Fission Fragment Mass Distributions Research Articles

Fabrication of 208Pb and 193Ir targets on Al-backing using high/ultra-high vacuum deposition technique for low-energy nuclear reaction experiments

Aiming to study the interplay of different nuclear reactions at low incident energies, thin targets of enriched stable isotopes, 208Pb and 193Ir, have been fabricated at the target development laboratory of the Inter-University Accelerator Centre (IUAC), New Delhi. As a means of preparing a large number of thin isotopically pure targets, physical vapour deposition techniques, (a) resistive heating, for fabrication of 208Pb targets of mass thickness 160-340μg/cm2, and (b) electron-gun evaporation technique, for fabrication of 193Ir targets of mass thickness 17-60μg/cm2, have been employed. The fabricated targets were characterized using Rutherford Backscattering Spectrometry (RBS) and Field Emission Scanning Electron Microscopy (FE–SEM). The thickness of targets was estimated based on the evaporation rate and measured by RBS. The RBS was employed to analyse the samples for trace- and heavy impurities, provided the measurements and analysis suggest that the targets prepared in the present work do not contain any impurities. FE-SEM was employed to examine the surface morphology and microstructure in high resolution. This technique enables the detection of any morphological changes induced by the irradiation process, offering insights into the effects of irradiation on the target films. The targets have been successfully deployed to measure complete and incomplete fusion excitation functions, mass distribution of fission fragments, and forward ranges of target-like recoils in 12C + 208Pb, 193Ir reactions from sub- to above barrier energies. Preliminary data analysis suggests the achievement of high-quality 208Pb and 193Ir target foils.

Fabrication and characterization of highly enriched thin 176Yb targets for the reaction dynamic studies on fission mass distribution

The fabrication, transfer, and storage of a highly enriched isotopic target is very much crucial because of air gases and moistures being chemically active at room temperature. One must limit the amount of time that the target surface is exposed to the atmosphere during the transfer of air sensitive targets. The highly interactive targets are not easy to store for longer time and demand an in-vacuum transfer facility. Getting an alternative technique to remove all such imperative is a need of the hour specially for the nuclear physics community, more particularly for reaction dynamic studies of fission fragment mass distribution induced by stable and radioactive ion beams. Highly enriched materials are rare and very expensive, so an effective and highly efficient technique is needed to deal with such materials. Here, an attempt has been made to fabricate 96.63% enriched thin targets of 176Yb by evaporation techniques enabling ≈129.1 μg/cm2 thickness with a ≈25.93 μg/cm2 carbon backing to increase its stability and durability. Numerous trials have been performed using natural Yb to optimize the evaporation setup and associated growth parameters for efficient Yb isotope deposition with thin film deposition unit and diffusion pump based coating unit at Inter University Accelerator Centre (IUAC), New Delhi. Thickness and elemental composition of the targets have been analysed using characterization techniques such as X-ray diffraction, α-energy loss technique, Rutherford backscattering spectroscopy and energy dispersive X-ray spectroscopy. Essentially no significant impurities are found by these measurements, describing high purity and robustness of the fabricated targets. During the nuclear experiments, 30Si and 16O beams of intensity of the order of ≈1010 particles per second were bombarded on the targets for 5–6 days and no ill effects on the target were observed. Isotopic abundance of the target has been obtained experimentally, which ensured the isotopic purity of the target required for the reaction studies. This article will admire our readers about the qualitative nature of the fabrication technique, measurement and characterization citing valuable insights, aiding in a comprehensive understanding of their quality.

Detailed modeling of odd–even staggering in fission-fragment charge distributions

During the last 10 years or so the Brownian shape-motion (BSM) model has been used in numerous calculations of fission-fragment mass and charge distributions with encouraging agreement with experimental measurements. In this model the structure obtained in the fission-fragment distributions is entirely a consequence of the structures in the calculated five-dimensional (5D) potential-energy surfaces. The potential-energy model until recently did not accommodate the influence of the emerging fragment properties on the calculated potential energy. Therefore there were no odd-even effects in the calculated fission-fragment distributions. Recent extensions of the potential-energy model allow properties of the nascent fragments to be included in the potential-energy model. Application of the BSM model to execute random walks on these more detailed potential-energy surfaces led to calculated fission-fragment yields that exhibited odd-even effects, which “by eye” indicated reasonable agreement with experimental data. The present work goes a step further with a quantitative comparison between experimental and theoretical results based on the global and local odd-even staggering observables. Theoretical calculations and experimental observations both show that pairing effects and enhancement of two-nucleon relative to one-nucleon transfer in heavy-ion collisions decrease with excitation energy and implementing a damping of these quantities with excitation energy leads to improved agreement with experiment. Characteristic variations of the local staggering with charge split seen in the experimental data are also present in the calculated results.

Fission fragment mass distribution of 187Ir

The fission fragment mass and total kinetic energy (TKE) distributions were studied for 187Ir populated using 12C + 175Lu reaction at excitation energies 24.8, 23.3, 20.8, and 16.7 MeV above the saddle point. Fission fragments (FF) were detected using position sensitive MWPC’s kept at a folding angle. The mass of the fragment was calculated using Time-of-Flight (TOF) difference method. The multi Gaussian fit to the extracted mass distributions shows the presence of narrower microscopic components corresponding to Z = 38 and 45 in addition to the liquid drop component at lower excitation energies. The GEF model predicts significant asymmetric components at all the measured energies. The TKE distribution investigations suggest the dominance of macroscopic liquid drop components at all energies.

Systematic study of the widths of fission fragment mass distributions of neutron deficient compound nuclei in the medium mass region

Systematic study of the widths of fission fragment mass distributions of neutron deficient compound nuclei in the medium mass region

SHELL EFFECTS IN THE FISSION OF <sup>236</sup>U* NUCLEI, FORMED IN THE REACTION <sup>232</sup>Th(α,f) AT INCIDENT ALPHA PARTICLES ENERGY OF 29 MeV

Mass and energy distribution of fission fragments of 236U* nuclei, formed in the reaction 232Th(α,f) at incident alpha energy of 29 MeV were studied to reveal the influence of shell effects. The experiment was carried out by 2E method at U-150M accelerator at Institute of Nuclear Physics, Almaty city. Acquired experimental data was decomposed into yields of separate shells, including deformed shells, assuming that the shell yield has the form of gauss distribution. The manifestation of deformed shells N84, Z52 and deformed shells Z36, Z38 was revealed.

Shell effect on fission fragment mass distribution at Ecn* up to 70 MeV: Role of multichance fission

Mass distributions of fission fragments produced from the $^{249}\mathrm{Bk}$ compound nucleus (CN), populated by complete fusion of $^{11}\mathrm{B}$ with $^{238}\mathrm{U}$, were measured for the excitation energies in the range of ${E}_{\mathrm{cn}}^{*}\ensuremath{\approx}36.7--69.7$ MeV. Nearly flat tops observed for these distributions indicate the presence of asymmetric fission, contrary to the pure Gaussian mass distributions expected at such high excitation energies. A fit to the mass distribution using three Gaussian functions provides an estimate of individual contributions from symmetric and asymmetric modes of fission. A significant contribution of the asymmetric component observed at CN excitation energies above 40 MeV can be understood only by invoking ``multichance fission'' in the calculations using the semiempirical model code gef. A systematic analysis of the mass distributions for several heavy transuranic nuclei reveals that the manifestation of the fragment shell effect in the integral mass distributions are visible even up to an initial compound nucleus excitation of ${E}_{\mathrm{cn}}^{*}\ensuremath{\approx}70$ MeV due to their influence on the distributions of higher chance fissions.

Fission dynamics of excited compound nuclei in a wide range of fissile parameters produced in O16,18+X reactions within a stochastic approach

A stochastic approach based on four-dimensional Langevin equations has been used to simulate the fission dynamics of the excited compound nuclei $^{256}\mathrm{Fm}, ^{254}\mathrm{Fm}, ^{224}\mathrm{Th}, ^{215}\mathrm{Fr}, ^{213}\mathrm{Fr}, ^{200}\mathrm{Pb}, ^{187}\mathrm{Ir}, ^{172}\mathrm{Yb}, ^{170}\mathrm{Yb}, ^{162}\mathrm{Yb}, ^{142}\mathrm{Ce}$, and $^{125}\mathrm{Cs}$ produced in ${}^{16,18}\mathrm{O}+X$ reactions with projectile energies 158.8 and 288 MeV. The effect of nuclear dissipation was investigated in the dynamical simulation of fission process of these nuclei. The average prescission neutron multiplicity, the fission time, the average total kinetic energy of fission fragments, the mass and energy distribution of fission fragments, and the variation of the prescission neutron multiplicity with total kinetic energy of fission fragments have been calculated for these nuclei. The dynamical calculations have taken into account not only three shape collective coordinates introduced on the basis of the ``funny hills'' shape parameters but also orientation degree of freedom (spin about the symmetry axis), and in the dynamical calculations dissipation generated through the chaos-weighted wall and window friction formula and also the sum of the chaos-weighted wall and window friction formula with the two-body nuclear friction. Furthermore, the effect of dissipation coefficient of $K, {\ensuremath{\gamma}}_{K}$, were also considered in the dynamical calculations. Comparison of the theoretical results with the experimental data shows that the experimental data for compound nuclei produced with the projectile energy 158.8 MeV can be reproduced satisfactorily with the results of calculations by using the chaos-weighted wall and window friction formula, although the results of calculations of the average prescission neutron multiplicity, the fission time, and the variation of the pre-scission neutron multiplicity with total kinetic energy of fission fragments lie somewhat below the experimental data for heavy nuclei produced in reactions with projectile energy ${E}_{\mathrm{lab}}=158.8\phantom{\rule{0.28em}{0ex}}\mathrm{MeV}$ and also for compound nuclei produced with the projectile energy 288 MeV. It was also shown that for reproducing experimental data for heavy nuclei produced in reactions with projectile energy ${E}_{\mathrm{lab}}=158.8\phantom{\rule{0.16em}{0ex}}\mathrm{MeV}$ and also for nuclei produced with projectile energy ${E}_{\mathrm{lab}}=288\phantom{\rule{0.16em}{0ex}}\mathrm{MeV}$ is suitable to use the sum of the chaos-weighted wall and window friction formula with the two-body nuclear friction in the Langevin equations. In other words, for simulations of fission dynamics of nuclei at high excitation energies and also for heavy nuclei it is suitable to use the sum of the chaos-weighted wall and window friction formula with the two-body nuclear friction in the Langevin equations.

Shell effect driven fission modes in fragment mass and total kinetic energy distribution of Hg*192

Background: Asymmetric fission in sublead nuclei, especially mercury and platinum isotopes, has generated keen interest in studying fission in $A<200$ mass region. The role of proton and neutron shells on asymmetric fission in these nuclei is still under test and requires more investigation.Purpose: This paper presents measurements aimed at studying the fission modes of $^{192}\mathrm{Hg}$ formed in the reaction $^{32}\mathrm{S}+^{160}\mathrm{Gd}$ in the excitation energy range 54--74 MeV.Method: Mass distributions have been determined from the fission fragments' (FF) time of flight (TOF). Two multiwire proportional chambers were placed symmetrically on opposite sides of the beam to cover the folding angle for symmetric fission. The measured TOF of the fission fragments was used to obtain their velocities, and after clean separation of compound nuclear fission events, the velocities were further used to obtain the fission fragment mass and total kinetic energy (TKE) distribution.Results: The fission fragment mass distributions at all beam energies were found to be characteristically flat topped, which deviates significantly from a single Gaussian behavior. The distinctive features of mass and energy distributions happen to be explained by the presence of a symmetric fission mode and three asymmetric fission modes, manifested by different total kinetic energies in varying mass regions. These asymmetric fission modes are a consequence of the stabilization role of proton shells at $Z\phantom{\rule{4pt}{0ex}}\ensuremath{\approx}$ 36, $Z\phantom{\rule{4pt}{0ex}}\ensuremath{\approx}$ 46, and $Z$ = 28/50. The most probable mass of light and heavy fragments are found to be around 86 and 106 u, respectively. The symmetric yield is found to increase with the excitation energy.Conclusions: The present study indicates a mixture of symmetric and asymmetric modes in the fission of $^{192}\mathrm{Hg}$ nuclei and points out the relevance of deformed proton shells at $Z\phantom{\rule{4pt}{0ex}}\ensuremath{\approx}$ 36 and 46.

The description of dynamical fission process using improved quantum molecular dynamics model incorporated with microscopic potential energy surface

A new method, namely the improved quantum molecular dynamics model incorporated with the microscopic potential energy surface, is proposed to simulate the dynamical evolution from saddle to scission point and to calculate the isotope yields of fission fragments in thermal neutron-induced fission of actinide nuclei near 236U. With this approach, the shell and pairing effects are introduced by the microscopic potential energy, while the isospin effect, the dynamical effects of fluctuations and correlations in fission process are automatically included in the improved quantum molecular dynamics model. These two aspects eventually influence the productions of fission fragments. The calculated both charge and mass distributions of fission fragments for a series of actinide nuclei are in overall agreement with the evaluated data from JEFF-3.3 and ENDF/B-VIII.0. The isotope distributions of fission fragments in the fission of 236U reproduce the data reasonable well, from which we extract the relationship between peak mass number of the isotope distributions and the charge number of fission fragments. We find that this relationship deviates linear relation slightly at certain cases, which indicates the isospin nonequilibrium effect. By tracking back to the fission process on the two dimension plane of elongation and mass asymmetry of prefragments, we find that the necked-in shape and the separation of two parts happen much faster for the 132Sn than 128,136Sn in the time evolution of the distance R between the centers of two parts of the fissioning system. The events producing 128,132,136Sn and 136,140,144Xe present two kinds of average fission paths with bypassing and going through the minimum-energy valley of the microscopic potential energy surface, respectively.

Microscopic study of neutron-induced fission process of 239Pu via zero- and finite-temperature density functional theory* *Supported by the National Natural Science Foundation of China (11790325, 12275081, 11790320, 11790321, 11961131010, 11605054, 12105369, 12147219, 12047568) and the Continuous Basic Scientific Research Project (WDJC-2019-09)

To study the neutron-induced fission of Pu, potential energy surface (PES) calculations were performed using zero and finite-temperature density functional theory (FT-DFT) with the Skyrme force. The energy of the incident neutron was simulated by the temperature of the FT-DFT. The variations of the least-energy fission path, fission barrier, total kinetic energy, scission line, and mass distribution of fission fragments with the incident neutron energy were analyzed. It was learned that an increase in the temperature lowers the barrier height, the isomeric-state energy, and the ridge between symmetric and asymmetric fission valleys. Additionally, the gaps of the single particle levels become smaller with an increase in the temperature. As the temperature increases, the pre-fission region shrinks, and the scission occurs at smaller deformation around the symmetric fission channel. At low temperatures, the pairing correlations in the collective space are similar to those in zero-temperature DFT, and when the temperature is 0.3 MeV, the pairing gaps decrease rapidly. Two different methods were used to calculate the fission yields of the neutron-induced fission Pu (n, f) with different incident neutron energies, in the framework of time-dependent generator coordinate method (TDGCM). One way to calculate the fission yield of Pu (n, f) is to solve the collective equation of the TDGCM by using the PES from the FT-DFT with the corresponding temperature. The other involves using the PES from the zero-temperature DFT and adjusting the initial collective energy of the wave packet in the TDGCM according to the incident neutron energy. For the cases of the lower incident neutron energies, these two methods gave similar results and reproduced the experimental peak and width of fission fragment distribution. However, for the highest incident neutron energy considered in this study, the results from the TDGCM using the PES from zero-temperature DFT deviated explicitly from the experimental data, whereas those obtained by using the PES from FT-DFT remained close to the experimental data. This indicated that, with the increase in the incident neutron energy, the shell structure of the compound nuclei changed explicitly; thus, it may not be effective to use the PES from zero-temperature to perform the fission dynamic calculation.

Four-dimensional Langevin approach to fission with Cassini shape parameterization

We apply the Cassini shape parameterization to the dynamical study of actinide fission using the multi-dimensional Langevin equation. The fragment mass distributions for fission of 236U and 256, 258 Fm are calculated with the 4D Langevin equation. The 4D collective space is formed by adding either α3 or α6 to the basic 3D {α, α1, α4} space. We investigate the role of α3 and α6 by comparing the results of 4D (α3 or α6) with that of 3D. For 236U and 256Fm, where asymmetric fission is predominant, the fragment mass distributions calculated in the 4D (α3) space are in good agreement with experimental data. On the other hand, for 258Fm, which is characterized by symmetric fission, 4D (α6) is effective for describing the shape of compact symmetric fragments. We demonstrate the advantages of the Cassini shape parameterization for the dynamical study of actinide fission.

Fission yield model for astrophysical use based on the four-dimensional Langevin model

Nuclear fission is expected to play an essential role in the r-process nucleosynthesis via the fission recycling process. However, the impact of the fission recycling process on the r-process is still unclear due to the ambiguity of the fission yields for nuclei far from the beta-stability line and superheavy nuclei. Our four-dimensional Langevin model can reproduce experimental nuclear fission properties, such as fission fragment mass distributions and total kinetic energies in a systematic manner including their sudden changes due to shell structure. Then we developed a phenomenological yield model for nuclei Z=92 to 122, ranging from neutron deficient to very neutron-rich ones, by fitting the fission fragment mass yield obtained by the Langevin model with five Gaussians. We use the charge distributions for each isobar based on a parametrization which we have evaluated based on thousands of experimental data for actinides, to obtain the fission yields in the form of (Z, N) distributions. What is unique here is the sudden change of the mass distribution from a 2-peak to 1-peak structure for the region of 256Fm, and the appearance of a 5-peak structure for superheavy nuclei predicted by the 4D Langevin calculation.

Determination of the Plasma Delay Time in PIPS detectors for fission fragments at the LOHENGRIN spectrometer

The VElocity foR Direct particle Identification spectrometer (VERDI) is a 2E-2v fission spectrometer that allows the measurement of the total mass distribution of secondary fission fragments with a resolving power of 1-2 u. It consists of two time-of-flight (ToF) arms, with one Micro Channel Plate (MCP) detector and up to 32 Silicon PIPS (Passive Implanted Planar Silicon) detectors per arm. The MCPs provide the start timing signals and the PIPS detectors provide both the energy and the stopping ToF signals. In real conditions, the PIPS signals are affected by the formation of plasma from the interaction between the heavy ions and the detector material. The plasma contributes to a reduction in signal amplitude, resulting in a Pulse Height Defect (PHD), and introduces a signal delay, known as Plasma Delay Time (PDT). An experiment to characterize the PDT and PHD was performed at the LOHENGRIN recoil separator of the Institut Laue Langevin (ILL). Characteristic fission fragments from the 239Pu(n,f) reaction were separated based on their A/Q and E/Q ratios, allowing the measurement of a wide range of energies from 21 to 110 MeV and masses between 80 and 149 u. Six PIPS detectors were characterized to study their individual responses to the PDT and PHD effects. The signals were recorded in a digital acquisition system to completely exploit the offline analysis capabilities. Achieved combined timing and energy resolutions for fission fragments varied between 72(2) ps and 100(4) ps and 1.4% - 2% (FWHM), respectively. Preliminary PHD and PDT data are presented from the masses A=85, 95, 130 and 143. The PHD trends are strongly correlated with both the ion energy and mass. The PDT, on the other hand, shows a strong variation as a function of the ion kinetic energy but a smaller dependence on the ion mass.

Understanding fission fragment mass distributions in a shape-modified random neck rupture model

Understanding fission fragment mass distributions in a shape-modified random neck rupture model

Development of Micro-Pattern Gaseous Detectors for Nuclear Reaction Studies

One of the frontiers of today’s nuclear physics research is the synthesis of Super Heavy Elements (SHE). Fusion-fission dynamics, namely the competition between quasi fission and fusion is one of the key challenges to optimize the SHE. To have an insight into the dynamics, one requires the study of fission fragment mass and angular distribution near barrier energies for heavy-ion induced fission reactions. Recent successful installation of linear accelerators in India offers a unique opportunity to study the dynamics of nuclear reactions and formation process of SHE. For the effective utilization of these current, as well as upcoming facilities, development of novel detectors to study reaction dynamics, formation process of SHE with heavier projectiles and higher beam energies is needed. Gaseous detectors have undergone a rapid improvement in terms of spatial, temporal and energy resolution, rate capability, radiation hardness, ion feedback etc., ushering in a new genre of micro-structured devices based on semi-conductor technology, commonly known as Micro-Pattern Gaseous Detectors (MPGDs). Although many of the MPGD structures were primarily developed for high-rate tracking of charged particles in high energy physics experiments, stability of operation, simplicity of construction and relatively low cost make these detectors suitable for other applications, such as low-energy nuclear physics experiments. The present activities encompass a detailed evaluation of the operational conditions of Micromesh-Multi Wire and THGEM-Multi Wire hybrid detector operated in low-pressure isobutane gas with a view to optimizing their use in the detection of charged particles and fission fragments.

The FFIS spectrometer for determination of fission fragment mass distribution with the energy–velocity method

The FFIS spectrometer (Fission Fragment Identification Spectrometer) for determining independent fission yields has been developed based on the E–v method. By directly measuring the time-of-flight (TOF) and kinetic energy of fission fragments on their flight path, the post-neutron-emission mass yield distributions can be obtained. With the aim of getting the highest possible mass resolution, an axial grid ionization chamber and a TOF measurement system based on microchannel plate (MCP) detectors were designed. The ionization chamber with a silicon nitride entrance window of 100 nm thickness was tested and calibrated with a 252Cf spontaneous fission source. The intrinsic energy resolution for 1.27 MeV/u 63Cu particles is measured to be 450 keV (FWHM). The time resolution was determined with two MCP timing detectors to be 157 ps (FWHM) for 241Am radiation source, implying that the intrinsic time resolution is about 110 ps (FWHM) for a single timing detector. Thermal-neutron-induced fission of 235U was studied at the In-hospital Neutron Irradiator (IHNI). An event-by-event energy loss addback technique was developed based on Geant4 calculation to minimize the correction error. The first results of mass yield distributions from 235U(nth,f) are reported.

Measurement of mass and total kinetic energy distributions for the C12+Lu175 system

Fission fragment mass and total kinetic energy (TKE) distributions were measured for $^{12}\mathrm{C}+^{175}\mathrm{Lu}$ system at excitation energies down to 16.7 MeV above the saddle point. The overall mass and TKE distributions could be fitted with single Gaussian functions. The observed width of the mass and TKE distributions agree well with the systematics based on liquid drop (LD) behavior. The average TKE also shows parabolic dependence on fragment mass, as expected from LD behavior. Small contributions due to microscopic corrections from $Z\ensuremath{\approx}38$ and 45 shells can be extracted, if the widths of the LD component are fixed from systematics. Contrary to the theoretical predictions of substantial contributions from microscopic corrections, dominance of liquid drop behavior was observed.

Impact of nuclear dissipation on the fission dynamics within the Langevin approach

The impact of nuclear dissipation on the dynamics of nuclear fission at low excitation energy is studied with a three-dimensional Langevin approach in which the two-center shell model is adopted to describe the nuclear shape and the single-particle potential. With three types of formulas for the friction tensor, such as the wall formula, the wall-and-window formula with and without the term related to the mass asymmetry change, the fragment mass and the total kinetic energy (TKE) distributions in low-energy fission are calculated. The calculated fragment mass distributions are almost identical for both with the wall formula and with the wall-and-window model without the term related to the mass asymmetry change, and both the results are consistent with the evaluated mass yields and the calculated results with the GEF model. Our study found that the term related to the mass asymmetry change in the window formula leads to an unreasonable shift of the peak position of the mass distribution. The TKE values calculated with the wall model are slightly larger than those with the wall-and-window model due to the more compact configuration obtained with the pure wall model. Moreover, the influences of the strength of friction tensor on the fragment distribution, the scission configuration, and the fission time distribution are investigated. With the present model the fission fragment mass distributions of U, Np, Pu, and Am isotopes are well reproduced and the systematic dependence of the averaged TKE on the Coulomb parameter is also well reproduced.

Fission mechanism inferred from nuclear shape fluctuation by the Langevin equation

One of the dynamical investigations foran understanding of the fission process is the analysis through Langevin trajectories in the nuclear deformation space. The main motivation of this analysis is to elucidate the characteristic nuclear-shape oscillation induced by a random force in the Langevin equation and its impact on the fission process. The characteristic random oscillation originates from the requirement of an overdamped condition. The properties of the friction tensor in the equation reveal that the directional nuclear shape fluctuations originate in the subspace of nuclear deformation. Our calculations exhibit a good agreement with the fission data for fermium isotopes, where the fission-fragment mass distribution changes dramatically from the asymmetric shape to the sharp symmetric shape as the masses of the fissioning Fm isotopes increase.