共查询到20条相似文献，搜索用时 31 毫秒
1.
I. V. Panov I. Yu. Korneev G. Martinez-Pinedo F. -K. Thielemann 《Astronomy Letters》2013,39(3):150-160
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
2.
The most recent fission-barrier calculations based on improved mass formulas indicate that the adopted values are underestimated. We analyze the dependence of the fission rates on the fission barrier and show that an increase in the fission barriers leads not so much to a decrease in the importance of fission as to the possible synthesis of heavier elements in the r-process. The rates of induced fission of most isotopes with Z>80 at astrophysical energies have been calculated for the first time for fission barriers obtained from different theoretical models. 相似文献
3.
John A. O'Keefe III 《Earth, Moon, and Planets》1974,9(1-2):219-225
Supporting evidence for the fission hypothesis for the origin of the Moon is offered. The maximum allowable amount of free iron now present in the Moon would not suffice to extract the siderophiles from the lunar silicates with the observed efficiency. Hence extraction must have been done with a larger amount of iron, as in the mantle of the Earth, of which the Moon was once a part, according to the fission hypothesis. The fission hypothesis gives a good resolution of the tektite paradox. Tektites are chemically much like products of the mantle of the Earth; but no physically possible way has been found to explain their production from the Earth itself. Perhaps they are a product of late, deep-seated lunar volcanism. If so, the Moon must have inside it some material with a strong resemblance to the Earth's mantle. Two dynamical objections to fission are shown to be surmountable under certain apparently plausible conditions. 相似文献
4.
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
5.
Abstract— Based on 43 documented drill core samples of the H5 chondrite Jilin, the depth profile of track densities was investigated. The study of olivine and pyroxene crystals extracted from the two drill cores A and B of the main fragment no. 1 yields a track background for the Jilin interior of 150 ± 90 cm?2. To estimate the contribution of different track origins, the U microdistribution of sample sections and single grain sections was mapped by neutron-induced 235U-fission using high-purity quartz glass detectors. The average U content of 0.59 ± 0.22 ppb for pyroxenes and 0.12 ± 0.06 ppb for olivines is consistent with the observed fission track background in these minerals. There are no indications of spontaneous fission of 244Pu, secondary mobilisation of U in the bulk sample, or substantial track annealing. As deduced from the calculated fission track density, the track contribution from galactic cosmic rays is negligible in the core samples investigated. 相似文献
6.
We show that for the discussed scenario of a neutron-star merger in highly neutronized ejecta (Y e ?0.1), neutron-induced fission plays a major role in the r-process cycling and is the main obstacle to the formation of superheavy elements. At the final stage of the r-process, when the free-neutron density is already too low to maintain rapid nucleosynthesis and only beta-decay and beta-delayed fission take place, the leading role in forming the final abundances of chemical elements passes to delayed fission. The latter ultimately changes the abundances of individual isotopes in the region before the second peak and heavier than lead, which, in particular, affects the determination of the age of the Galaxy. 相似文献
7.
Teruaki Ohnishi 《Astrophysics and Space Science》1975,34(2):321-345
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
8.
Abstract— Possible evidence for the presence of 248Cm in the early Solar System was reported from fission gas studies (Rao and Gopalan, 1973) and recently from studies of very high nuclear track densities (≥ 5 × 10g cm?2) in the merrillite of the H4 chondrite Forest Vale (F. V.) (Pellas et al., 1987). We report here an analysis of the isotopic abundances of xenon in F. V. phosphates and results of track studies in phosphate/pyroxene contacts. The fission xenon isotopic signature clearly identifies 244Pu as the extinct progenitor. We calculate an upper limit 248Cm/244Pu < 1.5 × 10?3 at the beginning of Xe retention in F. V. phosphates. This corresponds to an upper limit of the ratio 248Cm/235U ≤ 5 × 10?5, further constraining the evidence for any late addition of freshly synthesized actinide elements just prior to Solar System formation. The fission track density observed after annealing the phosphates at 290 °C (1 hr, which essentially erases spallation recoil tracks) is also in agreement with the 244Pu abundance inferred from fission Xe. The spallation recoil tracks produced during the 76 Ma cosmic-ray exposure account for the very high track density in merrillites. 相似文献
9.
The results of the determination of the fission track age of the Brahin pallasite are presented. Crystals of phosphates (stanfieldites), with an enhanced uranium concentration, were found directly in the investigated samples. Fossil tracks in them are represented by two generations. The distribution and partial annealing of first-generation track lengths point to a shock/thermal event at an early stage in the cosmic history of the pallasite. The value of the fission track age, registering the time of its passage, was computed from second-generation tracks. It was shown that the second generation is represented mainly by tracks of the spontaneous fission of 238U and 244Pu nuclei (Pu/U 4). The calculations performed made it possible to establish that the last intensive thermal event in the cosmic history of the Brahin pallasite occurred 4.26–4.20 billion years ago. The temperature of the reheating of its parent body did not rise above 500°C. The excellently preserved character of the second-generation tracks provides evidence that the pallasite underwent subsequent cosmic evolution in mild conditions. 相似文献
10.
L. Levsky 《Meteoritics & planetary science》1983,18(2):155-158
A new composition of primordial terrestrial xenon is derived, on the assumption that it lies on an extension of the mixing line between solar Xe and anomalous (CCF) Xe in carbonaceous chondrites. With this composition, the apparent fission components in atmospheric and well gas Xe become larger, and resemble 244Pu fission xenon. 相似文献
11.
D.J. Scheeres 《Icarus》2007,189(2):370-385
The energetics and dynamics of contact binary asteroids as they approach and pass the rotational fission limit is studied. We presume that the asteroids are subject to an external torque, such as from the YORP effect, that increases their angular momentum. Furthermore, we assume the asteroids can be described by a fairly minimal model comprised of a sphere and ellipsoid resting on each other. The minimum energy configurations for contact binary asteroids at different levels of angular momentum are computed and discussed. We find distinct transitions between different configurations as the angular momentum of the system is increased. These indicate that rapidly rotating contact binary asteroids may seek out clearly different relative configurations than slowly rotating systems. We find a single end state of the systems prior to rotational fission, and distinct dynamical outcomes as a function of mass distribution and shape when the rotational fission limit is exceeded. Our theoretical results agree qualitatively with observed properties of near-Earth asteroids, and can be used to help explain the spin-rate barrier, contact binaries, and the observed morphology of most NEO binaries. 相似文献
12.
We present a model of near-Earth asteroid (NEA) rotational fission and ensuing dynamics that describes the creation of synchronous binaries and all other observed NEA systems including: doubly synchronous binaries, high-e binaries, ternary systems, and contact binaries. Our model only presupposes the Yarkovsky-O’Keefe-Radzievskii-Paddack (YORP) effect, “rubble pile” asteroid geophysics, and gravitational interactions. The YORP effect torques a “rubble pile” asteroid until the asteroid reaches its fission spin limit and the components enter orbit about each other (Scheeres, D.J. [2007]. Icarus 189, 370-385). Non-spherical gravitational potentials couple the spin states to the orbit state and chaotically drive the system towards the observed asteroid classes along two evolutionary tracks primarily distinguished by mass ratio. Related to this is a new binary process termed secondary fission - the secondary asteroid of the binary system is rotationally accelerated via gravitational torques until it fissions, thus creating a chaotic ternary system. The initially chaotic binary can be stabilized to create a synchronous binary by components of the fissioned secondary asteroid impacting the primary asteroid, solar gravitational perturbations, and mutual body tides. These results emphasize the importance of the initial component size distribution and configuration within the parent asteroid. NEAs may go through multiple binary cycles and many YORP-induced rotational fissions during their approximately 10 Myr lifetime in the inner Solar System. Rotational fission and the ensuing dynamics are responsible for all NEA systems including the most commonly observed synchronous binaries. 相似文献
13.
D. A. Leich S. B. Kahl A. R. Kirschbaum S. Niemeyer D. Phinney 《Earth, Moon, and Planets》1975,14(3-4):407-444
Rare gas isotopic analyses have been performed on both pile-irradiated and unirradiated samples from Boulder 1, Station 2. Two samples from rock 72255, the Civet Cat clast and a sample of adjacent breccia, have concordant40Ar-39 Ar ages of 3.99±0.03 b.y. and 4.01±0.03 b.y., respectively. Several samples from rock 72275 have complex thermal release patterns with no datable features, but an intermediate-temperature plateau from the dark rim material of the Marble Cake clast yields an age of 3.99±0.03 b.y. - indistinguishable from the age of rock 72255. We regard these ages as upper limits on the time of the Serenitatis basin-forming event. The absence of fossil solar-wind trapped gases in the breccia samples implies that a prior existence for the boulder as near-surface regolith material can be regarded as extremely unlikely. Instead, the small trapped rare-gas components have isotopic and elemental compositions diagnostic of the terrestrial-type trapped component which has previously been identified in several Apollo 16 breccias and in rock 14321. Excess fission Xe is found in all Boulder 1 samples in approximately 1:1 proportions with Xe from spontaneous fission of238U. This excess fission Xe is attributed to spontaneous fission of244Puin situ. Cosmic-ray exposure ages for samples from rocks 72215 and 72255 are concordant, with mean81Kr-Kr exposure ages of 41.4±1.4 m.y. and 44.1±3.3 m.y., respectively. However a distinctly different81Kr-Kr exposure age of 52.5±1.4 m.y. is obtained for samples from rock 72275. A two-stage exposure model is developed to account for this discordance and for the remaining cosmogenic rare-gas data. The first stage was initiated at least 55 m.y. ago, probably as a result of the excavation of the boulder source-crop. A discrete change in shielding depths ～ 35 m.y. ago probably corresponds to the dislodgement of Boulder 1 from the South Massif and emplacement in its present position. 相似文献
14.
A new scenario is offered for the origin of the Moon. It is assumed that the Earth formed initially with about the maximum amount of angular momentum consistent with dynamical stability. This state is approximated by the secularly unstable Maclaurin spheroids (highly flattened, hamburger-shaped bodies). It is shown that the Earth cannot depart from this state at a resonable rate as long as its viscosity is in the range of liquid rock. Since core formation supplies about 1600 kJ kg?1 the Earth will not leave this state until core formation is complete. When cooling produces a rise in viscosity, the Earth will necessarily evolve along a path which is approximated by the Riemann ellipsoids (which have rapid internal motion). The evolution is toward a Jacobi ellipsoid, but it is intercepted by the development of a third-harmonic (pearshaped) instability, which is catastrophic and leads to fission. The process of fission itself may be fundamentally analogous to the breaking of a wave in water. We cannot exclude the possibility that some other planets evolved similarly. 相似文献
15.
Abstract— In a study of the isotopic signatures of trapped Xe in shock-produced glass of shergottites and in ALH 84001, we observe three components: (1) modern Martian atmospheric Xe that is isotopically mass fractionated relative to solar Xe, favoring the heavy isotopes, (2) solar-like Xe, as previously observed in Chassigny, and (3) an isotopically fractionated (possibly ancient) component with little or no radiogenic 129Xerad. In situ-produced fission and spallation components are observed predominantly in the high-temperature steps. Heavy N signatures in ALH 84001, EET 79001 and Zagami reveal Martian atmospheric components. The low-temperature release of ALH 84001 shows evidence for the presence of a light N component (δ15N ≤ -21%), which is consistent with the component observed in the other Shergotty, Nakhla and Chassigny (SNC) group meteorites. The highest observed 129Xe/130Xe ratio of 15.60 in Zagami and EET 79001 is used here to represent the present Martian atmospheric component, and the isotopic composition of this component is compared with other solar system Xe signatures. The 129Xe/130Xe ratios in ALH 84001 are lower but appear to reflect varying mixing ratios with other components. The consistently high 129Xe/130Xe ratios in rocks of different radiometric ages suggest that Martian atmospheric Xe evolved early on. As already concluded in earlier work, only a small fission component is observed in the Martian atmospheric component. Assuming that a chondritic 244Pu/129I initial ratio applies to Mars, this implies that either Pu-derived fission Xe is retained in the solid planet (in fact, in situ-produced fission Xe is observed in ALH 84001) or may reflect a very particular degassing history of the planet. 相似文献
16.
Nuclear explosions near the surface of a neutron star occur because of the nuclear fission of superheavy nuclei which is overabundant in neutrons. Such nuclei exist in the nonequilibrium layer of the neutron stars solid envelope and are transported close to the surface in starquake events. These explosions may be observed as γ-ray bursts. 相似文献
17.
D. J. Scheeres 《Celestial Mechanics and Dynamical Astronomy》2017,128(2-3):131-148
When gravitational aggregates are spun to fission they can undergo complex dynamical evolution, including escape and reconfiguration. Previous work has shown that a simple analysis of the full 2-body problem provides physically relevant insights for whether a fissioned system can lead to escape of the components and the creation of asteroid pairs. In this paper we extend the analysis to the full 3-body problem, utilizing recent advances in the understanding of fission mechanics of these systems. Specifically, we find that the full 3-body problem can eject a body with as much as 0.31 of the total system mass, significantly larger than the 0.17 mass limit previously calculated for the full 2-body problem. This paper derives rigorous limits on a fissioned 3-body system with regards to whether fissioned system components can physically escape from each other and what other stable relative equilibria they could settle in. We explore this question with a narrow focus on the Spherical Full Three Body Problem studied in detail earlier. 相似文献
18.
Model calculations show that the thermal history of a Moon which originated by fission from the proto-Earth is the same as that for the Moon as it is currently understood. In particular, a fissioned Moon currently has a small percent of partial melt or at least near solidus temperatures below depths of 800 km in accord with the seismic data which show that the deep interior of the Moon has a very lowQ. The models have moderate (20–50%) degrees of partial melting in the upper mantle (depths < 300 or 200 km) in the period between 3 to 4 × 109 years ago and, therefore, can account for the mare filling epoch. Finally the heat flow of the models is 18 ergs cm–2 s–1 which is close to the average of 19 ergs cm–2 s–1 derived from the Apollo heat flow experiments. These findings add further support for the fission origin of the Moon. 相似文献
19.
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
20.
V. G. Gorbatsky 《Astrophysics and Space Science》1975,33(2):325-332
Nova-like binary systems are similar to W UMa-systems in their basic physical characteristics. Outwardly such systems are different — nova-like systems contain a white dwarf as a component, while both components of a W UMa-system are near the Main Sequence. A hypothesis is proposed, seeking the origin of contact W UMa-type systems in a fission of rapidly-rotating helium isothermal core of an evolved giant star. The contraction of the more massive component leads to the formation of a white dwarf and, consequently, to a transformation of a W UMa-type system into a nova-like system. 相似文献