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1.
The steady flow approximation to ther-process has been used to evaluate the validity of the waiting point assumption (that, for a given charge numberZ, neutron captures become balanced by neutron photodisintegrations) under a variety of conditions. The criterion for the accuracy of the approximation for any set of nuclei with the same value ofZ is that the ratio of the total rate of neutron photodisintegration in those nuclei to the total rate of neutron captures should be close to unity. We have failed to find any set of conditions involving neutron production by helium thermonuclear reactions (at temperatures up to 109 K) for which the waiting point approximation is useful for all values ofZ, and we recommend that all suchr-process calculations not use this approximation. At higher temperatures, such as those characteristic of explosive carbon burning (2×109 K), the waiting point approximation is much better.  相似文献   

2.
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   

3.
The formation of heavy elements in the neutron star merger scenario is considered. In such a scenario, the duration of the r-process is long and when the nucleosynthesis wave passes through the region of actinides, beta-delayed, neutron-induced, and spontaneous fission are added to the main r-process reaction channels. The dependence of the formation of superheavy elements on spontaneous fission model is investigated numerically. The formation of nuclei lighter than the cadmium-peak elements and cosmochronometer nuclei is shown to depend on strongly on the spontaneous fission model used in nucleosynthesis calculations. The regions of nuclei with short spontaneous fission half-lives prevent the formation of superheavy elements in the r-process, but the prediction of their yields is so far inaccurate because of an insufficient accuracy of calculating a number of transactinide parameters. The relative contributions from neutron-induced, beta-delayed, and spontaneous fission have been determined for various spontaneous fission models in the nucleosynthesis scenario considered.  相似文献   

4.
Abstract— The classical model of s-process nucleosynthesis, based on the concept of a steady neutron flux under astrophysical conditions pertaining to the He-burning phase of red giant stars, has successfully described observed isotopic abundances and provided information on the physical conditions of the s-process environment. Because most of the isotopes on the s-process path are stable, their relevant nuclear parameters can be measured in the laboratory so that as more accurate elemental abundance and neutron capture cross-section data have become available, the classical model has been tested under increasingly stringent conditions. Accurate determinations of the neutron capture cross sections at appropriate astrophysical conditions for the Ba isotopes have shown that the abundance of the s-only isotope 136Ba is under-produced by ～20% according to the classical model. This paper describes the accurate assessment of the meteoritic abundance of Ba by the stable isotope dilution mass spectrometric technique, based on the Cl carbonaceous chondrites Orgueil and Ivuna. Repeated analyses of these two Cl chondrites give an abundance that is identical to the presently accepted solar system value for Ba within experimental errors, which indicates a deficiency in the classical model. When combined with similar data for the s-only nuclides 116Sn and 142Nd, it is apparent that the classical model, having served a valuable function for many years, must be replaced by stellar models that more accurately reflect the dynamic nature of the He-burning phase in red giant stars, in particular, during the thermal pulses of low-mass asymptotic giant branch (AGB) stars.  相似文献   

5.
The concept of the r-process path is considered from the standpoint of a dynamic model. Rapid nucleosynthesis is shown to proceed not along certain preferential lines called the r-process paths but in the region of nuclei bounded on the one side by the existing nuclei and on the other side by nuclei upon reaching which the r-process enters a cooling phase. This view is shown to account for the main heavy-element abundance patterns.  相似文献   

6.
During the formation of heavy elements in the neutron star merger (NSM) scenario with a fairly long duration of the r-process, most of the seed nuclei rapidly burn out at the initial stage. The nucleosynthesis wave rapidly reaches the region of actinoids, where beta-delayed, neutron-induced, and spontaneous fission are the main reaction channels. The fission products of transuranium elements are again drawn into the r-process as new seed nuclei to form the yields of elements with mass numbers A > 100. The contribution from the various types of fission to the formation of heavy and superheavy nuclei is investigated. The proposed r-process model applied to the NSM scenario describes well the observed abundances of chemical elements, which confirms the formation of the main r-process component in the NSM scenario. Simple extrapolations of the spontaneous fission half-lives are shown to be inapplicable for the region of nuclei with N ∼ 184, because the formulas do not reflect the increase in half-life when the shell structure changes as the number of neutrons approaches 184. The formation of superheavy elements in the r-process is possible, but their survival depends to a large extent on how reliable the predictions of nuclear parameters, including the half-lives of the forming nuclei from the island of long-lived isotopes, are. The contributions from various types of fission—neutron-induced, beta-delayed, and spontaneous one—to the formation of heavy elements in the main r-process have been determined.  相似文献   

7.
The rapid neutron capture process (r-process) is one of the major nucleosynthesis processes responsible for the synthesis of heavy nuclei beyond iron. Isotopes beyond Fe are most exclusively formed in neutron capture processes and more heavier ones are produced by the r-process. Approximately half of the heavy elements with mass number A > 70 and all of the actinides in the solar system are believed to have been produced in the r-process. We have studied the r-process in supernovae for the production of heavy elements beyond A = 40 with the newest mass values available. The supernova envelopes at a temperature >109 K and neutron density of 1024 cm?3 are considered to be one of the most potential sites for the r-process. The primary goal of the r-process calculations is to fit the global abundance curve for solar system r-process isotopes by varying time dependent parameters such as temperature and neutron density. This method aims at comparing the calculated abundances of the stable isotopes with observation. We have studied the r-process path corresponding to temperatures ranging from 1.0 × 109 K to 3.0 × 109 K and neutron density ranging from 1020 cm?3 to 1030 cm?3. With temperature and density conditions of 3.0 × 109 K and 1020 cm?3 a nucleus of mass 273 was theoretically found corresponding to atomic number 115. The elements obtained along the r-process path are compared with the observed data at all the above temperature and density range.  相似文献   

8.
The neutron capture cross-sections of124, 132, 134Xe have been measured by the activation technique at 25 keV neutron energy. These data were supplemented by calculated capture cross-sections for128, 129, 130, 131Xe via the statistical model. The complete set of capture cross-sections obtained in this way served to determine the solar xenon abundance throughs-process systematics and to study a variety of isotopic anomalies.  相似文献   

9.
Possibilities for the development of an r-process in the helium shell of a supernova under the action of free neutrons appearing as a result of inelastic neutrino scattering by 4He nuclei are investigated. The conditions in the outer part of the helium shell in metal-poor stars are shown to be favorable for the reproduction of a weak r-process component.  相似文献   

10.
We discuss the influence of nuclear masses and mass distributions of fission products on the formation of heavy elements at the final stages of the r-process recycled through fission on long duration timescales. The fission recycling is of great importance in an environment with a high density of free neutrons (e.g., in neutron star merger scenarios), when the r-process duration is long enough for most of the seed nuclei to be transformed into actinoids. The fission products of transuranium elements are again drawn into the r-process to produce the abundance curve beyond the iron peak. In this case, to explain the abundances of the A ～ 130 peak elements, not only the nuclear masses, fission barriers, and reaction rates, but also the fission product mass distribution must be predicted. Our r-process calculations using new nuclear masses and fission barriers and reaction rates based on them have shown that the simple two-fission-fragment model used previously in r-process calculations cannot describe adequately the position of the second peak in the observed abundance curve. We show that agreement between calculations and observations can be achieved only when we properly consider the mass distribution of fission products by taking into account the emission of instantaneous fission neutrons.  相似文献   

11.
The rapid neutron capture process (r-process) is one of the major nucleosynthesis processes responsible for the synthesis of heavy nuclei beyond iron. Isotopes beyond Fe are most exclusively formed in neutron capture processes and more heavier ones are produced by the r-process. Approximately half of the heavy elements with mass number A>70 and all of the actinides in the solar system are believed to have been produced in the r-process. We have studied the r-process in supernovae for production of heavy elements beyond A=40 with the newest mass values available. The supernovae envelopes at a temperature >109 K and neutron density of 1024 cm−3 are considered to be one of the most potential sites for the r-process. We investigate the r-process in a site-independent, classical approach which assumes a chemical equilibrium between neutron captures and photodisintegrations followed by a β-flow equilibrium. We have studied the r-process path corresponding to temperatures ranging from 1.0×109 K to 3.0×109 K and neutron density ranging from 1020 cm−3 to 1030 cm−3. The primary goal of the r-process calculations is to fit the global abundance curve for solar system r-process isotopes by varying time dependent parameters such as temperature and neutron density. This method aims at comparing the calculated abundances of the stable isotopes with observation. The abundances obtained are compared with supernova explosion condition and found in good agreement. The elements obtained along the r-process path are compared with the observed data at all the above temperature and density range.  相似文献   

12.
Empirical equations for the estimation of fission fragment yield are proposed. These equations can be applied to all nuclides in the nuclidic region of 208?A andZ 2/A<40.2, and they can explain experimental results within an accuracy of 20%. By applying these empirical equations to nuclear astrophysics, the contribution of fission fragments to nuclidic abundance in the mass region of 100?A?170 is estimated, in which mass region the fission fragments from fissioning nuclei withA>250 accumulate. These nuclei withA>250 are produced byr-process and make delayed spontaneous fissions duringr-process cooling. Furthermore, a possibility is proposed that the anomalously high abundance of medium weight elements observed in some peculiar A stars may be the accumulation of the fission fragments from the fissioning nuclei in 250?A?265.  相似文献   

13.
Neutron capture measurements made on a sample of fission-product zirconium containing 20%93Zr(t 1/2=1.5×106a) at the Oak Ridge Electron Linear Accelerator time-of-flight facility resulted in the identification of 138 resonance peaks for the93Zr isotope at neutron energies up to 21.5 keV. Average capture cross sections from 20 to 300 keV were derived by subtracting neutron capture yields of the stable zirconium isotopes90, 91, 92, 94, 96Zr and additional backgrounds. The average cross sections found were significantly less than those of JENDL-1. While generally 30% higher than those of ENDF/B —V below 60 keV, the binned data overlapped the smooth ENDF/B —V curve. The average for a Maxwellian neutron spectrum withkT=30 keV is (95±10) mb and the resonance contribution to the capture resonance integral is (15.0±0.5)b.Research sponsored by the U.S. Department of Energy, Office of Basic Energy Sciences, under contract DE-AC05-840R21400 with the Martin Marietta Energy Systems, Inc.  相似文献   

14.
Many unidentified lines in the spectrum of Przybylski’s star (HD 101065) match well the spectral lines of radioactive elements, including the lines of short-lived isotopes. The origin of such isotopes in the atmosphere of the star remains unknown. We discuss a scenario in which some heavy nuclei of radioactive elements are produced in Przybylski’s star atmosphere as a result of its permanent exposure to the gamma flux (photon-nuclear reactions) or of the direct interaction of the free neutrons which arise in the atmosphere with seed nuclei of lighter elements (r-process). In both cases the gamma quanta and the free neutrons originate due to the presence of the neutron star which forms a close binary system with Przybylski’s star. This neutron star is the source of the fast electrons and positrons which produce gamma quanta when they are decelerated in their interaction with the atomic nuclei of the atmospheric plasma, while free neutrons can originate in the reactions of the direct capture of fast electrons by atmospheric protons (nuclei of hydrogen atoms).  相似文献   

15.
A physical model based on the open‐source toolkit Geant4 for production rates of cosmogenic nuclei on the lunar surface is proposed and calibrated. The fluxes of proton and neutron beneath the lunar surface are obtained by simulating the physical processes between the cosmic‐ray particles and the lunar surface material. By combining the experimental proton cross sections and the a posteriori neutron cross sections, we calculate the production rate depth profiles of long‐lived nuclei (10Be, 14C, 26Al, 36Cl, and 53Mn). Through comparing experimental and theoretical data for these nuclei, we find that for all the selected nuclei, experimental and theoretical production rate depth profiles agree well with each other by introducing a single normalization factor. It means that the physical model based on Geant4 can also reproduce the depth profiles of cosmogenic nuclei, and that this model can be used by everyone worldwide. In addition, we predict the production rates of three stable nuclei (21Ne, 22Ne, and 38Ar).  相似文献   

16.
The implications of recent studies of the dynamics of the cores of highly evolved massive stars are considered with regard to the general problems of nucleosynthesis. The typical conditions estimated for these models are shown to be very promising for the process of element synthesis by neutron capture on a fast time scale (ther-process ofBurbidge et al., 1957).  相似文献   

17.
The effect of nuclear fissions occurring during ther-cooling stage is investigated. The initial conditions for ther-cooling process are derived from the final conditions of the dynamicalr-process. Neutron-excess nuclides in the region ofZ80 are all assumed to have finite probabilities for spontaneous fission, neutron-induced fission and -decay-induced fission. For the evaluation of the rates for delayed neutron emission, -decay-induced fission and -decay, three -decay theories—the Fermi theory, the Gross theory and the theory by Cameron, Delano and Truran—are used. It becomes clear that the -fissions and the symmetric spontaneous fissions duringr-cooling make a broad abundance peak of fission products, centring aroundA=128, whose effect is hidden by ther-abundance peak atA130. Asymmetric spontaneous fissions can form an apparent abundance peak atA105, with the position of this peak just corresponding to the position of the anomalous hump observed in ther-abundance curve. To explain the abundance of this observed hump by spontaneous fissions, ther-process must take place under conditions such that the number of free neutrons available for one seed nucleus is large enough to make the cyclicr-process, and that the final neutron-capture-path goes through the region of (neutron binding energy in the nuclei on the capture-path) 2 MeV.  相似文献   

18.
The enhanced -decay rate of ionized187Re in stars has been studied within the context of a detailed numerical model of the production ofr-process elements and their recycling through stars during the course of galactic evolution. It is concluded that the enhanced decay rate does not significantly reduce the Re-Os chronometer age for the Galaxy. Consequently, the187Re -decay half-life and the neutron cross-section ratio (186Os)/(187Os) remain as the significant uncertainties in this chronology. Unlike the uncertainties in other chronologies, both are amenable to laboratory measurements.  相似文献   

19.
The supernova yields of r-process elements are obtained as a function of the mass of their progenitor stars from the abundance patterns of extremely metal-poor stars on the left-side [{Ba/Mg}]--[{Mg/H}] boundary with a procedure proposed by Tsujimoto and Shigeyama. The ejected masses of r-process elements associated with stars of progenitor mass M ms ≤ 18 M ⊙ are infertile sources and the SNe II with 20 M ⊙ ≤ M ms ≤ 40 M ⊙ are the dominant source of r-process nucleosynthesis in the Galaxy. The ratio of these stars 20 M ⊙ ≤ M ms ≤ 40 M ⊙ with compared to the all massive stars is about∼ 18%. In this paper, we present a simple model that describes a star's [r/Fe] in terms of the nucleosynthesis yields of r-process elements and the number of SN II explosions. Combined the r-process yields obtained by our procedure with the scatter model of the Galactic halo, the observed abundance patterns of the metal-poor stars can be well reproduced.  相似文献   

20.
Recently synthesized (t109 yr)r-process material has actinide elemental and isotopic abundance characteristics which can be used to distinguish it from ancientr-process material. The time-dependence of relative actinide abundances provides detailed chronometric information. Also it is shown that, ifN=184 is a neutron magic number as predicted by nuclear thery, the resultingr-process production peak atA281 will in turn yield, due to symmetric fission, a broad abundance peak aroundA135–140. The present results can be used to help verify or refute some current hypotheses concerning the origin of UH cosmic rays and some of the abundance anomalies in peculiar stars.  相似文献   

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