Fission Fragment Kinetic Energy Distributions Research Articles

Theoretical investigation of fission fragment kinetic energy distributions in the symmetric mass region for233U(nth,f)

Fission yields are essential for nuclear reactor studies (decay heat, fuel inventory…) and constitute also one of the main observables needed to improve our understanding of the fission process. The symmetric mass region is of particular interest due to various intriguing properties of the fission fragments already reported in the literature : inversion of the nuclear charge polarization, large width of the fission fragment kinetic energy distribution, strong change of the prompt neutron multiplicity, etc. Recently, measurements of fission yields and kinetic energy distributions in the symmetric mass region were achieved at the LOHENGRIN mass spectrometer of the Institut Laue-Langevin (ILL). This experimental work is challenging due to the low counting rate and the appearance of contaminant masses, leading to pronounced components in the fission fragment kinetic energy distribution. Despite removing the undesirable contributions, the fission fragment kinetic energy distributions still show two components, indicating that the fission process could be modal. To go further and better characterize these components a comparison between our experimental data and Monte Carlo calculations (FIFRELIN code) simulating the de-excitation of the fission fragments for different fission channels will be presented and discussed.

Energy dependence of mass, charge, isotopic, and energy distributions in neutron-induced fission ofU235andPu239

The mass, charge, isotopic, and kinetic-energy distributions of fission fragments are studied within an improved scission-point statistical model in the reactions $^{235}\mathrm{U}+n$ and $^{239}\mathrm{Pu}+n$ at different energies of the incident neutron. The charge and mass distributions of the electromagnetic- and neutron-induced fission of $^{214,218}\mathrm{Ra},\phantom{\rule{4pt}{0ex}}^{230,232,238}\mathrm{U}$ are also shown. The available experimental data are well reproduced and the energy-dependencies of the observable characteristics of fission are predicted for future experiments.

Multimodal Analysis of Prompt and Delayed Neutron Emission inFission

The multimodal model of fission, in which the nucleus is considered to follow a distinctive path on the deformation energy surface, finally reaching a definite scission point configuration, has been proved to give a good description of the mass and kineticenergy distributions of fission fragments. The Madland-Nix model of prompt fission neutron spectra (PFNS) considered the cooling of the fission fragments by neutron emission and the energy dependence of the inverse reaction, providing a basic formalism for evaluation of the PFNS. Combination of the two models led to the multimodal Madland-Nix model. With further extension to consider the shell effects on the level density and asymmetry in neutron multiplicity and nuclear temperature of the fragments, it was successfully applied to evaluation of the PFNS. On the other hand, the summation method of the delayed neutron yields (DNY) is the most fundamental approach to the estimation of the quantity. Combination of the method with the multimodal fission model led to a new approach to interpret the energy dependence of the DNY. This physics-based approach has made it possible to understand the PFNS and DNY in a manner consistent with many other facets of fission physics. Some problems about possible effects of scission neutrons, anisotropy of neutron emission in the center-of-mass system of the fragments, and neutron emission during acceleration are discussed.

Nuclear scission and fission-fragment kinetic-energy distribution: Study within three-dimensional Langevin dynamics

A multi-dimensional stochastic approach based on three-dimensional Langevin equations is applied for calculation of the first four moments of the kinetic-energy distribution of fission fragments in a wide range of Coulomb parameter 700 < Z 2 / A 1 / 3 < 1700 . A modified one-body mechanism of nuclear dissipation with reduction factor k s = 0.25 and different scission criteria widely used in modern fission theory are applied: zero-neck-radius R N = 0 and finite-neck-radius R N = 0.3 R 0 . A probabilistic scission criterion has been proposed also for the description of the scission process of compound nucleus. All of these scission criteria result in a good description of experimental data of the first moment of the kinetic-energy distribution. At the same time all the scission criteria provide satisfactory description of the second, third and fourth moments of the energy distribution. On the basis of theoretical calculations we cannot discriminate scission criterion, which scission criterion gives the best agreement with the experimental data. A possible way to improve the theoretical description of the experimental data could be introduction of additional collective coordinates, which could provide more flexible shapes of compound nucleus during the descent from saddle to scission.

Possible explanation of fine structures in mass-energy distribution of fission fragments

Within an improved scission point model, experimental data on the relative yield, mean value and variance of the total kinetic-energy distribution of fission fragments are described. It is shown that for a fixed mass and charge fragmentation, the potential energy of the scission configuration has several minima as a function of the deformation parameters of the fragments. The scission at these minima leads to a relatively enhanced yield of the fragments with a certain TKE and creates fine structures in the TKE-mass distribution which are different from those produced by the odd-even effect.

Three-dimensional Langevin calculations of fission fragment mass-energy distribution from excited compound nuclei

A stochastic approach to fission dynamics based on three-dimensional Langevin equations was applied to calculate fission fragment mass-energy distribution from a number of excited compound nuclei formed in reactions induced by heavy ions. Evaporation of prescission light particles along Langevin fission trajectories from the ground state of the compound nucleus to its scission has been taken into account using a Monte Carlo simulation technique. Inclusion of the third collective coordinate in Langevin dynamics leads to a considerable increase of the variance of the mass and the kinetic-energy distributions of fission fragments as compared with two-dimensional Langevin calculations. A liquid-drop model with finite range of nuclear forces and a modified one-body mechanism for nuclear dissipation have been used in the calculations. The results of the calculations are compared with the available experimental data. The calculations performed using the three-dimensional Langevin dynamics reproduce sufficiently well all the parameters of the two-dimensional fission fragment mass-energy distribution and their dependence on various parameters of the compound nucleus. The mean prescission neutron multiplicities are also reproduced with good accuracy. In order to reproduce simultaneously the measured prescission neutron multiplicities and the variance of the fission fragment mass-energy distribution, the reduction coefficient of the contribution from a wall formula has to be decreased at least by half of the one-body dissipation strength $(0.25&lt;~{k}_{s}&lt;~0.5).$

Systematic studies of fission fragment kinetic energy distributions by Langevin simulations

Fission fluctuation-dissipation dynamics of heavy nuclei has been studied using Langevin Monte Carlo simulations. The covariant form of the fission transport equation and the coefficients related to it are investigated. To learn about the influence of the dynamics from the ground state to the saddle point on the kinetic energy distributions we have studied various systems and compared the calculations both starting from the ground state and from the saddle point. Both the mean total kinetic energy of the fission fragments and its variances can fit with the experimental values in terms of a finite neck radius as scission condition.

Resonance structure in the fission of ( 235U + n)

A new multilevel reduced R-matrix analysis of the neutron-induced resonance cross sections of 235U has been carried out. We used as a constraint in the analysis the angular anisotropy measurements of Pattenden and Postma, obtaining a Bohr-channel (or J, K channel) representation of the resonances in a two-fission vector space for each spin state. Hambsch et al., have reported definitive measurements of ( 235U the mass- and kinetic-energy distributions of fission fragments of ( 235U + n) in the resonance region and analyzed their results according to the fission-channel representation of Brosa et al., extracting relative contributions of the two asymmetric and one symmetric Brosa fission channels. We have explored the connection between Bohr-channel and asymmetric Brosa-channel representations. The results suggest that a simple rotation of coordinates in channel space may be the only transformation required; the multilevel fit to the total and partial cross sections is invariant to such a transformation.

A simplified two-dimensional diffusion model for calculating the fission-fragment kinetic-energy distribution

The variance\(\sigma _{E_k }^2\) of the fission-fragment kinetic-energy distribution is calculated in describing the dynamics of fission of an excited nucleus by a Fokker-Planck equation for the quantum distribution function of two collective coordinates (the distance between the centers of mass of the nascent fragments and the neck parameter) and their conjugate momenta. In the calculations fluctuations in collective coordinates near scission and prescission kinetic energy have been taken into account simultaneously. The results of statistical model calculations for the case of high friction in fission mode and those of the calculations in the framework of a dynamical model for zero nuclear viscosity are the limiting cases of the proposed model. The dependence of\(\sigma _{E_k }^2\) upon the fissility parameter is studied.

The influence of thermal fluctuations on the kinetic-energy distribution of fission fragments

Using a one-dimensional diffusion model, we show that thermal fluctuations (as opposed to fluctuations of the scission configuration) contribute significantly to the width of the kinetic-energy distribution of fission fragments.

Comparison of fragment kinetic energies from twoPu237fission isomers

Fission fragment kinetic energy distributions have been measured separately for two spontaneously fissioning isomers of $^{237}\mathrm{Pu}$ produced by the $^{237}\mathrm{Np}(d,2n)^{237}\mathrm{Pu}^{m}$ reaction at 13 MeV. Single fragment peak to valley ratios and heavy and light fragment peak positions were measured for each of the two isomers. Average fragment kinetic energies for each isomer were calculated and compared. Values of isomeric fission half-life are reported with increased accuracy.NUCLEAR REACTIONS, FISSION $^{237}\mathrm{Np}(d,2n)^{237}\mathrm{Pu}^{m}$, $E=13$ MeV. Spontaneously fissioning isomers. For each isomer: fragment $E$, ${T}_{\frac{1}{2}}$.

Thermal-Neutron-Induced Fission ofTh229

A back-to-back ionization chamber is used to measure the ${\mathrm{Th}}^{229}$ fission fragment kinetic energy distribution. This fission process is found to be the most asymmetric known, with a most probable mass ratio of 1.57. The measured total fragment energy is 160.2\ifmmode\pm\else\textpm\fi{}3 Mev. The experimental results are compared with the characteristics of other fission processes and with theory.