Electron Capture Rates Research Articles

Low energy electron attachment by bromoalkanes

Thermal electron attachment processes in the mixtures of CH3CH2CH2Br, CH3CHBrCH3, CH2BrCH2Br, CH2FCH2Br and CF3CH2Br with carbon dioxide have been investigated using an electron swarm method. It has been found that all the investigated compounds attach electrons only in a two-body process. Corresponding rate constants have been determined to be equal to 1.1×10−11, 1.4×10−12, 1.8×10−8, 5×10−11 and 1.4×10−9cm3molecule−1s−1, respectively. The dependence of the electron capture rate constants on electronic polarizability of the accepting center of the molecule has been demonstrated. The average rate constant for CH3Br, CH3CH2Br and CH3CH2CH2Br equal to (6±2)×10−12cm3molecule−1s−1 is recommended.

High-resolution determination of GT strength distributions relevant to the presupernova evolution using the (d,2He) reaction

Spin–isospin excitations of the medium-mass nucleus 58Co were studied using the (n,p)-type (d,2He) charge-exchange reaction at an incident energy Ed=170 MeV and small scattering angles. The achieved energy resolution of 130 keV, full width at half maximum, allowed the extraction of Gamow–Teller strength to 13 peaks at excitation energies lower than 4.1 MeV in the residual nucleus 58Co and is consistent with the results of a multipole-decomposition analysis. The identification of Gamow–Teller transitions is possible by comparison of the measured cross sections with DWBA calculations. The results are used to discuss the relevance of the Gamow–Teller strength distribution at low excitation energy with regard to electron-capture rates used in calculations for the final stage of a massive star.

Low-Energy Electron Attachment by Chloroalkanes

Thermal electron attachment rate constants for 1,1,2-trichloroethane, 1,1,2,2-tetrachloroethane, 1- and 2-chloropropanes, and 1,1-, 1,2-, 2,2-, and 1,3-dichloropropanes have been measured using an electron swarm method. It has been found that all the investigated compounds attach electrons only in a two-body process. Corresponding rate constants are equal to 1.4 × 10-10, 3.2 × 10-8, 2.7 × 10-13, 3.8 × 10-12, 5.7 × 10-11, 8.1 × 10-12, 6.3 × 10-12, and 1.2 × 10-11 cm3 molecule-1 s-1, respectively. The dependence of the electron capture rate constants on the electronic polarizability of the accepting center of the molecule and the vertical attachment energy has been demonstrated.

Estimates of Stellar Weak Interaction Rates for Nuclei in the Mass Range A = 65–80

We estimate lepton capture and emission rates, as well as neutrino energy loss rates, for nuclei in the mass range A = 65-80. These rates are calculated on a temperature/density grid appropriate for a wide range of astrophysical applications including simulations of late time stellar evolution and X-ray bursts. The basic inputs in our single-particle and empirically inspired model are (i) experimentally measured level information, weak transition matrix elements, and lifetimes, (ii) estimates of matrix elements for allowed experimentally unmeasured transitions based on the systematics of experimentally observed allowed transitions, and (iii) estimates of the centroids of the GT resonances motivated by shell model calculations in the fp shell as well as by (n, p) and (p, n) experiments. Fermi resonances (isobaric analog states) are also included, and it is shown that Fermi transitions dominate the rates for most interesting proton-rich nuclei for which an experimentally determined ground state lifetime is unavailable. For the purposes of comparing our results with more detailed shell model based calculations we also calculate weak rates for nuclei in the mass range A = 60-65 for which Langanke & Martinez-Pinedo have provided rates. The typical deviation in the electron capture and β-decay rates for these ≈30 nuclei is less than a factor of 2 or 3 for a wide range of temperature and density appropriate for presupernova stellar evolution. We also discuss some subtleties associated with the partition functions used in calculations of stellar weak rates and show that the proper treatment of the partition functions is essential for estimating high-temperature β-decay rates. In particular, we show that partition functions based on unconverged Lanczos calculations can result in errors in estimates of high-temperature β-decay rates.

Electron capture rates on nuclei and implications for stellar core collapse.

Supernova simulations to date have assumed that during core collapse electron captures occur dominantly on free protons, while captures on heavy nuclei are Pauli blocked and are ignored. We have calculated rates for electron capture on nuclei with mass numbers A=65-112 for the temperatures and densities appropriate for core collapse. We find that these rates are large enough so that, in contrast to previous assumptions, electron capture on nuclei dominates over capture on free protons. This leads to significant changes in core collapse simulations.

High resolution studies of low lying GT strength using the (d, 2He) reaction and its impact on electron-capture rates in stellar environments

Gamow-Teller transitions in fp-shell nuclei have been studied using the 58Ni( d, 2He) charge exchange reaction at an incident energy of 170 MeV and forward angles. The measurements have been performed at AGOR using the Big-Bite Spectrometer and the EuroSuperNova focal-plane detection system. The unprecedented energy resolution of about 130 keV FWHM for excitations in the residual nucleus 58Co allowed for the first time the extraction of single transitions in the β + direction up to about 4 MeV excitation. The impact on electron-capture rates in astrophysical environments has been evaluated.

The state of7Be in the core of the Sun and the solar neutrino flux

The exact ionization state of 7 Be in the solar core is crucial for the precise prediction of the solar 8 B neutrino flux. We therefore examine the effect of pressure ionization on the ionization state of 7 Be and all elements with 12 ≥Z≥4. We show that under the conditions prevailing in the solar core one has to consider the effect of the nearest neighbour on the electronic structure of a given ion. To this goal, we first solve the Schrodingerand then the Kohn-Sham equations for an ion immersed in a dense plasma under conditions for which the mean interparticle distance is smaller than the Debye radius. The question of which boundary conditions should be imposed on the wavefunction is discussed, examined and found to be crucial. Contrary to previous estimates showing that beryllium is partially ionized, we find that it is fully ionized.As a consequence, the predicted rate of the 7 Be + e- reaction is reduced by 20-30 per cent, depending on the details of the solar model. Because 7 Beis a trace element, its total production is controlled by the unchanged 4 He + 3 He reaction rate, and its destruction is determined by the rate of electron capture. As the latter rate decreases when the beryllium is fully ionized (relative to the case of partially ionized Be), the estimate for the abundance of 7 Beincreases and with it the 8 B neutrino flux. The increase inΦ v ( 8 B) is by about 20-30 per cent. The neutrino flux due to 7 Be electron capture remains effectively unchanged because the change in the rate is compensated for by a change in the abundance. Hence the prediction for the ratio of Φ v ( 8 B)/Φ v ( 7 Be) changes as well.

Electron capture and shock formation in core-collapse supernovae

New supernova progenitor models incorporating modern electron capture and beta decay rates have recently become available. One of the more notable differences in these new models is an increase in the electron fraction throughout the iron core when compared to earlier models. We describe fully self-consistent radiation hydrodynamic simulations of core collapse and shock formation using both sets of models, including both a standard set of electron capture rates for supernova simulations and a parameterized scheme meant to approximate the modern updates.

Electron capture rates for core collapse supernovae

Electron capture rates on neutron-rich nuclei (A>65) were calculated within the Random Phase Approximation with partial number formalism, including allowed and forbidden transitions. The partial occupation numbers were provided as a function of temperature by Shell-Model Monte Carlo calculations, including an pairing+quadrupole interaction. Capture rates on relevent nuclei were calculated for density and temperature conditions during the core collapse of a massive star. It was found that electron captures on nuclei can compete with electron captures on free protons. Furthermore, they produce neutrinos with average energies lower than neutrinos emitted from captures on free protons, with possible consequences on the cooling of the core.

Effects of Electron Screening on Electron Capture Rates inHigh Density Presupernova Core

Considering that the electron capture rate can be greatly reduced by theelectron screening effect in the regime of high temperature and highdensity, we calculate the effect of electron screening on the electroncapture rate for two important elements 56Ni and 55Co inthe high density condition of a pre-supernova star. The effect ofelectron screening is so effective that the electron capture rate may bereduced to about 20%.

Non-linear effects on the power dependent photocurrent of self-assembled InAs/GaAs quantum dots

We identify fundamental mechanisms of electron and hole dynamics from self-organized InAs/GaAs quantum dots subject to vertical electric fields by photocurrent and photoluminescence investigations. We observed a nonlinear power dependent photocurrent that is associated with charge accumulation into the dots. Steady-state rate equations have been used to simulate the charge accumulation process. The solution has shown that the charge accumulation should increase the capture rate of electrons into the dots.

Screening Corrections to the Electron Capture Rates in Dense Stars by the Relativistically Degenerate Electron Liquid

We calculate the screening corrections to the electron capture rates in dense stars by the relativistically degenerate electron liquid. In order to calculate the screening corrections we adopt the linear response theory which is widely used in the field of solid state physics and liquid metal physics. In particular, we use the longitudinal dielectric function for the relativistically degenerate electron liquid derived by Jancovici. We calculate the screening potential at the position of the nucleus. By using this screening potential one can calculate the screening corrections to the electron capture rates. We will present accurate analytic fitting formulae which summarize our numerical results. These fitting formulae will facilitate the application of the present results. The screening corrections to the electron capture rates are typically a few percent.

Photoluminescent and electronic properties of nanocrystalline silicon doped with gold

Doping nanocrystalline silicon (nc-Si) films grown by laser ablation with gold leads to a considerable suppression of the nonradiative recombination of the charge carriers and excitons, an increase in the intensity and stability of the visible photoluminescence, and enhancement of the low-energy (1.5–1.6 eV) photoluminescence band. In Au-doped samples, the magnitude of the photovoltage and the rate of electron capture by traps in the film are reduced, and the density of boundary electron states and the concentration of deep electron traps at the single-crystal silicon (c-Si) substrate are decreased as well. The observed effect of doping on the photoluminescent and electronic properties of nc-Si films and nc-Si/c-Si structures is caused by the passivation of dangling Si bonds with Au and by the further oxidation of silicon at the surface of nanocrystals, which results in the formation of high-barrier SiO2 layers.

Effect of host medium on theL/Kratio in7Beelectron capture

We discuss in the framework of linear muffin-tin orbital method and Bahcall's calculations how the ratio of L- to K-shell electron capture rates $(L/K$ ratio) in ${}^{7}\mathrm{Be}$ would be affected by the host medium in which ${}^{7}\mathrm{Be}$ is implanted. Our calculations show that the recently observed discrepancy between the measured and observed $L/K$ ratio in ${}^{7}\mathrm{Be}$ could be understood quantitatively as a result of in-medium effects distorting the L-shell electron orbital of ${}^{7}\mathrm{Be}.$

Practical formulations of the electron capture rate in quantum wells by phonon emission at low carrier density

In this article the Green’s functions formalism is exploited to describe the coupled phonon-plasmon system in the plasmon pole approximation for the longitudinal dielectric constant, for two-dimensional quantum wells. Electron capture time in quantum wells is obtained in a closed form, separating contributions coming from different two-dimensional plasmon modes. Moreover, useful criteria are pointed out, in order to decide under which conditions a simpler description in the static limit of the plasmon pole approximation may be enough accurate. In the limit of low carrier density, an analytical expression for the capture rate by LO-phonon emission has been given, in the two-dimensional static limit of the longitudinal dielectric constant. By means of this analytic description, effects of screening, Fermi filling, and electrons momentum distribution at finite temperature on capture time may be separately investigated in detail.

Ion mass effect on vacancy-related deep levels in Si induced by ion implantation

The ion mass effect on the dominating vacancy related deep levels in Cz-Si implanted with 4-MeV C ions and 6-MeV Si ions has been investigated by deep level transient spectroscopy (DLTS). It is found that the intensity of the DLTS signal for the doubly negative charge state of the divacancy $[{V}_{2}(=/\ensuremath{-})]$ deviates from a one-to-one correlation with that of the singly negative charge state of the divacancy $[{V}_{2}(\ensuremath{-}/0)]$ and decreases, compared to ${V}_{2}(\ensuremath{-}/0),$ with increasing ion mass. Capture kinetics studies reveal that the electron-capture rate for ${V}_{2}(=/\ensuremath{-})$ decreases with increasing ion mass, while that for ${V}_{2}(\ensuremath{-}/0)$ has a weaker dependence on ion mass. In this work, we suggest a model to explain most of the known experimental observations of the ion mass effect for ${V}_{2}(=/\ensuremath{-}).$ The model assumes a local compensation of the carrier concentration in highly disordered regions located within the collision cascades. In addition, it has been observed that the DLTS signal of the vacancy-oxygen pair exhibits a trend similar to that for ${V}_{2}(=/\ensuremath{-})$ regarding the ion mass effect.

The annealing of interstitial carbon atoms in high-resistivity n-type silicon after proton irradiation

The annealing of interstitial carbon C i after 7– 10 MeV and 23 GeV proton irradiations at room temperature in high-resistivity n-type silicon is investigated. Deep level transient spectroscopy is used to determine the defect parameters. The annealing characteristics of the impurity defects C i, C iC s, C iO i and VO i suggest that the mobile C i atoms are also captured at divacancy VV sites at the cluster peripheries and not only at C s and O i sites in the silicon bulk. The deviation of the electrical filling characteristic of C i from the characteristic of a homogeneously distributed defect can be explained by an aggregation of C i atoms in the environment of the clusters. The capture rate of electrons into defects located in the cluster environment is reduced due to a positive space charge region surrounding the negatively charged cluster core. The optical filling characteristic of C i suggests that the change of the triangle-shaped electric field distribution in a reverse biased p +n junction due to charged clusters is negligible.

Carrier dynamics in spatially ordered InAs quantum dots

AbstractSpatial ordering of InAs quantum dots was attained by using misfit dislocations generated in a metastable InGaAs layer by means of thermal annealing. Influence of quantum dot positional ordering and dot proximity to dislocation arrays on carrier dynamics was studied by timeresolved photoluminescence. Substantially narrower inhomogeneous broadening from the ordered quantum dots was observed. Excitation intensity dependence of the photoluminescence intensity and carrier lifetime indicates stronger influence of nonradiative recombination for the ordered quantum dot structures. Numerical simulations allow estimating electron and hole capture rates from the quantum dots to traps located either at the quantum dot interfaces or in the vicinity of the quantum dots.

Presupernova Evolution with Improved Rates for Weak Interactions

Recent shell-model calculations of weak-interaction rates for nuclei in the mass range A = 45-65 have resulted in substantial revisions to the hitherto standard set of Fuller, Fowler, & Newman (FFN). In particular, key electron-capture rates, such as that for 60Co, are much smaller. We consider here the effects of these revised rates on the presupernova (post-oxygen burning) evolution of massive stars in the mass range 11-40 M☉. Moreover, we include, for the first time in models by our group, the effects of modern rates for beta decay in addition to electron capture and positron emission. Stars of 15, 25, and 40 M☉ are examined in detail using both the full FFN rate set and the revised rates. An additional finely spaced (in mass) grid of 34 models is also calculated in order to give the systematics of iron core and silicon core masses. Values for the central electron mole number at the time of iron core collapse in the new models are typically larger, by ΔYe = 0.005-0.015, than those of Woosley & Weaver, with a tendency for the more massive models to display larger differences. About half of this change is a consequence of including beta decay; the other half, the result of the smaller rates for electron capture. Unlike what might be expected solely on basis of the larger Ye values, the new iron core masses are systematically smaller owing to a decrease in the entropy in the outer iron core. The changes in iron core mass range from zero to 0.1 M☉ (larger changes for high-mass stars). It would be erroneous however to estimate the facility of exploding these new models based solely upon their iron core mass since the entire core structure is altered and the density change is not so different as the adjustments in composition might suggest. We also observe, as predicted by Aufderheide et al., a tendency toward beta equilibrium just prior to the collapse of the core, and the subsequent loss of that equilibrium as core collapse proceeds. This tendency is more pronounced in the 15 M☉ model than in the heavier stars. We discuss the key weak reaction rates, both beta decay and electron capture, responsible for the evolution of Ye and make suggestions for future measurements.

Influence of physical and chemical environments on the decay rates of 7Be and 40K

We measured the electron-capture decay rate of 7Be implanted into hosts of graphite, boron nitride, tantalum, and gold. We have found that this decay rate varies by as much as 0.38% from one host to another. We also measured the electron-capture decay rate of 40K in four different chemical compounds and as both a solid and dissolved in solution. To within our measurement precision of ±1%, we have found no influence of the environment on the 40K decay rate. The implications of these results for the solar neutrino problem and for potassium/argon dating are discussed.