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1.
Jameel-Un Nabi 《Astrophysics and Space Science》2012,339(2):305-315
Electron capture and positron decay rates are calculated for neutron-deficient Kr and Sr waiting point nuclei in stellar matter.
The calculation is performed within the framework of pn-QRPA model for rp-process conditions. Fine tuning of particle-particle,
particle-hole interaction parameters and a proper choice of the deformation parameter resulted in an accurate reproduction
of the measured half-lives. The same model parameters were used to calculate stellar rates. Inclusion of measured Gamow-Teller
strength distributions finally led to a reliable calculation of weak rates that reproduced the measured half-lives well under
limiting conditions. For the rp-process conditions, electron capture and positron decay rates on 72Kr and 76Sr are of comparable magnitude whereas electron capture rates on 78Sr and 74Kr are 1–2 orders of magnitude bigger than the corresponding positron decay rates. The pn-QRPA calculated electron capture
rates on 74Kr are bigger than previously calculated. The present calculation strongly suggests that, under rp-process conditions, electron
capture rates form an integral part of weak-interaction mediated rates and should not be neglected in nuclear reaction network
calculations as done previously. 相似文献
2.
Jameel-Un Nabi 《Astrophysics and Space Science》2011,331(2):537-554
This paper reports on the microscopic calculation of ground and excited states Gamow-Teller (GT) strength distributions, both
in the electron capture and electron decay direction, for 54,55,56Fe. The associated electron and positron capture rates for these isotopes of iron are also calculated in stellar matter. These
calculations were recently introduced and this paper is a follow-up which discusses in detail the GT strength distributions
and stellar capture rates of key iron isotopes. The calculations are performed within the framework of the proton-neutron
quasiparticle random phase approximation (pn-QRPA) theory. The pn-QRPA theory allows a microscopic state-by-state calculation of GT strength functions and stellar capture rates which greatly increases the reliability of the results. For
the first time experimental deformation of nuclei are taken into account. In the core of massive stars isotopes of iron, 54,55,56Fe, are considered to be key players in decreasing the electron-to-baryon ratio (Y
e
) mainly via electron capture on these nuclide. The structure of the presupernova star is altered both by the changes in Y
e
and the entropy of the core material. Results are encouraging and are compared against measurements (where possible) and
other calculations. The calculated electron capture rates are in overall good agreement with the shell model results. During
the presupernova evolution of massive stars, from oxygen shell burning stages till around end of convective core silicon burning,
the calculated electron capture rates on 54Fe are around three times bigger than the corresponding shell model rates. The calculated positron capture rates, however,
are suppressed by two to five orders of magnitude. 相似文献
3.
《New Astronomy Reviews》2002,46(8-10):529-533
We discuss the prospects for direct measurements of nucleosynthesis yields of electron capture decay radioactive nuclei from supernovae, by detecting the characteristic K X-rays of the daughters. Several isotopes in galactic supernova remnants and SN 1987A appear to be detectable now or in the near future. Precise measurements of these nuclear yields, mostly achievable no other way, would greatly constrain nucleosynthesis theory. 相似文献
4.
The rates for a variety of beta decay processes have been determined as a function of temperature for nuclei which can participate in thes-process production of heavy elements, occurring in the presence of the22Ne(α,n)25 Mg neutron source operating in the convective helium shells of thermally pulsing stars. Specifically: calculated half-lives are presented for electron emission, positron emission, and electron capture over the temperature range 108–109 K. 相似文献
5.
Magnetars are a type of pulsars powered by magnetic field energy. Part of the X-ray luminosities of magnetars in quiescence have a thermal origin and can be fitted by a blackbody spectrum with the surface temperature, much higher than the typical values for rotation-powered pulsars. The persistent thermal emissions and bursts of magnetars indicate the presence of some internal heat sources in their outer crusts. In this work, we have formulated the energy balance equation and applied it to investigate the thermal evolution in the magnetar crust, taking into account the heating mechanisms of Ohmic decay and electron capture processes. This model can explain the changes in the X-ray luminosity of the magnetars. 相似文献
6.
7.
强磁场对非零温中子星壳层电子俘获反应的影响 总被引：6，自引：0，他引：6
本文讨论了强磁场作用下非零温电子气体的化学势，分析了磁场作用下电子气体屏蔽势的变化；以核素３３Ｓ为例，讨论了不同温度下，磁场对电子俘获率的影响，结果表明：在足够低的温度和密度下，足够强的磁场使电子俘获率显著降低，而就中子星表面存在的磁场强度（１０９－１０１３Ｇ）而言，磁场对其电子俘获率几乎没有影响． 相似文献
8.
We investigate the process of photon capture by strong magnetic fields, by transforming it into a positronium, and the subsequent decay of the positronium into two photons. We discuss the implications of this process for the polar gap models of pulsars. We find that the capture process is energy-dependent and photons above a certain energy (depending on the magnetic field) are not captured and can decay into electron-positron pairs. This leads to increased gap heights in the modle and leads to higher luminosities than found earlier. We also find that it may be possible for very high-energy positronia to escape the magnetosphere of the pulsar and be observed near the Earth as photons in agreement with the recent observations of 1012 eV gamma-rays from some pulsars. 相似文献
9.
One of the light elements created during the big bang nucleosynthesis is 7Be which then decays to 7Li by electron capture when recombination becomes effective but well before the Saha equilibrium recombination is reached.
This means that 7Be should wait until its recombination epoch even though the half-life of the hydrogenic beryllium atom is only 106.4 days.
We calculate when the conversion from primordial 7Be to 7Li occurs taking into account the population of the hyperfine structure sublevels and solving the kinetic equations for recombination,
photoionization and conversion rate. We also calculate the energies and the spectrum of two narrow neutrino lines from 7Be decay. 相似文献
10.
The rates of electron capture on heavier elements under the extreme conditions predicted for dwarf star supernovae have been computed, incorporating modifications that seem to be indicated by present experimental results. An estimate of the maximum possible value of such rates is also given. The distribution of nuclei in nuclear statistical equilibrium has been calculated for the range of expected supernovae conditions, including the effects of the temperature dependence of nuclear partition functions. These nuclide abundance distributions are then used to compute nuclear equilibrium thermodynamic properties. The effects of the electron capture on such equilibrium matter are discussed. The results of supernova numerical hydrodynamics incorporating the computed equilibrium properties and the influence of electron capture are presented. In the context of the ‘carbon detonation’ supernova model, the dwarf central density required to assure core collapse to a neutron star configuration is found to be slightly higher than that obtained by Bruenn (1972) with the electron capture rates of Hansen (1966). 相似文献
11.
A. G. W. Cameron J. J. Cowan H. V. Klapdor J. Metzinger T. Oda J. W. Truran 《Astrophysics and Space Science》1983,91(2):221-234
R-process yields for a helium layer have been calculated within a network of 6033 heavy nuclei using a steady flow approximation. The calculation of the neutron capture cross sections has been improved. The beta decay rates computed by Klapdor and his colleagues have been used in the calculation. We find that ther-process yield peaks near mass numbers 80 and 130 require a neutron number density of approximately 1020 cm?3 and a freezing time comparable to or less than 0.1 s. The peak near mass number 195 requires a neutron number density of about 1021 cm?3 and a freezing time comparable to or less than about 0.01 s. The individual yield features of the steady flowr-process depend entirely upon the neutron capture cross sections of the nuclei along the flow path and the beta decay rates, which can shift the flow path and thereby introduce inappropriate capture cross sections into the determination of the yields. 相似文献
12.
A resume of the main results obtained in the framework of fast electron hypothesis is presented. The fast electrons are possessed by strong constant properties, that is, nearly monoenergetic in spectrum and with the energy of 3. Fast electrons appear spontaneously in all kinds of cool dwarfs, young and old, and stimulate the generation of all types of flares, faint and powerful, in all ranges of electromagnetic waves — optical, ultraviolet, radio, X-ray. The general flare mechanisms are inverse Compton effect at the faint and moderate flares and nonthermal bremsstrahlung at powerfulness. This hypothesis explains an enormous number of observational facts and relationships — all these are illustrated by two tree-like diagrams in Figures 1 and 2.In the second part, the problem of the spontaneous appearance of fast electrons above the stellar atmosphere is discussed. The nature of the fast electrons in universal and is independent of the physical conditions of the star or its atmosphere. Their origin have nothing in common also with the well known acceleration processes, including the magnetic and hydrodynamic nature, in stellar atmospheres. At present nuclear decay processes seem to be most probable for the appearance of fast electrons in outer regions of stellar atmospheres. Some decay processes are examined with various probability of their realization. Creation of the radioactive nucleus13N as a result of capture of H by12C and its consequent decay with the escape of a positron seems most probable; as this fast positron will act just like a fast electron. It is outlined the high effectivity of this process in binary systems. 相似文献
13.
14.
《天文和天体物理学研究(英文版)》2017,(10)
Taking into account the effect of electron screening on electron energy and electron capture threshold energy, by using the method of Shell-Model Monte Carlo and random phase approximation theory, we investigate the capture rates of chromium isotopes with strong electron screening according to the linear response theory screening model. Strong screening rates can decrease by about 40.43%(e.g., for ~(60)Cr at T_9 = 3.44, Y_e = 0.43). Our conclusions may be helpful to researches on supernova explosions and related numerical simulation methods. 相似文献
15.
Based on the work of Wang et al. (Chin. Phys. Lett. 29:049701, 2012), we re-investigated electron capture on iron group nuclei in the outer crust of magnetars and studied magnetar evolution. Effects of ultra-strong magnetic field on electron capture rates for 57Co have been analyzed in the nuclear shell model and under the Landau-level-quantization approximation, and the electron capture rates and the neutrino energy loss rates on iron group nuclei in the outer crust of magnetar have been calculated. The results show that electron capture rates on 57Co are increase greatly in the ultra-strong magnetic field, and above 3 orders of magnitude generally; and the neutrino energy loss rates by electron capture on iron group nuclei increase above 3 orders of magnitude in the range from B=4.414×1013 G to B=4.414×1015 G. These conclusions play an important role in future studying the evolution of magnetar. Furthermore, we modify the expressions of the electron chemical potential (Fermi energy) and phase space factor by introducing Dirac δ-function, and select appropriate parameters of temperature T, magnetic field B and matter density ρ in the our crust, thus our results will be reliable than those of Wang et al. 相似文献
16.
C. C. Harvey 《Solar physics》1976,46(2):509-509
An attempt is made to explain the observed frequency-time profiles of type III solar radiobursts in terms of a rapid plasma wave decay rate combined with the exciter model recently proposed by the author. The decay rate is assumed to be sufficiently rapid for the plasma wave energy density profile to be similar to the excitor power density time profile; this is consistent with the exciter model, the rapid decay being caused by Landau damping on the electrons of the modified high energy tail of the ambient plasma electron velocity distribution. The model is compared with radio observations by making simple assumptions about the dependence of the radio intensity upon the plasma wave energy. A comparison is made with simultaneous radio and electron observations by further assuming a simple power-law velocity distribution for the electrons at their point of ejection from the Sun. 相似文献
17.
《天文和天体物理学研究(英文版)》2015,(9)
When a daughter nucleus produced by electron capture takes part in a level transition from an excited state to its ground state in accreting neutron star crusts, thermal energy will be released and heat the crust, increasing crust temperature and changing subsequent carbon ignition conditions. Previous studies show that the theoretical carbon ignition depth is deeper than the value inferred from observations because the thermal energy is not sufficient. In this paper, we present the de-excited energy from electron capture of rp-process ash before carbon ignition, especially for the initial evolution stage of rp-process ash, by using a level-to-level transition method. We find the theoretical column density of carbon ignition in the resulting superbursts and compare it with observations. The calculation of the electron capture process is based on a more reliable level-to-level transition, adopting new data from experiments or theoretical models(e.g., large-scale shell model and proton-neutron quasi-particle random phase approximation). The new carbon ignition depth is estimated by fitting from previous results of a nuclear reaction network. Our results show the average de-excited energy from electron capture before carbon ignition is ~0.026 Me V/u, which is significantly larger than the previous results. This energy is beneficial for enhancing the crust's temperature and decreasing the carbon ignition depth of superbursts. 相似文献
18.
It has been argued that the decay rates of several radioactive nuclides are slightly lower at Earth’s aphelion than at perihelion, and that this effect might depend on heliocentric distance. It might then be expected that nuclear decay rates be considerably lower at larger distances from the sun, e.g., in the asteroid belt at 2–3 AU from where most meteorites originate. If so, ages of meteorites obtained by analyses of radioactive nuclides and their stable daughter isotopes might be in error, since these ages are based on decay rates determined on Earth. Here we evaluate whether the large data base on nuclear cosmochronology offers any hint for discrepancies which might be due to radially variable decay rates. Chlorine-36 (t1/2 = 301,000 a) is produced in meteorites by interactions with cosmic rays and is the nuclide for which a decay rate dependence from heliocentric distance has been proposed, which, in principle, can be tested with our approach and the current data base. We show that compilations of 36Cl concentrations measured in meteorites offer no support for a spatially variable 36Cl decay rate. For very short-lived cosmic-ray produced radionuclides (half-lives < 10–100 days), the concentration should be different for meteorites hitting the Earth on the incoming vs. outgoing part of their orbit. However, the current data base of very short-lived radionuclides in freshly fallen meteorites is far from sufficient to deduce solid constraints. Constraints on the age of the Earth and the oldest meteorite phases obtained by the U–Pb dating technique give no hints for radially variable decay rates of the α-decaying nuclides 235U or 238U. Similarly, some of the oldest phases in meteorites have U–Pb ages whose differences agree almost perfectly with respective age differences obtained with “short-lived” radionuclides present in the early solar system, again indicating no variability of uranium decay rates in different meteorite parent bodies in the asteroid belt. Moreover, the oldest U–Pb ages of meteorites agree with the main-sequence age of the sun derived from helioseismology within the formal ∼1% uncertainty of the latter. Meteorite ages also provide no evidence for a decrease of decay rates with heliocentric distance for nuclides such as 87Rb (decay mode β−) 40K (β− and electron capture), and 147Sm (α). 相似文献
19.
P. Belli R. Bernabei W. Di Nicolantonio V. Landoni A. Incicchitti D. Prosperi C. J. Dai C. Bacci 《Astroparticle Physics》1996,5(3-4)
New limits on the mean lifetime of the electron for decay into non-ionizing particles have been achieved by using a low activity liquid xenon scintillator at the Gran Sasso National Laboratory. 相似文献
20.
Martin S. Spergel 《Earth, Moon, and Planets》1977,17(2):123-131
Modeling of the Jovian atmosphere shows that cosmic ray induced albedo neutron decay is inadequate to account for Pioneer 10 and 11 projected electron levels on Jupiter. High energy solar protons must also be excluded as an important neutron decay source. Analysis of neutron flux data near the top of the Jovian atmosphere can lead to the determination of He/H2 and3He/4He ratios for the Jovian atmosphere. 相似文献