Charge Distributions Of Fragments Research Articles

Complete kinematics studies of fission reactions induced by quasi-free nucleon scattering collisions

Measurements of spallation, fragmentation and Coulomb-induced fission reactions in inverse kinematics have provided valuable data in the last decades to accurately investigate the fission dynamics and nuclear structure at large deformations of a large variety of stable and non-stable heavy nuclei. To go further, quasi-free scattering (QFS) reactions in inverse kinematics are proposed as a new surrogate method to induce fission, which allows to reconstruct the excitation energy of compound fissioning systems by using the four-momenta of the two outgoing nucleons. This new approach would permit therefore to correlate the excitation energy with the charge and mass distributions of the fission fragments and with the fission probabilities, given for the first time direct access to the simultaneous measurement of the fission yield dependence on temperature and fission barrier heights of exotic heavy nuclei, respectively. The first experiment based on this approach was recently performed at the GSI/FAIR facility and a description of the methodology based on model calculations is given here.

The difference between charge polarizations of fission fragments deduced by the static theoretical model and in the current data library

In this paper, we propose a theoretical method to deduce the charge polarization (CP) and most probable charge for fission fragments for the selected range of mass numbers based on a quantum many-body framework, namely, a constrained Skyrme Hartree–Fock+BCS model. We investigate the CP on the low energy neutron-induced fission of [Formula: see text]U as a representative reaction. We have found that the calculated CP has a noticeable dip that is clearly affected by the spherical shell structure around [Formula: see text]Sn and deformed one around [Formula: see text]144 and a peak for their counterparts, which are common features to those of widely-used Wahl’s systematics, but otherwise, our CP values converge to zero. Especially, we could not see a deep valley and a big hump in the region of symmetric fission at [Formula: see text]112 and 124 present in Wahl’s systematics. The gradient of our CP near the symmetric fission is negative concerning the mass number of fission fragments, which is consistent with other previous theoretical studies; however, that is the opposite of the gradient in Wahl’s systematics. This paper shows the difference between the charge distribution of fission fragments, which has been used practically so far, in comparison with the calculation by microscopic nuclear theory.

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.

Comprehensive investigation of fission yields by using spallation- and (p,2p)- induced fission reactions in inverse kinematics

In the last decades, measurements of spallation, fragmentation and Coulex induced fission reactions in inverse kinematics have provided valuable data to accurately investigate the fission dynamics and nuclear structure at large deformations of a large variety of stable and non-stable heavy nuclei. To go a step further, we propose now to induce fission by the use of quasi-free (p,2p) scattering reactions in inverse kinematics, which allows us to reconstruct the excitation energy of the compound fissioning system by using the four-momenta of the two outgoing protons. Therefore, this new approach might permit to correlate the excitation energy with the charge and mass distributions of the fission fragments and with the fission probabilities, given for the first time direct access to the simultaneous measurement of the fission yield dependence on temperature and fission barrier heights of exotic heavy nuclei, respectively. The first experiment based on this methodology was realized recently at the GSI/FAIR facility and a detailed description of the experimental setup is given here.

A general framework for describing photofission observables of actinides at an average excitation energy below 30 MeV * *Supported by the National Natural Science Foundation of China (11675075), Independent research project of key laboratory of plasma physics, CAEP (JCKYS2021212009) and Hengyang Municipal Science and Technology Project (202150054076)

A reasonable prediction of photofission observables plays a paramount role in understanding the photofission process and guiding various photofission-induced applications, such as short-lived isotope production, nuclear waste disposal, and nuclear safeguards. However, the available experimental data for photofission observables are limited, and the existing models and programs have mainly been developed for neutron-induced fission processes. In this study, a general framework is proposed for characterizing the photofission observables of actinides, including the mass yield distributions (MYD) and isobaric charge distributions (ICD) of fission fragments and the multiplicity and energy distributions of prompt neutrons (np ) and prompt γ rays (γ p ). The framework encompasses various systematic neutron models and empirical models considering the Bohr hypothesis and does not rely on the experimental data as input. These models are then validated individually against experimental data at an average excitation energy below 30 MeV, which shows the reliability and robustness of the general framework. Finally, we employ this framework to predict the characteristics of photofission fragments and the emissions of prompt particles for typical actinides including 232Th, 235, 238U and 240Pu. It is found that the 238U(γ, f) reaction is more suitable for producing neutron-rich nuclei compared to the 232Th(γ, f) reaction. In addition, the average multiplicity number of both np and γ p increases with the average excitation energy.

Theoretical study of isotope production in the peripheral heavy-ion collision 136Xe + Pb at 1 GeV/nucleon

We have studied the fragment yields emitted from the fragmentation of excited projectile nuclei in peripheral collisions of X136e+Pb at 1 GeV/nucleon, and measured with the high-resolution magnetic spectrometer, the Fragment Separator (FRS) of GSI. The mass, charge and isotope distributions of nuclear fragments formed in the reactions were calculated within a statistical ensemble approach and compared to the experimental data. The ensemble of excited projectilelike source nuclei were created in the framework of a previous analysis of similar reactions performed at 600 MeV/nucleon (ALADIN-experiments, GSI). In addition, the ensemble of the low-excited compound nuclei is involved in the analysis. The overall agreement between theory and experiment was very satisfactory in reproducing the experimental data of isotope yields measured in the heavy-ion collisions. It is confirmed that a broad range of elements can be reproduced within a universal statistical disintegration of the excited projectile residues.

Multi-parameter global calculations of fission fragments using a simplified two-dimensional scission-point model * *Supported by National Natural Science Foundation of China (11875301, U1832167)

Exploiting the concept of the dinuclear system, the interaction potential energy of two fragments that are quite close to each other is analyzed. A semi-classical method is used to calculate fission fragment yields using a simplified two-dimensional scission-point model. By considering the tip-to-tip orientation at the scission point of the fission process, we investigate the mass, charge, and kinetic-energy distributions of the fission fragments, for excitation energies in the 0-20 MeV range. Our results show that the fission fragment distributions are reproduced quite well, including the recent experimental results for the isotone chain [D Ramos et al. Phys. Rev. C 97, 054612 (2018)]. Thus, the simplified model is useful for multi-parameter global measurements of fission products.

Fragment emission and critical behavior in light and heavy charged systems * *Supported Council of Scientific and Industrial Research (CSIR), Govt. of India (03 (1388)/16/EMR-II)

We study the emission of fragments in central collisions of light and heavily charged systems of 40Ar+45Sc and 84Kr+197Au, respectively, using the Quantum Molecular Dynamics (QMD) model as the primary model. The fragments are identified using an energy based clusterization algorithm, i.e., the Simulated Annealing Clusterization Algorithm (SACA). The charge distributions of intermediate mass fragments [3≤ ≤12] are fitted with power-law ( ) and exponential ( ) fits in order to extract the parameters τ and whose minimum values are also sometimes linked with the onset of fragmentation or the critical point for a liquid-gas phase transition. Other parameters such as the normalized second moment , , average size of the second largest cluster , phase separation parameter ( ), bimodal parameter (P), information entropy (H), and Zipf's law are also analyzed to find the exact energy of the onset of fragmentation. Our detailed analysis predicts that an energy point exists between 20-23.1 MeV/nucleon, which is very close to the experimentally observed value of 23.9 MeV/nucleon for the 40Ar+45Sc reaction. We also find that the critical energy deduced using Zipf's law is higher than those predicted from other critical exponents. Moreover, no minimum is found for τ values of the highly charged system of 84Kr+197Au, in agreement with experimental findings and various theoretical calculations. We observe that the QMD + SACA model calculations are in agreement with the experimental observations. This agreement supports our results regarding the energy point of the liquid-gas phase transition and the onset of fragmentation.

Study of emission characteristics of the projectile fragments produced in the interaction of 84Kr36 with nuclear emulsion detector at 1 GeV

This article is focused on the characteristics of the projectile fragments of charge 1 ≤ Z ≤ 10 produced in the interaction of the 84Kr36 with nuclear emulsion detector at 1 GeV per nucleon. We have studied the average charge distribution and multiplicity distribution of the projectile fragments having charge 1 ≤ Z ≤ 10. We have also studied the emission behavior of various projectile fragments produced from the interaction with different target groups of nuclear emulsion detector. From this study we have observed that the emission of projectile fragments is strongly dependent on the interaction with different type of the target groups of nuclear emulsion detector as well as on the mass of the projectile beam. The results are compared with other experimental observations carried out at relativistic energy and found to be consistent.

Examination of coexistence of symmetric mass and asymmetric charge distributions of fission fragments

Examination of coexistence of symmetric mass and asymmetric charge distributions of fission fragments

Change of the shape of mass and charge distributions in fission of Cf isotopes with excitation energy

Using the improved scission-point model, the mass and charge distributions of fragments resulting from the fission of californium isotopes are calculated and compared with the available experimental data. The change of the shape of mass and charge distributions with increasing excitation energy is predicted for future experiments.

Time-dependent generator-coordinate-method study of mass-asymmetric fission of actinides

Low-energy positive and negative parity collective states in the equilibrium minimum, and the dynamics of induced fission of actinide nuclei are investigated in a unified theoretical framework based on the generator coordinate method (GCM) with the Gaussian overlap approximation (GOA). The collective potential and inertia tensor, both at zero and finite temperature, are computed using the self-consistent multidimensionally constrained relativistic mean field model, based on the energy density functional DD-PC1. Pairing correlations are treated in the Bardeen-Cooper-Schrieffer approximation with a separable pairing force of finite range. A collective quadrupole-octupole Hamiltonian characterized by zero-temperature axially symmetric deformation energy surface and perturbative cranking inertia tensor, is used to model the low-lying excitation spectrum. The fission fragment charge distributions are obtained by propagating the initial collective states in time with the time-dependent GCM+GOA that uses the same quadrupole-octupole Hamiltonian, but with the collective potential and inertia tensor computed at finite temperature. The illustrative charge yields of $^{228}\mathrm{Th}$, $^{234}\mathrm{U}$, $^{240}\mathrm{Pu}$, $^{244}\mathrm{Cm}$, and $^{250}\mathrm{Cf}$ are in very good agreement with experiment, and the predicted mass asymmetry is consistent with the result of a recent microscopic study that has attributed the distribution (peak) of the heavier-fragment nuclei to shell-stabilized octupole deformations.

Influence of neutron emission on the charge distribution of final fragments from thermal-neutron-induced fission of 233U

Abstract We use a Monte Carlo simulation method to calculate the effect of neutron emission on the distribution of mass, charge and kinetic energy of final fragments (after they emit neutrons) from the thermal-neutron induced fission of 233U. For the simulation input data we assume (i) the distribution of mass, kinetic energy, neutron number of primary fragments previously used to reproduce the corresponding experimental distribution of final fragments (ii) a yield of charge (for a given mass) independent of kinetic energy and equal to the corresponding to the highest kinetic energy window (cold fission, where there is no neutron emission) measured in 1988 by U. Quade et al. From the output data of simulation we obtain a yield of charge as a function of kinetic energy of final fragment which approximately reproduce the trends of experimental data obtained by those authors at different kinetic energies.

Influence of neutron emission on the charge, mass and kinetic energy distribution of final fragments from 235U(nth, f) reaction

Using a Monte Carlo method we simulate an experiment that measures the mass, charge and kinetic energy of final fragments (after neutron emission) from the 235U(nth, f) reaction. As input data for the simulation, for primary fragments (before neutron emission), we assume (i) a distribution of mass and kinetic energy; (ii) an average number of emitted neutron as a decreasing linear function of kinetic energy and (iii) for each mass, a constant yield of charges as a function of kinetic energy, equal to that obtained by W Lang et al for the highest measured kinetic energy window (108.5 MeV) which corresponds to the lowest measured excitation energy, that corresponds to the so called cold fission. The output of the simulation is the distribution of mass, charge and kinetic energy of final fragments. From this output we obtain that, for a given mass, the charge that has the highest yield in cold fission region has a yield obeying an increasing function of kinetic energy in all other region. Conversely, the yield of the less probable charge in cold fission is a decreasing function of kinetic energy in all other region. Our results of simulation suggest that, for two primary isobaric fragmentations with similar Q-value, the preference for more asymmetric charge splits (called Coulomb effect), observed in cold fission, is valid in all region, but neutron emission shadows this property in final fragments distribution.

Toward an understanding of the anomaly in charge yield of Mo and Sn fragments in the fission reaction U238(n,f )

Using the improved scission-point model, the charge distribution of fission fragments is studied in neutron-induced fission of the $^{238}\mathrm{U}$ nucleus. A possible explanation of the anomaly in charge yield of Mo and Sn fragments is presented.

Suggestion for examination of a role of multi-chance fission

The mass distributions of fission fragments resulting from low- and high-energy fission of the even nuclei 218-228 Th, 240U , and 244Pu are studied with the statistical scission-point model. The calculated results are compared with the available experimental data. The method is suggested for the experimental verification of the models with opposite role of multi-chance fission. Based on the asymmetry of mass (charge) distribution of fission fragments of 228Th one can unambiguously distinguish the models and conclude on the nature of this asymmetry.

Quantal diffusion description of multinucleon transfers in heavy-ion collisions

Employing the stochastic mean-field (SMF) approach, we develop a quantal diffusion description of the multi-nucleon transfer in heavy-ion collisions at finite impact parameters. The quantal transport coefficients are determined by the occupied single-particle wave functions of the time-dependent Hartree-Fock equations. As a result, the primary fragment mass and charge distribution functions are determined entirely in terms of the mean-field properties. This powerful description does not involve any adjustable parameter, includes the effects of shell structure and is consistent with the fluctuation-dissipation theorem of the non-equilibrium statistical mechanics. As a first application of the approach, we analyze the fragment mass distribution in $^{48}\mathrm{Ca}+{}^{238}\mathrm{U}$ collisions at the bombarding energy $E_{\text{c.m.}}=193$ MeV and compare the calculations with the experimental data.

MODELING OF CHARGE TRANSPORT IN THE ACTIVE VOLUME OF THE IONIZATION FISSION CHAMBER IN CURRENT MODE

Charge transport in the active volume of the cylindrical ionization fission chamber (FC) in the current mode has been studied. The model is based on the continuity equations for ions and electrons, as well as on Poisson’s equation for the electric field. The source for the continuity equations is calculated taking into account a correct distribution of the initial ionization density in the active volume of the chamber and the charge, mass, and energy dependent distributions of the fission fragments. The distributions of the ion and electron density and electric fields inside the active volume are found for two types of chambers – miniature chambers and “large” chambers with the consideration of the space charge. The correct algorithm for calculation of the beginning of the plateau of the current-voltage characteristic – the minimum voltage on the FC to provide the stationary operation of the chamber is given. It is shown that the often used condition of the electric field absence at the anode E (ra ) = 0 to determine this value is incorrect, since it leads to complex values of the electric field inside the chamber active volume. Neglecting the processes of ion diffusion and recombination, the sensitivity and output current of the chamber in the stationary mode are calculated. Calculations have been carried out for miniature and “large” chambers. It has been shown that the use of the approximation for the generation density of ion pairs by the fission fragment along its track to be constant, often used in practice for “large” chambers, leads to significant errors when estimating the densities of ions, electrons and electric fields inside the FC; at that, the sensitivity may differ by an order of magnitude. Исследован перенос зарядов в активном объеме цилиндрической ионизационной камеры деления (ИКД), работающей в токовом режиме. В основу модели положены уравнения непрерывности для ионов и электронов и уравнение Пуассона для электрического поля. Источник для уравнений непрерывности рассчитан с учетом корректного распределения начальной плотности ионизации в активном объеме камеры и разброса фрагментов деления по их заряду, массе и энергии. Найдены распределения плотности ионов, электронов и электрического поля внутри активного объема для двух типов камер – миниатюрных и «больших» с учетом пространственного заряда. Дан правильный алгоритм для расчета начала плато вольтамперной характеристики – минимального напряжения на ИКД для обеспечения стационарной работы камеры. Показано, что часто используемое условие отсутствия электрического поля на аноде E(ra ) = 0 для определения этой величины является некорректным, поскольку приводит к комплексным значениям электрического поля внутри активного объема камеры. Пренебрегая процессами диффузии и рекомбинации ионов, рассчитаны чувствительность и выходной ток камеры при ее работе в стационарном режиме. Расчеты проведены для миниатюрных и «больших» камер. Показано, что часто применяемое на практике использование приближения постоянства плотности генерации пар ионов осколком деления вдоль его трека для «больших» камер приводит к существенным ошибкам в оценке плотностей ионов, электронов и электрических полей внутри ИКД, при этом чувствительности могут отличаться на порядок.

Charge distributions of fission fragments of low- and high-energy fission of Fm, No, and Rf isotopes

Charge distributions of fission fragments of low- and high-energy fission of Fm, No, and Rf isotopes

Role of the excitation energy of the compound nucleus in binary decay processes

Employing the improved scission-point model, the isotopic and excitation energy trends of the charge distribution of fission fragments are studied in fission of even-even Th isotopes at low and high excitation energies.