Electron Capture Rates Research Articles

Implications of the recent 59Co(n,p)59Fe experiment for stellar electron capture rates.

We study results from the recent measurement of the 0\ifmmode^\circ\else\textdegree\fi{} $^{59}\mathrm{Co}$(n,p${)}^{59}$Fe cross section in order to compute the stellar electron capture rate of $^{59}\mathrm{Co}$. This nucleus is the second most important electron capture nucleus in the core of a type II supernova progenitor just before core collapse. It is also the first nucleus which has a large effect on the stellar neutronization rate for which Gamow-Teller strength has been measured. The Gamow-Teller resonance has been observed experimentally at a higher energy above the daughter ground state than one would expect from independent particle shell model arguments, but large-basis shell model calculations employing configuration mixing are more successful in predicting the location of the strength. Because of the high energy of the Gamow-Teller resonance, the stellar electron capture rate and associated neutronization is much less than previous estimates had indicated. Similar trends are seen for $^{60}\mathrm{Co}$, the most important electron capture nucleus for supernova progenitors just before core collapse.

Photoelectrochemical catalysis of the oxidation of organic molecules by oxygen on small semiconductor particles with TiO 2 as an example

The kinetics of the photocatalytic oxidation of organic molecules at TiO 2 particles are analyzed with the aim of determining the conditions required to obtain high quantum yields. With the assumption that holes react at the interface faster than electrons, the particles contain, in the steady state, an excess of electrons under illumination. It is shown that the rate of electron capture by oxygen is a decisive parameter for the yield. It also transpires that only very small particles can reach a high yield.

The quasi-static evolution of ONeMg cores - Explosive ignition densities and the collapse/explosion alternative

The quasi-static evolution of an electron-degenerate ONeMg core is followed up to the point of explosive Ne-O ignition triggered by electron capture. Recently calculated electron-capture rates on 24Mg, 24Na, 20Ne, and 20F are used, and the ignition density is found to be about 8.5 x 10 exp 9 g/cu cm (when the Ledoux criterion for convection is adopted), instead of about 10 exp 10 g/cu cm as in earlier calculations. Such a comparatively low ignition density should lead to complete explosive disruption of the core even if the thermonuclear burning propagated all the way as a conductive, laminar flame. The dependence of the ignition density on the treatment adopted for mixing in the semiconvective region and the astronomical implications of our results are briefly discussed.

The fate of Be-7 in the sun

The electron- and proton-capture rates of 7 Be important to the solar neutrino «problem» are re-examinated. Although the assumptions implied by the traditional Debye approximation for plasma screening are not valid, a careful numerical study changes the electron capture rate by less than 2%. We extrapolate experimental data on the proton capture reaction to astrophysically relevant energies using an energy dependence that includes d-wave scattering and is shown to be relatively independent of the model space and interaction used. It is found that the solar proton capture rate is lowered by approximately 7% from the currently accepted value

Possibility of observing massive neutrino admixtures in nuclear orbital electron capture rates.

The unusual case in nuclear orbital electron capture whereby either a neutrino in a weak interaction eigenstate or in a mass eigenstate is emitted by the same nucleus is examined. The case of the low-energy Q value nuclear electron capture decays is considered where, for at least one electronic shell, the emission of a weak eigenstate neutrino having an admixture of a second mass eigenstate neutrino (with mass in the keV/${\mathrm{c}}^{2}$ range) is energetically prohibited. This feature, in particular, opens the possibility for a new approach to searching for massive neutrino admixtures using relative electron capture rates. Five candidates are specifically identified, of which one, $^{157}\mathrm{Tb}$, is conceivably of immediate interest to the 17 keV/${\mathrm{c}}^{2}$ neutrino mass question. Finally experimental prospects and limitations are described.

The influence of the DX centre C-V and I-V characteristics of Schottky barriers in n-type AlGaAs

Due to the vanishing of both electron emission and capture rates, the DX centre shows a non-equilibrium occupancy at low temperature. It is shown that this effect results in a non-uniform free-electron density profile that can be controlled by biasing the junction during cooling. Preliminary results are reported showing that the I-V characteristics of Schottky barriers on AlGaAs (x=0.25) are affected by the DX centre occupation.

DX-center-related features by capacitance measurements in AlGaAs

The role of quasistatic C-V measurements in investigating DX-center-related features in AlGaAs (x=0.25, 0.30, and 0.35) Schottky barriers has been reconsidered under different experimental conditions. The vanishing of the electron-capture rate by the DX center, at low temperature, is responsible for a frozen-step-like density profile of positively charged DX centers near the metal-semiconductor interface. This causes a knee-shaped 1/C2-vs-V plot and gives rise to an apparent built-in potential. The low-temperature freezing in of the free-electron density in the flatband region has been demonstrated through specific experiments of thermally stimulated capacitance and low-temperature C-V measurements performed on the sample cooled at different cooling rates.

Stellar electron capture rates and the 54Fe(n, p) experiment

Limits on stellar electron capture rates are estimated for 54Fe, 55Fe, 53Mn, 59Co, and 60Co using the results of the recent (n, p) experiment upon 54Fe. It is found that the rates currently used in stellar evolution codes of massive stars are not within these limits and can be different by as much as a factor of six in astrophysically relevant regions. These differences are the result of differing treatments of the Gamow-Teller strength in iron-peak elements. The sensitivity of the stellar electron capture rates to uncertainties in the Gamow-Teller strength function is explored. These uncertainties result from the paucity of information about Gamow-Teller strength functions and from experimental uncertainties.

Microscopic calculation of the rates of electron captures which induce the collapse of O+Ne+Mg cores

The rates of electron captures which induce the collapse of the stellar cores composed of 16O, 20Ne and 24Mg are calculated microscopically using the shell model wave functions of Wildenthal. Since these are the best wave functions available for sd-shell nuclei, the capture rates calculated in this paper are at present the most reliable ones. Our electron capture rates differ up to an order of magnitude from those previously calculated with the gross theory and FFN. The difference of the capture rates is analyzed and the effects on the evolution of the O + Ne + Mg core are discussed. In the appendix, we tabulate the capture rates as well as the associated neutrino energy loss rates and γ-ray heating rates in the form available for the calculation of the evolution of the O + Ne + Mg core.

Electron attachment reactions of perfluoroalkyl transition metal carbonyls: Rate constants and product analysis

Absolute rate constants for electron attachment reactions of CF3Mn(CO)5, CF3Co(CO)4, and C2F5Co(CO)4 have been measured in a gas flow system with electron cyclotron resonance (ECR) detection. Free electrons were generated by Penning ionization of argon gas by helium metastables produced in a low power radio-frequency discharge. The pressure was 2.3–2.5 Torr. The rate constants at 293 K in units of cm3 molecule−1 s−1 are (3.5±0.6)×10−7 for CF3Mn(CO)5; (2.0±0.4)×10−7 for CF3Co(CO)4; and (1.8±0.3)×10−7 for C2F5Co(CO)4. Negative ion mass spectra indicate that nondissociative attachment is important at thermal electron energy. The cross section for nondissociative capture falls rapidly with increasing electron energy. Dissociative product channels are also observed. Electron capture rate constants for several additional molecules are presented and compared with values from the literature. The rate constants in units of cm3 molecule−1 s−1 are 2.8×10−7 for SF6; 3.7×10−7 for CCl4; 3.1×10−7 for CFCl3; 1.8×10−9 for CF2Cl2; 1.5×10−8 for CF3Br; 3.8×10−9 for CHCl3; and 1.2×10−8 for C2F3Cl3.

Ultrafast studies of carrier relaxation in semiconductors and their microstructures

Recent advances in ultrafast lasers have made it possible to investigate fundamental properties of matter on femtosecond timescales using excite-and-probe as well as luminescence spectroscopy. We review here some recent studies of carrier relaxation in semiconductors and semiconductor microstructures using these ultrafast spectroscopic techniques. We discuss (1) hot carrier relaxation in bulk and 2D semiconductors, (2) intervalley scattering in GaAs, (3) carrier-carrier interactions in 2D systems and (4) capture rates of electrons and holes by quantum wells.

The DX Center: Evidence for Charge Capture Via an Excited Intermediate State

AbstractWe review three important experimental results which suggest that electron capture and emission by the DX center in AlxGa1-xAs proceeds via an excited intermediate state: the very different dependencies of the thermal capture and emission rates on alloy composition, the exponential dependence of the thermal capture rate on the quasi-equilibrium Fermi energy, and the thermal activation of the hot electron capture rate. None of these results is readily explained by a conventional lattice relaxation model, in which an electron is captured directly from the lowest lying band edge, but each can be simply explained if the dominant channel for multiphonon capture is via a transition state which lies well above the band edge. This picture is consistent with recent pseudopotential calculations which predict that the lattice relaxed state (the DX state) is stabilized by capture of more than one electron, since such a model naturally admits the possibility of an intermediate one-electron state.

Non-linearity in constant-current electron-capture detection

The effects of the ionization source activity, sample input rate, electron capture rate constant, ventilation rate constant and baseline frequency on the linearity, detectability and sensitivity of the constant-current electron-capture detector (CC-ECD) were investigated. The proposed model representing an extension of the Wenthworth-Lovelock model proved to be useful in demonstrating that the CC-ECD can be highly nonlinear for strong electron capturers, at high ionization source activities, at a low ventilation rate constant and at low baseline frequencies. Optimization of the CC-ECD operation is possible by a reasonable adjustment of the studied parameters.

Characterization of the Mn acceptor level in GaAs

We report on deep-level transient spectroscopy investigations of the Mn acceptor level in GaAs, including measurements of the emission and capture of holes and of the capture of electrons. A comparison is made between electron capture rates obtained from space-charge techniques and from photoluminescence decay measurements.

Far-infrared magnetooptical absorption in In0.53Ga0.47As

Abstract By observing far-infrared electron cyclotron resonance in photoexcited In x Ga 1−x As ( x = 0.53) at 4.2 K, as well as the Zeeman absorption signal due to donor electrons, the electron transfer between the conduction band and donor states is investigated. Impact ionization of donor electrons by photoexcited electrons has been confirmed. From the time resolved measurement, the apparent lifetime of conduction electrons is found to be 17 μs and the momentum relaxation time 6.7 × 10 −13 s. The time resolved absorption measurement gives a definite clue to the electron capture rate by a donor and the recombination rate at an acceptor center.

The physics of stellar core collapse

The hydrodynamics that develops during the infall of the core of a massive star at the end of its hydrostatic evolutionary epoch can have a profound effect on its subsequent dynamical history. In this paper the important processes governing the infall hydrodynamics are explored both semi-analytically and numerically with the aim of explaining the results of large-scale numerical calculations. We focus in particular on the factors responsible for reducing the mass of the inner core. These factors are found to be mainly associated with general relativistic effects and with the production, equilibration, and transport ofv e 's. Semi-leptonicv e emission and absorption are responsible for virtually all of thev e production. They dominate the energy transfer rate between matter andv e 's, but, with the rates being skewed to high energy, they are an importantv e -matter equilibrator only for the high-energy tail of thev e distribution. Neutrinoelectron scattering (NES) plays the dominant role inv e -matter equilibration with the important result that the equilibration is nearly complete by trapping. A careful examination of conditions where the onset of neutrino trapping is occurring indicates that the trapping density is a function of the enclosed mass, increasing with decreasing distance from the core center, and increasing with the extent of thev e -matter equilibration. Though not the dominant contributor to thev e -matter energy transfer rate, NES dominates the matter entropy production rate with another important result that the electron capture rate on free protons is significantly increased before trapping. The result of all of this is a greatly reduced inner core mass by the time of core turn-around. For numerical calculations employing the LLPR equation of state (or something similar), and for precollapse models having iron cores as small as 1.35M ⊙ (the smallest used by the author), this reduction of the inner core mass leads to the generation of a weak shock which fails to eject mass in a promptffashion. The effect of the small inner core mass on the subsequent dynamical history of the core is unknown.

Defect levels in electrodeposited n-type CdTe thin films

Admittance spectroscopy is used to measure electron traps near the conduction band of the n-type electrodeposited CdTe thin films. Several samples of Ni-CdTe Schottky-barrier and Ni-TeO2-CdTe devices are used in the measurement. Two donor levels with activation energies around 0.12 eV and between 0.19 and 0.26 eV are detected. In addition, there may be one or more shallow donor levels within 0.04 eV from the conduction band. Capacitance loops are observed in the capacitance-voltage measurements performed at different temperatures and bias sweep rates. The activation energies, densities, and electron-capture cross sections of the two slow electron-trap levels responding to the cyclical ramp bias are estimated. The rise and decay photocapacitance transient measurements at 295 K reveal two electron-trap levels with photoionization threshold energies of 0.8 and 1.3 eV. These two levels are suspected to correspond to the other two slow electron-trap levels estimated in the capacitance-voltage measurement and located around or above the midgap. The differences between thermal and optical ionization energies and the slow electron-capture rates of these two levels may be explained by the nonradiative, thermally activated capture processes with large lattice relaxation. Numerous defects around and below the midgap are detected by the steady-state photocapacitance measurement at 100 K. A band of seven defect levels spreading within 0.6 to 1.0 eV is observed. Large capacitance changes are recorded at 1.37, 1.41, and 1.45 eV. These three levels are speculated to be the major compensating acceptor levels. Based on both the photocurrent and photocapacitance measurements, electrons emitted from the 1.02-, 1.06-, 1.21-, and 1.28-eV levels are believed to arise from the defects at the TeO2-CdTe interface.

A comparison of recent numerical calculations of stellar core collapse

view Abstract Citations (23) References (40) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A Comparison of Recent Numerical Calculations of Stellar Core Collapse Bruenn, Stephen W. Abstract An analysis is given of the principal factors responsible for the large deleptonizations obtained during infall by the author's recent detailed modeling of stellar collapse. Comparisons are made with many other recent calculations, with the aim of understanding the reasons for the different deleptonizations obtained. It is found that the differences between the various models can be accounted for in detail by one of the following: (1) the particular neutrino transport algorithm used; (2) the values selected for adjustable parameters; (3) the choice of input physics, e.g., equation of state, nuclear electron capture rates, or initial precollapse model. Publication: The Astrophysical Journal Supplement Series Pub Date: October 1986 DOI: 10.1086/191143 Bibcode: 1986ApJS...62..331B Keywords: Gravitational Collapse; Leptons; Particle Motion; Stellar Cores; Density Distribution; Entropy; Equations Of State; Neutrinos; Stellar Models; Trapped Particles; Astrophysics; DENSE MATTER; EQUATION OF STATE; NEUTRINOS; STARS: COLLAPSED; STARS: INTERIORS full text sources ADS |

Hot-electron induced interface traps in metal/SiO2/Si capacitors: The effect of gate-induced strain

It is shown that the generation of interface traps by Fowler–Nordheim injection, like those generated by ionizing radiation, is a function of the mechanical strain at the silicon-silicon dioxide interface. However, because of the current enhancement at the edge of a metal-oxide-semiconductor gate when the gate serves as the hot-electron injector, this phenomenon may only be evident when the device is stressed with the gate biased positively. It is also shown that the capture rate of electrons in the silicon dioxide depends on the strain in the film.

On the carrier emission from donor-related centres in GaAs 1- xP x and Al xGa 1- xAs

From an experimental study of the electron thermal emission and capture rates of Te-related centres, donor-related (DX) traps have been identified in GaAsP and GaAlAs compounds. It is shown that under thermal excitation nonexponential transient waveforms markedly deviate from theory. The relation of the nonexponential degree with alloy composition and junction electric field has been studied. In addition, optical activated transients, a model based on trap charge dependent emission and capture coefficients, and the drawbacks of Deep Level Transient Spectroscopy (DLTS) techniques to study DX centres are presented.