Fission Fragment Angular Distributions Research Articles

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.

Simulation of the fission process of the excited compound nuclei Rn210 and Fr215 produced in fusion reactions within the framework of the modified statistical model

In the framework of the modified statistical model there have been simulated fission process of the excited compound nuclei $^{210}\mathrm{Rn}$ and $^{215}\mathrm{Fr}$ produced in $^{16}\mathrm{O}+^{194}\mathrm{Pt}$ and $^{19}\mathrm{F}+^{196}\mathrm{Pt}$ reactions and calculated the evaporation residue cross section, the fission cross section, the fission probability, the average prescission neutron multiplicity, the mean fission time, and the anisotropy of fission fragments angular distribution as a function of excitation energy. The classical collective motion of the excited compound nuclei about the ground state, the temperature dependence of the location, the height of fission transition points, and the projection of the total spin of the compound nucleus onto the symmetry axis $K$ have been considered in the statistical calculations. A constant nuclear dissipation was applied in the statistical calculations. In the statistical calculations, the temperature coefficient of the effective potential $k$ and the scaling factor of the fission-barrier height ${r}_{s}$ were considered as a free parameter and their magnitudes inferred by fitting measured data on the evaporation residue cross section and the fission cross section for the excited compound nuclei $^{210}\mathrm{Rn}$ and $^{215}\mathrm{Fr}$. It was shown that the results of calculations are in good agreement with the experimental data by using appropriate values for these parameters equal to $k=0.0160\ifmmode\pm\else\textpm\fi{}0.0050\phantom{\rule{0.28em}{0ex}}\mathrm{Me}{\mathrm{V}}^{\ensuremath{-}2}$ and ${r}_{s}=1.0030\ifmmode\pm\else\textpm\fi{}0.0020$ for $^{210}\mathrm{Rn}$ and $k=0.0065\ifmmode\pm\else\textpm\fi{}0.0040\phantom{\rule{0.16em}{0ex}}\mathrm{Me}{\mathrm{V}}^{\ensuremath{-}2}$ and ${r}_{s}=1.0040\ifmmode\pm\else\textpm\fi{}0.0015$ for $^{215}\mathrm{Fr}$. Furthermore, by using appropriate values of parameters $k$ and ${r}_{s}$ have been calculated the average prescission neutron multiplicity, the fission probability, the mean fission time, and the anisotropy of fission fragments angular distribution for the nuclei $^{210}\mathrm{Rn}$ and $^{215}\mathrm{Fr}$. Comparison of the theoretical data with the experimental data were shown that the modified statistical model is well able to reproduce different experimental data. Although, at high excitation energies the results of calculations for the anisotropy of fission fragments angular distribution and fission probability are slightly lower than the experimental data.

Angular momentum transfer in multinucleon transfer channels of O18+Np237

The angular momentum of a primary excited compound nucleus produced in multinucleon transfer reaction is an important quantity to evaluate cross sections to synthesize neutron-rich heavy-element nuclei as well as for surrogate reaction studies. The mechanism is, however, not enough understood due to the lack of detailed experimental data. In the present study, we determined the angular momentum of primary excited nuclei, $^{237,238,239}\mathrm{Np}, ^{238,239,240}\mathrm{Pu}$, and $^{239,240,241}\mathrm{Am}$, produced in the multinucleon transfer channels of the $^{18}\mathrm{O}+^{237}\mathrm{Np}$ reaction. With this aim, angular distributions of fission fragments with respect to the axis perpendicular to the reaction plane were measured for each compound nucleus. The distributions show an anisotropy exhibiting an enhanced yield on the reaction plane. They are well reproduced by a saddle-point model, from which the average angular momentum is derived in the model framework. The angular momentum increases with the compound-nucleus mass, thus the number of nucleons exchanged, but shows a saturating trend toward heavier compound nuclei. These results are the first ones to point to the dependence of the angular momentum on the transfer channels.

Measurement of fission excitation function for 19F + 194, 196, 198Pt reactions

Experimental fission cross-sections are reported for the 19F + 194,196,198Pt reactions populating an isotopic chain of compound nuclei comprising both closed and non-closed shell nuclei. The fission cross-sections are obtained at near and above Coulomb barrier energies by measuring fission fragment angular distributions. The present work aims to estimate nuclear dissipation and find its isotopic and shell closure dependence from statistical model (SM) analysis of experimental data. Pre-scission neutron multiplicity data for the same systems is also included in the SM analysis. An updated version of SM is used in the present analysis, which includes shell corrections in level density and fission barrier as well as the effect of collective enhancement of level density and orientation effect of the compound nucleus along the symmetry axis. An isotopic dependence of the dissipation strength fitting the fission excitation functions is observed. The pre-scission neutron multiplicity, however, is underestimated by the SM.

Angular distribution of fragments in neutron-induced nuclear fission at energies 1−200 MeV: Data, theoretical models and relevant problems

In recent years, investigations of angular distributions of fragments in neutron-induced nuclear fission have been extended to intermediate energies, up to 200 MeV, as well as to a wide range of target isotopes. Using as an example the latest data obtained by our group for the reaction 237Np(n; f), we discuss the specific features of fission fragment angular distribution and present a method for their simulation based on the code TALYS. It is shown that a simplified model reasonably describes energy dependence of the angular distribution in the whole range 1–200 MeV. The ways to improve the model are discussed along with the possibilities to use it for obtaining new information on fission and pre-equilibrium processes in neutron-nucleus interaction. We consider also the relevant problems of describing fission fragment angular distributions.

238U FISSION NEAR THE THRESHOLD

Using the data on angular distribution of fission fragments, a threshold has been determined in a dipole fission channel with J = 1 spin projection to the nucleus symmetry axis. It has been shown that the peak observed in the 238U fission cross section is determined by the contribution from the quadrupole excitation.

Measurement of the Angular Distributions of Fission Fragments from the Neutron-Induced Fission of 240Pu Nuclei in the Energy Range of 1–200 MeV and Their Model Analysis

Measurement of the Angular Distributions of Fission Fragments from the Neutron-Induced Fission of 240Pu Nuclei in the Energy Range of 1–200 MeV and Their Model Analysis

Neutron-induced fission fragment angular distributions, anisotropy, and linear momentum transfer measured with the NIFFTE fission time projection chamber

The Neutron Induced Fission Fragment Tracking Experiment (NIFFTE) collaboration has performed measurements with a fission time projection chamber (fissionTPC) to study the fission process by reconstructing full three-dimensional tracks of fission fragments and other ionizing radiation. The amount of linear momentum imparted to the fissioning nucleus by the incident neutron can be inferred by measuring the opening angle between the fission fragments. Using this measured linear momentum, fission fragment angular distributions can be converted to the center-of-mass frame for anisotropy measurements. Angular anisotropy is an important experimental observable for understanding the quantum mechanical state of the fissioning nucleus and vital to determining detection efficiency for cross section measurements. Neutron linear momentum transfer to fissioning $^{235}$U, $^{238}$U, and $^{239}$Pu and fission fragment angular anisotropy of $^{235}$U and $^{238}$U as a function of neutron energies in the range 130 keV--250 MeV are presented.

Effects of isospin and dissipation coefficient of the K coordinate in simulation of fission dynamics of excited compound nuclei

A stochastic approach based on four-dimensional (4D) dynamical model has been used to simulate the fission process of the excited compound nuclei [Formula: see text]Fr, [Formula: see text]Fr and [Formula: see text]Fr produced in fusion reactions. Effects of isospin and dissipation coefficient of the [Formula: see text] coordinate, [Formula: see text], on estimation of the evaporation residue (ER) cross-section, the prescission neutron multiplicity, the variance of the mass and energy distributions of fission fragments and the anisotropy of fission fragments angular distribution have been investigated for the excited compound nuclei [Formula: see text]Fr, [Formula: see text]Fr and [Formula: see text]Fr. Three collective shape coordinates [Formula: see text] plus the projection of total spin of the compound nucleus to the symmetry axis, [Formula: see text], were considered in the 4D dynamical model. In the 4D dynamical model, the magnitude of the dissipation coefficient of [Formula: see text], [Formula: see text], was considered as a free parameter and its magnitude inferred by fitting measured data on the ER cross-section. Results of the extracted dissipation coefficients of [Formula: see text] for different isotopes of Fr were shown that the magnitude of the dissipation coefficient of [Formula: see text] increases with decreasing isospin of fissioning compound nucleus. It was also shown that the prescission neutron multiplicity and the anisotropy of fission fragments angular distribution increase with increasing isospin whereas the variance of the mass and energy distributions of fission fragments decrease with increasing isospin of fissioning compound nucleus. Furthermore, it was shown that the calculated values of prescission neutron multiplicity and the variance of the mass distribution of fission fragments for the excited compound nuclei [Formula: see text]Fr, [Formula: see text]Fr and [Formula: see text]Fr decrease with the dissipation strength of [Formula: see text], whereas the variance of the energy distribution of fission fragments and the anisotropy of fission fragments angular distribution increase with the dissipation strength of [Formula: see text].

Angular distributions and anisotropy of fission fragments from neutron-induced fission of239Pu,237Np, andnatPb in energy range 1–200 MeV

This work summarizes some results of a series of experiments aimed at the investigation of energy dependence of anisotropy of fission fragments (FFs) in (n,f) reactions for neutron energies from low to intermediate. Angular distributions of FFs from the neutron-induced fission of239Pu,237Np, andnatPb have been measured in the energy range 1–200 MeV at the neutron TOF spectrometer GNEIS based on the spallation neutron source at 1 GeV proton synchrocyclotron of the Petersburg Nuclear Physics Institute (Gatchina, Russia). The anisotropies of FFs deduced from the measured angular distributions are presented. In the neutron energy range above 20 MeV the results have been obtained for the first time in our works. The experimental data for FF anisotropy in237Np(n,f) are compared with calculations based on "adapted" TALYS software.

Angular Distributions and Anisotropy of Fragments from the Neutron-Induced Fission of 237Np in the Energy Range of 1–200 MeV: Measurement Data and Model Calculations

The experimental data on the angular distributions of fragments from the fission of 237Np nuclei induced by 1–200 MeV neutrons have been presented. Such measurements for neutron energies above 16 MeV have been performed for the first time. These distributions have been analyzed and the angular anisotropy in the center-of-mass of fragments has been determined in the entire energy range under study. A method involvng the complex dynamics of the formation and decay of highly excited nuclei and based on the TALYS program has been proposed to calculate the angular distributions of fission fragments in a wide energy range for the collision of nuclei with neutrons. It has been shown that the developed model describes well the main features of the energy dependence of the angular anisotropy for the 237Np target nuclei and can be used to extract new information on the reaction and fission process.

Fission fragment angular anisotropy in neutron-induced fission of U235 measured with a time projection chamber

Fission fragment angular distributions can provide an important constraint on fission theory, improving predictive fission codes, and are a prerequisite for a precise ratio cross section measurement. Available anisotropy data is sparse, especially at neutron energies above 5 MeV. For the first time, a three-dimensional tracking detector is employed to study fragment emission angles and provide a direct measurement of angular anisotropy. The Neutron Induced Fission Fragment Tracking Experiment (NIFFTE) collaboration has deployed the fission time projection chamber (fissionTPC) to measure nuclear data with unprecedented precision. The fission fragment anisotropy of $^{235}$U has been measured over a wide range of incident neutron energies from 180 keV to 200 MeV; a careful study of the systematic uncertainties complement the data.

Fission program at n_TOF

Since its start in 2001 the n_TOF collaboration developed a measurement program on fission, in view of advanced fuels in new generation reactors. A special effort was made on measurement of cross sections of actinides, exploiting the peculiarity of the n_TOF neutron beam which spans a huge energy domain, from the thermal region up to GeV. Moreover fission fragment angular distributions have also been measured. An overview of the cross section results achieved with different detectors is presented, including a discussion of the 237Np case where discrepancies showed up between different detector systems. The results on the anisotropy of the fission fragments and its implication on the mechanism of neutron absorption, and in applications, are also shown.

Prompt Fission Neutron Investigation in 235U(nth,f) and 252Cf(sf) Reactions

The prompt neutron emission in thermal neutron induced fission of 235U and spontaneous fission of 252Cf was investigated by using digital signal electronics. The goal was to check a new revised data analysis software with fission fragment (FF) kinetic energy corrections after prompt fission neutron (PFN) emission. The revised software was used to reanalyze old data measured in EC-JRC-IRMM, where 252Cf(sf) reaction was investigated. Both measurements were done using similar twin Frisch grid ionization chamber for fission fragment detection with equivalent NE213 fast neutron detector. About 0.5*106 FF with PFN coincidences have been analyzed in both measurements. The fission fragment kinetic energy, mass and angular distribution were investigated along with prompt neutron time of flight and pulse shape analysis using a six channel synchronous waveform digitizer (WFD) with sampling frequency of 250 MHz and 12 bit resolution in the 235U(nth,f) reaction. Similar WFD with sampling frequency of 100 MHz was used for PFN investigation in 252Cf(sf) reaction. These two experiments were considered as a reference for further investigations with a new setup composed of position sensitive ionization chamber to detect FF and an array of 32 liquid scintillators recently constructed in Dubna to detect neutrons.

Study of mass yield and kinetic energy distribution of fission fragments from fission of the excited compound nucleus 254Fm produced in fusion reaction

A stochastic approach based on four-dimensional Langevin fission dynamics has been used to calculate the anisotropy of fission fragment angular distribution, the mass and energy distribution of fission fragments, the yield and variance of the mass distribution of fission fragments, the fission cross section and the pre-scission neutron multiplicity for the compound nucleus 254Fm produced in the reaction 16O + 238U. In the dynamical calculations dissipation coefficient of K , \( \gamma_k\) , was considered as a free parameter and its magnitude inferred by fitting measured data on the anisotropy of fission fragments angular distribution. It was shown that the results of the anisotropy of fission fragments angular distribution for the compound nucleus 254Fm are in good agreement with the experimental data by using values of dissipation coefficient of K, equal to \(\gamma_{k}=(0.076-0.088)\) (MeV zs)-1/2. We also calculated the above-mentioned experimental data for the compound nucleus 254Fm by using \(\gamma_{k}=(0.076-0.088)\) (MeV zs)-1/2 and results of the calculations compared with the experimental data. A comparison of the theoretical results with the experimental data showed that the results of calculations are in good agreement with the experimental data, although at high energy the difference between the results of calculations with the experimental data for the variance of the mass distribution of fission fragments and the pre-scission neutron multiplicity is slightly high. It was also shown that at high energy the variance of the mass distribution of fission fragments and the pre-scission neutron multiplicity can be reproduced for the compound nucleus 254Fm by using values of dissipation coefficient of K equal to \(\gamma_{k}=0.098\) (MeV zs)-1/2 and \(\gamma_{k}=0.065\) (MeV zs)-1/2, respectively.

Unified Mechanism behind the Appearance of T-Odd TRI and ROT Asymmetries in Actinide Fission Induced by Cold Polarized Neutrons

Some shortcomings of the approaches that are used to describe T-odd ROT and TRI asymmetries in true ternary fission via reactions involving the emission of prescission alpha particles and which are based on employing the classical method of trajectory calculations are analyzed. These shortcomings are caused by the disregard of the interference between the fission widths of different sJs neutron resonance states formed in the first well of the deformation potential of fissile compound nuclei. It is shown that the method used in some studies to determine T-odd TRI-asymmetries for prescission alpha particles is at odds with basic concepts of the generalizedmodel of the nucleus and approaches to constructing collective (for example, bending) vibrations of a fissile compound nucleus. Quantum-mechanical fission theory is generalized via employing a unified mechanism of formation of T-odd TRI and ROT asymmetries for prescission alpha particles and evaporated photons (neutrons). The proposed mechanism takes correctly into account the effect of quantum rotation of a fissile compound nucleus on the angular distributions of fission fragments and alpha particles for true ternary fission, as well as on the angular distribution of prompt photons (neutrons) emitted by fragments originating from the delayed fission of the aforementioned nuclei.

Statistical and dynamical modeling of heavy-ion fusion–fission reactions

A modified statistical model and a four dimensional dynamical model based on Langevin equations have been used to simulate the fission process of the excited compound nuclei 207At and 216Ra produced in the fusion 19F + 188Os and 19F + 197Au reactions. The evaporation residue cross section, the fission cross section, the pre-scission neutron, proton and alpha multiplicities and the anisotropy of fission fragments angular distribution have been calculated for the excited compound nuclei 207At and 216Ra. In the modified statistical model the effects of spin K about the symmetry axis and temperature have been considered in calculations of the fission widths and the potential energy surfaces. It was shown that the modified statistical model can reproduce the above mentioned experimental data by using appropriate values of the temperature coefficient of the effective potential equal to λ=0.0180±0.0055, 0.0080±0.0030 MeV−2 and the scaling factor of the fission barrier height equal to rs=1.0015±0.0025, 1.0040±0.0020 for the compound nuclei 207At and 216Ra, respectively. Three collective shape coordinates plus the projection of total spin of the compound nucleus on the symmetry axis, K, were considered in the four dimensional dynamical model. In the dynamical calculations, dissipation was generated through the chaos weighted wall and window friction formula. Comparison of the theoretical results with the experimental data showed that two models make it possible to reproduce satisfactorily the above mentioned experimental data for the excited compound nuclei 207At and 216Ra.

Experimental study of the transfer-induced fission fragment angular distribution in the Li6+U238 reaction

Transfer-induced fission fragment angular distribution measurements have been carried out in $^{6}\mathrm{Li}+^{238}\mathrm{U}$ reaction at beam energies of 36 and 40 MeV in coincidence with projectile-like fragments. The fission fragment anisotropies for $\ensuremath{\alpha}$- and deuteron-transfer-induced fission reactions are obtained with respect to the recoil direction. The anisotropy values for transfer-induced fissions are observed to be either similar or lower than the inclusive measurement. In order to quantitatively understand the present experimental data on transfer-induced fission, anisotropy values have been compared with the preequilibrium fission model considering ground-state spin of the projectile and/or target. It is found that the model predicts the inclusive and exclusive anisotropy data reasonably well.

Pu242 neutron-induced fission cross-section measurement from 1 to 2 MeV neutron energy

Relative values of the neutron-induced fission cross section $\ensuremath{\sigma}(n,f)$ of $^{242}\mathrm{Pu}$ have been measured with respect to the standard $^{1}\mathrm{H}(n,p)$ elastic scattering cross section, at average energies of 1.0, 1.4, and $1.9\phantom{\rule{4pt}{0ex}}\mathrm{MeV}$. The measurements are part of an international effort to reduce uncertainties and provide independent nuclear data relevant for fast-spectrum reactors. The shape of the measured cross section is in good agreement with data from Tovesson et al. [Phys. Rev. C 79, 014613 (2009)] and with the most recent data from Matei et al. [Phys. Rev. C 95, 024606 (2017)], but disagrees with the shapes of ENDF/B-VII.1 and JEFF3.2 evaluations. Absolute values of $\ensuremath{\sigma}(n,f)$, obtained under some assumptions, indicate an overestimation of $\ensuremath{\sigma}(n,f)$ in the evaluated libraries at 1.0 and $1.4\phantom{\rule{4pt}{0ex}}\mathrm{MeV}$, while a good agreement is found with ENDF/B-VII.1 at $1.9\phantom{\rule{4pt}{0ex}}\mathrm{MeV}$. A careful analysis of the impact of scattered neutrons and anisotropy of the fission fragment angular distribution has been performed. The measurement of the neutron flux by means of a proton-recoil detector is discussed. A comprehensive study of corrections applied to the data, of associated uncertainties, and of correlations between the measurements at the different energies is presented.

Mass distribution and mass-resolved angular distribution in the O16+U238 reaction at sub-barrier energy

Background: The angular distribution of fission fragments or products is a sensitive probe to investigate non-compound-nucleus (NCN) fission. However, the gross angular distribution cannot distinguish between different NCN fission processes. Both preequilibrium fission and quasifission contribute to NCN fission and would both result in deviations from the statistical saddle point model (SSPM). Mass-resolved angular distributions of the fission product offers the possibility to distinguish between the two mechanisms [Sodaye et al., Phys. Rev. C 95, 014612 (2017); Tripathi et al., Phys. Rev. C 88, 024603 (2013); Vorkapic and Ivanisevic, Phys. Rev. C 55, 2711 (1997)].Purpose: Mass and mass-resolved angular distributions of fission products have been measured in the $^{16}\mathrm{O}+^{238}\mathrm{U}$ reaction at ${E}_{\mathrm{lab}}=85.3\phantom{\rule{0.16em}{0ex}}\mathrm{MeV}$ to investigate the dominant NCN fission mechanism at sub-barrier energy.Method: Mass and mass-resolved angular distributions were measured by recoil catcher technique followed by offline \ensuremath{\gamma}-ray spectrometry.Results: The mass distribution was observed to be predominantly asymmetric. The angular anisotropy of the fission products was observed to be independent of their mass.Conclusions: The mass independence of the angular anisotropy suggested quasifission to be the dominant NCN fission process. This was further confirmed from the underestimation of angular anisotropy by theoretical calculations even after including the contribution from preequilibrium fission.