Two-body Mechanism Research Articles

Transport coefficients for modeling fission dynamics

Transport coefficients are important ingredients in dynamical investigations of a nuclear fission process, which determine the dynamical evolution of the fissioning system. The inertia (mass) and friction (dissipation) tensors influence substantially on different observables. The proper calculation of these tensors is a necessary condition for correct modeling of the fission process. The calculation of the inertia and friction tensors is presented for a large variety of nuclear shapes generated by a {c,h,α} parametrization. The inertia tensor is calculated within the Werner-Wheeler approximation for incompressible irrotational flow. The friction tensor is calculated within the one-body and two-body mechanisms of nuclear dissipation. A numerical code is made available, which calculates all these quantities. Program summaryProgram Title: Mass_DissCPC Library link to program files:https://doi.org/10.17632/rcndf6sbns.1Licensing provisions: CC BY NC 3.0Programming language: C++Nature of problem: The modeling of the nuclear fission within dynamical consideration requires calculation of the inertia and friction coefficients. This calculation should be performed for a large variety of the nuclear shapes, which describe the evolution of the shape of compound nucleus from the ground state till the separated fission fragments.Solution method: The inertia tensor is calculated within the Werner-Wheeler approximation for incompressible irrotational flow. The friction tensor is calculated within the one-body and two-body mechanisms of the nuclear dissipation. The shape of the fissioning nucleus is described by the {c,h,α} parametrization. This parametrization determines the axially symmetric shapes in cylindrical coordinates. The inertia and friction tensors are calculated using numerical integration procedure as a function of the three shape parameters. All subroutines, which define the nuclear shape, are separated from the code, which calculates the inertia and friction tensors. Thus, the program could be adopted for any other shape parameterization in case if it is given in cylindrical coordinates.

Cross section asymmetry of two-body carbon disintegration [formula omitted] with polarized photons at energy 40–50 MeV

The cross section asymmetry of C12(γ,p01)B11 and C12(γ,p2−6)B11 reactions has been studied at the energy range 40–55 MeV, using linearly polarized tagged photons of the MAX-lab facility. The asymmetry of the C12(γ,p01)B11 processes, which assume the one-body mechanism of the reaction, is Σ≈0.82±0.05 for photon energies 45–50 MeV. The asymmetry for the C12(γ,p2−6)B11 reactions, which produce a maximum at excitation energy ∼6 MeV, is Σ≈0.53±0.13 for a photon energy 49 MeV. It is close to the asymmetry of reaction of the free deuteron photodisintegration, and can be resulted from the two-body mechanism of the photon absorption.

The relative dynamic resilience of masonry collapse mechanisms

Masonry structures have exhibited recurrent collapse mechanisms during past earthquakes, and building codes now require that the seismic capacities of typical mechanisms are assessed. In this paper, a new framework is proposed for predicting out-of-plane seismic collapse of masonry walls. The method remains relatively simple by utilizing linearized equations of motion, while improving upon previous methods by considering the dynamic rocking response. The equations of motion for the rocking response of masonry walls with three fundamental scenarios of loading are first presented. Both applied external forces and external inertial forces are considered, in addition to the possible formation of a two-body (three-hinge) mechanism if the wall is restrained at its top and bottom. After linearization, these fundamental loading scenarios are all described by a single equation of motion, allowing them to be combined to consider a much wider variety of loading scenarios. The proposed framework is then used to investigate the dynamic resilience of different collapse mechanisms by considering the response to pulse-type ground motions. For this purpose, an analytical solution which describes the collapse envelope for pulse-type ground motions and one-sided rocking is derived. The comparison between collapse envelopes makes it possible to describe the relative resilience of each mechanism compared to the others. Eventually, the proposed framework is used to investigate the relative resilience of the mechanisms observed in a church damaged in the 2009 L’Aquila earthquake.

Effective merging dynamics of two and three fluid vortices: Application to two-dimensional decaying turbulence

We present a kinetic theory of two-dimensional decaying turbulence in the context of two-body and three-body vortex merging processes. By introducing the equations of motion for two or three vortices in the effective noise due to all the other vortices, we demonstrate analytically that a two-body mechanism becomes inefficient at low vortex density n<<1. When the more efficient three-body vortex mergings are considered (involving vortices of different signs), we show that n~t(-ξ), with ξ=1. We generalize this argument to three-dimensional geostrophic turbulence, finding ξ=5/4, in excellent agreement with direct Navier-Stokes simulations [McWilliams et al., J. Fluid Mech. 401, 1 (1999)].

Langevin description of charge fluctuations in fission of highly excited nuclei

A stochastic approach to fission dynamics based on three-dimensional Langevin equations is used to study charge fluctuations in fission of highly excited nuclei. Elongation, neck and charge asymmetry parameters are chosen as relevant collective coordinates. Using as an example 4He + 232Th induced fission of 236U in a broad range of excitation energies (from 60 to 160 MeV), the isobaric charge variance is investigated to obtain information on nuclear dissipation. The friction parameter of the charge mode is calculated under the assumption of both the one-body and two-body mechanisms of nuclear viscosity. The results obtained for the variances of the charge distribution within the applied approach reveal that the optimal reproduction of the available data is achieved with the value of the two-body viscosity coefficient (0.6≤ν≤1.8)×10-23MeV s fm-3 that is close to those deduced earlier from the description of the fission fragment mass-energy distribution. The expression for the friction parameter of the charge mode is derived within the one-body mechanism of nuclear dissipation. The one-body mechanism of nuclear dissipation also provides a good reproduction of data on the isobaric charge variance without any adjustable strength parameter.

Fission of targetlike fragments populated in the multinucleon transfer reactions of 340 MeV28Sion232Th

The transfer-induced fission channel has been studied in the collision of 340 MeV ${}^{28}\mathrm{Si}$ on ${}^{232}\mathrm{Th}$ as a function of the atomic number of the projectilelike fragments (PLF's) by using a $4\ensuremath{\pi}$ detector array. It is found that the energy loss of the transfer reaction increases as a function of the net charge transfer $\ensuremath{\Delta}Z$ from the projectile to the target nucleus, going from quasielastic to deep-inelastic regimes. The average excitation energy of the targetlike fragment (TLF) is derived from the measured energy loss, whereas its angular momentum has been obtained from the angular distribution of fission fragments. It is found that the populated TLF nuclei with ${Z}_{\mathrm{TLF}}=90--96 (\ensuremath{\Delta}Z=0--6)$ have average excitation energies up to about 100 MeV and angular momenta up to about $40\ensuremath{\Elzxh}.$ The measured ratio of transfer-fission yield to PLF singles, ${Y}_{f},$ first increases with increasing net charge transfer up to $\ensuremath{\Delta}Z=4$ and then shows a plateau around the values ${Y}_{f}=0.4--0.6$ followed by a decrease for higher $\ensuremath{\Delta}Z$ transfers. This ratio can be identified as the cumulative fission probability of the populated nuclei for net charge transfers up to $\ensuremath{\Delta}Z&lt;~6,$ where a two-body mechanism for the first reaction step is supported by the experimental data. This result suggests a significant survival probability against fission of these TLF nuclei, in marked disagreement with the standard statistical model predictions. The observed survival probability implies that there is a strong hindrance to fission in the early stages of deexcitation, as also indicated by the large fission times ${(t}_{f}=10--100 \mathrm{zs})$ derived from earlier neutron measurements in fusion-fission reactions. The importance of such effects in the population of nuclei in the heavy and superheavy mass regions by transfer reactions is discussed.

A close-coupling study of collision-induced dissociation: three-dimensional calculations for the He+H 2→He+H+H reaction

Three-dimensional quantum mechanical close-coupling calculations using discretized continuum states have been carried out for the He+H 2→He+H+H collision-induced dissociation reaction in order to study the contribution of sequential two-body and direct three-body collisions just above the dissociation threshold. Although the calculations were done only for zero total angular momentum, we have found that the nonresonant, direct three-body mechanism is generally dominant for collisions of H 2 having a low rotational quantum number and that the two-body mechanism via quasibound resonance states is important only for H 2 having a higher rotational quantum number. We have also found that the magnitude of the total collision-induced dissociation probabilities is exclusively determined by the energy difference between the rovibrational energy level of H 2 and the dissociation threshold. This indicates that collision-induced dissociation can be described with a simple collisional energy transfer process. The semiclassical calculations, in which the relative radial motion is treated in classical mechanics while the other motions are treated quantum mechanically, have also been performed for comparison. We found that the semiclassical method does not work well for dissociation processes with small probabilities, as expected.

The role of test conditions on the microabrasive wear behaviour of soda-lime glass

In a recently developed microabrasive wear test, specimens are abraded as a fine abrasive slurry is drawn into a gap between the specimen and a rotating ball. In this work, the microabrasive wear behaviour of an ideally brittle material, soda-lime glass with 4 μm SiC particles has been examined as a function of applied load with polymeric, metallic and ceramic counterfaces. It has been shown that due to the nature of the processes occurring in the contact zone, wear does not always proceed as predicted by the Archard wear equation. Although abrasion by a particle in the contact zone becomes more severe as the load is increased, the ease of entrainment of the abrasive slurry into the contact zone is reduced. In certain circumstances, ribbons of material form within the wear scar which support the ball and reduce the wear of the material while in other cases, elastic deformation of the ball may lead to anomalously shaped wear scars. The intrinsic mechanisms of wear depend largely upon the nature of the counterface ball; stiff metallic and ceramic counterfaces resulted in chipping and fracture of the glass by three-body abrasion whereas the compliant polymeric counterface resulted in ductile cutting of the glass by a two-body mechanism. Care must be taken to ensure that when comparing test results the mechanisms of wear are known and the operation of other phenomena (such as ribbon formation) which will affect the results must be noted.

A direct comparison of wet and dry abrasion behaviour of mild steel

A rubber wheel type test apparatus has been constructed which allows abrasion testing to be conducted in wet or dry environments in otherwise identical conditions. It has been shown that the environment is a significant parameter in the abrasive wear rate of mild steel with alumina and silica abradants over a range of loads and particle sizes. The wear rate is significantly (in some cases, orders of magnitude) less in wet conditions than in the dry, with the greatest differences being observed for small and/or rounded abrasives. Wear in dry conditions occurred predominantly by a three-body mechanism, whilst wear in wet conditions occurred mainly by a two-body mechanism. The water may affect the wear test in a number of ways, but it is proposed that it is its role as a lubricant which is most significant. Since wear rates and mechanisms are so dependent on the test environment, care needs to be exercised in choosing the correct test type to simulate service conditions.

Coherent pion photoproduction on the deuteron in the Δ resonance region

Abstract Coherent pion photoproduction on the deuteron in the Δ resonance region is calculated adopting a nonrelativistic model based on time-ordered perturbation theory without rescattering. Results for cross sections and polarization observables corresponding to the polarized photon beam and/or the deuteron target are presented for which the influence of the nonresonant Born amplitudes and the sensitivity to several multipoles of the elementary photoproduction on the nucleon are studied in detail. Furthermore, the role of a two-body mechanism where photon absorption and pion emission take place at different nucleons is discussed.

Two-body mechanisms in pion photoproduction on the trinucleon.

A breakdown of the Impulse Approximation is studied in pion photoproduction on $^3$He at high momentum transfers. The usual DWIA formalism with Faddeev wave functions which works well for small momentum transfers deviates from experimental measurements by up to two orders of magnitude for $Q^2>6 fm^{-2}$. It is found that the explicit inclusion of two-body mechanisms, where the photon is absorbed on one nucleon and the pion is emitted from another nucleon can restore agreement with the data.

Reaction mechanisms in two-body photodisintegration and electrodisintegration of 4He

One- and two-body mechanisms alone cannot reproduce the experimental values of the 4He( γ,p)T reaction cross-section in the Δ region. Meson double scattering is shown to account for the major part of these discrepancies. All together these mechanisms account for the near equality of the cross-section of the 4He( γ,p)T and 4He( γ,n) 3He channels. Consequences on the 4He(e,e′p)T and the 4He(e,e′n) 3He channels are investigated, with a special emphasis on the determination of the 4He wave function and the study of the behavior of bound nucleon form factors. The sensitivity of spin observables, to various interaction effects, is investigated.

Measurement of the reaction 3He( gamma,pp)n and its relation to three-body forces.

The three-body photodisintegration of $^{3}\mathrm{He}$ has been measured at photon energies ranging from 90 to 250 MeV, in kinematic regions where three-body forces effects are expected to be maximized, and two-body mechanisms suppressed. The differential cross sections as a function of neutron momentum demonstrate that calculations using only one-body and two-body photoabsorption mechanisms cannot describe the data and that a two-pion-exchange, three-body absorption mechanism is needed to adequately describe the data.

Three-body force effects in the reaction 3He(γ,pp)n

The three-body photodisintegration of 3He has been measured, at energies below the pion-production threshold, in kinematic regions where contributions from three-body forces are expected to be maximized, and two-body mechanisms are suppressed. The measured differential cross sections show an enhancement over calculations using one-body and two-body photo-absorption mechanisms only. The inclusion of a two-pion-exchange three-body absorption mechanism is needed to describe the data.

Three-body exchange mechanisms in the3He( γ,2p)n reaction

Owing to a strong suppression of two-body mechanisms, the 3He( gamma ,2p)n reaction is dominated by three-body exchange currents, and appears to be the best place to study them. Relevant kinematics are proposed and discussed.

Excitation of the Delta resonance in the 12C(e,e') reaction.

The inclusive (e,e') scattering from nuclei is studied with the assumption that the electromagnetic production of pions from the nucleon can be described by a model consisting of the conventional Born term and a \ensuremath{\Delta}-excitation term. We show that the global features of the recent data of the inclusive $^{12}\mathrm{C}$(e,e') reaction in the GeV energy region can be described by the following procedures: (1) the parameters of the constructed one-body and two-body current operators are fitted to the total cross sections of \ensuremath{\gamma}N\ensuremath{\rightarrow}\ensuremath{\pi}N, p(e,e'), and \ensuremath{\gamma}d\ensuremath{\rightarrow}np; (2) the medium effect on the \ensuremath{\Delta} propagation is treated according to the information extracted from the \ensuremath{\Delta}-hole model calculation. It is found that the two-body mechanism of \ensuremath{\Delta} annihilation by the nuclear medium improves the fit to the data in the ``dip'' region. The predicted magnitudes in the \ensuremath{\Delta} region are lower than the data, indicating the importance of more complicated multinucleon mechanisms.

Three-body mechanisms in coherent meson production by pd capture

Meson double scattering is shown to be the good candidate for accounting for the discrepancies left out by the conventional analysis of pd mesonic capture in terms of one-and two-body mechanisms.

(p, pi -) continuum spectra at intermediate energy in the local Fermi gas model.

We study the (p,π-) continuum spectra at intermediate energy in the local Fermi gas model, where the pion production process is described in the standard p- and s-wave two-body mechanisms. The continuum spectra are very sensitive to the surface properties of nuclei and the local Fermi gas model seems to provide a good description of the (p,π-) spectra.

Comparison of a two-body threshold (π,2π) reaction mechanism with the usual one-body mechanism in the deuteron

A two-body threshold (${\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}},{\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}}{\ensuremath{\pi}}^{\ensuremath{\mp}}$) reaction mechanism is suggested in direct analogy with pion absorption. The mechanism involves boson rescattering via $\ensuremath{\Delta}$ excitation. The relative importance of this mechanism and the ordinary one-body mechanism in nuclei is studied in the particular case of $S$-wave deuteron targets. The contribution of the two-body mechanism to the threshold reaction cross section is found to be less than 1% of the simple one-body estimate.

Integral-equation method for partially ionized plasmas

A pseudoclassical integral-equation method for obtaining the equation of state, pair-distribution functions, and static structure factors for partially ionized plasmas is presented. The electron-ion charge distribution is separated into a localized, quantum-mechanical part and a delocalized, nearly classical part. The Planck-Larkin method is used to separate the electron distribution. Localized-electron distributions are obtained from solutions of the Schroedinger equation. Distribution functions for ions and free electrons, including weakly bound electrons, are obtained from solutions to the classical hypernetted-chain (HNC) integral equation. Pseudopotentials that have two-body quantum mechanics built in, but which preclude strong bound states, are used as input to the HNC. The results are applied to H and Ar plasmas.