Analysis of experimental data on the key thermonuclear reactions going within the low energy range

A comparative analysis of experimental data on hydrogen and helium isotope interaction with deuterons and tritons at low energy is performed. If the energy of an incident particle falls to several keV, the astrophysical factor S(E) rises sharply, indicating that there is an electron screening effect for such interaction. The values of electron screening potentials and enhancement factors are given for key thermonuclear reactions. It is important to allow for the electron screening effect in calculations performed during astrophysical research and in designing different thermonuclear facilities.     

Rate of excited-nucleus fission within a multidimensional stochastic approach

A stochastic approach to describing the dynamics of the nuclear-fission process is applied to study the effect of the number of dimensions on the fission rate within the dynamical model used. The time dependence of the fission rate is calculated on the basis of a multidimensional Langevin equation without taking into account particle evaporation. The one-, two-, and three-dimensional cases are considered for the example of the “c, h, α” parametrization of nuclear-surface shapes. The calculations are performed for a large number of compound nuclei whose parameter Z ²/A falls within the range 20 ≤ Z ²/A ≤ 40. The stationary level of the fission rate is found to increase considerably upon going over from the one- to the three-dimensional case. This increase is especially pronounced for light fissile nuclei in the vicinity of the Businaro-Gallone point. Also, the stationary fission-rate level obtained from our dynamical simulation is compared with its counterpart calculated by the Kramers formula generalized to the multidimensional case.     

Inversion of Robin coefficient by a spectral stochastic finite element approach

This paper investigates a variational approach to the nonlinear stochastic inverse problem of probabilistically calibrating the Robin coefficient from boundary measurements for the steady-state heat conduction. The problem is formulated into an optimization problem, and mathematical properties relevant to its numerical computations are investigated. The spectral stochastic finite element method using polynomial chaos is utilized for the discretization of the optimization problem, and its convergence is analyzed. The nonlinear conjugate gradient method is derived for the optimization system. Numerical results for several two-dimensional problems are presented to illustrate the accuracy and efficiency of the stochastic finite element method.     

Study of different features offission dynamics of ~(224)Th produced in fusion reactions within a stochastic approach

The evaporation residue cross section anisotropy of the fission fragment angular distribution, pre-scission neutron multiplicity and the pre-saddle and post-saddle contributions of the pre-scission neutron multiplicity were analyzed within a stochastic approach based on one-, two- and three-dimensional Langevin equations for the compound nucleus224 Th formed via a complete fusion. In these calculations, dissipation was generated through the chaos weighted wall and window friction formula. Comparison of the theoretical results with the experimental data showed that three-dimensional Langevin equations with dissipation generated through the chaos weighted wall and window friction formula make it possible to reproduce satisfactorily the above-mentioned experimental data.     

Spin distribution of evaporation residue cross section within a stochastic approach

The Langevin equation including particle emission was used to reproduce the recently measured spin distribution of evaporation residue cross sections in the reaction 16O+184W at beam energies of 84, 92,100, 108, 116 and 120 MeV. By comparing the theoretical calculations with the experimental data, the validity of the stochastic approach to dissipative fission is verified. Moreover, a pre-saddle nuclear viscosity coefficient of 5 x 1021 S-1 is extracted.     

Spin distribution of evaporation residue cross section within a stochastic approach

The Langevin equation including particle emission was used to reproduce the recently measured spin distribution of evaporation residue cross sections in the reaction ¹⁶O+¹⁸⁴W at beam energies of 84, 92, 100, 108, 116 and 120MeV. By comparing the theoretical calculations with the experimental data, the validity of the stochastic approach to dissipative fission is verified. Moreover, a pre-saddle nuclear viscosity coefficient of 5×10²¹s^－1 is extracted.     

Application of a two-step dynamical model to calculating properties of fusion-fission reactions

The two-step model of fusion-fission reactions that was proposed previously by the present authors is extended in such a way as to describe the multiplicity of light particles emitted from nuclearfission fragments. Calculations are performed for the reaction induced by ⁴⁸Ca + ²⁴⁴Pu collisions. The mass distribution of fragments, their mass-energy distribution, and the total multiplicity of neutrons and gamma rays are obtained for E _lab = 230, 238, 249, and 255 MeV. It is shown that the model reproduces qualitatively relevant experimental data. In order to attain quantitative agreement, it is necessary to take into account the angular momentum carried away by particles from the nucleus undergoing fission and various types of gamma rays emitted by the nucleus and its fission fragments.     

Study of fusion cross-section in heavy-ion fusion-fission reactions at around fusion barrier energies using the Langevin dynamical approach

The neutron detector with ³He -filled counters placed in the focal plane of the VASSILISSA separator is used for measuring the average number and determining the multiplicity distribution of prompt neutrons from the spontaneous fission of heavy short-lived isotopes. The test reaction $\ensuremath \mathrm{{}^{48}Ca}+\mathrm{{}^{206}Pb}=2{\rm n}+\mathrm{{}^{252}No}$ is used for tuning the separator settings and calibrating the detector system with the spontaneous fission of the ²⁵²No . The average neutron number per ²⁵²No spontaneous fission event is as large as $\ensuremath \bar{\nu}=4.06 \pm 0.12$ . The short-lived heavy isotope ²⁴⁴Fm , produced in the complete fusion reaction ⁴⁰Ar + ²⁰⁶Pb , is investigated. The average number of neutrons per spontaneous fission of ²⁴⁴Fm from the experimental data ( $\ensuremath \bar{\nu}=3.3 \pm 0.3$ is determined for the first time.     

Influence of nuclear dissipation on fission dynamics of the excited nucleus <Superscript>248</Superscript>Cf within a stochastic approach

A stochastic approach to fission dynamics based on two-dimensional Langevin equations was applied to calculate the anisotropy of the fission fragments angular distribution and average pre-scission neutron multiplicities for the compound nucleus ²⁴⁸Cf formed in the ¹⁶O+²³²Th reactions. Postsaddle nuclear dissipation strength of (12–14) × 10²¹ s^?1 was extracted for Cf nucleus by fitting the results of calculations with the experimental data. Furthermore, it was found that the results of calculations for the anisotropy of the fission fragments angular distribution and pre-scission neutron multiplicities are very sensitive to the magnitude of post-saddle nuclear dissipation.     

Dependence of nuclear dissipation upon deformation or temperature: analysis of the data using a Langevin-Monte-Carlo approach

We investigate the dynamical formation of clusters in nuclear matter. The model is based on classical nucleation theory and describes the time evolution of clusters by repeated condensation and evaporation of nucleons. We numerically calculate the growth of clusters with A <- 10 at temperatures of T = 2 MeV and T = 6 MeV and densities of ρ = 0.01fm^?3 and ρ = 0.05fm^{t -.} Equilibration times are rather long, 30 -2 100fm/c, and it is unlikely that clusters with A >- 10 observed in heavy-ion collisions arise from the nucleation process.     

Hydrodynamic cavitation: from theory towards a new experimental approach

Hydrodynamic cavitation is analysed by a global thermodynamics principle following an approach based on the maximum irreversible entropy variation that has already given promising results for open systems and has been successfully applied in specific engineering problems. In this paper we present a new phenomenological method to evaluate the conditions inducing cavitation. We think this method could be useful in the design of turbo-machineries and related technologies: it represents both an original physical approach to cavitation and an economical saving in planning because the theoretical analysis could allow engineers to reduce the experimental tests and the costs of the design process.      

Geoacoustic inversion of short range source data using a plane wave reflection coefficient approach

Acoustic time series data were collected in a shallow, hard bottom lake environment located in central Texas using both short range (2 m) implosive data, obtained with the source and a single hydrophone located near mid-depth in the waveguide, along with longer range implosive and explosive data from a near surface source to a bottom mounted hydrophone. Matched field inversions using simulated annealing were performed with a ray trace plus complex plane wave reflection coefficient forward propagation model that was validated in previous work. Isolating bottom interacting paths to perform the inversions is shown to be essential to reduce parameter uncertainties in the hard bottom environment and enables a systematic approach to the inversions which establishes the number of layers needed to represent the lake environment. Measured transmission loss data from a towed source were compared through a RMS error analysis to modeled transmission loss, constructed with the parameters from inversions of data from several source types, to further establish the validity of the inversion approach for this environment. Geoacoustic parameters obtained by inversions of short range, low frequency impulsive data are used to predict transmission loss at longer ranges and higher frequencies. The range dependence of the global minimum is discussed.     

Analysis of experimental data on the neutron yield from muons

Experimental data accumulated over 60 years of studying the yield of cosmogenic neutrons in a liquid scintillator (Y _LS), iron (Y _Fe), and lead (Y _Pb) were analyzed. This analysis revealed that the main part of the results on the yield Y _LS were overestimated by about 30%. With allowance for this circumstance, all experimental data can be described by the dependence Y(ē _µ, A) = b _n A ^β ē _µ ^α , where the product b _n ē _µ ^α stands for the energy spent by a muon on neutron production. The exponents of α = 0.78 and β = 0.95 are determined by the properties of the medium and by neutron production in showers.     

Comparison of the characteristics of the fusion-fission and quasifission products in reactions with heavy ions

Overlap of quasifission and fusion-fission product mass distributions measured for the ⁴⁸Ca + ¹⁵⁴Sm reaction at near-barrier energies is analyzed. As the incident ion energy increases, the angular distribution of quasifission fragments becomes narrower and they are concentrated near the forward and backward angles. The observed decrease in the yield of quasifission products with increasing collision energy is caused by noncoincidence of the product emission and detector position directions (the latter direction lags behind the beam direction by more than 15°) rather than by a decrease in the number of quasifission events. Our evaluations show that the excitation functions of evaporation residues in the ⁵⁴Cr + ²⁴⁸Cm reaction are several orders of magnitude larger than the excitation functions for the other reactions, ⁵⁸Fe + ²⁴⁴Pu and ⁶⁴Ni + ²³⁸U, used to synthesize a new superheavy element with Z = 120 (A = 302).     

Applying a new method for analyzing experimental data on nuclear reactions at high energies

A new statistical method is proposed for the analysis of experimental data obtained in nucleus-nucleus collisions at high energies which borrows from ideas developed using the Random Matrix Theory. The method allows us to detect regions with correlation effects in the momentum distributions of secondary particles.     

Vacancies in metals: from first-principles calculations to experimental data

We have revealed, and resolved, an apparent inability of density functional theory, within the local density and generalized gradient approximations, to describe vacancies in Al accurately and consistently. The shortcoming is due to electron correlation effects near electronic edges and we show how to correct for them. We find that the divacancy in Al is energetically unstable and we show that anharmonic atomic vibrations explain the non-Arrhenius temperature dependence of the vacancy concentration.     

Inflaton field governed universe from NKK theory of gravity: stochastic approach

We study a non-perturbative single field (inflaton) governed cosmological model from a 5D non-compact Kaluza-Klein (NKK) theory of gravity. The inflaton field fluctuations are estimated for different epochs of the evolution of the universe. We conclude that the inflaton field has been sliding down its (quadratic) potential hill along the whole evolution of the universe and a mass is involved of the order of the Hubble parameter. In the model here developed the only free parameter is the Hubble parameter, which could be reconstructed in the future from Super Nova Acceleration Probe (SNAP) data. Received: 17 August 2005, Revised: 12 September 2005, Published online: 14 October 2005 PACS: 04.20.Jb, 11.10.kk, 98.80.Cq     

Correlations in stochastic theories of chemical reactions

A comprehensive study of correlations in linear and nonlinear chemical reactions is presented using coupled chemical and diffusion master equations. As a consequence of including correlations in linear reactions the approach to the steady-state Poisson distribution from an initial non-Poissonian distribution is given by a power law rather than the exponential predicted by neglecting correlations. In nonlinear reactions we show that a steadystate Poisson distribution is achieved in small volumes, whereas in large volumes a non-Poissonian distribution is built up via the correlation. The spatial correlation function is calculated for two examples, one which exhibits an instability, the other which exhibits a second-order phase transition, and correlation length and correlation time are calculated and shown to become infinite as the critical point is approached. The critical exponents are found to be classical.