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Prompt-fission-neutron spectra from $^{239}\mathrm{Pu}$ ($\mathit{n},\phantom{\rule{0.16em}{0ex}}\mathit{f}$) were measured with respect to $^{252}\mathrm{Cf}$ spontaneous fission for incident neutron energies from 0.7 to $700\phantom{\rule{0.16em}{0ex}}\mathrm{MeV}$ at the Weapons Neutron Research facility (WNR) of the Los Alamos Neutron Science Center. A newly designed high-efficiency fission chamber was coupled to the highly segmented Chi-Nu neutron liquid scintillator array to detect neutrons emitted in fission events. The double time-of-flight technique was used to deduce the incident neutron energies from the spallation target and the outgoing-neutron energies from the fission chamber. Prompt-fission-neutron spectra (PFNS) were measured with respect to $^{252}\mathrm{Cf}$ spontaneous fission down to $200\phantom{\rule{0.16em}{0ex}}\mathrm{keV}$ and up to about $12\phantom{\rule{0.16em}{0ex}}\mathrm{MeV}$ for all the incident neutron energies with typical total uncertainties well below $2%$ up to about 7-MeV outgoing-neutron energy. The general trend of PFNS is well reproduced by JEFF3.3 and ENDF evaluations, although a better agreement is found with JEFF3.3. Discrepancies were observed for the low-energy part of the spectra, especially around the opening of the second-, third- and fourth-chance fission. Neutron average kinetic energies as a function of incident neutron energy are obtained experimentally with reported total uncertainties below $0.5%$. The measured values agree with the most recent data. The trend is fairly well reproduced by the JEFF3.3 evaluation, although it fails to reproduce the experimental values within their uncertainties.

International audience; In nuclear thermodynamics, the determination of the excitation energy of hot nuclei is a fundamental experimental problem. Instrumental physicists have been trying to solve this problem for several years by building the most exhaustive 4π detector arrays and perfecting their calorimetry techniques. In a recent paper, a proposal for a new calorimetry, called “3D calorimetry”, was made. It tries to optimize the separation between the particles and fragments emitted by the quasiprojectile and the other possible contributions. This can be achieved by determining the experimental probability for a given nucleus of a nuclear reaction to be emitted by the quasiprojectile. It has been developed for the INDRA data. In the present work, we wanted to dissect and validate this new method of characterization of a hot quasiprojectile. So we tried to understand and control it completely to determine these limits. Using the Heavy Ion Phase Space Exploration (HIPSE) event generator and a software simulating the functioning of INDRA, we were able to achieve this goal and provide a quantitative estimation of the quality of the quasiprojectile characterization.

Prompt-fission $\ensuremath{\gamma}$-ray spectra (PFGS) have been measured for the $^{238}\mathrm{U}(n,f$) reaction using fast neutrons produced by the LICORNE directional neutron source. Fission events were detected with an ionization chamber containing actinide samples placed in the neutron beam, and the coincident prompt-fission $\ensuremath{\gamma}$ rays were measured using a number of ${\mathrm{LaBr}}_{3}$ scintillation detectors and a cluster of nine phoswich detectors from the PARIS array. Prompt-fission $\ensuremath{\gamma}$ rays (PFGs) were discriminated from prompt-fission neutrons using the time-of-flight technique over distances of around 35 cm. PFG emission spectra were measured at two incident neutron energies of 1.9 and 4.8 MeV for $^{238}\mathrm{U}(n,f$) and also for $^{252}\mathrm{Cf}(sf$) as a reference. Spectral characteristics of PFG emission, such as mean $\ensuremath{\gamma}$ multiplicity and average total $\ensuremath{\gamma}$-ray energy per fission, as well as the average $\ensuremath{\gamma}$-ray energy, were extracted. The sensitivity of these results to the width of the time window and the type of spectral unfolding procedure used to correct for the detector responses was studied. Iteration methods were found to be more stable in low-statistics data sets. The measured values at ${E}_{n}=1.9\phantom{\rule{4.pt}{0ex}}\text{MeV}$ were found to be the mean $\ensuremath{\gamma}$ multiplicity ${\overline{M}}_{\ensuremath{\gamma}}=6.54\ifmmode\pm\else\textpm\fi{}0.19$, total released energy per fission ${E}_{\ensuremath{\gamma},\mathrm{tot}}=5.25\ifmmode\pm\else\textpm\fi{}0.20$ MeV, and the average $\ensuremath{\gamma}$-ray energy ${\ensuremath{\epsilon}}_{\ensuremath{\gamma}}=0.80\ifmmode\pm\else\textpm\fi{}0.04$ MeV. Under similar conditions, the values at ${E}_{n}=4.8\phantom{\rule{4.pt}{0ex}}\text{MeV}$ were measured to be ${\overline{M}}_{\ensuremath{\gamma}}=7.31\ifmmode\pm\else\textpm\fi{}0.46, {E}_{\ensuremath{\gamma},\mathrm{tot}}=6.18\ifmmode\pm\else\textpm\fi{}0.65$ MeV, and ${\ensuremath{\epsilon}}_{\ensuremath{\gamma}}=0.84\ifmmode\pm\else\textpm\fi{}0.11$ MeV.

Background: The isovector part of the nuclear equation of state remains partly unknown and is the subject of many studies. The degree of equilibration between the two main collision partners in heavy ion reactions may be used to study the equation of state since it is connected to isospin (N/Z) transport properties of nuclear matter. Purpose: We aim to test chemical equilibrium attainment by measuring isotopic characteristics of emitted elements as a function of impact parameter. Method: We study four Xe136,124+Sn124,112 reactions at 32 MeV/nucleon. The data were acquired with the INDRA detector at the GANIL (Caen, France) facility. Combined (projectile+target) systems are identical for two studied reactions, therefore it is possible to study the path towards chemical equilibrium from different neutron to proton ratio (N/Z) entrance channels. The study is limited to identified isotopes detected in the forward part of the center of mass in order to focus on the evolution of projectile-like fragment isotopic content and the benefit of excellent detection performances of the forward part of the apparatus. Results: Light charged particle productions, multiplicities, and abundance ratios dependence against impact parameter are studied. It is measured to almost identical mean characteristics for the two Xe124+Sn124 and Xe136+Sn112 systems for central collisions. Comparing all four studied systems it is shown that mean values evolve from projectile N/Z to projectile+target N/Z dependence. Those identical mean characteristics concern all light charged particles except He3 whose mean behavior is strongly different. Conclusions: Our inclusive analysis (no event selection) shows that N/Z equilibration between the projectile-like and the target-like is realized to a high degree for central collisions. The light charged particle production mean value difference between Xe124+Sn124 and Xe136+Sn112 systems for central collisions is of the order of a few %. This slight difference could be explained by pre-equilibrium particle emission whose intensity may differ for the two reactions. This point is demonstrated using He3 mean characteristics whose production takes place before chemical equilibrium attainment. The realized N/Z balance between projectile-like and target-like does not imply a pure two-body mechanism. Indeed a midrapidity production of light charged particle does exist and its N/Z is different as compared to the projectile-like one: it is n enriched. This point is touched using He6 midrapidity production which is favored by the drift phenomenon.

Relative values of the neutron-induced fission cross section $\ensuremath{\sigma}(n,f)$ of $^{242}\mathrm{Pu}$ have been measured with respect to the standard $^{1}\mathrm{H}(n,p)$ elastic scattering cross section, at average energies of 1.0, 1.4, and $1.9\phantom{\rule{4pt}{0ex}}\mathrm{MeV}$. The measurements are part of an international effort to reduce uncertainties and provide independent nuclear data relevant for fast-spectrum reactors. The shape of the measured cross section is in good agreement with data from Tovesson et al. [Phys. Rev. C 79, 014613 (2009)] and with the most recent data from Matei et al. [Phys. Rev. C 95, 024606 (2017)], but disagrees with the shapes of ENDF/B-VII.1 and JEFF3.2 evaluations. Absolute values of $\ensuremath{\sigma}(n,f)$, obtained under some assumptions, indicate an overestimation of $\ensuremath{\sigma}(n,f)$ in the evaluated libraries at 1.0 and $1.4\phantom{\rule{4pt}{0ex}}\mathrm{MeV}$, while a good agreement is found with ENDF/B-VII.1 at $1.9\phantom{\rule{4pt}{0ex}}\mathrm{MeV}$. A careful analysis of the impact of scattered neutrons and anisotropy of the fission fragment angular distribution has been performed. The measurement of the neutron flux by means of a proton-recoil detector is discussed. A comprehensive study of corrections applied to the data, of associated uncertainties, and of correlations between the measurements at the different energies is presented.

Directed and elliptic flows of neutrons and light charged particles were measured for the reaction 197Au+197Au at 400 MeV/nucleon incident energy within the ASY-EOS experimental campaign at the GSI laboratory. The detection system consisted of the Large Area Neutron Detector LAND, combined with parts of the CHIMERA multidetector, of the ALADIN Time-of-flight Wall, and of the Washington-University Microball detector. The latter three arrays were used for the event characterization and reaction-plane reconstruction. In addition, an array of triple telescopes, KRATTA, was used for complementary measurements of the isotopic composition and flows of light charged particles. From the comparison of the elliptic flow ratio of neutrons with respect to charged particles with UrQMD predictions, a value \gamma = 0.72 \pm 0.19 is obtained for the power-law coefficient describing the density dependence of the potential part in the parametrization of the symmetry energy. It represents a new and more stringent constraint for the regime of supra-saturation density and confirms, with a considerably smaller uncertainty, the moderately soft to linear density dependence deduced from the earlier FOPI-LAND data. The densities probed are shown to reach beyond twice saturation., Comment: 25 pages, 22 figures, accepted for publication in Physical Review C

We investigated the U238(d,p) reaction as a surrogate for the n+U238 reaction. For this purpose we measured for the first time the γ-decay and fission probabilities of U∗239 simultaneously and compared them to the corresponding neutron-induced data. We present the details of the procedure to infer the decay probabilities, as well as a thorough uncertainty analysis, including parameter correlations. Calculations based on the continuum-discretized coupled-channels method and the distorted-wave Born approximation (DWBA) were used to correct our data from detected protons originating from elastic and inelastic deuteron breakup. In the region where fission and γ emission compete, the corrected fission probability is in agreement with neutron-induced data, whereas the γ-decay probability is much higher than the neutron-induced data. We have performed calculations of the decay probabilities with the statistical model and of the average angular momentum populated in the U238(d,p) reaction with the DWBA to interpret these results.

Prompt fission gamma-ray (PFG) spectra have been measured in a recent experiment with the novel directional fast-neutron source LICORNE at the ALTO facility of the IPN Orsay. These first results from the facility involve the comparative measurement of prompt. emission in fast-neutron-induced fission of U-235 and U-238. Characteristics such as. multiplicity and total and average radiation energy are determined in terms of ratios between the two systems. Additionally, the average photon energies were determined and compared with recent data on thermal-neutron-induced fission of U-235. PFG spectra are shown to be similar within the precision of the present measurement, suggesting that the extra incident energy does not significantly impact the energy released by prompt. rays. The origins of some small differences, depending on either the incident energy or the target mass, are discussed. This study demonstrates the potential of the present approach, combining an innovative neutron source and new-generation detectors, for fundamental and applied research on fission in the near future.

We have measured the fission probabilities of $^{242}\mathrm{Am},^{243}\mathrm{Cm}$, and $^{244}\mathrm{Cm}$ induced by the transfer reactions $^{243}\mathrm{Am}(^{3}\mathrm{He},^{4}\mathrm{He}),^{243}\mathrm{Am}(^{3}\mathrm{He},t)$, and $^{243}\mathrm{Am}(^{3}\mathrm{He},d)$, respectively. The details of the experimental procedure and a rigorous uncertainty analysis, including a correlation matrix, are presented. For $^{243}\mathrm{Cm}$ our data show clear structures well below the fission threshold. To our knowledge, it is the first time that these structures have been observed for this nucleus. We have compared the measured fission probabilities to calculations based on the statistical model to obtain information on the fission barriers of the produced fissioning nuclei.

Isotopic ratios for light fragments ($Z\ensuremath{\le}4$) emitted by the quasiprojectile (QP) and the mid-rapidity (MR) sources are investigated by the use of a slightly asymmetric system ($^{36}\mathrm{Ar}+^{58}\mathrm{Ni}$) and a symmetric one ($^{58}\mathrm{Ni}+^{58}\mathrm{Ni}$) for six energies between 32A and 84A MeV and three semiperipheral centrality range selections. Experimental data come from the INDRA $4\ensuremath{\pi}$ multidetector. The results show a clear neutron-rich isotope production from the MR region as compared to the QP source. The beam energy and the centrality also show interesting different trends depending on the charge of the fragments and the emission source. Experimental results are compared to antisymmetrized molecular dynamics simulations.

Odd-even staggering effects on charge distributions are investigated for fragments produced in semiperipheral and central collisions of ${}^{112}\text{Sn}+{}^{58}\text{Ni}$ at 35 MeV/nucleon. For fragments with $Z\ensuremath{\le}16$ one observes a clear overproduction of even charges, which decreases for heavier fragments. Staggering persists up to $Z\ensuremath{\sim}30$. The staggering appears to be substantially independent of the centrality of the collisions, suggesting that it is mainly related to the last few steps in the decay of hot nuclei.