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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 $\mathtt{FREYA}$ 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 $\mathtt{FREYA}$ 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 $^{235}$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, $^{235 {\rm m}}$U($n_{\rm th}$,f), may enhance the symmetric yields and can thus result in higher neutron multiplicities for low total fragment kinetic energy., Comment: 11 pages, 9 figures

The transformation of an atomic nucleus into two excited fission fragments is modeled as a strongly damped evolution of the nuclear shape, until scission occurs at a small critical neck radius, at which point the mass, charge, and shape of each fragment are extracted. The available excitation energy then is divided statistically on the basis of the microscopic level densities. This approach takes account of the important (and energy-dependent) finite-size effects. After the fragments have been fully accelerated and their shapes have relaxed to their equilibrium form, they undergo sequential neutron evaporation. The dependence of the resulting mean neutron multiplicity on the fragment mass, $\bar{\nu}$(A), including the dependence on the initial excitation energy of the fissioning compound nucleus, is in good agreement with the observed behavior, as demonstrated here for $^{235}$U(n,f).

© 2017 American Physical Society. 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.

The event-by-event fission model 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 FREYA to these variables. We then compare the results to the available photon data from spontaneous and thermal neutron-induced fission., Comment: 25 pages, 31 figures, version as accepted in Phys. Rev. C

© 2016 American Physical Society. The dependence of correlations of neutron multiplicity ν and γ-ray multiplicity Mγ in spontaneous fission of Cf252 on fragment mass A∗ and total kinetic energy (TKE) have been investigated by employing the ratio of Mγ/ν and the form of Mγ(ν). We show for the first time that Mγ and ν have a complex correlation for heavy fragment masses, while there is a positive dependence of Mγ for light fragment masses and for near-symmetric mass splits. The ratio Mγ/ν exhibits strong shell effects for neutron magic number N=50 and near doubly magic number shell closure at Z=50 and N=82. The γ-ray multiplicity Mγ has a maximum for TKE=165-170 MeV. Above 170 MeV Mγ(TKE) is approximately linear, while it deviates significantly from a linear dependence at lower TKE. The correlation between the average neutron and γ-ray multiplicities can be partly reproduced by model calculations.

The dependence of correlations of neutron multiplicity {\nu} and gamma-ray multiplicity Mg in spontaneous fission of 252Cf on fragment mass A* and total kinetic energy TKE has been investigated employing the ratio of Mg/{\nu} and the form of Mg({\nu}). We show for the first time that Mg and {\nu} have a complex correlation for heavy fragment masses, while there is a positive dependence of Mg({\nu}) for light fragment masses and for near-symmetric mass splits. The ratio Mg/{\nu} exhibits strong shell effects for the neutron magic number N = 50 and near the doubly magic number shell closure at Z = 50 and N = 82. The gamma-ray multiplicity Mg has a maximum for TKE = 165-170 MeV. Above 170 MeV Mg(TKE) is approximately linear, while it deviates significantly from a linear dependence at lower TKE. The correlation between the average neutron and gamma-ray multiplicities can be partly reproduced by model calculations., Comment: 14 pages,8 figures

"Collision Dynamics" is part of the CBM Physics Book.

Employing a recently developed Monte Carlo model, we study the fission of 240Pu induced by neutrons with energies from thermal to just below the threshold for second chance fission. Current measurements of the mean number of prompt neutrons emitted in fission, together with less accurate measurements of the neutron energy spectra, place remarkably fine constraints on predictions of microscopic calculations. In particular, the total excitation energy of the nascent fragments must be specified to within 1 MeV to avoid disagreement with measurements of the mean neutron multiplicity. The combination of the Monte Carlo fission model with a statistical likelihood analysis also presents a powerful tool for the evaluation of fission neutron data. Of particular importance is the fission spectrum, which plays a key role in determining reactor criticality. We show that our approach can be used to develop an estimate of the fission spectrum with uncertainties several times smaller than current experimental uncertainties for outgoing neutron energies up to 2 MeV., Comment: 17 pages, 20 figures

The factorial moments of the pion multiplicity distributions are calculated with HIJING and UrQMD and found to be independent of the pT range included, in contrast to recent simulations with the linear sigma model which leads to large enhancements for pions with transverse kinetic energies below 200 MeV. This supports the use of the ratio of the factorial moments of low and high pT pions as a signal of ``new'' physics at low momentum scales, such as the formation of disoriented chiral condensates., 4 pages total, incl 4 eps figures (Randrup@LBL.gov)

The formation and evolution of compressed nuclear matter is studied in heavy ion collisions at relativistic energies. Neutral meson (pi(0), eta) yields from baryon resonance decays have been measured for a number of colliding systems and bombarding energies. The energy dependence of the meson yields suggests the formation of an equilibrated baryon system with an initial thermal energy of roughly 85% of the available energy. Collisions of resonances and mesons with nucleons cause absorption and redistribution of mesons in phase space. This results in an azimuthal asymmetry of pions. The energy spectra of pi(0) and eta mesons reveal a scaling with the transverse mass similar to observations at ultrarelativistic energies. This underlines the importance of individual resonances for the spectral distribution of mesons. The azimuthal dependence of baryon energy spectra reveals the influence of shadowing by spectator matter.

With a view towards implementation in microscopic transport simulations of heavy-ion collisions, the properties of spin-isospin modes are studied in nuclear matter consisting of nucleons and Delta isobars that interact by the exchange of pi and rho mesons. For a standard p-wave interaction and an effective g' short-range interaction, the dispersion relations for the spin- isospin modes, and the associated amplitudes, are calculated at various nuclear densities and temperatures, within the random-phase approximation. Quantities of physical interest are then extracted, including the total and partial Delta decay widths and the Delta cross sections in the nuclear medium. The self-consistent inclusion of the Delta width has a strong effect on the Delta cross sections at twice normal nuclear density, as compared with the result of ignoring the width. Generally, the obtained quantities exhibit a strong density dependence, but are fairly insensitive to the temperature, at least up to T=25 MeV. Finally, it is described how these in-medium effects may be consistently included into microscopic transport simulations of nuclear collisions, and the improvements over previous approaches are discussed., LaTeX 47 pages, 17 postscript figures in accompanying uuencoded file

Dynamic calculations of the spontaneous fission half-lives are performed for doubly even nuclei of actinide and transactinide regions. No adjustable parameters are introduced. Fairly good agreement with experiment is obtained. (14 refs).

Mass and charge distributions have been measured for damped projectile-like fragments in the reaction /sup 74/Ge+/sup 165/Ho at 8.5 MeV per nucleon bombarding energy. Coincidences were measured between angle-correlated reaction partners in order to derive the primary mass distribution and the excitation energy distribution for projectile-like fragments. The evolution of the primary N and Z distributions as a function of energy loss is found to deviate significantly from predictions of the nucleon exchange transport model. The fraction of excitation energy residing in the projectile-like fragment is shown to depend both upon energy loss and the direction of the nucleon flow.

International audience; Vaporization events, where all species have atomic numbers lower than 3, and deexcitation properties of quasi-projectiles involves in binary dissipative collisions between 36 Ar and 58 Ni are studied with the multidetector INDRA. Kinematical properties and chemical composition (mean values and variances) of vaporizing sources are derived over the excitation energy per nucleon range 8-28 MeV. These data are found in good agreement with the results of a model describing a gas of fermions and bosons in thermal and chemical equilibrium, which strongly suggests that thermodynamical equilibrium has been reached even for such sources produced in very extreme conditions of collisions. Finally, removing the constraint on atomic numbers lower than 3, the evolution of the chemical composition of quasi-projectiles is presented over the excitation energy range 0-25 AMeV.