Search

The physics of nuclear fission will be briefly illustrated, from the basic mechanism behind this phenomenon to the relevant physical quantities like nuclear cross sections, neutron flux and reaction products, together with the accompanying phenomenon of neutron capture and its role in determining how the fuel transforms in a nuclear reactor. The basic concepts underlying the operation of different types of nuclear reactors will be illustrated, along with the concept of fuel cycle. The aspects of radioactive waste, fuel resources and safety will also be briefly illustrated. [ABSTRACT FROM AUTHOR]

In the first part of the paper, the basic features of nuclear fission and fusion energy are addressed. In the second part, an overview of energy consumption in the world is presented, which highlights the growing need for electricity from non-CO2-emitting sources, among which nuclear power is expected to play a role. The perspectives for the future of nuclear energy are then discussed, showing that it develops mostly in emerging countries, and highligting the role that small modular reactors are likely to have in the future. The third part is devoted to the question of nuclear waste management and to the non-radioactive environmental impact of nuclear energy in comparison with other sources of energy. [ABSTRACT FROM AUTHOR]

It is known that the kinetic energy of individual fragments for fission of actinides is constant at about 100 MeV for light fragments and that for heavy fragments decreases with mass number. The kinetic energy is, for example, important for evaluating the prompt-neutron spectrum in the laboratory system. However, its theoretical study has not been attempted in detail so far. We have calculated them in thermal-neutron induced fission of 235U using the dynamical model, considering the Coulomb energy at the scission point and the prescission kinetic energy. The calculated results reproduce the experimental data. It is found that the pre-scission kinetic energy has about 5% contribution in the kinetic energy. [ABSTRACT FROM AUTHOR]

We found that certain three-body excited states of 55133Cs13H2 in the Pd12 cuboctahedron molecule could interfere with the three-nucleon resonance states of Cs+t+t or 57139 La∗ which transfer from the 7/2+ state of CsH2 to 7/2+ state of La∗ by the E2 transition, and down to the La ground state, although the molecular ground state and the first excited state of 55133Cs13H2 are very stable. The three-body calculation is performed by using six potentials. We also found that the critical fusion value of t × ρ × T (t:duration time, ρ: plasma density, T : temperature or energy) for the ultra low energy molecular synthesis is almost the same as that for the thermal nuclear fusion, or higher. It is claimed that the radioactive nuclear waste products of the nuclear fission, could form via nuclear fusion a stable nucleus due to our ultra-low energy molecule-nucleus transition. [ABSTRACT FROM AUTHOR]

This research analyzed trajectories of nuclear fission leading to symmetric or assymmetric mass division, obtained by a four-dimensional Langevin-model, using machine learning models. A hybrid neural network, combining Long Short-Term Memory (LSTM) and Gated Recurrent Unit (GRU), both of which were types of Recurrent Neural Networks (RNN), was utilized to classify whether each Langevin trajectory led to symmetric or asymmetric mass division. It was found that the current model could classify fate of these trajectories before reaching to the final destination (symmetric or assymmetric mode) with an accuracy of over 70%, clearly overestimating the asymmetric data. [ABSTRACT FROM AUTHOR]

The quadrupole (β2) deformed targets in the actinide region are found to be of great relevance in the production of superheavy nuclei (SHN), in the compact fusion mechanism. Recently, the application of "elongated" and "compact" configurations of pear-shape octupole (up to β3) deformed nuclei was found to play a significant role in the fusion-fission dynamics of heavy-ion induced reactions. In the present work, we intend to explore the relevance of higher-order deformed (up to β4) actinides and their compact configuration in the production cross-section as well as de-excitation of SHN (dynamics of 48Ca + 238U →286Cn∗ → A1 + A2 reaction). For the above analysis, we have calculated σER within the framework of dynamical cluster-decay model developed on the basis of quantum mechanical fragmentation theory, which has been extended with the inclusion of deformations up to β4 and corresponding compact optimum orientation. Besides, on the basis of this theory, the fragmentation structure of 286Cn∗ superheavy nucleus is examined in terms of fragmentation potential (Vη) and preformation probability (P0) as a function of fragment mass number. [ABSTRACT FROM AUTHOR]

In the framework of the dinuclear system model, we introduce a formula accounting for temperaturedependent shell corrections to assess their influence on the fission barrier calculations and, consequently, the survival probability of excited nuclei. Our approach considers the excitation energy's effect on the fission barrier, a crucial aspect in determining the likelihood of a nucleus persisting. Leveraging experimental data with associated errors, we constrain the region of the shell correction damping factor. Employing this refined methodology, we predict the production cross sections for the novel superheavy element (SHE) with atomic number Z=119. Specifically, at the 54Cr(243Am,3n)294119 reaction, cross sections are anticipated to range between 3-27 femtobarns (fb), while for the 51V(248Cm,3n)296119 reaction, they are projected to span 21-180 fb. These predictions offer valuable insights into the formation dynamics of this exotic superheavy element. [ABSTRACT FROM AUTHOR]

The formation of superheavy elements (SHEs) by nuclear fusion can be conceptually divided into two steps: capture, and compound nucleus formation. Once captured, the two nuclei may reseparate before fusing to form a compact compound nucleus. This outcome is called quasifission. Fusion-fission, in many cases, leads to reactions outcomes inseparable from quasifission. Evaporation residue (ER) measurements are therefore the most reliable, direct experimental signature of fusion. ER cross section measurements forming the same compound nucleus, 220Th, using 16O, 40Ar, 48Ca, 82Se and 124Sn-induced reactions [1–4] revealed [1] that fusion was severely suppressed for the more symmetric reactions relative to the 16O-induced reaction. Here two new reactions forming 220Th using 28Si, 34S projectiles provide conclusive evidence that the ER cross section is exponentially suppressed as a function of ZpZt. The fission characteristics show no mass-angle correlation, demonstrating that here ER cross sections are suppressed by slow quasifission. [ABSTRACT FROM AUTHOR]

Even though more than 80 years have passed since the discovery of fission, its microscopic understanding has still been unclear. To clarify the underlying mechanics of induced fission, we analyze the distribution of a fission width using a miscropic framework based on a configuration-interaction approach. The distribution is known to follow a chi-squared distribution, which is characterized by the effective number of decay channels, ν. We introduce an effective Hamitonian for the space of compound nucleus states and estimate ν from the rank of the imaginary part of the effective Hamiltonian. Applying the model to 235U(n,f), we succesfully reproduce the empirical value of ν = 2.3 ± 1.1. We also find that ν is insensitve to the number of fission channels, which is consistent with an experimental finding. [ABSTRACT FROM AUTHOR]

This contribution provides an overview of the systematic β-delayed fission (βDF) studies performed by our collaboration at the ISOLDE facility at CERN. Measurements of βDF properties of eight isotopes, including the first identification of this process for six of them, were carried out over the years. Systematic studies of fission fragment mass distributions, which led to the discovery of the new region of asymmetric fission and to identification of multimodal fission in several isotopes, are revisited. Motivation and description of the most recent experiments aimed at 178Au and neutron rich actinium and francium isotopes are introduced. The detection system used until recently as well as the two newly-developed detection systems are described. [ABSTRACT FROM AUTHOR]

Fission Fragment Spectroscopy (FFS) technique has been utilized to make simultaneous measurements of both the relative charge and mass yield distributions of even-even fission fragments produced from the fission of 236U at two different excitation energies, Eex. The necessary analysis has been carried out on the measured yield distributions by employing the Multimodal Random Neck Rupture Model (MM-RNRM). The relative contribution of two different types of asymmetric mode of fission – Standard I and Standard II has been extracted at Eex = 6.5 and 21.5 MeV. An attempt has been made to study the evolution of these two asymmetric fission modes with respect to the values of Eex of the concerned fissioning system at low-excitation regime. The probable evidence, although at the primitive stage, for an oscillatory behavior of the relative contributions of different low-energy fission modes with increasing values of Eex has been presented. [ABSTRACT FROM AUTHOR]

The fission fragment angular momenta can generally be expressed in terms of the six normal modes of rotation for the dinucleus and different models populate those modes to different degrees reflecting the mechanisms involved. For the nucleon exchange mechanism, the characteristic time scales for the various modes are discussed, leading to the expectation that the wriggling mode is fully populated, while twisting is unlikely to play a major role; bending probably has some presence which increases with mass asymmetry. Subsequently it is discussed how information about the fragment spin directions can be determined by observation of E2 emissions in even-even nuclei. It is shown how the yield ratio Y (0°)/Y (90°) in a modern Wilhelmy-type experiment can reveal the degree of twisting, a measurement that could be readily carried out now. Furthermore, with a view towards the future when the required technology has been developed, it is illustrated how the relative role of positive modes (wriggling) and negative modes (bending and twisting) can be determined by measuring the opening angle between two E2 photons whose helicities are also identified. [ABSTRACT FROM AUTHOR]

Nuclear fission is still one of the most complex physical processes due to the interplay between macroscopic and microscopic nuclear properties that decide the output. An example of this coupling is the presence of nuclear dissipation as an important ingredient that contributes to drive the dynamics and has a clear impact on the time of the process. However, different theoretical interpretations and scarce experimental data make it poorly understood. At low excitation energy, the relative yields of fragments even and odd atomic numbers show a clear difference, which can be quantified with the so-called even-odd effect. This seemingly mundane property can be used to obtain information about the energy dissipated during the process and the role of structure in its dynamics. In this paper, the study of the even-odd effect for elasticand transfer-induced fission data is discussed. A clear connection with particular fragment shells and the dissipation energy is found, as detailed in Ref. [1]. In addition, preliminary results from quasi-fission data show the formation of a relatively large even-odd effect, which suggests a process with low dissipation mainly consisting in the exchange of nucleon pairs. [ABSTRACT FROM AUTHOR]

Quantum shell effects are responsible for asymmetric fission. They are also expected to affect the formation of fission fragments in quasi-fission reactions occurring in heavy-ion collisions. Shell effects in fission are studied with the single-particle energy level density near the Fermi level. In particular, shell effects in the pre-fragments and their role in fixing the final mass asymmetry of the fission fragments are discussed. Systematic time-dependent Hartree-Fock simulations of heavy-ion collisions show that quasi-fission fragment properties share strong similarities with fragments formed in fission of the compound nuclei. This is an indication that similar shell effects are responsible for the final asymmtery in both mechanisms. [ABSTRACT FROM AUTHOR]

Measured binary quasifission mass spectra in reactions with actinide nuclides show a large peak in yield near the doubly-magic 208Pb, generally attributed to enhanced binding energy causing a valley in the potential energy surface, which attracts quasifission trajectories. Measurements of binary quasifission mass spectra and cross-sections have been made for reactions of 50Ti with actinide nuclides from 232Th to 249Cf. Cross-sections have also been deduced for sequential fission (a projectile-like nucleus and two fragments from fission of the complementary target-like nucleus). Binary cross-sections fall from 70% of calculated capture cross-sections for 232Th to only 40% for 249Cf, with a compensating increase in sequential fission cross-sections. The data are consistent with the peak in yield near 208Pb originating largely from sequential fission of heavier fragments produced in more mass-asymmetric primary quasifission events. These are increasingly suppressed as the heavy quasifission fragment mass increases above 208Pb. The important role of sequential fission calls for re-interpretation of quasifission observables and dynamics in superheavy element synthesis reactions. [ABSTRACT FROM AUTHOR]

We utilize the macroscopic-microscopic approach to fission to calculate nascent fragment distributions. Assuming strongly damped shape motion, we run many iterations of a Metropolis random walk across nuclear potential-energy surfaces to obtain sufficient scission statistics. Our nuclear potential surfaces consist of a macroscopic energy from the Finite-Range Liquid-Drop Model (FRLDM) and microscopic terms that arise from the single-particle spectra. We compare our predictions for two major actinides to experimental data. We present global trends from this modeling that manifest as a function of mass number of the fissioning species. We discuss the impact of fission yield modeling on nucleosynthetic outcomes. [ABSTRACT FROM AUTHOR]

The neutron Time-of-Flight (TOF) research facility at CERN, n_TOF, has been a pioneering platform for neutron cross-section measurements since its inception in 2001. It boasts three distinct experimental areas, each tailored to address a specific range of neutron energies. This paper delves into the intricacies of the n_TOF facility, including its recent upgrade during the Long Shutdown 2 (LS2) at CERN. Additionally, it highlights the key characteristics of the detectors employed for capture and fission cross-section measurements, paving the way for future research endeavors. [ABSTRACT FROM AUTHOR]

The process in superheavy element synthesis in which two nuclei collide and fission before forming a compound nucleus is called quasi-fission. The dynamical model used in our calculations is highly reproducible in calculating the total kinetic energy (TKE) of actinide fission. However, there is a problem in the reproducibility of TKE of quasi-fission in the current calculations using the dynamical model. Our calculations show that the TKE of mass asymmetry is lower than the experimental data. The reason for this is that the shell effect of fission fragments is not considered. In this study, we attempted to reproduce mass-asymmetric quasi-fission with shell effects by using a dynamical model. The objective is to analyze the calculation results and follow the time evolution of the shape of the nucleus to deepen our understanding of the dynamics of quasi-fission. [ABSTRACT FROM AUTHOR]

The neutron induced nuclear reaction cross sections of fission products are related with the neutron fiux and the reactor burnup, which are important for the accurate of nuclear engineering design. To predict the (n,2n) reaction cross section, especially those lack of experimental measurements, we analyzed the relevant features and establish the experimental data set on the basis of sorting out the experimental data recorded in EXFOR library. The back propagation artificial neural network (ANN) and decision tree (DT) models are built to learn the experimental data set, respectively, adopting PyTorch and XGBOOST toolboxes. we report that machine learning models are applied to analysis and predicate (n,2n) reaction cross section. [ABSTRACT FROM AUTHOR]

One of the most important sources of systematic uncertainties in the evaluation of measured cross sections is the absolute normalization of every dataset, which were often performed by measuring simultaneously the reference cross-section of the standard isotope. In other experiments the shape of the cross-section spectrum is normalized using as reference the integral value in a certain energy interval taken from an evaluated library. The choice of the energy interval used as reference has been often left up to the experimentalist criteria, leading to inconsistent normalizations and hardly assessable uncertainties. In this work the experimental datasets of the (n,f) cross section of many actinides are reviewed looking for the best suited energy interval to be recommended for renormalization purposes. Using standard integration intervals, wide enough to get very low statistical uncertainties, should improve the normalization of every experimental dataset, reducing so the associated total uncertainty when making the evaluation. A common integration range from 8 to 10 MeV is proposed for the whole set of actinides needed in fission applications. This energy range, which falls between the second and the third fission-chance thresholds, is characterized by a flat behaviour of the fission cross sections. [ABSTRACT FROM AUTHOR]

This paper presents work that has been done on the FIFRELIN Monte-Carlo code. The purpose of the code is to simulate the de-excitation process of fission fragments. Numerous quantity of insterest are calculated (mass yields, prompt particle spectra, mulitiplicities ...). Up to now the code relies on four free parameters which control the initial excitation and total angular momentum of fission fragment. Finding the good set of the free parameters is a diffucult task. In this work, we have developed an optimization algorithm based on Gaussian Process regression. [ABSTRACT FROM AUTHOR]

Neutron emission is a physical process that can occur at various stages of nuclear fission. The main source of neutron emission comes from primary fission fragments after full acceleration. Another source of neutron emission comes from the de-excitation of the nucleus prior to scission (scission neutrons are neglected in this work). This type of event can occur if the energy of the incoming neutron is above a threshold energy dependent on nuclear properties (neutron binding energy Sn and fission barrier height Bf). This type of de-excitation initiates a competition between fission and neutron emission. This physical mechanism is referred to as multi-chance fission and becomes significant for incident neutron kinetic energies higher than the fission barrier height of the residual nucleus (approximately 5 MeV). Until now, this phenomenon has not been considered in the FIFRELIN code, limiting its scope of application. In this context, we propose a method to model multi-chance fission for the 235U(n,f) reaction. It is based on evaluated microscopic partial fission cross-sections used to calculate multi-chance fission probabilities. [ABSTRACT FROM AUTHOR]

Accurate multiplicities of prompt fission neutrons emitted in neutron-induced fission on a large energy range are essential for fundamental and applied nuclear physics. Measuring them to high precision for radioactive fissioning nuclides is, however, an experimental challenge. In this work, we extract the average prompt-neutron multiplicity emitted in the 239 Pu (n, f) reaction as a function of the incident-neutron energy, over the range 0.7-700 MeV. We used a novel technique, which allowed us to minimize and correct for the main sources of bias and thus achieve unprecedented precision. At low energies, our data validate, for the first time, the ENDF/B-VIII.0 nuclear data evaluation with an independent measurement and reduce the evaluated uncertainty by up to 60%. This work opens up the possibility of measuring, with high precision, prompt fission neutron multiplicities on highly radioactive nuclei relevant for energy production. [ABSTRACT FROM AUTHOR]

The experimental setup of the new measurement of 239Pu fission and capture cross-section in the n_TOF time-of-flight facility at CERN is presented. The measurement aims to address the needs and demands of nuclear data users. The experiment incorporates an innovative fast Fission Fragment Detector and the n_TOF Total Absorption Calorimeter, enabling the implementation of the fission tagging technique. Preliminary results exhibit the robust performance of the detector systems, along with the high quality of the new 239Pu samples. These samples were exclusively produced for this measurement by the European Commission's Joint Research Centre in Geel. [ABSTRACT FROM AUTHOR]

The nuclear reaction code TALYS adopts the Hauser-Feshbach statistical decay theory, to de-excite fission fragments. This involves for instance the evaporation of prompt fission neutrons and γ-rays. TALYS incorporates databases of primary fission fragment distribution which consists of primary fission fragment yield and data for excitation energy distribution of fission fragments. We conducted a sensitivity study on three parameters in TALYS and fitted them in order to reproduce experimental and evaluated data, in thermal neutron-induced fission of 235U. Moreover, we demonstrate a large-scale calculation of average prompt neutron and γ-ray multiplicities for 243 selected actinide isotopes. [ABSTRACT FROM AUTHOR]

Nuclear fission produces fragments whose spins are coupled to each other as well as to the relative fragment motion due to angular momentum conservation. Statistical ensembles of such spins can readily be obtained by either direct microcanonical sampling or by sampling of the associated normal modes of rotation. The spin-spin opening angle distribution is illustrated for both 3D and 2D spin scenarios and it is demonstrated how recent calculations with various models can be well reproduced by the statistical results under different assumptions about the scission geometry. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The evaluated nuclide yields, fragment mass and charge distributions, and averaged total kinetic energy of fission fragments for the neutroninduced fission of 30 actinide nuclei are well described in the new model, which considers the fissioning scission system consisting of two heavy fragments and an α-particle between them. The α-particle has its origin in the neck nucleons. The yield of fission fragments in the model is linked to the number of states over the barrier of the saddle point, which is between the contacting and well-separated fission fragments. The α-particle is fused with the nearest heavy fragment after passing the saddle point, therefore, two final fragments appear during fission. The quadrupole deformations of heavy fragments are taken into account in the model. The fragment yields depend on the heights of corresponding saddle points and the values of the equilibrium quadrupole deformation of fragments. [ABSTRACT FROM AUTHOR]

The energy dependence of high-yield fission products has been measured using quasi-monoenergetic neutron beams at energies between 5.5 and 11.0 MeV. The absolute number of fissions during the irradiation period was determined via dual-fission ionization chambers, while the fission products were measured via direct g-ray spectroscopy. This paper presents absolute fission product yields from neutron-induced fission of 235U, 238U, and 239Pu isotopes for five incident energies in the second chance fission region. [ABSTRACT FROM AUTHOR]

The quantum mechanical fragmentation theory (QMFT) based dynamical cluster-decay model (DCM) is applied to analyze the probable fission decay modes of 254Fm* compound nucleus produced in 16O+238U nuclear reaction at excitation energy EC*N =45.9 MeV. The fission valley of collective fragmentation potential and the multi-humped peaks of preformation probability P0 profile are analyzed by considering compact as well as elongated configurations of quadrupole (β2) deformed fragments. The competitive emergence of different symmetric [symmetric superlong (SL), symmetric supershort (SS)] and asymmetric [standard 1 (S1), standard 2 (S2), standard 3 (S3)] fission modes have been observed for the case of elongated configuration. The division of mass and charge in nuclear fission of 254Fm* depicts the importance of spherical and deformed magic shell closures. The most energetic light (AL and heavy (AH) decay fragments of aforementioned fission modes are identified. Moreover, the DCM-calculated fission cross-sections (σfission) show reasonable agreement with the experimental measurements [24]. [ABSTRACT FROM AUTHOR]

Nuclear fission is a complex dynamical process, whose description involves the coupling between intrinsic and collective degrees of freedom, as well as different quantum-mechanical phenomena. For this reason, to this day it still lacks a satisfactory and complete microscopic description. In addition to the importance of describing fission itself, studies of the r-process in astrophysics depend on fission observables to constrain the theoretical models that explain the isotopic abundances in the Universe. To improve on the existing data, fission reactions of heavy nuclei in inverse kinematics are produced in quasi-free (p,2p) scattering reactions, which induce fission through particle-hole excitations that can range from few to tens of MeV. In order to study the evolution of the fission yields with temperature, the excitation energy of the fissioning system must be reconstructed, which is possible by measuring the four-momenta of the two outgoing protons. Performing this kind of experiment requires a complex experimental setup, providing full isotopic identification of both fission fragments and an accurate measurement of the momenta of the two outgoing protons. This was realized recently at the GSI/FAIR facility and some of the results obtained for the charge distributions are presented in this work. [ABSTRACT FROM AUTHOR]

A new experimental fission approach is presented in the context of the R3B (Reactions with Relativistic Radioactive Beams) collaboration, at the GSI/FAIR facility, in which knockout reactions in inverse kinematics are used to induce fission of 238U that will allow to characterise the excitation energy of the fission process and all the fission products. The CALIFA (CALorimeter for In-Flight detection of γ-rays and high energy charged pArticles) calorimeter, a key part of the R3B set-up, is used to reconstruct the momenta of the two protons from the (p, 2p) reactions. Preliminary results show that kinematic variables and first estimates for nucleon-removal cross sections are well reconstructed and in good agreement with other experimental measurements. [ABSTRACT FROM AUTHOR]