In this work, we have calculated a complete set of primary fission fragment mass yields, Y(A), for heavy nuclei across the chart of nuclides, including those of particular relevance to the rapid ...neutron capture process (r process) of nucleosynthesis. We assume that the nuclear shape dynamics are strongly damped, which allows for a description of the fission process via Brownian shape motion across nuclear potential-energy surfaces. The macroscopic energy of the potential was obtained with the Finite-Range Liquid-Drop Model (FRLDM), while the microscopic terms were extracted from the single-particle level spectra in the fissioning system by the Strutinsky procedure for the shell energies and the BCS treatment for the pairing energies. For each nucleus considered, the fission fragment mass yield, Y(A), is obtained from 50 000 to 500 000 random walks on the appropriate potential-energy surface. The full mass and charge yield, Y(Z,A), is then calculated by invoking the Wahl systematics. With this method, we have calculated a comprehensive set of fission-fragment yields from over 3800 nuclides bounded by 80 ≤ Z ≤ 130 and A ≤ 330; these yields are provided as an ASCII formatted database in the Supplemental Material. We compare our yields to known data and discuss general trends that emerge in low-energy fission yields across the chart of nuclides.
FREYA (Fission Reaction Event Yield Algorithm) is a fission event generator which models complete fission events. As such, it automatically includes fluctuations as well as correlations between ...observables, resulting from conservation of energy and momentum. The purpose of this paper is to present the main differences between FREYA versions 1.0 and 2.0.2 : additional fissionable isotopes, angular momentum conservation, Giant Dipole Resonance form factor for the statistical emission of photons, improved treatment of fission photon emission using RIPL database, and dependence on the incident neutron direction. FREYA 2.0.2 has been integrated into the LLNL Fission Library 2.0.2, which has itself been integrated into MCNP6.2, TRIPOLI-4.10, and can be called from Geant4.10.
Program title:FREYA 2.0.2
Program Files doi: ” http://dx.doi.org/10.17632/2mssy7r3gt.1”
Licensing provisions: BSD 3-clause
Programming language: Fortran 90, C++
Journal Reference of previous version: J. M Verbeke, J. Randrup, R. Vogt, “Fission Reaction Yield Algorithm FREYA for Event-by-Event Simulation of Fission,” Comp. Phys. Comm. 191, pp. 178–202; doi:10.1016/j.cpc.2015.02.002 (2015).
Does the new version supersede the previous version? Yes.
Reasons for the new version: New physics and more fissionable isotopes.
Summary of revisions: Additional fissionable isotopes, angular momentum conservation, Giant Dipole Resonance form factor for the statistical emission of photons, improved treatment of fission photon emission using RIPL database, and dependence on the incident neutron direction.
Nature of problem: Modeling of fission events.
Solution method: Simulation of complete fission events, production of secondary fission fragments, fission neutrons and photons.
Restrictions: Restricted to spontaneous fission of 238U, 238Pu, 240Pu, 242Pu, 244Cm, 252Cf; neutron-induced fission of 233U, 235U, 238U, 239Pu, 241Pu, for incident neutron energies less than 20 MeV.
The correlation between baryon number and strangeness elucidates the nature of strongly interacting matter, such as that formed transiently in high-energy nuclear collisions. This diagnostic can be ...extracted theoretically from lattice QCD calculations and experimentally from event-by-event fluctuations. The analysis of present lattice results above the critical temperature severely limits the presence of qq bound states, thus supporting a picture of independent (quasi)quarks.
The role of angular momentum in fission has generated a great deal of attention recently. Recent data has shown that, while the fission fragment spins may be generated by highly correlated processes, ...the resulting measured fragment spins were shown to be largely uncorrelated. This proceedings will summarize some of the advances made with the fission simulation model FREYA which is well suited for studying the role of angular momentum in fission because it can easily simulate a variety of scenarios for generating fragment spin and determine the observational consequences.
.
Detailed information on the fission process can be inferred from the observation, modeling and theoretical understanding of prompt fission neutron and
γ
-ray observables. Beyond simple average ...quantities, the study of distributions and correlations in prompt data,
e.g.
, multiplicity-dependent neutron and
γ
-ray spectra, angular distributions of the emitted particles,
n
-
n
,
n
-
γ
, and
γ
-
γ
correlations, can place stringent constraints on fission models and parameters that would otherwise be free to be tuned separately to represent individual fission observables. The FREYA and CGMF codes have been developed to follow the sequential emissions of prompt neutrons and
γ
rays from the initial excited fission fragments produced right after scission. Both codes implement Monte Carlo techniques to sample initial fission fragment configurations in mass, charge and kinetic energy and sample probabilities of neutron and
γ
emission at each stage of the decay. This approach naturally leads to using simple but powerful statistical techniques to infer distributions and correlations among many observables and model parameters. The comparison of model calculations with experimental data provides a rich arena for testing various nuclear physics models such as those related to the nuclear structure and level densities of neutron-rich nuclei, the
γ
-ray strength functions of dipole and quadrupole transitions, the mechanism for dividing the excitation energy between the two nascent fragments near scission, and the mechanisms behind the production of angular momentum in the fragments, etc. Beyond the obvious interest from a fundamental physics point of view, such studies are also important for addressing data needs in various nuclear applications. The inclusion of the FREYA and CGMF codes into the MCNP6.2 and MCNPX - PoliMi transport codes, for instance, provides a new and powerful tool to simulate correlated fission events in neutron transport calculations important in nonproliferation, safeguards, nuclear energy, and defense programs. This review provides an overview of the topic, starting from theoretical considerations of the fission process, with a focus on correlated signatures. It then explores the status of experimental correlated fission data and current efforts to address some of the known shortcomings. Numerical simulations employing the FREYA and CGMF codes are compared to experimental data for a wide range of correlated fission quantities. The inclusion of those codes into the MCNP6.2 and MCNPX - PoliMi transport codes is described and discussed in the context of relevant applications. The accuracy of the model predictions and their sensitivity to model assumptions and input parameters are discussed. Finally, a series of important experimental and theoretical questions that remain unanswered are presented, suggesting a renewed effort to address these shortcomings.
Spinodal multifragmentation in nuclear physics is reviewed. Considering first spinodal instability within the general framework of thermodynamics, we discuss the intimate relationship between ...first-order phase-transitions and convexity anomalies in the thermodynamic potentials, clarify the relationship between mechanical and chemical instability in two-component systems, and also address finite systems. Then we analyze the onset of spinodal fragmentation by various linear-response methods. Using the Landau theory of collective modes in bulk matter as a starting point, we first review the application of mean-field methods for the identification of the unstable collective modes and the determination of their structure and the associated dispersion relations yielding their growth rates. Subsequently, the corresponding results for finite nuclei are addressed and, within the random-phase approximation, we establish the connection between unstable modes in dilute systems and giant resonances in hot nuclei. Then we turn to the temporal evolution of the unstable systems, discussing first how the dynamics changes its character from being initially linear towards being chaotic and then considering the growth of initially agitated instabilities within the framework of one-body dynamics. We review especially the body of work relating to the Boltzmann–Langevin model, in which the stochastic part of the residual two-body collisions provides a well-defined noise that may agitate the collective modes. We seek to assess the utility of various approximate treatments, including brownian one-body dynamics, and discuss the many possible refinements of the basic treatment. After these primarily formal or idealized studies, we turn to the applications to nuclear multifragmentation and review the various investigations of whether the bulk of the collision zone becomes spinodally unstable. Fragmentation studies with both many-body and stochastic one-body models are discussed and we address the emerging topic of isospin fractionation. We then make contact with experimental data which indicates that the spinodal region is being entered under suitable conditions and we discuss in particular recent results on multifragment size correlations that appear to present signals of spinodal fragmentation. It is demonstrated how various aspects of the data can be understood both qualitatively and quantitatively within the stochastic one-body framework, thus strongly suggesting that nuclear spinodal fragmentation indeed occurs. We finally outline perspectives for further advances on the topic and make connections to current progress on related issues.
The possibility that nucleosynthesis in neutron star mergers may reach fissioning nuclei introduces significant uncertainties in predicting the relative abundances of r-process material from such ...events. We evaluate the impact of using sets of fission yields given by the 2016 GEF code for spontaneous (sf), neutron-induced ((n, f)), and β-delayed (βdf) fission processes which take into account the approximate initial excitation energy of the fissioning compound nucleus. We further explore energy-dependent fission dynamics in the r process by considering the sensitivity of our results to the treatment of the energy sharing and de-excitation of the fission fragments using the FREYA code. We show that the asymmetric-to-symmetric yield trends predicted by GEF 2016 can reproduce the high-mass edge of the second r-process peak seen in solar data and examine the sensitivity of this result to the mass model and astrophysical conditions applied. We consider the effect of fission yields and barrier heights on the nuclear heating rates used to predict kilonova light curves. We find that fission barriers influence the contribution of 254Cf spontaneous fission to the heating at ∼100 d, such that a light curve observation consistent with such late-time heating would both confirm that actinides were produced in the event and imply the fission barriers are relatively high along the 254Cf β-feeding path. We lastly determine the key nuclei responsible for setting the r-process abundance pattern by averaging over thirty trajectories from a 1.2-1.4 M neutron star merger simulation. We show it is largely the odd-N nuclei undergoing (Z, N)(n, f) and (Z, N)βdf that control the relative abundances near the second peak. We find the 'hot spots' for β-delayed and neutron-induced fission given all mass models considered and show most of these nuclei lie between the predicted N = 184 shell closure and the location of currently available experimental decay data.
Excitation energy partition in fission Albertsson, M.; Carlsson, B.G.; Døssing, T. ...
Physics letters. B,
04/2020, Letnik:
803, Številka:
C
Journal Article
Recenzirano
Odprti dostop
The transformation of an atomic nucleus into two excited fission fragments is modeled as a strongly damped evolution of the nuclear shape. As in previous studies, it is assumed that the division of ...mass and charge is frozen in at a critical neck radius of c0=2.5fm. In order to also determine the energetics, we follow the system further until scission occurs at a smaller neck radius, at which point the shapes of the proto-fragments are extracted. The statistical energy available at scission is then divided on the basis of the respective microscopic level densities. This approach takes account of important (and energy-dependent) finite-size effects. After the fragments have been fully accelerated and their shapes have relaxed to their equilibrium forms, they undergo sequential neutron evaporation. The dependence of the resulting mean neutron multiplicity on the fragment mass, ν¯(A), including the dependence on the initial excitation energy of the fissioning compound nucleus, agrees reasonably well with observations, as demonstrated here for 235U(n,f), and the sawtooth appearance of ν¯(A) can be understood from shell-structure effects in the level densities.
The CBM Physics Book Friman, Bengt; Höhne, Claudia; Leupold, Stefan ...
2011, 20110217, 2014-07-30, Letnik:
814
eBook, Book
This exhaustive survey is the product of a four-year effort by leading researchers in the field to produce both a readable introduction and a yardstick for the upcoming experiments using heavy ion ...collisions that will examine the properties of nuclear matter.
Background: The role of angular momentum in fission has long been discussed but the observable effects are difficult to quantify. Purpose: Here we discuss a variety of effects associated with angular ...momentum in fission and present quantitative illustrations. Methods: We employ the fission simulation model freya , which is well suited for this purpose because it obeys all conservation laws, including linear and angular momentum conservation at each step of the process. We first discuss the implementation of angular momentum in freya and then assess particular observables, including various correlated observables. We also study potential effects of neutron-induced fission of the low-lying isomeric state of U 235 relative to the ground state. Results: The fluctuations inherent in the fission process ensure that the spin of the initial compound nucleus has only a small influence on the fragment spins, which are therefore nearly uncorrelated. There is a marked correlation between the spin magnitude of the fission fragments and the photon multiplicity. We also consider the dynamical anisotropy caused by the rotation of an evaporating fragment and study especially the distribution of the projected neutron-neutron opening angles, showing that while it is dominated by the effect of the evaporation recoils, it is possible to extract the signal of the dynamical anisotropy by means of a Fourier decomposition. Finally, we note that the use of an isomeric target, U 235 m ( nth ,f), may enhance the symmetric yields and can thus result in higher neutron multiplicities for low total fragment kinetic energies. Conclusions: While the initial angular momentum of the fissioning nucleus tends to have little effect on the observables, those of the produced fragments influence the emitted neutrons and photons in a significant and correlated manner which may be exploited experimentally to elucidate the fission process.
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