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Nuclear fission produces fragments whose spins are coupled to the relative angular motion via angular momentum conservation. It is shown how ensembles of such spins can readily be obtained by either direct microcanonical sampling or by sampling of the associated normal modes of rotation. The resulting distribution of the spin-spin opening angle is illustrated in various three- and two- dimensional scenarios and it is demonstrated how recent mutually conflicting model calculations can be well reproduced with different assumptions about the scission geometry., 5 pages, 5 figures

We discuss how the measurement of photon angular correlations can reveal information about the orientation of the fission fragment angular momenta. Photons from identified stretched E2 collective transitions in even-even fission product nuclei are particularly suitable because they do not affect the orientation of the nuclear spin. Their angular distribution relative to the direction of a fission fragment may reveal the orientation of the fragment spins relative to the fission axis. A novel means of probing the correlated fission fragment spins is the distribution of the opening angle between E2 photons from even-even partner fragments which reveals the mutual correlation of the fragment spins, if the photon helicities can be determined, demonstrating the potential power of helicity measurements in fission.

As a fissioning nucleus approaches scission, the angular-momentum bearing modes in the evolving dinuclear complex may be agitated by multiple transfers of individual nucleons. It is discussed how this mechanism populates the various rotational modes at different rates and leads to fragment angular momenta that are preferentially perpendicular to the fission axis but mutually largely uncorrelated. Using the fission simulation code FREYA, it is demonstrated how a measurement of the angular distribution of photons from identified collective transitions in the product nuclei can provide quantitative information on the relative importance of the twisting mode in fission.

The emission of neutrons and $\ensuremath{\gamma}$ rays by fission fragments reveal important information about the properties of fragments immediately following scission. The initial fragment properties, correlations between fragments, and emission competition give rise to correlations in neutron-$\ensuremath{\gamma}$ emission. Recent theoretical and experimental advances have been proposed to explain the mechanism of angular momentum generation in fission, which would result in observable signature in neutron-$\ensuremath{\gamma}$ emission correlations. In this paper, we present a novel analysis method of neutrons and $\ensuremath{\gamma}$ rays emitted by fission fragments that allows us to discern structure in the observed correlations. We have analyzed data collected on $^{252}\mathrm{Cf}$(sf) at the Chi-Nu array at the Los Alamos Neutron Science Center. Through our analysis of the energy-differential neutron-$\ensuremath{\gamma}$ multiplicity covariance, we have observed enhanced neutron-$\ensuremath{\gamma}$ correlations, corresponding to rotational band $\ensuremath{\gamma}$-ray transitions, at $\ensuremath{\gamma}$-ray energies of 0.7 and 1.2 MeV. To shed light on the origin of this structure, we compare the experimental data with the predictions of three model calculations. The origin of the observed correlation structure is understood in terms of a positive spin-energy correlation in the generation of angular momentum in fission.

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

Prompt fission $\ensuremath{\gamma}$ rays (PFGs) resulting from the $^{240}\mathrm{Pu}(d,p\mathrm{f})$ reaction have been measured as a function of fissioning nucleus excitation energy ${E}_{\text{x}}$ at the Oslo Cyclotron Laboratory. We study the average total PFG multiplicity per fission, the average total PFG energy released per fission, and the average PFG energy. No significant changes in these characteristics are observed over the range $5.75l{E}_{\text{x}}l8.25$ MeV. The physical implications of this result are discussed. The experimental results are compared to simulations conducted using the computational fission model FREYA. We find that FREYA reproduces the experimental PFG characteristics within $8%$ deviation across the ${E}_{\text{x}}$ range studied. Previous excitation energy-dependent PFG measurements conducted below the second-chance fission threshold have large uncertainties, but are generally in agreement with our results within a $2\ensuremath{\sigma}$ confidence interval. However, both a published parametrization of the PFG energy dependence and the most recent PFG evaluation included in ENDF/B-VIII.0 were found to poorly describe the PFG excitation-energy dependence observed in this and previous experiments.

Background: The role of angular momentum in fission has long been discussed but the observable effects are difficult to quantify.Purpose: 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 $^{235}\mathrm{U}$ 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, $^{235\mathrm{m}}\mathrm{U}$(${n}_{\mathrm{th}}$,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.

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

We calculate neutron multiplicities from fission fragments with specified mass numbers for events having a specified total fragment kinetic energy. The shape evolution from the initial compound nucleus to the scission configurations is obtained with the Metropolis walk method on the five-dimensional potential-energy landscape, calculated with the macroscopic-microscopic method for the three-quadratic-surface shape family. Shape-dependent microscopic level densities are used to guide the random walk, to partition the intrinsic excitation energy between the two proto-fragments at scission, and to determine the spectrum of the neutrons evaporated from the fragments. The contributions to the total excitation energy of the resulting fragments from statistical excitation and shape distortion at scission is studied. Good agreement is obtained with available experimental data on neutron multiplicities in correlation with fission fragments from $^{235}$U(n$_{\rm th}$,f). At higher neutron energies a superlong fission mode appears which affects the dependence of the observables on the total fragment kinetic energy., Comment: 12 pages, 10 figures

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

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 $^{235}$U(n,f), and the sawtooth appearance of ν¯(A) can be understood from shell-structure effects in the level densities.

Fission-fragment mass and total-kinetic-energy (TKE) distributions following fission of even-even nuclides in the region $74 \leq Z \leq 126$ and $92 \leq N \leq 230$, comprising 896 nuclides have been calculated using the Brownian shape-motion method. The emphasis is the region of superheavy nuclei. To show compatibility with earlier results the calculations are extended to include earlier studied regions. An island of asymmetric fission is obtained in the superheavy region, $106\leq Z\leq114$ and $162\leq N\leq 176$, where the heavy fragment is found to be close to $^{208}$Pb and the light fragment adjusts accordingly. Most experimentally observed $\alpha$-decay chains of superheavy nuclei with $Z > 113 $ terminate by spontaneous fission in our predicted region of asymmetric fission. In these cases, the pronounced large asymmetry is accompanied by a low TKE value compatible with measurements., Comment: 6 pages, 10 figures, published version in EPJA

Neutron star mergers (NSMs) are the first verified sites of rapid neutron capture (r-process) nucleosynthesis, and could emit gamma rays from the radioactive isotopes synthesized in the neutron-rich ejecta. These MeV gamma rays may provide a unique and direct probe of the NSM environment as well as insight into the nature of the r process, just as observed gammas from the 56Ni radioactive decay chain provide a window into supernova nucleosynthesis. In this work, we include the photons from fission processes for the first time in estimates of the MeV gamma-ray signal expected from an NSM event. We consider NSM ejecta compositions with a range of neutron richness and find a dramatic difference in the predicted signal depending on whether or not fissioning nuclei are produced. The difference is most striking at photon energies above ~3.5 MeV and at a relatively late time, several days after the merger event, when the ejecta is optically thin. We estimate that a Galactic NSM could be detectable by a next generation gamma-ray detector such as AMEGO in the MeV range, up to ~10^4 days after the merger, if fissioning nuclei are robustly produced in the event., Comment: 9 pages, 5 figures, v2 matches version to appear in ApJL

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.

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 . New version program summary 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 238 U, 238 Pu, 240 Pu, 242 Pu, 244 Cm, 252 Cf; neutron-induced fission of 233 U, 235 U, 238 U, 239 Pu, 241 Pu, for incident neutron energies less than 20 MeV.

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 -- 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 3,800 nuclides bounded by $80\leq Z \leq 130$ and $A\leq330$; 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., 22 pages, 15 figures

Within the framework of $\mathtt{FREYA}$ event-by-event fission simulations, we study the sensitivity of various neutron observables to the input yield function $Y(A,Z,\mathrm{TKE})$ on which the fission event sampling is based. We first perform a statistical analysis of the available fission data in order to determine the distribution of possible yield functions $Y(A,Z,\mathrm{TKE})$ and we then construct a sample of 15 000 such yield functions. For each of these, $\mathtt{FREYA}$ is used to generate one million fission events, leading to a corresponding ensemble of fission observables, including the neutron multiplicity distribution and its factorial moments, the neutron energy spectrum, and the neutron-neutron angular correlation. This procedure allows us to study the sensitivity of those neutron observables to the uncertainty in the experimental data. Particular attention is given to the pronounced anticorrelation between the mean neutron multiplicity $\overline{\ensuremath{\nu}}$ and the mean total fragment kinetic energy $\overline{\mathrm{TKE}}$. Because the former observable is very well determined experimentally, it is possible to exploit that inherent anticorrelation to derive a significantly stricter tolerance on $\overline{\mathrm{TKE}}$. In addition, we study the sensitivity to the various $\mathtt{FREYA}$ parameters and we introduce a method for determining a $A$-dependent $x$ parameter, $x(A)$, based on the measured $A$-dependent neutron multiplicity, $\overline{\ensuremath{\nu}}(A)$.

Fission-fragment mass and total-kinetic-energy distributions following fission of the fermium isotopes $^{256,258,260}\text{Fm}$ at low excitation energy have been calculated using the Brownian shape-motion model. A transition from asymmetric fission in $^{256}\text{Fm}$ to symmetric fission in $^{258}\text{Fm}$ is obtained. The total-kinetic-energy distributions for the three isotopes show radically different behaviour due to varying contributions from different fission modes, with a double-humped distribution for $^{258}\text{Fm}$., 2 pages, 3 figures, Contribution to the Proceedings of the IV International Conference on Nuclear Structure and Dynamics (NSD2019), Venice, Italy, 2019

Possible observables of baryon number clustering due to the instabilities occurring at a first order QCD phase transition are discussed. The dynamical formation of baryon clusters at a QCD phase transition can be described by numerical fluid dynamics, augmented with a gradient term and an equation of state with a mechanically unstable region. It is shown that the dynamical description of this phase transition, in nuclear collisions, will lead to the formation of dense baryon clusters at the phase boundary. State-of-the-art machine learning methods find that the coordinate space clumping leaves characteristic imprints on the spatial net density distribution in almost every event. On the other hand the momentum distributions do not show any clear event-by-event features. It is shown that the 'third order' cumulant, the skewness, shows a peak at the beam energy where the system, created in the heavy ion collision, reaches the deconfinement phase transition.

For many years, the state of the art for simulating fission in transport codes amounted to sampling from average distributions. However, such "average" fission models have limited capabilities. Energy is not explicitly conserved and no correlations are available because all particles are emitted independently. However, in a true fission event, the emitted particles are correlated. Recently, Monte Carlo codes generating complete fission events have been developed, thus allowing the use of event-by-event analysis techniques. Such techniques are particularly useful because the complete kinematic information is available for the fission products and the emitted neutrons and photons. It is therefore possible to extract any desired observables, including correlations. The fast event-by-event fission code FREYA (Fission Reaction Event Yield Algorithm) generates large samples of complete fission events, employing only a few physics-based parameters. A recent optimization of these parameters for the isotopes in FREYA that undergo spontaneous fission is described and results are presented. The sensitivity of neutron observables in FREYA to the input yield functions is also discussed and the correlation between the average neutron multiplicity and fragment total kinetic energy is quantified.

Neutrons and photons are characteristically emitted during the nuclear fission process when a deformed, neutron-rich nucleus divides into two fragments that then deexcite. During deexcitation, neutrons are emitted first, followed by photons; this process gives rise to correlated emissions. Few data exist on event-by-event neutron-photon correlation. In this work, $^{252}\mathrm{Cf}(\mathrm{s}\mathrm{f})$ neutron and photon correlations were measured with an array of 45 liquid organic scintillation detectors and a fission chamber. The measured correlations are compared with MCPNX-PoliMi simulations using the built-in model and two event-by-event fission models, CGMF and FREYA, which predict correlations in prompt emissions from fission. Experimental results suggest weak neutron-photon competition during fragment deexcitation.

We evaluate the impact of using sets of fission yields given by the GEF code for spontaneous (sf), neutron-induced ((n,f)), and beta-delayed (betadf) 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 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 days, 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 beta-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_odot neutron star merger simulation. We show it is largely the odd-N nuclei undergoing (Z,N)(n,f) and (Z,N)betadf that control the relative abundances near the second peak. We find the "hot spots" for beta-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.

The event-by-event fission model $\mathtt{FREYA}$ has been improved, in particular to address deficiencies in the calculation of photon observables. We discuss the improvements that have been made and introduce several new variables, some detector dependent, that affect the photon observables. We show the sensitivity of $\mathtt{FREYA}$ to these variables. We then compare the results to the available photon data from spontaneous and thermal neutron-induced fission.

From nuclear materials accountability to detection of special nuclear material, SNM, the need for better modeling of fission has grown over the past decades. Current radiation transport codes compute average quantities with great accuracy and performance, but performance and averaging come at the price of limited interaction-by-interaction modeling. For fission applications, these codes often lack the capability of modeling interactions exactly: energy is not conserved, energies of emitted particles are uncorrelated, prompt fission neutron and photon multiplicities are uncorrelated. Many modern applications require more exclusive quantities than averages, such as the fluctuations in certain observables (e.g. the neutron multiplicity) and correlations between neutrons and photons. The new computational model, FREYA (Fission Reaction Event Yield Algorithm), aims to meet this need by modeling complete fission events. Thus it automatically includes fluctuations as well as correlations resulting from conservation of energy and momentum. FREYA has been integrated into the LLNL Fission Library, and will soon be part of MCNPX2.7.0 , MCNP6 , TRIPOLI-4.9 , and Geant4.10 . Program summary Program title: FREYA 1.0 Catalogue identifier: AEVS_v1_0 Program summary URL: http://cpc.cs.qub.ac.uk/summaries/AEVS_v1_0.html Program obtainable from: CPC Program Library, Queen’s University, Belfast, N. Ireland Licensing provisions: Special LLNL licence No. of lines in distributed program, including test data, etc.: 187792 No. of bytes in distributed program, including test data, etc.: 2585089 Distribution format: tar.gz Programming language: Fortran 90, C++. Computer: Any computer with a Fortran 90 and a C++ compiler, tested with (a) Intel Xeon CPU X5660, 2.8 GHz, 48 GB RAM, (b) Intel(R) Xeon(R) CPU E5-2620, 2 GHz, 64 GB RAM, (c) Intel Xeon CPU W3520, 2.67 GHz, 6 GB RAM. Operating system: (a) Red Hat Enterprise Linux Server release 6.5 (GNU Fortran and g++ (GCC) 4.4.7 20120313 (Red Hat 4.4.7-4)), (b) CentOS release 6.4 (GNU Fortran and g++ (GCC) 4.4.7 20120313 (Red Hat 4.4.7-3)), (c) MacBook Pro OSX 10.6.8 (GNU Fortran and g++-mp-4.7 (MacPorts gcc47 4.7.3 5) 4.7.3). RAM: 6 GB Classification: 17.8. 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 238 U, 240 Pu, 244 Cm, 252 Cf; neutron-induced fission of 233 U, 235 U, 239 Pu, for incident neutron energies less than 20 MeV. Running time: 8 s for 1M events

The event-by-event fission model FREYA generates large samples of complete fission events from which any observable can extracted, including fluctuations of the observables and the correlations between them. We describe here how FREYA was recently refined to include angular momentum throughout. Subsequently we present some recent results for both neutron and photon observables.

Author(s): Clarke, SD; Wieger, BM; Enqvist, A; Vogt, R; Randrup, J; Haight, RC; Lee, HY; Perdue, BA; Kwan, E; Wu, CY; Henderson, RA; Pozzi, SA | Abstract: For the first time, the complete neutron multiplicity distribution has been measured from the photofission of U235 induced by high-energy spallation γ rays arriving ahead of the neutron beam at the Los Alamos Neutron Science Center. The resulting average neutron multiplicity 3.80±0.08 (stat.) neutrons per photofission is in general agreement with previous measurements. In addition, unique measurements of the prompt fission energy spectrum of the neutrons from photofission and the angular correlation of two-neutron energies emitted in photofission also were made. The results are compared to calculations with the complete event fission model freya.

© 2017 American Physical Society. By combining microscopically calculated level densities with the Metropolis walk method, we develop a consistent framework for treating the energy and angular-momentum dependence of the nuclear shape evolution in the fission process. For each nucleus under consideration, the level density is calculated microscopically for each of more than five million shapes with a recently developed combinatorial method. The method employs the same single-particle levels as those used for the extraction of the pairing and shell contributions to the macroscopic-microscopic potential-energy surface. Containing no new parameters, the treatment is suitable for elucidating the energy dependence of the dynamics of warm nuclei on pairing and shell effects. It is illustrated for the fission fragment mass distribution for several uranium and plutonium isotopes of particular interest.

Detailed information on the fission process can be inferred from the observation, modeling and theoretical understanding of prompt fission neutron and $\gamma$-ray~observables. Beyond simple average quantities, the study of distributions and correlations in prompt data, e.g., multiplicity-dependent neutron and \gray~spectra, angular distributions of the emitted particles, $n$-$n$, $n$-$\gamma$, and $\gamma$-$\gamma$~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 $\gamma$-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 $\gamma$~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 $\gamma$-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. (See text for full abstract.), Comment: 39 pages, 57 figure files, published in Eur. Phys. J. A, reference added this version

We discuss the effect of spinodal instabilities on the fluctuations of conserved flavors in nuclear collisions at NICA. We find that, when the system undergoes a phase transformation, baryon number clumping due to the mechanical instabilities in the spinodal phase occurs. This dynamical clumping enhances the cumulants of the net baryon number residing in a finite test volume of the total collision system.

The dependence of correlations of neutron multiplicity $\ensuremath{\nu}$ and $\ensuremath{\gamma}$-ray multiplicity ${M}_{\ensuremath{\gamma}}$ in spontaneous fission of $^{252}\mathrm{Cf}$ on fragment mass ${A}^{*}$ and total kinetic energy (TKE) have been investigated by employing the ratio of ${M}_{\ensuremath{\gamma}}/\ensuremath{\nu}$ and the form of ${M}_{\ensuremath{\gamma}}(\ensuremath{\nu})$. We show for the first time that ${M}_{\ensuremath{\gamma}}$ and $\ensuremath{\nu}$ have a complex correlation for heavy fragment masses, while there is a positive dependence of ${M}_{\ensuremath{\gamma}}$ for light fragment masses and for near-symmetric mass splits. The ratio ${M}_{\ensuremath{\gamma}}/\ensuremath{\nu}$ exhibits strong shell effects for neutron magic number $N=50$ and near doubly magic number shell closure at $Z=50$ and $N=82$. The $\ensuremath{\gamma}$-ray multiplicity ${M}_{\ensuremath{\gamma}}$ has a maximum for $\text{TKE}=165--170$ MeV. Above 170 MeV ${M}_{\ensuremath{\gamma}}$(TKE) is approximately linear, while it deviates significantly from a linear dependence at lower TKE. The correlation between the average neutron and $\ensuremath{\gamma}$-ray multiplicities can be partly reproduced by model calculations.

Spinodal amplification provides a mechanism by which the spatial symmetry of an unstable system is broken as it spontaneously separates into its thermodynamically coexisting phases. This general phenomenon has been exploited to signal the nuclear liquid-gas phase structure in nuclear collision experiments at medium energies and it may also be of use in relativistic collisions for exploring the expected first-order confinement transition.

This paper reports on initial comparisons between the LANL CGMF and LBNL/LLNL FREYA codes, which both aim at computing prompt fission neutrons and gammas. While the methodologies used in both codes are somewhat similar, the detailed implementations and physical assumptions are different. We are investigating how some of these differences impact predictions.

Working within the Langevin framework of nuclear shape dynamics, we study the dependence of the evolution on the degree of excitation. As the excitation energy of the fissioning system is increased, the pairing correlations and the shell effects diminish and the effective potential-energy surface becomes ever more liquid-drop like. This feature can be included in the treatment in a formally well-founded manner by using the local level densities as a basis for the shape evolution. This is particularly easy to understand and implement in the Metropolis treatment where the evolution is simulated by means of a random walk on the five-dimensional lattice of shapes for which the potential energy has been tabulated. Because the individual steps between two neighboring lattice sites are decided on the basis of the ratio of the statistical weights, what is needed is the ratio of the local level densities for those shapes, evaluated at the associated local excitation energies. For this purpose, we adapt a recently developed combinatorial method for calculating level densities which employs the same single-particle levels as those that were used for the calculation of the pairing and shell contributions to the macroscopic-microscopic deformation-energy surface. For each nucleus under consideration, the level density (for a fixed total angular momentum) is calculated microscopically for each of the over five million shapes given in the three-quadratic-surface parametrization. This novel treatment, which introduces no new parameters, is illustrated for the fission fragment mass distributions for selected uranium and plutonium cases.

The event-by-event fission model FREYA is employed to study photon observables. The model has been expanded beyond the simple statistical photon emission reported previously to include the discrete RIPL-3 lines. We update the earlier results and discuss the sensitivity of the results to the FREYA input parameters.

The interest in hot and baryon-dense matter has recently intensified because of the prospects for exploring this phase region by a downgraded RHIC and the future FAIR. The baryon excess, the spatial irregularity of the system and its rapid evolution, and particularly the presence of a phase transition, pose interesting theoretical challenges that must be addressed in the development of the transport models required for the planning and interpretation of the experiments.

Background: In the seminal experiment by Schmidt et al. [Nucl. Phys. A 665, 221 (2000)] in which fission-fragment charge distributions were obtained for 70 nuclides, asymmetric distributions were seen above nucleon number $A\ensuremath{\approx}226$ and symmetric ones below. Because asymmetric fission had often loosely been explained as a preference for the nucleus to always exploit the extra binding of fragments near $^{132}\mathrm{Sn}$ it was assumed that all systems below $A\ensuremath{\approx}226$ would fission symmetrically because available isotopes do not have a proton-to-neutron $Z/N$ ratio that allows division into fragments near $^{132}\mathrm{Sn}$. But the finding by Andreyev et al. [Phys. Rev. Lett. 105, 252502 (2010)] did not conform to this expectation because the compound system $^{180}\mathrm{Hg}$ was shown to fission asymmetrically. It was suggested that this was a new type of asymmetric fission, because no strong shell effects occur for any possible fragment division.Purpose: We calculate a reference database for fission-fragment mass yields for a large region of the nuclear chart comprising 987 nuclides. A particular aim is to establish whether $^{180}\mathrm{Hg}$ is part of a contiguous region of asymmetric fission, and if so, its extent, or if not, in contrast to the actinides, there are scattered smaller groups of nuclei that fission asymmetrically in this area of the nuclear chart.Methods: We use the by now well benchmarked Brownian shape-motion method and perform random walks on the previously calculated five-dimensional potential-energy surfaces. The calculated shell corrections are damped out with energy according to a prescription developed earlier.Results: We have obtained a theoretical reference database of fission-fragment mass yields for 987 nuclides. These results show an extended region of asymmetric fission with approximate extension $74\ensuremath{\le}Z\ensuremath{\le}85$ and $100\ensuremath{\le}N\ensuremath{\le}120$. The calculated yields are highly variable. We show 20 representative plots of these variable features and summarize the main aspects of our results in terms of ``nuclear-chart'' plots showing calculated degrees of asymmetry versus $N$ and $Z$.Conclusions: Experimental data in this region are rare: only ten or so yield distributions have been measured, some with very limited statistics. We agree with several measurements with higher statistics. Regions where there might be differences between our calculated results and measurements lie near the calculated transition line between symmetric and asymmetric fission. To draw more definite conclusions about the accuracy of the present implementation of the Brownian shape-motion approach in this region experimental data, with reliable statistics, for a fair number of suitably located additional nuclides are clearly needed. Because the nuclear potential-energy structure is so different in this region compared to the actinide region, additional experimental data together with fission theory studies that incorporate additional, dynamical aspects should provide much new insight.

The production yield of exotic pentaquarks in high-energy nuclear collisions is estimated on the basis of the standard statistical description of the hadronic freeze-out.

The various dynamical models for fragment formation in nuclear collisions are discussed in order to bring out their relative advantages and shortcomings. After discussing the general requirements for dynamical models that aim to describe fragment formation, we consider the various mean-field models that incorporate fluctuations and then turn to models based on molecular dynamics.

Nuclear fission generates neutrons whose correlations may reveal at which stage they appear during the fission process, whether from the fully formed fission fragments or from the scission process itself. The authors study the sensitivity of the neutron-neutron angular correlations to the parameters of their fission model, and they find that the available experimental data can be predicted without the need for any scission neutrons.

We summarize recent progress in utilizing the spinodal decomposition mechanism to probe the liquid-gas phase transition in nuclear matter, which occurs at subsaturation densities and moderate temperatures, and then discuss the prospects for exploiting this mechanism as a probe of the transition from a quark-gluon plasma to a hadron gas.

High-energy nuclear collisions produce quark-gluon plasmas that expand and hadronize. If the associated phase transition is of first order then the hadronization should proceed through a spinodal phase separation. We explore here the possibility of identifying the associated clumping by analysis of suitable N-particle momentum correlations.

It is expected that chiral symmetry is approximately restored in the very excited matter produced in a high-energy nuclear collision and that the chiral order parameter will thereafter quickly relax towards its aligned vacuum value. This non-equilibrium evolution of the environment endows the pionic degrees of freedom with a time-dependent effective mass and this agency, in turn, may create correlated soft pions that could provide specific observable signatures of the chiral dynamics that are based solely on charged pions.

Within the framework of the fission event generator FREYA, we are studying the sensitivity of various neutron observables to the yield distribution Y (A ,Z ,TKE) used as input to the code. Concentrating on spontaneous fission of 252 Cf, we have sampled a large number of different input yield functions based on χ2 fits to the experimental data on Y (A ) and Y (TKE|A ). For each of these input yield distributions, we then use FREYA to generate a large sample of complete fission events from which we extract a variety of neutron observables, including the multiplicity distribution, the associated correlation coefficients, and its factorial moments, the dependence of the mean neutron multiplicity on the total fragment kinetic energy TKE and on the fragment mass number A , the neutron energy spectrum, and the two-neutron angular correlation function. In this way, we can determine the variation of these observables resulting from the uncertainties in the experimental mesurements. The imposition of a constraint on the resulting mean neutron multiplicity reduces the variation of the calculated neutron observables and provides a means for shrinking the uncertainties associated with the measured data.