Kinetic Energy Of Fission Fragments Research Articles

Fission-fragment total kinetic energy and mass yields for neutron-induced fission of 235U and 238U with En =200 keV – 30 MeV

The average Total Kinetic Energy (TKE ) release and fission-fragment yields in neutron-induced fission of 235 U and 238 U was measured using a Frisch-gridded ionization chamber. These observables are important nuclear data quantites that are relevant to applications and for informing the next generation of fission models. The measurements were performed a the Los Alamos Neutron Science Center and cover E n = 200 keV – 30 MeV. The double-energy (2E) method was used to determine the fission-fragment yields and two methods of correcting for prompt-neutron emission were explored. The results of this study are correlated mass and TKE data.

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.

Post-scission dissipative motion and fission-fragment kinetic energy

A dynamic model developed earlier is used to describe the last stage of nuclear fission: the postscission motion of fission fragments. In this process, Coulomb repulsive energy turns into the kinetic energy of fission fragments measured by experimenters. It is shown that the dissipated energy can be as high as 10% of the average experimental kinetic energy.

Asymmetry of fission fragment mass distribution for Po and Ir isotopes

Using the improved scission-point model, the mass distributions are calculated for induced fission of several Po and Ir isotopes. The calculated mass distributions and mean total kinetic energies of fission fragments are compared with the existing experimental data. The revealed coexistence of both symmetric and asymmetric modes in the $\ensuremath{\beta}$-delayed fission of $^{194,196}\mathrm{At}$ is in agreement with the experimental observations. The change of the shape of mass distribution with increasing $A$ of fissioning $^{A}\mathrm{Ir}$ nucleus from asymmetric for $^{185}\mathrm{Ir}$ to symmetric for $^{193}\mathrm{Ir}$ is found.

Fission modelling with FIFRELIN

The nuclear fission process gives rise to the formation of fission fragments and emission of particles $ (n,\gamma , e^{-})$ . The particle emission from fragments can be prompt and delayed. We present here the methods used in the FIFRELIN code, which simulates the prompt component of the de-excitation process. The methods are based on phenomenological models associated with macroscopic and/or microscopic ingredients. Input data can be provided by experiment as well as by theory. The fission fragment de-excitation can be performed within Weisskopf (uncoupled neutron and gamma emission) or a Hauser-Feshbach (coupled neutron/gamma emission) statistical theory. We usually consider five free parameters that cannot be provided by theory or experiments in order to describe the initial distributions required by the code. In a first step this set of parameters is chosen to reproduce a very limited set of target observables. In a second step we can increase the statistics to predict all other fission observables such as prompt neutron, gamma and conversion electron spectra but also their distributions as a function of any kind of parameters such as, for instance, the neutron, gamma and electron number distributions, the average prompt neutron multiplicity as a function of fission fragment mass, charge or kinetic energy, and so on. Several results related to different fissioning systems are presented in this work. The goal in the next decade will be i) to replace some macroscopic ingredients or phenomenological models by microscopic calculations when available and reliable, ii) to be a support for experimentalists in the design of detection systems or in the prediction of necessary beam time or count rates with associated statistics when measuring fragments and emitted particle in coincidence iii) extend the model to be able to run a calculation when no experimental input data are available, iv) account for multiple chance fission and gamma emission before fission, v) account for the scission neutrons. Several efforts have already been made to replace macroscopic ingredients and phenomenology by microscopic ingredients provided in various nuclear parameter libraries such as electric dipole photon strength functions or HFB level densities. First results relative to theses aspects are presented in this work.

Theoretical investigation of shape parametrization’s effects on characteristics of fission fragments in 18O-induced fission of 154Sm, 197Au, 238U and 246Cm

The multi-dimensional Langevin equations are employed to investigate mass and energy distributions of fission fragments. The calculations have been performed with two different shape classes of parametrization, namely Funny Hills and Cassinian Ovaloids. It was shown that inclusion of the Funny Hills parameterizations in the dynamical model produced considerable increase in neutron multiplicity as compared with available experimental data for 18O-induced fission of 154Sm, 197Au, 238U and 246Cm. The proposed shape-dependent multi-dimensional dynamical model reproduces well experimental data for mass distribution, neutron multiplicity and average kinetic energy of fission fragments over a wide range of mass–energy regions.

Description of induced nuclear fission with Skyrme energy functionals. II. Finite temperature effects

Understanding the mechanisms of induced nuclear fission for a broad range of neutron energies could help resolve fundamental science issues, such as the formation of elements in the universe, but could have also a large impact on societal applications in energy production or nuclear waste management. The goal of this paper is to set up the foundations of a microscopic theory to study the static aspects of induced fission as a function of the excitation energy of the incident neutron, from thermal to fast neutrons. To account for the high excitation energy of the compound nucleus, we employ a statistical approach based on finite-temperature nuclear density functional theory with Skyrme energy densities, which we benchmark on the 239 Pu(n,f) reaction. We compute the evolution of the least-energy fission pathway across multidimensional potential energy surfaces with up to five collective variables as a function of the nuclear temperature, and predict the evolution of both the inner and outer fission barriers as a function of the excitation energy of the compound nucleus. We show that the coupling to the continuum induced by the finite temperature is negligible in the range of neutron energies relevant for many applications of neutron-induced fission. We prove that the concept of quantum localization introduced recently can be extended to T > 0, and we apply the method to study the interaction energy and total kinetic energy of fission fragments as a function of the temperature for the most probable fission. While large uncertainties in theoretical modeling remain, we conclude that finite-temperature nuclear density functional may provide a useful framework to obtain accurate predictions of fission fragment properties.

Study of fission dynamics of the excited nuclei produced in fusion reactions in the framework of the four-dimensional Langevin equations

The dynamics of fission of excited nuclei has been studied by solving four-dimensional Langevin equations with dissipation generated through the chaos-weighted wall and window friction formula. The projection of the total spin of the compound nucleus to the symmetry axis, K, was considered as the fourth dimension in Langevin dynamical calculations. The average pre-scission neutron multiplicities, mean kinetic energy of fission fragments and the variances of the mass and kinetic energy have been calculated in a wide range of fissile parameter for compound nuclei 162Yb, 172Yb, 215Fr, 224Th, 248Cf, 260Rf and results compared with the experimental data. Calculations were performed with a constant dissipation coefficient of K, γK (MeV zs)-1/2, and with a non-constant dissipation coefficient. Comparison of the theoretical results for the average pre-scission neutron multiplicities, mean kinetic energy of fission fragments and the variances of the mass and kinetic energy with the experimental data showed that the results of four-dimensional Langevin equations with a non-constant dissipation coefficient are in better agreement with the experimental data. Furthermore, the difference between the results of two models for compound nuclei with low fissile parameter is low whereas, for heavy compound nuclei, is high.

Ionization Chamber for Prompt Fission Neutron Investigations

In this work we report recent achievements in design of twin back-to-back ionization chamber (TIC) for fission fragment (FF) mass and kinetic energy measurement. Correlated FF kinetic energies, their masses and the angle of FF in respect to the axes in 3D Cartesian coordinates can be determined from analysis of the heights and shapes of the pulses induced by the fission fragments on the anodes of TIC. Anodes of TIC were designed as consisting of isolated strips each having independent electronic circuitry and special multi-channel pulse processing apparatus. Mathematical formulae provided for FF angles measured in respect to the coordinate axes. It was shown how the point of fission fragments origin on the target plane may be determined using the same measured data. The last feature made the TIC a rather powerful tool for prompt fission neutron (PFN) emission investigation in event-by-event analysis of individual fission reactions from non- point fissile source. Position sensitive neutron induced fission detector for neutron-imaging applications with both thermal and low energy neutrons was found as another possible implementation of the designed TIC.

Cluster aspects of binary fission

With the improved scission-point model the mass distributions are calculated for induced fission of different Hg isotopes with even mass numbers A =180, 184, 188, 192, 196, 198. The calculated mass distribution and mean total kinetic energy of fission fragments are in a good agreement with the existing experimental data. The change in the shape of the mass distribution from asymmetric to more symmetric is revealed with increasing A of the fissioning AHg nucleus, and the reactions are proposed to verify this prediction experimentally.

Point-by-Point model description of average prompt neutron data as a function of total kinetic energy of fission fragments

The experimental data of average prompt neutron multiplicity as a function of total kinetic energy of fragments (TKE) exhibit, especially in the case of 252 Cf(SF), different slopes dTKE/dν and different behaviours at low TKE values. The Point-by-Point (PbP) model can describe these different behaviours. The higher slope dTKE/dν and the flattening of at low TKE exhibited by a part of experimental data sets is very well reproduced when the PbP multi-parametric matrix ν(A,TKE) is averaged over a double distribution Y(A,TKE). The lower slope and the almost linear behaviour over the entire TKE range exhibited by other data sets is well described when the same matrix ν(A,TKE) is averaged over a single distribution Y(A). In the case of average prompt neutron energy in SCM as a function of TKE, different dTKE/de slopes are also obtained by averaging the same PbP matrix e(A,TKE) over Y(A,TKE) and over Y(A). The results are exemplified for three fissioning systems benefiting of experimental data as a function of TKE: 252 Cf(SF), 235 U(nth,f) and 239 Pu(nth,f). In the case of 234 U(n,f) for the first time it was possible to calculate (TKE) and (TKE) at many incident energies by averaging the PbP multi-parametric matrices over the experimental Y(A,TKE) distributions recently measured at IRMM for 14 incident energies in the range 0.3- 5 MeV. The results revealed that the slope dTKE/dν does not vary with the incident energy and the flattening of at low TKE values is more pronounced at low incident energies. The average model parameters dependences on TKE resulted from the PbP treatment allow the use of the most probable fragmentation approach, having the great advantage to provide results at many TKE values in a very short computing time compared to PbP and Monte Carlo treatments.

Influence of shell effects on mass asymmetry in fission of different Hg isotopes

With the improved scission-point model mass distributions are calculated for induced fission of Hg isotopes with even mass numbers A = 174 – 198. The calculated mass distributions and mean total kinetic energy of fission fragments are in good agreement with the existing experimental data. The asymmetric mass distribution of fission fragments of 180 Hg observed in the recent experiment is explained. The change in the shape of the mass distribution with increasing A of the fissioning A Hg nucleus from symmetric for 174 Hg to asymmetric around 180 Hg, and to more symmetric for 192−198 Hg is revealed.

Corrections of Prompt-neutron Emission in Fission-fragment Experiments

The number of emitted prompt neutrons in the fission process is strongly dependent on the fragment mass. The deformation of the fragments as well as shell effects give the characteristic “sawtooth” shape. It is well known that the total average neutron emission (ν̄tot) increases as a function of incident-neutron energy. But do these extra emitted neutrons also show a particular dependence on the fragment mass? Some experiments have shown that the additional neutrons are emitted from the heavy fragments only. Recent theoretical studies provided an explanation for this, thus emphasising the validity of these observations. Despite this, in various experiments and calculations an average increase of ν̄ ( A ) was and is still assumed. Experimental data on 234U ( n , f) , have been used in this work to study the influence of different neutron-multiplicity shapes. Based on the double-energy technique one cannot validate the one or the other method, because no prompt neutrons are detected in coincidence with the fission-fragment kinetic energies. However one may investigate the impact of the ν̄ ( A ) choice on the fission observables. Two methods were used in the analysis of the experimental data. In one case a higher ν̄ ( A ) was assumed for all fragment masses and in the other case a higher ν̄ ( A ) was only assumed for the heavy fragments. By comparing the two methods, the choice of ν̄ ( A ) was found to be important in the analysis of fission-fragments with relatively strong implications on the mass- and energy distributions. The results stress the need to determine ν̄ ( A ) by measuring fission fragments in coincidence with prompt-fission neutrons.

Point-by-Point model description of experimental average prompt neutron multiplicity as a function of total kinetic energy of fission fragments

The present Point by Point (PbP) model calculations of average prompt neutron multiplicity as a function of total kinetic energy 〈ν〉(TKE), performed for three fissioning systems 252Cf(SF), 239Pu(nth, f) and 235U(nth, f), describe very well the experimental data over the entire TKE range, including not only the slope dTKE/dν but also the flattening of experimental 〈ν〉 data at low TKE values.Present deterministic PbP model results of 〈ν〉(TKE) are analyzed comparatively with previous PbP 〈ν〉(TKE) results based on the same multi-parametric matrix ν(Z, A, TKE) and different manners of averaging this matrix over the fission fragment distributions, as well as with previous and recent results of probabilistic Monte Carlo treatments.The average model parameters as a function of TKE provided by the PbP treatment exhibit nice and regular behaviours and can be fitted well. The resulted model parameter dependences on TKE allow the use of the most probable fragmentation approach (Los Alamos model with subsequent improvements) having the advantages to provide results at many TKE values in a very short calculation time (compared to the PbP model and Monte Carlo treatments) and also outside the TKE range where the PbP model is applicable.Possible explanations of discrepancies between the slopes of different sets of experimental 〈ν〉(TKE) data are mentioned, too.

Mass distributions for induced fission of different Hg isotopes

With the improved scission-point model mass distributions are calculated for induced fission of different Hg isotopes with even mass numbers $A=180,$ 184, 188, 192, 196, and 198. The calculated mass distribution and mean total kinetic energy of fission fragments are in good agreement with the existing experimental data. The asymmetric mass distribution of fission fragments of ${}^{180}$Hg observed in the recent experiment is explained. The change in the shape of the mass distribution from asymmetric to more symmetric is revealed with increasing $A$ of the fissioning ${}^{A}$Hg nucleus, and reactions are proposed to verify this prediction experimentally.

Mass-energy distribution of fragments within Langevin dynamics of fission induced by heavy ions

A stochastic approach based on four-dimensional Langevin fission dynamics is applied to calculating mass-energy distributions of fragments originating from the fission of excited compound nuclei. In the model under investigation, the coordinate K representing the projection of the total angular momentum onto the symmetry axis of the nucleus is taken into account in addition to three collective shape coordinates introduced on the basis of the {c, h, α} parametrization. The evolution of the orientation degree of freedom (K mode) is described by means of the Langevin equation in the overdamped regime. The tensor of friction is calculated under the assumption of the reducedmechanismof one-body dissipation in the wall-plus-window model. The calculations are performed for two values of the coefficient that takes into account the reduction of the contribution from the wall formula: k s = 0.25 and k s = 1.0. Calculations with a modified wall-plus-window formula are also performed, and the quantity measuring the degree to which the single-particle motion of nucleons within the nuclear system being considered is chaotic is used for k s in this calculation. Fusion-fission reactions leading to the production of compound nuclei are considered for values of the parameter Z 2/A in the range between 21 and 44. So wide a range is chosen in order to perform a comparative analysis not only for heavy but also for light compound nuclei in the vicinity of the Businaro-Gallone point. For all of the reactions considered in the present study, the calculations performed within four-dimensional Langevin dynamics faithfully reproduce mass-energy and mass distributions obtained experimentally. The inclusion of the K mode in the Langevin equation leads to an increase in the variances of mass and energy distributions in relation to what one obtains from three-dimensional Langevin calculations. The results of the calculations where one associates k s with the measure of chaoticity in the single-particle motion of nucleons within the nuclear system under study are in good agreement for variances of mass distributions. The results of calculations for the correlations between the prescission neutron multiplicity and the fission-fragment mass, 〈n pre(M)〉, and between, this multiplicity and the kinetic energy of fission fragments, 〈n pre(E k )〉, are also presented.

Study of different features of fission dynamics in heavy-ion-induced reactions

A stochastic approach based on three-dimensional Langevin equations was used to calculate the average pre-scission neutron multiplicities, fission probabilities and the mean kinetic energy of fission fragments. The chaos-weighted wall and window friction formula was applied to determine the dissipative forces in Langevin equations. The analysis of the results shows that the above-mentioned experimental data can be reproduced using the chaos-weighted wall and window friction formula. However, for heavy nuclei the results of calculations for the average pre-scission neutron multiplicities and the variances of the mass and kinetic energy distributions are lower than the experimental data.

Project 242: Fission fragments direct heating for space propulsion—Programme synthesis and applications to space exploration

The status and the main results achieved by Project 242 are presented. Project 242 is a programme (funded by ASI—1999/2002 from an idea of Carlo Rubbia) that studied a new concept of space propulsion motor by using direct conversion of the kinetic energy of fission fragments into increasing of enthalpy of a propellant gas. Project 242 studied the application of this propulsion system to a manned mission to Mars. Preliminary results were very satisfactory and it has been observed that a propulsion system with these characteristics could make the mission feasible. Results for other unmanned missions to the outer solar system are also presented.

Mass-energy distribution of fragments in Langevin dynamics of fission induced by heavy ions

A stochastic approach based on four-dimensional Langevin fission dynamics is applied to calculating mass-energy distributions of fragments originating from the fission of excited compound nuclei. In the model under investigation, the coordinate K representing the projection of the total angular momentum onto the symmetry axis of the nucleus is taken into account in addition to three collective shape coordinates introduced on the basis of the {c, h, α} parametrization. The evolution of the orientation degree of freedom (K mode) is described by means of the Langevin equation in the overdamped regime. The tensor of friction is calculated under the assumption of the reducedmechanismof one-body dissipation in the wall-plus-window model. The calculations are performed for two values of the coefficient that takes into account the reduction of the contribution from the wall formula: ks = 0.25 and ks = 1.0. Calculations with a modified wall-plus-window formula are also performed, and the quantity measuring the degree to which the single-particle motion of nucleons within the nuclear system being considered is chaotic is used for ks in this calculation. Fusion-fission reactions leading to the production of compound nuclei are considered for values of the parameter Z2/A in the range between 21 and 44. So wide a range is chosen in order to perform a comparative analysis not only for heavy but also for light compound nuclei in the vicinity of the Businaro-Gallone point. For all of the reactions considered in the present study, the calculations performed within four-dimensional Langevin dynamics faithfully reproduce mass-energy and mass distributions obtained experimentally. The inclusion of the K mode in the Langevin equation leads to an increase in the variances of mass and energy distributions in relation to what one obtains from three-dimensional Langevin calculations. The results of the calculations where one associates ks with the measure of chaoticity in the single-particle motion of nucleons within the nuclear system under study are in good agreement for variances of mass distributions. The results of calculations for the correlations between the prescission neutron multiplicity and the fission-fragment mass, 〈npre(M)〉, and between, this multiplicity and the kinetic energy of fission fragments, 〈npre(Ek)〉, are also presented.

A new approach to prompt fission neutron TOF data treatment

Abstract The prompt neutron emission in spontaneous fission of 252Cf has been investigated applying digital signal electronics along with associated digital signal processing algorithms. A new mathematical approach, applicable to single events, was developed for prompt fission neutron (PFN) time-offlight distribution unfolding. The main goal was to understand the reasons of the long existing discrepancy between theoretical calculations and the measurements of prompt fission neutron (PFN) emission dependence on the total kinetic energy (TKE) of the fission fragments (FF). Since the 252 Cf (sf) reaction is one of the main references for nuclear data the understanding of the PFN emission mechanism is very important both for nuclear fission theory and nuclear data. The experimental data were taken with a twin Frisch-grid ionization chamber and a NE213-equivalent neutron detector in an experimental setup similar to the well known work of C. Budtz-Jorgensen and H.-H. Knitter. About 2.5 × 10 5 coincidences between fission fragment (FF) and neutron detector response to prompt fission neutron detection have been registered ( ∼ 1.6 × 10 7 of total recorded fission events). Fission fragment kinetic energy, mass and angular distribution, neutron time-of-flight and pulse shape have been investigated using a 12-bit waveform digitizer. The signal waveforms have been analyzed using digital signal processing algorithms. The main goal of this work was a detailed description of the prompt fission neutron treatment.