Search

A neutron-physical calculation is performed for neutron-producing lead, tungsten, and natural uranium targets for a subcritical assembly controlled by a proton accelerator at 0.6, 1, and 2.5 GeV. At these high proton energies, the interaction of protons with target atoms is accompanied by a splitting reaction. In terms of the neutron yield per primary proton, this reaction is more advantageous than the fission reaction. The calculation results are used to estimate the neutron-flux densities in a subcritical multiplying assembly.

Nuclear fission is one of the most complex processes in nuclear physics, involving a strong interplay between nuclear structure and dynamics, as well as various many-body correlations and fluctuation effects. Systematically theoretical and experimental research on the fission problems is of great significance to clarify the fission mechanism, reveal the complexity of low-energy nuclear dynamics, and test the nuclear model theory based on the nuclear many-body theory. However, despite the huge theoretical effort pursued by several generations of physicists, many crucial facets of this process are still not understood at a fundamental level, warranting further exploration of its dynamics. This is more so when viewed from the microscopic theoretical point of view. In the subject of study on nuclear fission, scission dynamics represents one of the most challenging problems in many-body theory as it plays a crucial role for determining the main properties of fission fragments and the emitted light particles. In the scission and post-scission processes, the fragments can experience a rapid change in shape and thus trigger the non-adiabatic effects coming from couplings between collective and intrinsic degrees of freedom. Therefore, from microscopic point of view, a reasonable description of the entire fission process could be achieved with an adiabatic model describing the slow evolution across the barrier, followed by a non-adiabatic treatment of the scission and post-scission dynamics. In this paper, with the aid of the nuclear mean-field model and Gogny effective nucleon-nucleon interaction, a three-dimensional Cartesian mesh code of time-dependent Hartree-Fock-Bogoliubov theory (Gogny-TDHFB Code for Fission) has been developed for solving the static and/or dynamic equations with isolated or periodic boundary conditions and no further symmetry assumptions. In contrast with the Skyrme HFB, the particle-hole channel and the particle-particle channel are treated on an equal footing in the HFB calculations with the Gogny interaction (Gogny HFB). The code consists of two parts: One is the constrained HFB for calculating the adiabatic fission potential energy surface in the multi-dimensional collective phase space, the other is TDHFB for studying the dynamical process of nuclear fission. In order to describe the extremely large deformation of nuclei in the direction of fission, the basis functions in the spatial grid points of the Lagrange mesh are introduced with respect to the direction of the z -axis, and the deformation harmonic oscillator eigenfunctions are used as the basis functions in the ( x , y ) directions. In the calculation of multi-dimensional potential energy surfaces, the constraints are imposed on expectation values of the multi-pole moments Q lm and the augmented Lagrangian method (ALM) is exploited for multi-constrained calculations. A parallel interface using the message passing interface (MPI) library has been implemented for large-scale calculations on massively parallel computers. Under the help of Gogny-TDHFB Code for Fission and by mainly focusing on the scission process, various aspects including the nuclear fission processes, the multi-dimensional potential energy surface, the configuration of scission point, more importantly, the non-adiabatic dynamics in the course of deformation from saddle to scission and also in post-scission are microscopically investigated in order to make clarification on the fission mechanisms, the nature of scission and some other basic problems. As an example, the spontaneous fission process of 240Pu has been studied. The results on the non-adiabatic dynamics in the course of deformation from saddle to scission and also in post-scission show that the pairing correlation plays crucial roles in the scission dynamics of nuclear fission.

Problems of describing experimental data on the angular distributions of fragments of nuclear fission induced by intermediate-energy neutrons are discussed. A way of calculating angular distributions based on using the TALYS program is proposed. The capabilities of the technique for obtaining new information on the fission process and reactions at intermediate energies are shown by the example of describing data obtained for the $${}^{{237}}{\text{Np}}$$ nucleus.

A time-of-flight spectrometer was used to compare the masses of nuclear fission fragments before and after their passage through the carbon foil, event by event. For all the registered fragments, there was a significant loss of mass of the fragment, and the registered fragment-residue turned out to be a magic nucleus. For spontaneous fission of 252Cf nuclei, the effect was observed for both light and heavy mass peaks. This gives grounds to assume that the fragment of the conventional binary fission is born in the shape isomer state, which looks like a di-nuclear system consisting of a magical core and a light cluster.

The expression of dependence of the average total kinetic energy of fission fragments on the excitation energy of the fissioning system is obtained in a simple model, which takes into account the statistical and quantum properties. The experimental data of the average total kinetic energy of fragments formed in the fission of nuclei $^{234,236,239}$U and $^{240}$Pu are well described using the obtained expression in the wide range of the excitation energy of these nuclei. The origin of the energy dependence of the average total kinetic energy is discussed in details.

Background: $\ensuremath{\beta}$-delayed neutron emission and fission are essential in $r$-process nucleosynthesis. Although the number of experimental studies covering $r$-process nuclei has recently increased, the uncertainties of $\ensuremath{\beta}$-delayed neutron emission and fission are still large for $r$-process simulations.Purpose: Our aim is to introduce a theoretical framework for the description of $\ensuremath{\beta}$-delayed neutron-emission and fission rates based on relativistic nuclear energy density-functional and statistical models and investigate their properties throughout the nuclide map.Methods: To obtain $\ensuremath{\beta}$ strength functions, the relativistic proton-neutron quasiparticle random-phase approximation is employed. Particle evaporations and fission from highly excited nuclear states are estimated by the Hauser-Feshbach statistical model. $\ensuremath{\beta}$-delayed neutron branching ratios ${P}_{n}$ are calculated and compared with experimental data, and the $\ensuremath{\beta}$-delayed fission branching ratio ${P}_{f}$ are also assessed by using different fission barrier data.Results: Calculated ${P}_{n}$ are in a good agreement with the experimental data and the root mean square deviation is comparable to results of preceding works. It is found that energy withdrawal by $\ensuremath{\beta}$-delayed neutron-emission sensitivity varies ${P}_{n}$, especially for nuclei near the neutron drip line. ${P}_{f}$ depend sensitively on fission barrier data. It is found that not only the barrier height but also the number of barrier humps is important to evaluate ${P}_{f}$.Conclusions: The framework introduced in this work provides an improved theoretical description of the $\ensuremath{\beta}$-delayed neutron emission and fission. Since ${P}_{f}$ as well as ${P}_{n}$ depend strongly on fission barrier information, four kinds of fission barrier data are used in this work to allow further sensitivity studies of the $r$-process nucleosynthesis on the nuclear fission. More studies on fission barrier are highly requested to assess the role of $\ensuremath{\beta}$-delayed fission in the $r$-process study. A complete set of calculated data for $\ensuremath{\beta}$-delayed neutron emission and fission are summarized as a table in supplemental material for its use in $r$-process studies as well as to complement a part of nuclear data in which no experimental data are available.

This work is a next step toward the study of physical properties of the Accelerator Driven System (ADS) in which a deeply subcritical active core (512 kg natural uranium target) is irradiated by a pulsed beam of relativistic particles. The measurement of the total number of fission reactions is a fundamental task in the given research. The estimated method of determining the number of fission reactions based on activation detectors is analyzed. Our interest has been focused on the study of the proton- and neutron-induced fission reaction inside the high-energy particle beam volume. The fission products of fission reactions have been measured employing natural uranium activation detectors and germanium gamma detectors (HPGe). Experimental data are taken from the QUINTA experiment performed at JINR, Dubna, in November 2014. The presented method gives an effective possibility of estimating the measured number of fission reactions induced by neutrons and high-energy protons simultaneously.

The fission of the 181Re∗ nucleus formed via the 12C +169Tm reaction is studied at ELab= 77–89 MeV within the dynamical cluster model (DCM) for the choices of spherical as well as deformed fragments originated in binary decay channels of the of collective clusterization approach. The mass distribution of the compound nucleus 181Re∗ is analyzed for both choices and compared with the available data, and consequently the probable fission fragments are identified. The barrier height, barrier modification, and barrier penetrability of the fission fragments are investigated. The total fission cross-sections are calculated for spherical as well as deformed choices of fragmentation, and they are in nice agreement with experimental data. In addition, the fission emission times and the decay widths of the fission fragments are predicted for comparison with future experiments. Finally, the preformation behavior of spontaneous and induced fission paths are compared to give a better insight of the decay mechanism of 181Re nucleus.

A comparison is made of two theoretical approaches to describing the coefficients of T-odd asymmetries in the angular distributions of prompt γ-quanta and neutrons evaporated from thermalized fragments of the binary fission of non-oriented target nuclei by cold polarized neutrons, and in angular distributions of pre-scission α particles emitted in the same ternary nuclear fission: the classical approach, which is based on trajectory calculations, and the quantum approach, which is based on quantum fission theory. It is shown that the coefficients for all of the considered nuclei are of a quantum nature and vanish in the classical approach when there is no interference from the fission amplitudes of the different neutron resonances of a compound nucleus. The advantages of the quantum approach versus the classical one are demonstrated.

The proposed Compact Intense Fast NEutron Facility (CIFNEF) will have a wide range of applications, including the investigation of the exotic structure of neutron-rich nuclei, data for the nuclear fission , and the simulation of the neutron field in a star. To satisfy aims of producing high-intensity fast neutrons and forward neutrons with very low environmental background, the CIFNEF linac should have the capacity of accelerating continuous wave (CW) deuteron (D + ), hydrogen ( H 2 + ) and lithium (7Li 3 + ) beams to 2.5 MeV/u with maximum beam currents of 10 mA, 5 mA and 10 uA, respectively. Based on the above requirements, we proposed a novel compact linac using a combination of RFQ and DTL structures. The dynamics of RFQ and DTL are completed to meet all requirements and start-to-end simulation results show that the three ion species can be accelerated to the final energy with transmission efficiency above 99% as well as good beam quality with lower emittance growth. In addition, we performed error sensitivity analysis and combined error study to evaluate the error tolerance limits of the obtained design.

In this paper we report on an effort to determine an optimal parameter set for the complete event fission model F R E Y A to reproduce spontaneous fission of 252 Cf(sf), 244 Cm(sf), 238 Pu(sf), 240 Pu(sf), 242 Pu(sf), and 238U(sf). Earlier studies have partially optimized the event-by-event fission model F R E Y A with respect to the available experimental data using brute force computational techniques. We have confirmed and expanded these results using a least-squares minimization based on the simulated annealing approach. We have also developed a more complete statistical picture of this optimization, consisting of a full correlation matrix for the parameters utilized by F R E Y A . The newly improved parameter values themselves, along with this correlation matrix, have led to a more well-developed physical picture of the fission process.

A recent analysis of experimental data [J. Wilson $et. al$, Nature $\mathbf 590$, 566 (2021)] found that the angular momenta of nuclear fission fragments are uncorrelated. Based on this finding, the authors concluded that the spins are therefore determined only $after$ scission has occurred. We show here that the nucleon-exchange mechanism, as implemented in the well-established event-by-event fission model $\mathtt{FREYA}$, while agitating collective rotational modes in which the two spins are highly correlated, nevertheless leads to fragment spins that are largely uncorrelated. This fact invalidates the reasoning of those authors. Furthermore, it was reported [J. Wilson $et. al$, Nature $\mathbf 590$, 566 (2021)] that the mass dependence of the average fragment spin has a sawtooth structure. We demonstrate that such a behavior naturally emerges when shell and deformation effects are included in the moments of inertia of the fragments at scission., 5 pages, 2 figures

When a heavy atomic nucleus splits (fission), the resulting fragments are observed to emerge spinning1; this phenomenon has been a mystery in nuclear physics for over 40 years2,3. The internal generation of typically six or seven units of angular momentum in each fragment is particularly puzzling for systems that start with zero, or almost zero, spin. There are currently no experimental observations that enable decisive discrimination between the many competing theories for the mechanism that generates the angular momentum4–12. Nevertheless, the consensus is that excitation of collective vibrational modes generates the intrinsic spin before the nucleus splits (pre-scission). Here we show that there is no significant correlation between the spins of the fragment partners, which leads us to conclude that angular momentum in fission is actually generated after the nucleus splits (post-scission). We present comprehensive data showing that the average spin is strongly mass-dependent, varying in saw-tooth distributions. We observe no notable dependence of fragment spin on the mass or charge of the partner nucleus, confirming the uncorrelated post-scission nature of the spin mechanism. To explain these observations, we propose that the collective motion of nucleons in the ruptured neck of the fissioning system generates two independent torques, analogous to the snapping of an elastic band. A parameterization based on occupation of angular momentum states according to statistical theory describes the full range of experimental data well. This insight into the role of spin in nuclear fission is not only important for the fundamental understanding and theoretical description of fission, but also has consequences for the γ-ray heating problem in nuclear reactors13,14, for the study of the structure of neutron-rich isotopes15,16, and for the synthesis and stability of super-heavy elements17,18. γ-ray spectroscopy experiments on the origin of spin in the products of nuclear fission of spin-zero nuclei suggest that the fission fragments acquire their spin after scission, rather than before.

Nuclear fission is hindered by dissipation. Using the stochastic Langevin model, we calculate the drop of fission cross sections caused by friction with respect to its standard statistical-model values, ${\ensuremath{\sigma}}_{f}^{\mathrm{drop}}$, as a function of the presadde dissipation strength $(\ensuremath{\beta})$ for $^{226}\mathrm{U}$, $^{234}\mathrm{U}$, and $^{242}\mathrm{U}$. It is shown that the sensitivity of ${\ensuremath{\sigma}}_{f}^{\mathrm{drop}}$ to $\ensuremath{\beta}$ is substantially enhanced with increasing the isospin of the U fissioning system. Furthermore, we find that under typical conditions of excitation energy and angular momentum populating $^{240}\mathrm{U}$ (via radioactive beams) and $^{200}\mathrm{Tl}$ (via stable beams), the fission cross section of the high-isospin heavy $^{240}\mathrm{U}$ demonstrates a greater sensitivity to friction than that of the light $^{200}\mathrm{Tl}$. Our findings suggest that on the experimental side, to accurately probe presaddle dissipation with fission cross sections of heavy nuclei, it is useful to choose radioactive nuclear beams induced reactions as a way to populate excited heavy fissioning systems with high isospin.

During nuclear fission, a heavy nucleus splits into two rotating fragments. The associated angular momentum is large, yet the mechanism of its generation and its dependence on the mass of fragments remain poorly understood. In this Letter, we provide the first microscopic calculations of angular momentum distributions in fission fragments for a wide range of fragment masses. For the benchmark case of $^{239}$Pu($n_{\text{th}}$,f), we find that the angular momentum of the fragments is largely determined by the nuclear shell structure and deformation, and that the heavy fragments therefore typically carry less angular momentum than their light partners. We use the fission model $\tt{FREYA}$ to simulate the emission of neutrons and photons from the fragments. The dependence of the angular momenta on fragment mass after the emission of neutrons and statistical photons is linear for the heavy fragments and either constant or weakly linear for the light fragments, consistent with the universal sawtooth pattern suggested by recent experimental data. Finally, we observe that using microscopic angular momentum distributions modifies the number of emitted photons significantly., 7 pages, 2 figures

Fission properties of the actinide nuclei are deduced from theoretical analysis. We investigate potential energy surfaces and fission barriers and predict the fission fragment mass-yields of actinide isotopes. The results are compared with experimental data where available. The calculations were performed in the macroscopic-microscopic approximation with the Lublin-Strasbourg Drop (LSD) for the macroscopic part and the microscopic energy corrections were evaluated in the Yukawa-folded potential. The Fourier nuclear shape parametrization is used to describe the nuclear shape, including the non-axial degree of freedom. The fission fragment mass-yields of considered nuclei are evaluated within a 3D collective model using the Born-Oppenheimer approximation., Comment: 11 pages, 12 figures, submitted to the Chinese Physics C. arXiv admin note: text overlap with arXiv:2001.08652

In the exothermic process of fission decay, an atomic nucleus splits into two or more independent fragments. Several aspects of nuclear fission are not properly understood, in particular the formation of the neck between the nascent fragments, and the subsequent mechanism of scission into two or more independent fragments. Using an implementation of time-dependent density functional theory, based on a relativistic energy density functional and including pairing correlations, we analyze the final phase of the process of induced fission of $^{240}$Pu, and show that the time-scale of neck formation coincides with the assembly of two $\alpha$-like clusters (less than 1 zs = 10$^{-21}$ s). Because of its much larger binding energy, the dynamical synthesis of 4He in the neck predominates over other light clusters, e.g., $^3$H and $^6$He. At the instant of scission the neck ruptures exactly between the two $\alpha$-like clusters, which separate because of the Coulomb repulsion and are eventually absorbed by the two emerging fragments. The newly proposed mechanism of light charged clusters formation at scission provides a natural explanation of ternary fission., Comment: 5 pages, 4 figures, Final version for publication

Potential energy surfaces and fission barriers of superheavy nuclei are analyzed in the macroscopic-microscopic model. The Lublin-Strasbourg Drop (LSD) is used to obtain the macroscopic part of the energy, whereas the shell and pairing energy corrections are evaluated using the Yukawa-folded potential. A standard flooding technique has been used to determine the barrier heights. It was shown the Fourier shape parametrization containing only three deformation parameters reproduces well the nuclear shapes of nuclei on their way to fission. In addition, the non-axial degree of freedom is taken into account to describe better the form of nuclei around the ground state and in the saddles region. Apart from the symmetric fission valley, a new very asymmetric fission mode is predicted in most superheavy nuclei. The fission fragment mass distributions of considered nuclei are obtained by solving the 3D Langevin equations., Comment: 20 pages, 17 figures

Rapid-advancing intense laser technologies enable the possibility of a direct laser-nucleus coupling. In this paper the effect of intense laser fields on a series of nuclear fission processes, including proton emission, $\ensuremath{\alpha}$ decay, and cluster decay, is theoretically studied with the help of nuclear double folding potentials. The results show that the half-lives of these decay processes can be modified by non-negligible amounts, for example on the order of 0.01 or 0.1% in intense laser fields available in the forthcoming years. In addition to numerical results, an approximate analytical formula is derived to connect the laser-induced modification to the decay half-life and the decay energy.

Prompt and delayed γ -ray spectroscopy of the neutron-rich 94 Kr was performed, as part of the fission campaign at the ALTO facility of the IPN Orsay, using the fast-neutron-induced fission reaction 238 U ( n , f ) in combination with the ν -Ball array, a novel hybrid γ spectrometer for energy and lifetime measurements. Several new yrast and nonyrast transitions were observed for the first time, extending the previously known level scheme. Additionally, we report on the observation of a new short-lived isomer at 3444 keV with a half-life of 32(3) ns. The analysis of the Nilsson orbitals obtained from Gogny cranked Hartree-Fock-Bogoliubov calculations suggests a ( 9 − ) spin and an oblate deformation for this isomer corresponding to a two-quasineutron state, indicating an isomeric structure very similar to that of the neighboring isotones 96 Sr and 92 Se

The competition between the dominant mass-asymmetric and rarer narrow mass-symmetric fission modes in actinide nuclei are controlled by deformed and spherical shell effects. The low energy fission of 80 180 Hg was recently observed to be strongly mass-asymmetric, indicating that despite spherical shell gaps in fragments around 40 90 Zr, the system does not fission mass-symmetrically. Several theoretical approaches have been used to explain this unexpected result. To investigate the underlying mechanism, systematic measurements of fission mass distributions for isotopes of Os, Pt, Hg and Pb, formed in fusion reactions with p, 12C, 32S, 40,48Ca projectiles, have been made for excitation energies above the fission saddle-point (E eff ⁎ ) between 2.8 and 28.2 MeV. Evidence for mass-asymmetric fission is widespread, manifested as flat topped mass distributions or significant deviations from a single Gaussian shape. The systematic trends seen cannot be attributed to quasifission. Comparing two-Gaussian fits at a wide range of E⁎, it is concluded that the fit centroids reflect the low energy character of mass-asymmetric fission in the sub-lead region. Quantitative comparisons were made with microscopic calculations by Scamps and Simenel (2019) [33] of fission mass-asymmetries attributed to the influence of shell gaps in both neutrons (N=52, 56 for compact octuple deformations) and protons (Z=34 and Z=42, 44, 46 with large quadrupole deformations). For the predominant fission mode in the calculations, having one elongated and one compact fragment, the results are in extremely good agreement with all experimental values. This provides strong support for both the calculations, and the exploration of mass-asymmetric fission systematics through heavy ion fusion reactions. The total kinetic energy distributions for 176Pt and 180Pt do not show any evidence of a low TKE mass-symmetric fission mode, as had been reported for 178Pt by Tsekhanovich et al. (2019) [39] .

There has been much recent interest in nuclear fission, due in part to a new appreciation of its relevance to astrophysics, stability of superheavy elements, and fundamental theory of neutrino interactions. At the same time, there have been important developments on a conceptual and computational level for the theory. The promising new theoretical avenues were the subject of a workshop held at the University of York in October 2019; this report summarises its findings and recommendations., SCOPUS: re.j, info:eu-repo/semantics/published

Nuclear fission is a fascinating field of research that involves large-amplitude collective dynamics of a microscopic many-body system. Specifically, the process of spontaneous-fission decay can only be explained within the quantum tunneling phenomenon. The present review discusses recent advancements in the theoretical understanding of spontaneous fission. These concern precise prediction of the spontaneous fission observables like fission lifetime and distribution of fragment yields. The theoretical developments presented here are based on a coherent coupling between the adiabatic collective dynamics and the static inputs obtained from the nuclear energy density functional formalism.

Safe and long term operation of nuclear reactors is one of the most discussed challenges in nuclear power engineering. The radiation degradation of nuclear design materials limits the operational lifetime of all nuclear installations or at least decreases its safety margin. This paper is a review of experimental PALS/PLEPS studies of different nuclear reactor pressure vessel (RPV) steels investigated over last twenty years in our laboratories. Positron annihilation lifetime spectroscopy (PALS) via its characteristics (lifetimes of positrons and their intensities) provides useful information about type and density of radiation induced defects. The new results obtained on neutron-irradiated and hydrogen ions implanted German steels were compared to those from the previous studies with the aim to evaluate different processes (neutron flux/fluence, thermal treatment or content of selected alloying elements) to the microstructural changes of neutron irradiated RPV steel specimens. The possibility of substitution of neutron treatment (connected to new defects creation) via hydrogen ions implantation was analyzed as well. The same materials exposed to comparable displacement damage (dpa) introduced by neutrons and accelerated hydrogen ions shown that in the results interpretation the effect of hydrogen as a vacancy-stabilizing gas must be considered, too. This approach could contribute to future studies of nuclear fission/fusion design steels treated by high levels of neutron irradiation.

Fission barriers for a large number of nuclei have been extracted from Gogny D1M potential energy surfaces and compared to experimental data. The same data have been used to estimate spontaneous fission half-lives on the basis of the least-action path. An upgraded version of the SPY model for the calculation of the main observables related to fission fragments is also presented. Fission path and yields are key ingredients to estimate the composition of matter synthesized by the r-process nucleosynthesis in the ejecta of binary neutron star mergers., SCOPUS: cp.p, info:eu-repo/semantics/published

Nuclear fission plays an important role in fundamental and applied science, from astrophysics to nuclear engineering, yet it remains a major challenge to nuclear theory. Theoretical methods used so far to compute fission observables rely on symmetry-breaking schemes where basic information on the number of particles, angular momentum, and parity of the fissioning nucleus is lost. In this Letter, we analyze the impact of restoring broken symmetries in the benchmark case of $^{240}\mathrm{Pu}$.

The well-established macroscopic-microscopic (mac-mic) description of nuclear fission enables the prediction of fission-fragment yields for a broad range of fissioning systems. In this work, we present several key enhancements to this approach. We improve upon the microscopic sector of nuclear potential-energy surfaces by magnifying the resolution of the Lipkin-Nogami equations and strengthening the Strutinsky procedure, thus reducing spurious effects from the continuum. We further present a novel deterministic method for calculating fission dynamics under the assumption of strongly damped nucleonic motion. Our technique directly determines the evolution of the scissioned shape distribution according to the number of random-walk steps rather than the statistical accumulation of fission events. We show that our new technique is equivalent to the Metropolis random walk pioneered over the past decade by Randrup and colleagues. It further improves upon it because we remove the need for altering the nuclear landscape via a biased potential. With our final improvement, we calculate fission fragments mass and charge distributions using particle number projection, which affords the simultaneous calculation of both mass- and charge-yield distributions. Fission fragments are thus calculated from the quantum-mechanical $A$-body states of the potential-energy surface rather than from the collective mass asymmetry variable ${\ensuremath{\alpha}}_{\mathrm{g}}$ of the finite-range liquid-drop model used in past work. We highlight the success of our enhancements by predicting the odd-even staggering and the charge polarization for the neutron-induced fission of $^{233}\mathrm{U}$ and $^{235}\mathrm{U}$.

$\textbf{Background}$ More than half of all the elements heavier than iron are made by the rapid neutron capture process (or r process). For very neutron-rich astrophysical conditions, such at those found in the tidal ejecta of neutron stars, nuclear fission determines the r-process endpoint, and the fission fragment yields shape the final abundances of $110\le A \le 170$ nuclei. The knowledge of fission fragment yields of hundreds of nuclei inhabiting very neutron-rich regions of the nuclear landscape is thus crucial for the modeling of heavy-element nucleosynthesis. $\textbf{Purpose}$ In this study, we propose a model for the fast calculation of fission fragment yields based on the concept of shell-stabilized prefragments defined with help of the nucleonic localization functions. $\textbf{Methods}$ To generate realistic potential energy surfaces and nucleonic localizations, we apply Skyrme Density Functional Theory. The distribution of the neck nucleons among the two prefragments is obtained by means of a statistical model. $\textbf{Results}$ We benchmark the method by studying the fission yields of $^{178}$Pt, $^{240}$Pu, $^{254}$Cf, and $^{254,256,258}$Fm and show that it satisfactorily explains the experimental data. We then make predictions for $^{254}$Pu and $^{290}$Fm as two representative cases of fissioning nuclei that are expected to significantly contribute during the r-process nucleosynthesis occurring in neutron star mergers. $\textbf{Conclusions}$ The proposed framework provides an efficient alternative to microscopic approaches based on the evolution of the system in a space of collective coordinates all the way to scission. It can be used to carry out global calculations of fission fragment distributions across the r-process region., 10 pages, 9 figures

Recent developments in theoretical modeling and in computational power have allowed us to make significant progress on a goal not achieved yet in nuclear theory: a fully microscopic theory of nuclear fission. The complete microscopic description remains a computationally demanding task, but the information that can be provided by current calculations can be extremely useful to guide and constrain phenomenological approaches. First, a truly microscopic framework that can describe the real-time dynamics of the fissioning system can justify or rule out assumptions and approximations incompatible with an accurate quantum treatment or with our understanding of the inter nucleon interactions. Second, the microscopic approach can be used to obtain trends such as: the excitation energy sharing mechanism between fission fragments (FFs) with increasing excitation energy of the fissioning system, the angular momentum content of the FFs, or even to compute observables that cannot be otherwise calculated in phenomenological approaches or even measured, as in the case of astronomical environments. Merely the characterization of the trends would be of great importance for various application. We present here arguments that a truly microscopic approach to fission does not support the assumption of adiabaticity of the large amplitude collective motion in fission, particularly starting from the outer saddle down to the scission configuration., Comment: 14 pages, 7 figures,, published version

Presently, it is well established that fission of excited nuclei is a dynamical process being a subject of fluctuations and dissipation. In the literature, there are indications that, at the compact nucleus shapes, the strength of nuclear friction is significantly reduced in comparison with the prediction of the one-body approach. Thus, one can expect that at small deformations the nuclear fission process occurs in the so-called "energy diffusion regime". The purpose of our present work is to compare an approximate analytical formula for the fission rate in this regime with the quasistationary numerical rate which is exact within the statistical errors. Our calculations demonstrate relatively good agreement between these two rates provided the friction parameter is deformation independent. If one accounts for its deformation dependence, the agreement becomes significantly poorer., INPC2019 contribution

A dynamical model is employed to determine some peculiar properties of the scission neutrons, i.e., those emitted at the rupture of the neck between the fragments during low energy nuclear fission. Thus, by a specially adapted procedure, we calculate the time-dependent decay rate. This, defines the character of the emission process: pulsed or exponentially. From the time dependence of the survival probability, we deduced the “half-life” of the scission neutron emission. The angular distribution of the scission neutrons with respect to the fission axis is calculated as well. This is obtained for sets of neutron wave functions defined by a given quantum number [Formula: see text] (projection of the total angular momentum on the symmetry axis). A strong dependence on [Formula: see text] was found, namely, wave functions with different [Formula: see text] have different most probable emission angles: from emission along the fission axis (1/2) to emission perpendicular to the fission axis (9/2). This result leads to a new interpretation of the measured angular distribution of the prompt fission neutrons. Finally, an extension of the dynamical model (i.e., with a time-dependent potential also after scission) is presented.

Subcritical assembly for Mo-99 production (SAMOP) has been developed at The Center for Accelerator Science and Technology (CAST), National Nuclear Energy Agency (SATAN). The objective of the SAMOP development is to provide a prototype of a subcritical assembly which produces Mo-99 isotope. In the nuclear medical application, Mo-99 plays an important role as a generator of isotope Tc-99m which is the most widely used radioisotope for diagnostic purposes. SAMOP use uranyl nitrate solution UO2(NO3)2 as fuel as well as target material for the process of Mo-99 production. This process is based on the fission reaction of uranium U-235 contained in the uranyl nitrate solution. The U-235 fission reactions occur as long as there are still neutrons from external neutron source and it will stop whenever the external neutron source is removed. Presently SAMOP uses external neutron source from the radial beam port of the Kartini research reactor which emerges thermal neutrons in the order of 108 n/cm2s. For further development, SAMOP will be equipped with a compact neutron generator as an external neutron source. Compact neutron generator is a neutron source device that contains a compact ion accelerator and that produces neutrons by fusing isotopes of hydrogen. The fusion reactions take place in the device by accelerating either deuteron, triton, or a mixture of these two isotopes into a metal hydride target which also contains deuterium, tritium or a mixture of these isotopes. The fusion of deuterium atoms (D + D) results in the formation of a He-3 ion and a neutron with a kinetic energy of approximately 2.5 MeV. The fusion of a deuterium and a tritium atom (D + T) results in the formation of a He-4 ion and a neutron with a kinetic energy of approximately 14.1 MeV. SAMOP needs an external neutron source which can provide neutrons with an average neutron flux of 108 n/cm2s for maintaining the sustainability of U-235 fission reactions. The results of the study on compact neutron generator as an external neutron source of SAMOP show that a compact neutron generator has a chance to substitute a nuclear reactor as external neutron source of SAMOP. A compact neutron generator can produce fast neutrons which neutron yield is sufficient to meet the need of neutron for SAMOP. It depends on the type of the compact neutron generator, one of which is the compact neutron generator produced by LSNL i.e. the axial compact neutron generator which can produce neutron yield up to 109 n/s. The benefit of using a compact neutron generator as an external neutron source of SAMOP is that the operational cost will reduce since the neutron production cost using a compact neutron generator is cheaper than a nuclear reactor. In addition, the operational procedure of a compact neutron generator is simpler than a nuclear reactor.

Collective mass tensors derived in the cranking approximation to the adiabatic time-dependent Hartree-Fock-Bogoliubov (ATDHFB) method are employed in a study of induced fission dynamics. Together with a collective potential determined in deformation-constrained self-consistent mean-field calculations based on nuclear energy density functionals, the mass tensors specify the collective Hamiltonian that governs the time evolution of the nuclear wave function from an initial state at equilibrium deformation, up to scission and the formation of fission fragments. In an illustrative calculation of low-energy induced fission of $^{228}$Th, $^{230}$Th, $^{234}$U, and $^{240}$Pu, we compare the non-perturbative and perturbative cranking ATDHFB mass tensors in the plane of axially-symmetric quadrupole and octupole deformations, as well as the resulting charge yields., Comment: 7 pages, 7 figures. arXiv admin note: text overlap with arXiv:1902.09535

Nuclear power plants produce energy basically through nuclear fission reactions in nuclear fuel. When fission reactions occur in the fuel material, a large amount of heat is produced along with several neutrons and fission fragments, thus leading to a fission chain reaction and energy production. This chapter provides a basic understanding of nuclear reactor principles. The first part begins with a basic concept of nuclear power plants beginning with the fundamentals and practical applications related to the utilization of nuclear energy from fission. It explains the different types of neutron interaction with matter including neutron attenuation, scattering, and fission. The concepts of neutron diffusion and reactor theory are explained starting with equation of continuity and Fick's law approximation in one-energy group, multigroup, and two-group contexts. Then, a brief introduction to nuclear reactor kinetics including reactor reactivity, neutron life, reactor feedback coefficients, and neutron poisons is explained. The second part of the chapter explains the basic principles of nuclear system thermal hydraulics with single-phase fluid mechanics and heat transfer relevant to nuclear systems. This part includes topics ranging from basic thermal hydraulic characteristics of power reactors to the main classification of nuclear power plants, power cycles and reactor designs, and primary coolant system in various reactor designs. The chapter then discusses thermal design principles and reactor heat generation, which leads to a more focused thermal analysis of nuclear fuel elements including a brief description of the single-phase heat transfer for laminar and turbulent flow in both circular and noncircular subchannels.

The Bohr hypothesis, one of the most fundamental assumptions in nuclear fission theory, states that the decay of a compound nucleus with a given excitation energy, spin and parity is independent of its formation. Using fission product yields (FPYs) as a sensitive probe, we have performed new high precision test of the combined effects of the entrance channel, spin and parity on the fission process. Two different reactions were used in a self-consistent manner to produce a compound 240Pu nucleus with the same excitation energy: neutron induced fission of 239Pu at En = 4.6 MeV and photon-induced fission of 240Pu at Eγ = 11.2 MeV. The FPYs from these two reactions were measured using quasimonoenergetic neutron beams from the TUNL's FN tandem Van de Graaff accelerator and quasimonenergetic photon beams from the High Intensity γ-ray Source (HlγS) facility. The first results comparing the FPYs from these two reactions will be presented. Implications for validating the Bohr hypothesis will be discussed.

A global potential-driving model with well-determined parameters is proposed by uniting the empirical asymmetric fission potential and the empirical symmetric fission potential, which can precisely calculate the pre-neutron-emission mass distributions for neutron-induced actinide nuclei fission. Based on the developed potential-driving model, Monte-Carlo code calculates the characteristics of fission reaction process for neutron-induced 241 Am fission. Typical calculated results, including yields, kinetic energy distributions, fission neutron spectrum and decay γ-ray spectrum, are compared with experimental data and evaluated data. It shows that the Monte-Carlo calculated results agree quite well with the experiment data, which indicate that Monte-Carlo code with the developed potential-driving model can reproduce and predict the characteristics of fission reaction process at reasonable energy ranges. Given the well predictions on the characteristics of fission reaction process, Monte-Carlo code with the developed potential-driving model can guide for the physical design of nuclear fission engineering.

The fast event-by-event fission code FREYA (Fission Reaction Event Yield Algorithm) generates large samples of complete fission events while employing only a few physics-based parameters. Not only is FREYA fast, it is also flexible, able to employ a variety of input formats to test the implications of various fission yield evaluations on neutron and photon observables. We describe how FREYA was applied to the neutron-rich nuclei needed for r-process nucleosynthesis calculations as an example of this flexibility. Finally, we discuss how we plan to make use of this flexibility to extend FREYA to calculations of cumulative fission product yields to aid evaluations of these yields in the future.

Fission product yields (FPYs) are a uniquely sensitive probe of the fission process, with well established dependence on the species of nucleus undergoing fission, its excitation energy and spin. Thus FPYs are well suited for testing Bohr’s hypothesis in the context of nuclear fission, which states that the decay of a compound nucleus with a given excitation energy, spin and parity is independent of its formation. Using FPYs, we have performed a new highprecision test of the combined effects of the entrance channel, spin and parity on the fission process from two of the most commonly used particles to induce fission neutrons and photons. The 239 Pu(n,f) reaction at En = 4.6 MeV and the 240 Pu(γ,f) reaction at Eγ = 11.2 MeV were used to produce a 240 Pu∗ compound nucleus with the same excitation energy. The FPYs from these two reactions were measured using quasimonoenergetic neutron beams from the TUNL’s FN tandem Van de Graaff accelerator and quasimonenergetic photon beams from the High Intensity γ-ray Source (HIγS) facility. The FPYs from these two reactions are compared quantitatively for the first time.

The design principles and parameters of a promising neutron source, based on the fission reaction of the isotope 237Np, for use in research in extracted beams are presented. Such a source can operate as a pulsing reactor or super-booster or as a super-booster multiplying the neutrons from the target of a proton accelerator by using a reactivity modulator. Owing to the threshold character of 237Np fission such a neutron source will possess significantly higher characteristics than a similar setup with plutonium, and in terms of utilization efficiency it will be the leading source in the world for research employing neutron spectroscopy.

Monte Carlo simulation is a powerful tool used to model neutron coincidence detectors for international safeguards. The simulation has typically sampled properties such as the fission neutron multiplicity, energy, and direction, from independent probability density functions. However, multiplicity counters detect event-based neutron correlations and thus more accurate fission event modeling is needed. To respond to this need, the Fission Reaction Event Yield Algorithm (FREYA) and the Cascading Gamma-ray Multiplicity with Fission (CGMF) models were added in the newest version of MCNP, MCNP6.2. The models simulate individual fission events conserving momentum, energy, and angular momentum such that correlated particles are emitted. The effects of the new models on simulations of safeguards neutron coincidence counters were studied and compared to standard MCNP simulations. The MCNPX-PoliMi model was also included in the comparison. The properties of fission neutrons from safeguards relevant isotopes were compared to literature references. Then a hypothetical simplified detector was modeled to isolate the effects of specific differences between models. Experimental measurements from previous work were modeled and agreements were compared. Finally, the probabilities of correlated events occurring in the experimental measurements were calculated with the different models. For example, the probability was calculated of detecting neutrons from both induced fission in uranium and spontaneous fission of Cf-252 in the same fission chain.

Using a Monte Carlo simulation, we study the correlations between the average prompt neutron multiplicities as a function of the primary (pre-neutron) mass ν ¯ s ( A ) and the corresponding to the final (post-neutron) mass n ¯ ( m ) and the pseudo mass (obtained by the 2 E method) N ¯ ( μ ) of fragments from the spontaneous fission of 252Cf. We interpret the results in terms of the ratio of neighboring mass yields R A (= Y A + 1 / Y ( A ) ). We find that; (i) n ¯ - ν ¯ s is correlated with R - 1 , and (ii) N ¯ - ν ¯ s is correlated (anticorrelated) with R - 1 when μ A ( μ > A ).

Neutrons are an exceptional diagnostic signature in magnetic fusion experiments with high temperature plasma. A part of neutron detection methods relies on detecting nuclear reactions products, initiated by neutrons. Among different possibilities, the neutron-induced fission reactions can be used. These methods use the specific properties of the radioactive decay phenomena induced by neutrons of heavy elements like uranium or thorium. The neutron-induced fission reactions produce energetic fission fragments that can be detected in a special type of ionization chambers that has one of electrode covered with fissionable material. A fission reaction may be also adapted to neutron detection by activation method. A dedicated target-sample made from fissionable isotope (like 235U, 238U, 232Th), irradiated by neutrons, is the source of prompt and delayed neutrons. The decay curve of delayed neutrons can be registered almost immediately after irradiation by fusion neutrons, with a delay resulting exclusively from the time of transport of the samples to the measuring set-up. The yield of a fusion neutron source can be calculated from that measurement. Fission phenomena and fissionable materials are applicable in plasma neutron diagnostics. The lecture gives the introduction to the nuclear fission process and the particular attention is done to the phenomenon and physics of delayed neutrons which are generated during the fission reaction. Examples of delayed neutrons activation set-up and fission chambers dedicated for tokamaks are presented.

Chemicals enriched in lithium isotopes are of paramount importance in the current nuclear fission technology, in future nuclear fusion technology, and in neutron detectors. Currently, lithium isotopes are obtained by using mercury amalgams, which is a nonenvironmentally friendly procedure. An interesting alternative is to take advantage of the electrochemical isotope effect. Here, we take, as the departure point, experimental data that were already reported for a prospective first stage of an electrochemical process, and we numerically simulate different scale-up possibilities. We calculate the minimum number of repetitive stages needed to reach a certain degree of isotopic enrichment. Second, we improve our simulations, considering that different operating conditions are used while keeping the same fundamental electrochemical process: a minimum of 145 stages are necessary to produce a sample with 90% enrichment in 6Li. We conclude with a comparison on the different electrochemical technologies, in view of the complementary steps necessary for scaling up. Fil: Acosta, Lautaro Nicolás. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro de Investigacion y Desarrollo En Materiales Avanzados y Almacenamiento de Energia de Jujuy. - Universidad Nacional de Jujuy. Centro de Investigacion y Desarrollo En Materiales Avanzados y Almacenamiento de Energia de Jujuy. - Gobierno de la Provincia de Jujuy. Centro de Investigacion y Desarrollo En Materiales Avanzados y Almacenamiento de Energia de Jujuy; Argentina Fil: Flexer, Victoria. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro de Investigacion y Desarrollo En Materiales Avanzados y Almacenamiento de Energia de Jujuy. - Universidad Nacional de Jujuy. Centro de Investigacion y Desarrollo En Materiales Avanzados y Almacenamiento de Energia de Jujuy. - Gobierno de la Provincia de Jujuy. Centro de Investigacion y Desarrollo En Materiales Avanzados y Almacenamiento de Energia de Jujuy; Argentina

The activation experiment was performed using the accelerated beam of Phasotron accelerator at the Joint Institute for Nuclear Research (JINR). The natural uranium spallation target QUINTA was irradiated with protons with energy 660 MeV. Monte Carlo simulations of neutron production were performed using the Geant4 code. The number of leakage neutrons from the sections of the uranium target surrounded by the lead shielding and the number of leakage neutrons from lead were determined. The total number of fissions in the setup QUINTA was determined. Experimental values of reaction rates for the produced nuclei in the 127I sample were obtained and several values of reaction rates were compared with the results of simulations. Experimentally determined fluence of neutrons in energy interval 10–175 MeV using the (n,xn) reactions in the 127I(NaI) sample was compared with the results of simulations. Possibility of transmutation of the long-lived radionuclide 129I in the QUINTA setup was estimated.

We have developed a Hauser–Feshbach fission fragment decay model, HF3D, which can be applied to the statistical decay of more than 500 primary fission fragment pairs (1,000 nuclides) produced by the neutron induced fission of 235U. The HF3D model numerically integrates fission fragment yield and their initial excitation energy, spin and parity distributions for these 1,000 nuclides. The fission fragment yield Y(A) and the total kinetic energy are model inputs, and we estimate them from available experimental data for the 235U(nth,f) system. The model parameters in the statistical decay calculation are adjusted to reproduce some fission observables, such as the neutron emission multiplicity vˉ, its distribution P(ν), and the mass dependence vˉ(A). The calculated fission product yield (FPY) and isomeric ratio (IR) are compared with experimental data. The calculated independent FPY YI(A) at the thermal energy reproduces the experimental data well, while the calculated IRs tend to be lower than the Ma...

To enhance the capabilities of the DANCE array, a new detector array NEUANCE was developed to enable simultaneous measurements of prompt fission neutrons and γ rays. NEUANCE was designed and constructed using 21 stilbene organic scintillator crystals. It was installed in the central cavity of the DANCE array. Signals from the 160 BaF 2 detectors of DANCE and the 21 detectors of NEUANCE were merged into a newly designed high-density high-throughput data acquisition system. The excellent pulse shape discrimination properties of stilbene enabled detection of neutrons with energy thresholds as low as 30–40 keVee. A fission reaction tagging method was developed using a NEUANCE γ -ray or neutron signal. The probability of detecting a neutron from the spontaneous fission of 252Cf using NEUANCE is ∼ 47%. New correlated data for prompt fission neutrons and prompt fission γ rays were obtained for 252 Cf using this high detection efficiency experimental setup. Average properties of prompt fission neutron emission as a function of prompt fission γ -ray quantities were also obtained, suggesting that neutron and γ -ray emission in fission are correlated.

Ideas, solely related on the nuclear shell model, fail to give an interpretation of the experimental central role of 54Xe in the asymmetric fission of actinides. The same is true for the β-delayed fission of 180Tl to 80Kr and 100Ru. The representation of the natural isotopes, in the Z-Neutron Excess plane, suggests the importance of the of the Neutron Excess evolution mode in the fragments of the asymmetric actinide fission and in the fragments of the β-delayed fission of 180Tl. The evolution mode of the Neutron Excess, hinged at Kr and Xe, is directed by the 50 and 82 neutron magic numbers. The present isotope representation offers a frame for the interpretation of the post fission evaporation of neutrons, higher for the AL compared to the AH fragments, a tenet in nuclear fission. Further enlightened is the functional meaning of the 50 proton magic number, marking the start of the yield rise of the AH fragments in actinide fission.

The CGMF code implements the Hauser-Feshbach statistical nuclear reaction model to follow the de-excitation of fission fragments by successive emissions of prompt neutrons and γ rays. The Monte Carlo technique is used to facilitate the analysis of complex distributions and correlations among the prompt fission observables. Starting from initial configurations for the fission fragments in mass, charge, kinetic energy, excitation energy, spin, and parity, Y(A,Z,KE,U,J, π ), CGMF samples neutron and γ-ray probability distributions at each stage of the decay process, conserving energy, spin and parity. Nuclear structure and reaction input data from the RIPL3 library are used to describe fission fragment properties and decay probabilities. Characteristics of prompt fission neutrons, prompt fission γ rays, and independent fission yields can be studied consistently. Correlations in energy, angle and multiplicity among the emitted neutrons and γ rays can be easily analyzed as a function of the emitting fragments. In its current version, CGMF is limited to certain spontaneous fission reactions on Pu and Cf isotopes, as well as neutron-induced fission reactions up to 20 MeV on U, Np, and Pu isotopes. Program summary Program Title: CGMF 1.1 CPC Library link to program files: https://doi.org/10.17632/jrtt73npzw.1 Licensing provisions: BSD-3 clause Programming language: C++ Nature of problem: Modeling of fission fragment decay Solution method: Monte Carlo implementation of the Hauser-Feshbach statistical theory of nuclear reactions to describe the de-excitation of fission fragments on an event-by-event basis. Additional comments including restrictions and unusual features: spontaneous fission reactions for 238,240,242,244Pu and 252,254Cf; neutron-induced fission reactions from thermal up to 20 MeV for n+233,234,235,238U, n+237Np, and n+239,241Pu. Binary fission only (no ternary fission).

The scission point model is improved by considering the excitation-dependent liquid drop model to calculate mass distributions for neutron-induced actinide nuclei fission. Excitation energy effects influence the deformations of light and heavy fragments. The improved scission point model shows a significant advance with regard to accuracy for calculating pre-neutron-emission mass distributions of neutron-induced typical actinide fission with incident-neutron-energies up to 99.5 MeV. The theoretical frame assures that the improved scission point model is suitable for evaluating the fission fragment mass distributions, which will provide guidance for studying fission physics and designing nuclear fission engineering and nuclear transmutation systems.