an O-Ne-Mg core. The simulations were continued until nearly one second after core bounce and were performed with the Prometheus/Vertex code with a variable Eddington factor solver for the neutrino transport, including a state-of-the-art treatment of neutrino-matter interactions. Particular effort was made to implement nuclear burning and electron capture rates with sufficient accuracy to ensure a smooth continuation, without transients, from the progenitor evolution to core collapse. Using two different nuclear equations of state (EoSs), a soft version of the Lattimer & Swesty EoS and the significantly stiffer Wolff & Hillebrandt EoS, we found no prompt explosions, but instead delayed explosions, powered by neutrino heating and the neutrino-driven baryonic wind which sets in about 200 ms after bounce. The models eject little nickel (< 0.015 solar masses), explode with an energy of about or slightly more than 10**50 erg, and leave behind neutron stars (NSs) with a baryonic mass near 1.36 solar masses. Different from previous models of such explosions, the ejecta during the first second have a proton-to-baryon ratio of Ye > 0.46, which suggests a chemical composition that is not in conflict with galactic abundances. No low-entropy matter with Ye << 0.5 is ejected. This excludes such explosions as sites of a low-entropy r-process. The low explosion energy and nucleosynthetic implications are compatible with the observed properties of the Crab supernova, and the small nickel mass supports the possibility that our models explain some subluminous Type II-P supernovae.

Electron capture rates in strong electron screening

The effect of electron screening on electron capture reactions in the environments of pre-supernova objects is discussed, using the strong electron screening potential derived recently from the linear response theory. Our calculations of the electron capture rates for the nuclei 56Co, 56Fe and 56Mn show that in low-temperature and high-density circumstances, the effect of electron screening on the electron capture reaction is a little less than what was previously given, but it is still quite obvious.

Effect of electron screening on the prompt explosion energy for type Ⅱ supernova

The effect of electron screening on prompt explosion energy of supernova is discussed in regard to progenitor model WS15M⊙. The average heavy nuclei model is used for calculating the electron capture rates. The simulation results show that electron capture rate is decreased and the collapsing time-scale is prolonged by the electron screening, and the total energy of neutrino leakage increases. Acordingly, by the energy of the shock wave also decreases appreciably. The effect of electron screening on other supernova simulations is estimated.

Influence of plasmon-assisted charge exchange processes on ion-induced electron emission from metals

Charge changing processes (electron capture and loss) of light ions moving inside metals contribute significantly to the electron emission at intermediate and low impact velocities. An improved method for the description of screening of the moving ion in the electron gas developed by Nagy et al. [Nucl. Instrum. Methods 1996;B115:58.] and Lifshitz et al. [Phys. Rev. A 1998;57:200] will be used to determine the velocity-dependent bound-state wave functions and binding energies related to the moving projectile. These wave functions and binding energies are the basic ingredients for the calculation of the electron capture and loss rates, which determine the equilibrium charge state fractions of the different projectile species, as well as for the calculation of the electron excitation during the charge transfer processes. Explicit calculations were performed for Mg. We compare the contribution of electron emission by decay of plasmons generated during the charge transfer processes with that of plasmon excitation by fast secondary electrons generated by binary ion–electron collisions. The total plasmon-mediated electron yield, obtained by summing the contributions of different mechanisms of each projectile species, weighted with the respective charge state fraction, will be compared with available experimental data.

Electron-positron capture rates and a steady state equilibrium condition for an electron-positron plasma with nucleons

The reaction rates of the beta processes for all particles at arbitrary degeneracy are derived, and an {\it analytic} steady state equilibrium condition $\mu_n=\mu_p+2\mu_e$ which results from the equality of electron and positron capture rates in the hot electron-positron plasma with nucleons is also found, if the matter is transparent to neutrinos. This simple analytic formula is valid only if electrons are nondegenerate or mildly degenerate, which is generally satisfied in the hot electron-positron plasma. Therefore, it can be used to efficiently determine the steady state of the hot matter with plenty of positrons. Based on this analytic condition, given the baryon number density and the temperature, if the nucleons are nondegenerate, only one algebraic equation for determining the electron fraction is obtained, which shows the great advantage of the analytic equilibrium condition.

Shell-model applications in supernova physics

This manuscript reviews recent applications of the nuclear shell-model for the calculation of several quantities relevant for the core-collapse supernova dynamics and nucleosynthesis. These include electron capture rates and neutrino-nucleus cross sections for inelastic scattering. It is shown that electron capture rates on nuclei dominates over capture on free protons during the collapse leading to significant changes in the hydrodynamics of core collapse and bounce. Neutrino-nucleus cross sections at supernova neutrino energies can be determined from precise data on the magnetic dipole strength. The results agree well with large-scale shell-model calculations, validating this model.

Microscopic Calculations of Weak Interaction Rates

Progress in nuclear structure calculations have allowed for detailed calculations of weak interaction rates for astrophysical environments, including electron capture rates and neutrino-nucleus scattering cross sections. The theoretical models can be validated using high resolution charge-exchange reactions for the calculation of electron capture rates, and electron-scattering data for the calculation of neutrino scattering cross sections. We show that electron capture rates on nuclei with A > 60 are large enough that, in contrast to previous assumptions, electron capture on nuclei dominates over capture on free protons.

Electron capture in fp shell at presupernova stage

The electron capture of neutron-rich nuclei in fp shell in massive stars at presupernova stage is discussed based on the nuclear shell model. The Gamow-Teller resonance transition strength is modified by introducing a Gaussian function. As a result, the electron capture rate in high density is evidently larger than the previous results given by some authors.

Hopping photoconductivity and its long‐time relaxation in two‐dimensional array of Ge/Si quantum dots

Photoconductivity excitation kinetics has been studied in a two-dimensional array of Ge/Si quantum dots under illumination with different light wavelength. Both negative and positive photoeffects depending on dot occupations with holes were observed. Long-time conductivity dynamics (typically, 102–104 sec at T = 4.2 K) has been revealed as well as after switch on and switch off the illumination, displaying a sluggish temporal dependence. The observed effects were not suppressed by decreasing of the excitation energy below the silicon band-gap. For electronic glasses it was discovered that the more time under excitation the faster relaxation rate. Our results are explained by the different capture rate of electrons and holes by quantum dots, due to the presence of potential barriers created by positively charged Ge quantum dots. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Exploiting the neutronization burst of a galactic supernova

One of the robust features found in simulations of core-collapse supernovae (SNe) is the prompt neutronization burst, i.e. the first $\sim 25$ milliseconds after bounce when the SN emits with very high luminosity mainly $\nu_e$ neutrinos. We examine the dependence of this burst on variations in the input of current SN models and find that recent improvements of the electron capture rates as well as uncertainties in the nuclear equation of state or a variation of the progenitor mass have only little effect on the signature of the neutronization peak in a megaton water Cherenkov detector for different neutrino mixing schemes. We show that exploiting the time-structure of the neutronization peak allows one to identify the case of a normal mass hierarchy and large 13-mixing angle $\theta_{13}$, where the peak is absent. The robustness of the predicted total event number in the neutronization burst makes a measurement of the distance to the SN feasible with a precision of about 5%, even in the likely case that the SN is optically obscured.

Gamow–Teller strength distributions and electron capture rates for 55Co and 56Ni

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 (0.01×109–30×109 K) and density (10–1011 gcm−3) 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.

Enhanced electron-capture decay rate of 7Be encapsulated in C60 cages.

The decay rate of 7Be electron capture was measured in C60 and Be metal with a reference method. The half-life of 7Be endohedral C60 ((7)Be@C(60)) and 7Be in Be metal (Be metal (7Be)) is found to be 52.68+/-0.05 and 53.12+/-0.05 days, respectively. This amounts to a 0.83% difference in electron-capture decay half-life between (7)Be@C(60) and Be metal (7Be). Our result is a reflection of the different electron wave functions for (7)Be@C(60) inside C60 compared to the situation when 7Be is in a Be metal.