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1.
A probabilistic criterion is proposed for the scission of a fissile nucleus into fragments. The probability of the rupture of the neck between would-be fragments is estimated by considering scission as a fluctuation. The energy of the prescission configuration and the energy of the separated-fragment configuration are computed on the basis of a macroscopic model that takes into account a finite range of nuclear forces and the diffuseness of the nuclear surface. The effect of the probabilistic criterion of nuclear scission on fission-process observables, such as the moments of the mass-energy distribution of fission fragments, the mean multiplicity of prescission neutrons, and mean fission times, is demonstrated. It is shown that the Strutinsky criterion, according to which nuclear scission occurs at a finite neck radius of 0.3R0, is a rather good approximation to the probabilistic scission criterion in Langevin dynamical calculations employing the one-body nuclear-viscosity mechanism modified in such a way that the wall-formula contribution is reduced, the reduction factor satisfying the condition k s <05.  相似文献   

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We evaluate the temperature Tscis at the scission point and the saddle-to-scission time τscis for the fission of heated nuclei. We use classical Lagrange-like equations of motion within the liquid-drop model. The nuclear surface is parameterized by a two-parameter family of the Lawrence shapes. Conservative forces are defined through the free energy of the nucleus at finite temperatures. We use the friction tensor that is derived from the Navier-Stokes momentum-flux tensor and which takes into account the boundary conditions at the nuclear surface. The scission line is determined from the instability condition of the nuclear shape with respect to variations of the neck radius. A numerical solution to the dynamical equations is obtained for the 236U nucleus. The viscosity coefficient μ is deduced from a comparison of experimental data on the kinetic energy of fission fragments with the computed one. It is found that μ obtained by using our approach deviates significantly from μ of the standard hydrodynamic model.  相似文献   

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An experiment has been carried out to study neutron-neutron angular correlations in spontaneous fission of 252Cf. Angular dependences of the number of neutron-neutron coincidences obtained in the experiment were compared with the results of the Monte Carlo calculations for various neutron detection thresholds in the range 425–1600 keV. It was inferred that 10–11% of the total number of prompt neutrons from 252Cf (s.f.) in the laboratory system were emitted isotropically and may probably be interpreted as neutrons directly associated with the instant of scission of the nucleus. The analysis allowed their energy distribution to be determined as well. A similar method was also used to describe the angular correlation of prompt neutrons that accompanied the reaction 235U(n th ,f).  相似文献   

5.
The anisotropy of angular distributions of fission fragments and the average multiplicity of prescission neutrons were calculated within a stochastic approach to fission dynamics on the basis of three-dimensional Langevin equations. This approach was combined with a Monte Carlo algorithm for the degree of freedom K (projection of the total angular momentum I onto the fission axis). The relaxation time τ K in the coordinate K was considered as a free parameter of the model; it was estimated on the basis of a fit to experimental data on the anisotropy of angular distributions. Specifically, the relaxation time τ K was estimated at 2 × 10?21 s for the compound nuclei 224Th and 225Pa and at 4 × 10?21 s for the heavier nuclei 248Cf, 254Fm, and 264Rf. The potential energy was calculated on the basis of the liquid-drop model with allowance for finiteness of the range of nuclear forces and for the diffuseness of the nuclear surface. A modified one-body viscosity mechanism featuring a coefficient k s that takes into account the reduction of the contribution from the wall formula was used to describe collective-energy dissipation. The coefficient k s was also treated as a free parameter and was estimated at 0.5 on the basis of a fit to experimental data on the average prescission multiplicity of neutrons.  相似文献   

6.
The fragment mass yields in fission of 235U induced by thermal neutrons for A = 145–160 and EK = 50–75 MeV were measured using a mass spectrometer. The fine structure is observed at A = 153, 154 and EK = 50–60 MeV. The obtained results were described in the framework of a model based on the dinuclear system concept. The analyzed correlation between the total kinetic energy and mass distribution of fission fragments is connected with the shell structure of the formed fragments of fission. From this correlation and the time dependence of the calculated mass distribution of the binary reaction products, one can conclude that the descent time from a saddle point to a scission point for the more deformed fragments is longer than that for fragments of more compact shape.  相似文献   

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A unified mechanism of the emergence of T-odd ROT- and TRI-asymmetries is proposed for describing experimental T-odd asymmetry coefficients D(θ) in the angular distributions of prescission alphaparticles that are emitted in true ternary and quaternary nuclear fission reactions induced by cold polarized neutrons. The mechanism is related to the different ways in which the Coriolis interaction of the total spin of a polarized compound fissile nucleus with the orbital moment of alpha-particles affects even (for ROT-asymmetries) and odd (for TRI-asymmetries) components of the amplitude of an undisturbed angular distribution of emitted alpha-particles. Coefficients DROT(θ) and DTRI(θ) derived with this mechanism for T-odd ROT- and TRI-asymmetries successfully describe the dependences of corresponding experimental coefficients for 235U and 239Pu nuclei over the range of angles θ, and for the 233U nucleus in the angular range of 60° < θ < 110°. It is explained why only ROT-type T-odd asymmetries emerge for evaporated neutrons and γ-quanta emitted by fission fragments in similar reactions if we allows for the Coriolis interaction of the total spin of the compound fissile nucleus with the orbital moments of the fission fragments and the wriggling vibrations of the above nucleus near its scission point.  相似文献   

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Using multi-dimensional Langevin equations for the probability distribution of the distance between the surfaces of two approaching nuclei, we have studied the formation of superheavy elements via calculation of evaporation and fission cross sections of these elements. Evaporation residue cross sections have been calculated for the 1n, 2n, 3n, 4n, and 5n evaporation channels using one and four dimensional Langevin equations for the 48Ca+226Ra, 232Th, 238U, 237Np, 239,240,242,244Pu, 243Am, 245,248Cm, 249Bk, and 249Cf reactions. Our results show that with increasing dimension of Langevin equations the evaporation residue cross section is increased. Also, obtained results based on fourdimensional Langevin are in better agreement with experimental data in comparison with one-dimensional model.  相似文献   

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The various masses of fission products of U235-thermal fission were spacially separated by a helium filled magnetic mass separator. The number and energy of theβ-decays to the stable nuclides were counted and measured by a scintillation spectrometer. The time dependance of theβ-activity andβ-energy of the total of the U235 fission products was measured. The number ofβ-decays/fission was found to be (6,9±0,4)β-particles fission, the energy liberated byβ-decays (8,1±0,4) MeV/fission. A calculation of the totalβ-decay energy is given and compared with experimental data. The chainlengths ofβ-decay chains as a function of mass of the fission products were measured. Data on prompt neutron emission from fission products combined with the chainlength measurement give the mass dependance of the most probable charge of the primarily formed fission products. A recent model of asymmetric fission is shown to agree with the experimental data. The data give strong evidence that the filled nucleon shells ofZ=50 andN=50 are responsible for the asymmetric fission mechanism.  相似文献   

10.
It was shown previously that the fission lifetime of a nucleus excited to about 100 MeV depends strongly and nonmonotonically on the initial value of its angular momentum L 0. This result was obtained on the basis of a refined version of the combined dynamical and statistical model. The present study is devoted to a theoretical analysis of the dependence of the fission time on the nucleonic composition of the nucleus involved. The respective calculations were performed within the same model. The dependence of the average fission time 〈t f 〉 on the initial fissility parameter (Z 2/A)0 appears to be of a resonance type and is similar to its dependence on L 0. This dependence of the average fission time on (Z 2/A)0 stems both from statistical calculations and from a dynamical simulation of the fission mode with allowance for friction. The conditions under which the average fission time reaches a maximum are specified. The dependence of the average fission time on (Z 2/A)0 remains nonmonotonic in the fusion-fission reaction as well, in which case the distribution of compound nuclei with respect to the initial angular momentum is broad.  相似文献   

11.
The time-of-flight technique is used to measure the ratios R(E, E n )=N(E, E n )/NCf(E) of the normalized (to unity) spectra N(E, E n ) of neutrons accompanying the neutron-induced fission of 238U at primary-neutron energies of E n =6.0 and 7.0 MeV to the spectrum NCf(E) neutrons from the spontaneous fission of 252Cf. These experimental data and the results of their analysis are discussed together with data that were previously obtained for the neutron-induced fission of 238U at the primary energies of E n =2.9, 5.0, 13.2, 14.7, 16.0, and 17.7 MeV.  相似文献   

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The sensitivity of the muon attachment probabilities to the light fragments of prompt fission W l on the fission dynamics is discussed. It is shown that the final W l value consists of two contributions: one from the point of scission and a second from avoided crossing of the levels. The relative contribution from the prescission phase turns out to be higher for more asymmetric fission.  相似文献   

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It has been shown that, at the initial stage of the magnetization curve, the magnetic susceptibility of magnetic liquid determined as χ = Mμ0/B (M is the magnetization, B is the magnetic induction in a sample) obeys the Curie law, and the magnetic susceptibility determined as χ = M/H (H is the magnetic field intensity in a sample) obeys the Curie–Weiss law. Since the Curie law is a particular case of the Langevin dependence, it is assumed that an experimental magnetization curve is described by the Langevin formula with a Langevin parameter ξ = PB/kT, where P is the magnetic moment of a particle and T is the temperature. Experimental verification has shown that, at parameter ξ, the mean relative deviation between the values of M measured and calculated by the Langevin formula is 5%. This deviation can be accounted for by the influence of dispersion of the magnetic moments of nanoparticles.  相似文献   

14.
It is shown that, because of sufficiently large energy spacing between neutron resonance states (NRS-II) in the second well of the deformation potential for actinide nuclei, the Coriolis interaction, mixing the states of an axially symmetric deformed nuclei with different values of the projection K of the nuclear spin J onto the symmetry axis, is week, and the K value in the wave functions of NRS-II is a good quantum number. It is concluded that the K distribution for the states of fissile actinide nuclei in the vicinity of their scission point into fission fragments is determined by simultaneous influence of the internal and external fission barriers, which allows to coordinate the experimental data on subbarrier photofission with the P-odd and P-even correlations in the angular distributions of fission fragments.  相似文献   

15.
The fragment mass and energy distributions from the proton-induced fission of compound nuclei 233Pa, 234,236,237,239Np, 239,240,241,243Am, and 245Bk at proton energy E p =10.3 and 22.0 MeV have been experimentally studied. It was revealed that the shapes of the asymmetric fission mass distributions are mainly defined by the proton numbers of compound nuclei and demonstrate only a weak dependence on the neutron ones. The detailed study of the fission fragment mass yields for compound nuclei Np and Am isotopic chains has shown that the asymmetric fission fragment charge distributions calculated within the unchanged charge density hypothesis for nuclei with equal Z C practically coincide.  相似文献   

16.
The transition dipole moments for the transition T1(ππ*) → S0 to vibrational energy levels of the nontotally symmetric vibrational modes of 2,3,7,8-tetrachloro-and 1,2,3,7,8-pentachlorodibenzo-p-dioxins are calculated. The interpretation of the fine-structure phosphorescence spectrum of the first of these compounds is refined, and the radiative deactivation rate constants for the s sublevels of the lowest triplet state T1 are estimated. For a number of polychlorinated compounds, the effect of chlorine atoms occupying the α and β positions in a molecule on the TsS0 transition dipole moments is discussed.  相似文献   

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On the basis of data obtained by the incomplete fusion reactions 7Li(43A MeV)+232Th and 14N(34A MeV)+197Au, the energy dependence of the variance (σ M 2 ) of the fragment mass in fission of highly heated nuclei has been investigated for total excitation energies E tot * ranging from 50 up to 350 MeV. The dependence σ M 2 E tot * shows some unexpected features when E tot * exceeds a value of about 70 MeV. After this value, the steady increase of σ M 2 expected from its temperature dependence changes to some kind of plateau between 100 and 200 MeV. Further on, at E tot * in excess of about 250 MeV, the variance is found to increase again sharply. In order to analyze this behavior quantitatively, a dynamical stochastic model has been developed. The model employs the one-body dissipation mechanism and describes the decay of highly excited and rotating nuclei by fission and light-particle evaporation. It satisfactorily explains the measured prior-to-scission neutron multiplicities and the experimental mass variances up to E tot * ?250 MeV, but the stochastic treatment does not reveal any increase in σ M 2 at higher excitation energies in contradiction with the data.  相似文献   

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