Dynamical study of fission process and estimation of prescission neutron multiplicity (original) (raw)
Related papers
On Fission Fragment De-Excitation at Scission Point
Dynamical Aspects of Nuclear Fission, 2002
The number of prompt neutrons emitted in the fission event have been measured separately for each complementary fragment in coincidence with fragment mass and kinetic energies in spontaneous fission of 252 Cf, 244 Cm and 2 4 8 Cm. Two high efficient Gd-loaded liquid scintillator tanks were used for the neutron registration . Approximately 3 · 10 6 fission events coincident with prompt neutron emission have been accumulated for each isotope. The mean neutron multiplicity, the dispersion and the covariance of the multiplicity distributions have been obtained as a function of fission fragment mass and kinetic energy. Dependencies of the moments of the multiplicity distributions on the fragment mass and total kinetic energy for different mass bins, as well as mass and total kinetic energy distributions of the fission fragments are presented, discussed and compared for the different isotopes investigated. The results showed a different behavior of the moments of the multiplicity distribution depending on the fragment mass asymmetry that reflects changes in the dynamical effects for different fission modes. Possible reasons for the decrease of the neutron yield at low TKE of fission fragments are discussed .
Fission rate of excited nuclei at variable friction in the energy diffusion regime
Journal of Physics: Conference Series
Presently, it is well established that fission of excited nuclei is a dynamical process being a subject of fluctuations and dissipation. In the literature, there are indications that, at the compact nucleus shapes, the strength of nuclear friction is significantly reduced in comparison with the prediction of the one-body approach. Thus, one can expect that at small deformations the nuclear fission process occurs in the so-called "energy diffusion regime". The purpose of our present work is to compare an approximate analytical formula for the fission rate in this regime with the quasistationary numerical rate which is exact within the statistical errors. Our calculations demonstrate relatively good agreement between these two rates provided the friction parameter is deformation independent. If one accounts for its deformation dependence, the agreement becomes significantly poorer.
Chinese Physics C, 2022
Dissipative dynamics of nuclear fission is a well confirmed phenomenon described either by a Kramers-modified statistical model or by a dynamical model employing the Langevin equation. Though dynamical models as well as statistical models incorporating fission delay are found to explain the measured fission observables in many studies, it nonetheless shows conflicting results for shell closed nuclei in the mass region 200. Analysis of recent data for neutron shell closed nuclei in excitation energy range 40−80 MeV failed to arrive at a satisfactory description of the data and attributed the mismatch to shell effects and/or entrance channel effects, without reaching a definite conclusion. In the present work we show that a well established stochastic dynamical code simultaneously reproduces the available data of pre-scission neutron multiplicities, fission and evaporation residue excitation functions for neutron shell closed nuclei 210 Po and 212 Rn and their isotopes 206 Po and 214,216 Rn without the need for including any extra shell or entrance channel effects. The calculations are performed by using a phenomenological universal friction form factor with no ad-hoc adjustment of model parameters. However, we note significant deviation, beyond experimental errors, in some cases of Fr isotopes.
Proceedings of the Conference on Advances in Radioactive Isotope Science (ARIS2014), 2015
The pre-and post-scission neutron multiplicities have been measured for the 220,222,224 Th isotopes which were populated from the fusion reaction of 16 O+ 204,206,208 Pb systems at the excitation energy range of 40MeV to 64MeV using the facility of National Array of Neutron Detectors (NAND). It is observed that experimentally measured value of neutron multiplicities are under-predicted by the predictions of the standard statistical model, showing that fission is a slow process dominated by nuclear dissipation. This enhanced yield of neutron multiplicity has been analyzed within the framework of a statistical model containing the nuclear dissipation as a free parameter.
Fission dynamics in the proton induced fission of heavy nuclei
Nuclear Physics A, 2004
Multi-parameter correlation study of the reaction 242Pu(p, f) at E,, = 13, 20 and 55 MeV has been carried out. Fission fragment mass and kinetic energy distributions and the double differential neutron spectra have been measured. It was observed that the two-humped shape of mass distributions prevailed up to highest proton energy. Manifestation of the nuclear shell 2 = 28 near fragment mass Af, = 70 has been detected. The experimental results were analyzed in the framework of a time-dependent statistical model with inclusion of nuclear friction effects in the fission process. The multi-parameter correlation study of the reaction During the past two decades the number and the scope of studies on fission dynamics involving heavy ion reactions have increased significantly [l]. Nevertheless, the use of light projectiles for investigation of fission dynamics of heavy compound nuclei retains at least two important advantages: (i) the contribution of the fast fission will be excluded due to small angular momenta, and (ii) it is possible to study the fission dynamics at the compound nuclei excitation energy below 50 MeV where the fusion-fission heavy ion reaction cross sections are small. The contradictions between the pre-and postscission neutron multiplicities measured in the heavy ion reactions and with light particle projectiles were already discussed [2] but data about the pre-and post-scission neutron multiplicities from light particle induced fission remain scarce and are mainly at excitation energies below 30 MeV. The other important aspects of fission dynamics are the interplay *email rubchen@phys.jyu.fi 0375-9474/$ ~ see front matter 0 2004 Elsevier B.V. All rights reserved
Physical Review C, 2019
The multiplicities of α particles and neutrons have been measured simultaneously for the reaction 16O+196Pt forming 212Rn compound nucleus at excitation energies of 56 MeV, 61 MeV, and 68 MeV. Neutrons and α particles were detected at various angles in coincidence with the fission fragments. To extract the contribution of pre- and postmultiplicities using the total α-particle and neutron spectra, moving source formalism was implemented. In the case of α particle, near scission contribution has also been extracted. Study of the fission mechanism using light particle emissions are helpful in understanding the detailed fusion-fission reaction dynamics. The statistical model code joanne2, which includes deformation-dependent particle transmission coefficients, binding energies and level densities, has been used to reproduce the measured multiplicities of neutrons and α particles by varying the transient (τtr) and saddle to scission (τssc) times. It is found that the fission time scales of the order of 50–70×10−21 sec are required to reproduce the neutron and α-particles multiplicities simultaneously. The fission time scales are the measure of the nuclear viscosity, which is responsible for the dynamic hindrance of the fission process.
The influence of the boundary conditions on characteristics of nuclear fission
arXiv (Cornell University), 2023
In this paper, using a quasi-classical statistical approach based on the Langevin equation, we simulate the fission dynamics of selected even-even U, Pu, Cm, Cf and Fm actinide nuclei. As a preparatory part of the work, before solving the Langevin equations, the determination of transport parameters such as inertia and friction tensors within the hydrodynamic approach is performed. Potential energy surfaces are calculated within a macroscopic-microscopic approach in a threedimensional space of deformation parameters defined within the Fourier decomposition of the surface radius function in cylindrical coordinates. Using the Lublin-Strasbourg drop model, Strutinsky shell correction and BCS-like pairing energy model with the projection onto good particle number, we calculate the nuclear total potential energy surfaces (PES). The restoration of the particle number in the superfluid approach is realized within the Generator Coordinate Method (GCM) with the so called Gaussian Overlap Approximation (GOA). The final study is concerned with the effect of the starting point of the stochastic Langevin trajectory on its time evolution and, more importantly, the conditions for judging whether such a trajectory for a given time moment describes an already passed fission nucleus or not. Collecting a large number of such stochastic trajectories allows us to assess the resulting fragment mass distributions, which appear to be in good agreement with their experimental counterparts for light and intermediate actinides. More serious discrepancies are observed for single isotopes of californium and fermium. II. QUASI-CLASSICAL STOCHASTIC LANGEVIN APPROACH The exact determination of the relevant fission process deformation parameters and the collective inertia
Zeitschrift f�r Physik A Atoms and Nuclei, 1984
The time-of-flight technique was used to measure the mass and kinetic energy distribution of fragments from fission of 233U, 235U and 239pu, induced by thermal neutrons at the Grenoble High Flux Reactor. The data array is presented as equal probability lines in the high kinetic energy regions. The fluctuations observed in those experimental lines are explained by a static scission configuration model, in which the most important influence comes from the Coulomb interaction energy between the two fragments. The highest values of total kinetic energy are obtained for fragmentations with heavy fragments Z = 50-52, N = 80-82 and light fragments Z = 40-42, N = 60-64.