arXiv subjects

S. V. Radionov

Publications and source records attributed to S. V. Radionov.

14 recordsLinked to original sources

arXiv2026

Shell effects and the neutron emission within the multi-dimensional Langevin model for nuclear fission

We solve the Langevin equations for the time evolution of parameters that describe the shape of fissioning system. On each integration step, we calculate the probability of neutron emission and estimate whether a neutron would be emitted or not. If yes, we decrease the excitation energy of the nucleus by the neutron separation energy plus the average energy of the emitted neutron, switch to the layer of potential energy surface with a smaller number of neutrons and continue the process of integration. If the trajectory reaches the scission point, we check how many neutrons were emitted along this trajectory. The pre-scission neutron multiplicity $M_{pre}$ is defined by the ratio of the total number of emitted neutrons to the total number of fission trajectories. Besides $M_{pre}$, the mass distribution of fission fragments, the distribution of emitted neutrons with respect to the fission stage (deformation of system) and the distribution of emitted neutrons with respect to their energies are calculated. The calculated quantities are compared with the available experimental data.

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arXiv2023

Nuclear descent from the fission barrier in the presence of long--range memory effects

We have investigated the peculiarities of nuclear descent from a parabolic fission barrier within a generalized Langevin equation with power--law $f(t-t')=(|t-t'|/τ)^{-α}$ memory function. We have observed much stronger slowing down of the nuclear descent in the presence of long--range memory effects, caused by the power--law memory function at $0<α<1$, than in the presence of short--range memory effects, generated by exponential $f(t-t')={\rm exp}(-|t-t'|/τ)$ memory function. At a specific value of the exponent $α=1/2$ of the power--law memory function, it turned out possible to find analytically the trajectory of the descent and demonstrate that the long--range memory effects give rise to complex time oscillations of nuclear shape, becoming more frequent and damped with the correlation time $τ$. We have found fairly long ($>10^{-20}~{\rm s}$) times of the descent of $^{\rm 236}{\rm U}$ at the values of the correlation time $τ\sim [10^{-24}÷10^{-23}]~{\rm s}$.

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arXiv2023

The Langevin approach for fission of heavy and super-heavy nuclei

In this contribution, we present the main relations of the Langevin approach to the description of fission or fusion-fission reactions. The results of Langevin calculations are shown for the mass distributions of fission fragments of super-heavy elements and used for the investigation of memory effects in nuclear fission.

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arXiv2021

Stochastic features of dissipative large-amplitude dynamics and nuclear fission

Within a density matrix approach for nuclear many--body system, it is derived non--Markovian Langevin equations of motion for nuclear collective parameters, where memory effects are defined by memory time. The developed stochastic approach is applied to study both the nuclear descent from fission barrier to a scission point and thermal diffusive overcoming of the barrier. The present paper is partly a review of our results obtained earlier and contains new results on the non--Markovian generalization of Kramers' theory of escape rate and on time features of the collective dynamics in the presence of periodic external modulation.

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arXiv2021

Memory effects in Langevin approach to the nuclear fission process

We present the schematic calculations within the Langevin approach in order to investigate the dependence of fission width on the memory time and the excitation energy at low temperatures where the quantum fluctuations play an important role. For this we consider the simple one-dimensional case with the potential energy given by two parabolic potentials (Kramers potential). For friction and the mass parameters we use the deformation independent values fitted to the results obtained earlier within the microscopic linear response theory. We have found out that at small excitation energies (comparable with the fission barrier height) the memory effects in the friction and random force acts on the fission width in opposite direction. The total effect is not so large, but still quite noticeable (depending on the value of the relaxation time). The use of effective temperature in the diffusion coefficient turns out to be much more important compared with the memory effects. The calculated fission width at very low excitation energies is unrealistically too big.

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arXiv2019

Non-Markovian response of complex quantum systems

We study the perturbative response of a complex quantum system on time changes of an external parameter $X$. The driven dynamics is treated in adiabatic basis of the system's Hamiltonian $\hat{H}[X]$. Within a random matrix approach we obtained non--Markovian Fokker--Planck equation for the occupancy of given adiabatic state. We observed normal diffusion regime of the driven quantum dynamics at quite small values of the memory time defined by the time scales of the $X$--correlations and energy--distribution of the coupling matrix elements $(\partial \hat{H}/\partial X)_{nm}$. Here the normal energy diffusion was found to drop out with the width of the matrix elements' energy--distribution and the diffusion may be significantly suppressed with the decrease of the correlations between the matrix elements. In the opposite limit of relatively large memory times we obtained ballistic regime of the dynamics.

cond-mat.stat-mech↗

arXiv2019

The effect of the Landau--Zener transitions on nuclear fission dynamics

In the paper, it is studied the influence of Landau-Zener transitions between nuclear many-body states on the dissipative properties of nuclear large--amplitude collective motion. Within the cranking-like approach, we describe the time evolution of a nuclear many-body system as the self-consistent motion in a space of intrinsic excitations and in a space of a single collective (deformation) parameter. By that we measure how the spectral statistics of the nuclear energy levels affects the fission rate at quite large initial temperatures of heavy nuclei.

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arXiv2018

A simple approach to the chaos-order contributions in nuclear spectra

The simple one-parameter nearest neighbor-spacing distribution (NNSD) is suggested for statistical analysis of nuclear spectra. This distribution is derived within the Wigner-Dyson approach in the linear approximation for the level repulsion density of quantum states. The obtained NNSD gives the individual information on the Wigner and Poisson contributions in agreement with that of the statistical experimental distributions of collective states in deformed nuclei. Using this NNSD, one finds that the symmetry breaking due to the fixing of projections of the angular momentum of collective states enhances a chaos as a shift of the NNSD from the Poisson to Wigner distribution behavior.

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arXiv2017

Statistical analysis of excitation energies in the actinide and rare-earth nuclei

Statistical analysis of distributions of the collective states in the actinide and rare-earth nuclei is performed in terms of the nearest neighbor spacing distribution (NNSD). Several approximations, such as the linear approach to the level repulsion density and that suggested by Brody to the NNSDs were applied for the analysis. We found an intermediate character of the experimental spectra between the order and the chaos for a number of the rare-earth and actinide nuclei. They are more close to the Wigner distribution for energies limited by 3 MeV, and to the Poisson distribution for data including higher excitation energies and higher spins. The latter is in agreement with the theoretical calculations. These features are confirmed by the cumulative distributions, where the Wigner contribution dominates at smaller spacings while the Poisson one is more important at larger spacings.

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arXiv2013

Gamma-radiation in non-Markovian Fermi systems

The gamma-quanta emission is considered within the framework of the non--Markovian kinetic theory. It is shown that the memory effects have a strong influence on the spectral distribution of gamma-quanta in the case of long-time relaxation regime. It is shown that the gamma-radiation can be used as a probe for both the time-reversible hindrance force and the dissipative friction caused by the memory integral.

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arXiv2011

Non-Markovian diffusion over potential barrier in the presence of periodic time modulation

The diffusive non-Markovian motion over a single-well potential barrier in the presence of a weak sinusoidal time-modulation is studied. We found non-monotonic dependence of the mean escape time from the barrier on a frequency of the periodic modulation that is character to the stochastic resonance phenomenon. The resonant acceleration of diffusion over the barrier occurs at the frequency inversely proportional to the mean first-passage time for the motion in the absence of the time-modulation.

cond-mat.stat-mech↗

arXiv2008

Non-Markovian large amplitude motion and nuclear fission

The general problem of dissipation in macroscopic large-amplitude collective motion and its relation to energy diffusion of intrinsic degrees of freedom of a nucleus is studied. By applying the cranking approach to the nuclear many body system, a set of coupled dynamical equations for the collective classical variables and the quantum mechanical occupancies of the intrinsic nuclear states is derived. Different dynamical regimes of the intrinsic nuclear motion and its consequences on time properties of collective dissipation are discussed. The approach is applied to the descant of the nucleus from the fission barrier.

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arXiv2007

Collective motion in quantum diffusive environment

The general problem of dissipation in macroscopic large-amplitude collective motion and its relation to energy diffusion of intrinsic degrees of freedom of a nucleus is studied. By applying the cranking approach to the nuclear many-body system, a set of coupled dynamical equations for the collective classical variable and the quantum mechanical occupancies of the intrinsic nuclear states is derived. Different dynamical regimes of the intrinsic nuclear motion and its consequences on time properties of collective dissipation are discussed.

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arXiv2001

Memory effects on descent from nuclear fission barrier

Non-Markovian transport equations for nuclear large amplitude motion are derived from the collisional kinetic equation. The memory effects are caused by the Fermi surface distortions and depend on the relaxation time. It is shown that the nuclear collective motion and the nuclear fission are influenced strongly by the memory effects at the relaxation time $τ\geq 5\cdot 10^{-23}{\rm s}$. In particular, the descent of the nucleus from the fission barrier is accompanied by characteristic shape oscillations. The eigenfrequency and the damping of the shape oscillations depend on the contribution of the memory integral in the equations of motion. The shape oscillations disappear at the short relaxation time regime at $τ\to 0$, which corresponds to the usual Markovian motion in the presence of friction forces. We show that the elastic forces produced by the memory integral lead to a significant delay for the descent of the nucleus from the barrier. Numerical calculations for the nucleus $^{236}$U shows that due to the memory effect the saddle-to-scission time grows by a factor of about 3 with respect to the corresponding saddle-to-scission time obtained in liquid drop model calculations with friction forces.

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