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The pairing correlations of neutrons in 236U deformed at maximum was studied as function of the mass asymmetry of the fission fragments. The renormalized BCS equations were used. The nuclear shapes involved were described by modified Cassini ovals. A strong dependence of the pairing gap Δ on the fission‐fragment mass ratio AH/AL was found with a pronounced minimum at 1.5. This is in agreement with the most probable mass division measured in 235U(nth,f). Since the lowest value of Δ corresponds to the most stable configuration, we offer a simple explanation of mass asymmetry in low‐energy nuclear fission.

Fission-fragment properties have been calculated for thermal neutron-induced fission on a $^{239}\textrm{Pu}$ target, using constrained Hartree-Fock-Bogoliubov calculations with a finite-range effective interaction. A quantitative criterion based on the interaction energy between the nascent fragments is introduced to define the scission configurations. The validity of this criterion is benchmarked against experimental measurements of the kinetic energies and of multiplicities of neutrons emitted by the fragments., 8 page, 4 figures, to be published in Proceedings of the 4th International Workshop on Fission and Fission Product Spectroscopy

International audience; A quantitative comparison regarding absolute cross sections associated to Statistical and Dynamical Fission processes in the 124Sn+64Ni and 112Sn+58Ni collisions experimentally investigated at 35 AMeV beam energy using multi-detector CHIMERA is presented. It results that Dynamical Fission process is about two times more probable in the neutron rich 124Sn+64Ni system than in the 112Sn+58Ni neutron poor one. The observed difference could indicate an influence of the entrance channel Isospin (N/Z) content on the reaction mechanisms.

The decay of a new 3.2 μs, (4−) isomeric state at 1298.0 keV has been observed using γ‐ray spectroscopy at the Lohengrin mass spectrometer of the Institut Laue‐Langevin. Prompt γ‐ray data from a spontaneous fission experiment have also been analyzed allowing the observation of a collective band on top of this isomeric state, the identification of a new band on top of the previously reported (5−) isomer of 156Nd and the extension of collective bands on top of (5−) isomers of the neighboring 156,158Sm nuclei. Quasiparticle rotor model calculations correctly predict the energies and decay patterns of these bands.

International audience; Adding an extra nucleon having a definite K-value to an axially symmetrical even-even core generates a breaking of the time-reversal symmetry. Within the Skyrme-Hartree-Fock approach the effects of the time-odd mean-field terms resulting from the presence of non-vanishing current and spin-vector densities are investigated. In this preliminary study, pairing correlations are not taken into account. The associated removal of the Kramers degeneracy is interpreted in terms of diamagnetic/paramagnetic types of polarization of the even-even core in the field of an odd particle. This general approach is illustrated here by calculations in the region of light fragments of heavy (e.g. actinide) nuclei.

The spectroscopy of fission fragments, obtained by the 18O+208Pb fusion‐fission reaction at 85 MeV bombarding energy, has been performed with the Euroball IV γ‐array. High‐spin states in many neutron‐rich Se, Br, Kr and Rb of the 78Ni region nuclei have been identified for the first time. Several physical aspects which govern the nuclear structure in this mass region are underlined thanks to the new results. Among those, empirical values of the residual proton‐neutron interactions have been extracted for the πf5/2⊗vd5/2 and πp3/2⊗vd5/2 configurations.

A renewed interest in photonuclear reactions was stimulated by applications as radioactive ion beam production, irradiation stations by high energy photons, shielding of electron accelerators, etc. Today, a particular attention is paid to the non‐destructive characterization of waste barrels and the detection of nuclear materials, both based on photofission process and the associated delayed neutron (DN) and delayed photon (DP) emissions. The need of accurate and complete data for DN and DP yields and time characteristics of actinides was the motivation for an experimental campaign, started in 2004. In this paper, the experimental setup and the data analysis method will be presented and the modeling work will be described. Experimental results for DN and DP characteristics will be compared to calculations in the case of photofission of 232Th, 235,238U, and 237Np.

Theoretical and numerical studies of prompt fission neutrons are presented. The main results of the Los Alamos model often used in nuclear data evaluation work are reviewed briefly, and a preliminary assessment of uncertainties associated with the evaluated prompt fission neutron spectrum for n (0.5 MeV)+239Pu is discussed. Advanced modeling of prompt fission neutrons is done by Monte Carlo simulations of the evaporation process of the excited primary fission fragments. The successive emissions of neutrons are followed in the statistical formalism framework, and detailed information, beyond average quantities, can be inferred. This approach is applied to the following reactions: 252Cf(sf), nth+239Pu, n(0.5 MeV)+235U, and 236Pu(sf). A discussion on the merits and present limitations of this approach concludes this presentation.

Angular distributions of light charged particles relative to the momentum of the light fission fragment were studied in ternary fission induced by cold polarized neutrons. It was discovered that the angular distribution is asymmetric with respect to a plane determined by the direction of incident neutron spin and the light fragment momentum. The violation of the mirrorlike symmetry was established at the level of ∼10−2–10−4 for three reactions studied: 233U(n,f), 235U(n,f) and 239Pu(n,f). In spite of the smallness of the effects the neutron spin flip method allowed to make reliable measurements. The current status of the experimental studies of the effect and its interpretation are presented. Perspectives for further studies of the new phenomenon are discussed.

Well‐characterized neutron fields are a prerequisite for the investigation of neutron detectors. Partly in collaboration with external partners, the PTB neutron metrology group makes available for other users neutron reference fields covering the full energy range from thermal to 200 MeV. The specification of the neutron fluence in these beams is traceable to primary standard cross sections.

In an experiment carried out at the 4π Ge detector facility Gammasphere the yields of ternary fission events of 252Cf accompanied by the emission of He, Be and C LCPs were measured. For the first time independent yields were measured for 59 pairs of secondary fragments of the He ternary fission and for many fission fragments detected together with the He, Be and C LCPs. With a reasonable assumption that the detected LCPs predominantly are 4He, 10Be and 14C their yields and the charge distributions of fission fragments were conjointly analyzed in terms of the “energy cost” concept first suggested by Halpern.

Over the last five years, new evaluations of plutonium and uranium have been performed at Bruyeres‐le‐Châtel from the resolved resonance region up to 30 MeV. Only nuclear reactions models have been used to build these evaluations. Total, shape elastic and direct inelastic cross sections are obtained from a coupled channel model using a dispersive optical potential devoted to actinides. All the other cross sections are calculated owing to the Hauser‐Fesbach theory. We take particular care over the fission channel. For uranium isotopes, a triple‐humped barrier is required in order to reproduce accurately the variations of the experimental fission cross sections. As not commonly expected, we show that the effect of the class II or class III states located in the wells of the aforementioned fission barrier provide sometimes an anti‐resonant transmission rather than a resonant. With increasing neutron incident energy, a lot of residual nuclei produced by nucleon emission lead to fission also. All available experimental data assigned to the various fission mechanisms of the same nucleus are used to define its fission barrier parameters. As a result of this approach, we are now able to provide consistent evaluations for a large series of isotopes. Integral data testing results of our new evaluations will be presented.

Some neutrons are emitted from fission fragments several seconds to several minutes after fission occurs. These delayed neutrons play a key role for the conduct and in safety aspects of nuclear reactors [1]. But the probabilities to emit such neutrons (Pn) are not well known. A summary of different database and compilation of Pn values is presented to show these discrepancies and uncertainties. Experiments are carried out at the Lohengrin mass spectrometer (at Institut Laue Langevin in Grenoble) and at the ISOLDE facility (CERN) in order to measure some Pn values. Two different techniques are used: either by using gamma‐rays detection or neutron emission detection. These two techniques and some preliminary results are presented.

We are carrying out a research program with the 8πLP apparatus at the Laboratori Nazionali di Legnaro (Italy), aimed at studying fission dynamics in systems of intermediate fissility. These systems, compared to the heavier ones, are characterized by larger pre‐scission charged particle multiplicities as well as by comparable fission and evaporation residue cross sections. Therefore, the measurement of observables in both channels allows putting severe constraints on the models, in order to obtain more reliable information on the dynamics of the process. We present the results for the composite system 122Ce, produced at Ex = 122 MeV in 200 MeV 32S+100Mo reaction. The comparison of the data with the predictions of a dynamical model, based on 3‐D Langevin equations, indicates that full one‐body dissipation plays a dominant role in the fission process, with a viscosity parameter independent from the temperature and a fission delay of ∼25*10−21 s.

The fine structure in the Th neutron induced fission cross section is analyzed in terms of the Landau Zener effect. An excellent agreement with experimental data was obtained. It was shown that the fine structure can be explained by the promotion of the unpaired neutron from one single‐particle level to another in the region of the second barrier, where the level density is very large. Partial cross sections for different values of the spin can be predicted for each resonance.

The observation and prediction of unusually depressed first excited 2^+_1 states in even-A neutron - rich isotopes of semi-magic Sn above 132Sn provide motivations for reviewing the problems related to the nuclear astrophysics in general. In the present work, the beta-decay rates of the exotic even Sn isotopes (134,136Sn) above the 132Sn core have been calculated as a function of temperature (T). In order to get the necessary ft values, B(GT) values corresponding to allowed Gamow Teller (GT-) beta-decay have been theoretically calculated using shell model. The total decay rate shows decrease with increasing temperature as the ground state population is depleted and population of excited states with slower decay rates increases. The abundance at each Z value is inversely proportional to the decay constant of the waiting point nucleus for that particular Z. So the increase in half-life of isotopes of Sn, like 136Sn, might have substantial impact on the r-process nucleosynthesis.

A program to measure fission neutron spectra from neutron‐induced fission of actinides is underway at the Los Alamos Neutron Science Center (LANSCE) in a collaboration among the CEA laboratory at Bruyeres‐le‐Châtel, Lawrence Livermore National Laboratory and Los Alamos National Laboratory. The spallation source of fast neutrons at LANSCE is used to provide incident neutron energies from less than 1 MeV to 100 MeV or higher. The fission events take place in a gas‐ionization fission chamber, and the time of flight from the neutron source to that chamber gives the energy of the incident neutron. Outgoing neutrons are detected by an array of organic liquid scintillator neutron detectors, and their energies are deduced from the time of flight from the fission chamber to the neutron detector. Measurements have been made of the fission neutrons from fission of 235U, 238U, 237Np and 239Pu. The range of outgoing energies measured so far is from 0.7 MeV to approximately 8 MeV. These partial spectra and average fiss...

The fission fragment separator Lohengrin was used to find new nanosecond isomers in exotic neutron‐rich nuclei. The technique is described. It provides a rough estimate of the level lifetime. First results are shown on isotopes with masses 137, 138, 142 and 144.

Using the statistical model, we calculate the induced fission cross sections for the nuclei 211–223Ra in their peripheral collisions with 208Pb. The role of closed shell N = 126 is studied. Level densities of the Fermi‐gas model and of the model with collective enhancement are used. Taking into account the particle‐hole excitation in addition to the collective Coulomb excitation, we obtain satisfactory agreement with the experimental data.

Potential energy surfaces are calculated in the elongation‐asymmetry plane, with nuclear shapes ranging from sphericity to very large deformations, using the Hartree‐Fock‐Bogoliubov method and the Gogny nucleon‐nucleon effective interaction (parameterization D1S), for the 226Th and 256,258,260Fm Fissioning systems. In order to discriminate between pre‐ and post‐scission configurations, we define a criterion based on the nuclear density. Using this criterion, a big number of scission configurations are identified, and several fragment properties are extracted from them, namely fragment deformations, deformation energies, energy partitioning, neutron binding energies at scission, charge polarization, total fragment kinetic energies and neutron emission multiplicities.

On the base of modified trajectory calculations the shift of angular distribution of α‐particles accompanied the reaction 235U(n,f) induced by cold polarized neutrons is evaluated. It was supposed that angular distribution shift is caused by the rotation of nuclear system before scission. The orientation of a rotation motion is determined by the neutron spin polarization along and opposite to the beam direction. For the first time the estimation was done in the frame of trajectory calculations assuming the rotation motion of scissioning nucleus [1]. The result of the calculation is in a good agreement with experimental data of paper [2], where this new phenomenon was named as ROT‐effect.

Ground‐state masses, fission barrier heights and α lifetimes for actinide and trans‐actinide nuclei are determined in the framework of the macroscopic‐microscopic model with the Lublin‐Strasbourg Drop, the Strutinsky shell‐correction method and the Modified Funny‐Hills shape parametrization accounting for elongation, neck formation, left‐right asymmetry and non‐axality together with the Yukawa folding procedure. Fission barrier height are nicely reproduced in our approach which contains no adjustable parameter.

Systems of intermediate fissility are characterized by an evaporation residues cross section comparable or larger than the fission cross section, and by a relatively higher probability for charged particle emission in the pre‐scission channel. In a theoretical framework in which time scale estimates of the fission process rely on statistical model calculations, the analysis of particle emission in the evaporation residues channel is the source of additional constraints on the statistical and dynamical models. This contribution will focus on our statistical and dynamical analysis of a more complete set of data from the system 32S+100Mo at ELab = 200 MeV. Statistical model fails in reproducing the whole set of data and no convincing estimate is possible of the fission time scale. In particular, while pre‐scission multiplicities can be reproduced without delay, the model strongly overestimates proton and alpha particle multiplicities in the evaporation residues channel irrespective of the statistical model input parameters and prescriptions used for the level density and the transmission coefficients. The analysis of the same set of data with a three‐dimensional Langevin dynamical model produces a very good agreement with the full set of data and indicates that one‐body dissipation plays a dominant role in the fission process, implying a fission delay 23–25×10−21 s.

Mass and energy distributions of fission and quasifission fragments in the reactions 22Ne+249Cf, 26Mg+248Cm, 36S+238U and 58Fe+208Pb leading to the formation of the Z = 108 composite nucleus are reported. In this series of experiments it is shown for the first time that the quasifission process leads to the formation of not only asymmetric fragments but also gives a contribution to the symmetric mass region of fission fragments, most probably caused by the influence of nuclear shells with Z = 50 and N = 82.

Fragment mass distributions from neutron-induced fission of /sup 232/Th and /sup 238/U have been measured at quasi-monoenergetic neutron beam of the Louvain-la-Neuve cyclotron facility CYCLONE. The measurements have been carried out making use of a multi-section Frisch-gridded ionization chamber. The measurement technique as well as the data processing is described. Preliminary data on primary fragment mass yields are given for an incident neutron energy of 32.8 MeV.

A methodology to compute fission prompt neutron and γ multiplicities for neutron‐induced fission reactions is presented in this article. The basic tool of this procedure is the TALYS code, which follows the statistical decay of all fission fragments by using neutron‐γ emission competition. The neutron multiplicity probability distribution P(v) as well as the average prompt neutron energy are also computed. We present some hypothesis concerning Total Kinetic Energy distribution and the partitioning energy of the Total Excitation Energy.

The spontaneous fission half‐lives of heavy and superheavy nuclei from 232Th to 286114 are calculated within the parabolic potential approximation by taking nuclear structure effects into account [Phys. Rev. C 78, 044329 (2008)]. The agreement between theoretical results and the newly observed data is very good in our calculations. The average deviation between data and theory is within a factor of 10 for total 45 spontaneous fission nuclei. This level of agreement is very satisfactory because the spontaneous fission is much more complex than other decay modes such as light cluster radioactivities.

A new measurement of the fast neutron‐induced fission cross section of 238Pu in the 2–200 MeV domain is presented. Preliminary results confirm theoretical indications that above 6 MeV this cross section was, up to now, overestimated by 20 to 30%.

We discuss several aspects of the fission phenomenon studied within a mean‐field approach with two incarnations of the Gogny force. Results for the fission of odd‐A nuclei obtained in the framework of the equal‐filling approximation are discussed. We also analyze the impact of a finite but moderate temperature in the fission process and finally we discuss fission properties of neutron rich uranium isotopes studied with two different parametrizations of the Gogny force, namely D1S and D1N.

We have developed a new method to propagate the uncertainties of fundamental nuclear physics models and parameters used in nuclear data evaluation to the design and performance of future nuclear energy systems. Using Monte Carlo simulation, it is for the first time possible to couple these two fields at the extremes of nuclear science without any loss of information in between. With the help of a large database of nuclear reaction measurements, we have determined the uncertainties of theoretical nuclear reaction models such as the optical, compound nucleus, pre‐equilibrium and fission models. A similar assessment is done for the parameters that describe the resolved resonance range. We are now able to quantify the required quality of theoretical nuclear reaction models and measurements directly from the reactor design requirements. Examples will be presented for actinides using criticality‐safety benchmarks with feedback on experimental requirements.

The excitation energy and nucleon composition dependence of the transition from asymmetric to symmetric scission of fission observables of Th(Pa) and U(Np) nuclei is interpreted for nucleon‐induced fission cross sections of 232Th(p,F)(232Th(n,F)) and 238U(p,F) (238U(n,F)) reactions at En(p) = 1–200 MeV. Predominantly symmetric fission in 232Th(p,F) and roughly equal contributions of symmetric and asymmetric modes in 238U(n,F) at En(p) = 200 MeV as revealed by experimental branching ratios is reproduced. Steep transition from asymmetric to symmetric fission with increase of nucleon incident energy is due to fission of neutron‐deficient Th(Pa) (A

The properties of cylindrical 1.5//×1.5//LaCl3:Cescintillation detectors were determined. For the energy resolution values between 3.8 and 4.2% at 662 keV were observed, following the expected E-1/ ...

The cross sections for neutron induced fission of 235U, 238U, 209Bi and natPb were measured in the energy range from 30 MeV to 200 MeV using parallel‐plate ionization chambers. Proton recoil telescopes and a 238U ionization chamber were employed for the determination of the incident neutron fluence. With the exception of a few deviating data points, the experimental results for 235U(n,f), 238U(n,f) and 209Bi(n,f) are close to the recommended cross sections.

The systematic behaviour of the average parameters required for the Los Alamos model is verified a) in the case of 238U(n,f) by using recent experimental Y(A) and TKE(A) distributions in the frame of the Point by Point treatment, the results confirming the parameter values provided by the systematics. b) In the case of 233U(n,f) the prompt neutron spectra and multiplicities calculated up to 30 MeV incident energy by using exclusively model parameter values given by the systematics are obtained in very good agreement with the existing experimental data, c) For light actinides the unique experimental P(Z) and TKE(Z) data obtained for the electromagnetic induced fission can be exploited in the frame of an original procedure to obtain Y(A) and TKE(A) corresponding to the neutron induced reaction leading to the same fissioning nucleus. The procedure, verified for the 234U fissioning nucleus case, is applied to the 233U fissioning nucleus, the corresponding average model parameter values obtained by the Point b...

In spite of the huge amount of fission yield data available in different libraries, more accurate values are still needed for nuclear energy applications and to improve our understanding of the fission process. Thus measurements of fission yields were performed at the mass spectrometer Lohengrin at the Institut Laue‐Langevin in Grenoble, France. The mass separator Lohengrin is situated at the research reactor of the institute and permits the placement of an actinide layer in a high thermal neutron flux. It separates fragments according to their atomic mass, kinetic energy and ionic charge state by the action of magnetic and electric fields. Coupled to a high resolution ionization chamber the experiment was used to investigate the mass and isotopic yields in the light mass region. Almost all fission yields of isotopes from Th to Cf have been measured at Lohengrin with this method. It has been extended in this work to the heavy mass region for the reactions 235U(nth,f), 239Pu(nth,f), and 241Pu(nth,f). For t...

Fission fragment mass distribution has been measured from the decay of 246Bk nucleus populating via two entrance channels with slight difference in mass asymmetries but belonging on either side of the Businaro Gallone mass asymmetry parameter. Both the target nuclei were deformed. Near the Coulomb barrier, at similar excitation energies the width of the fission fragment mass distribution was found to be drastically different for the 14N+232Th reaction compared to the 11B+235U reaction. The entrance channel mass asymmetry was found to affect the fusion process sharply.

The prompt neutron emission in spontaneous fission of 252Cf has been investigated applying digital signal electronics. The goal was to compare the results of this digital data acquisition and digital signal processing analysis to the results of the pioneering work of Budtz‐Jo/rgensen and Knitter. Using a twin Frisch‐grid ionization chamber for fission fragment (FF) detection and a NE213 equivalent neutron detector in total about 10 neutron coincidences have been registered. Fission fragment kinetic energy, mass and angular distribution have been investigated using a 10 bit waveform digitizer. Neutron time‐ of‐ flight and pulse shape have been measured using analogue CAMAC modules, a 1 ns TDC and a pair of 12 bit charge‐to‐digital‐converters. The fission fragment signals have been analyzed using digital signal processing algorithms. The results are in very good agreement with literature. For the first time the dependence of the number of neutrons as a function of total kinetic energy (TKE) of the fragments...

The results of experimental and theoretical studies of the deuteron‐induced fission of 238U at Ed = 65 and 124 MeV are presented. The nucleon spectra measured in the coincidence with fission fragments were analyzed taking into account the direct (d, pf) and (d, nf) channels, pre‐compound emission and the pre‐ and post‐scission evaporation. It was demonstrated that the (d, pf) channels can be used as a surrogate method for measuring the characteristics of (n, f) reaction.

An innovative approach for studying transient effects in nuclear fission is proposed. Fragmentation of radioactive actinides is employed to prepare fissile nuclei in well‐defined initial conditions, and the fission‐fragment nuclear charge distribution is used to establish a clock for the dynamical evolution of the system. Based on a so‐far un‐reached large number of fissioning systems, the study demonstrates the undeniable manifestation of transient effects at high excitation energy and addresses the question about the influence of initial deformation.

An overview of the future FAIR facility is given. A special accent is given on the present status and future perspectives of the NuSTAR community.

The production cross sections of neutron‐rich fission residues produced in reactions induced by a 238U beam impinging onto Pb and Be targets were investigated at the Fragment Separator (FRS) at GSI. These data allowed us to discuss the optimum energies in fission for producing the most neutron‐rich residues.

Effects of nuclear orientation on fission fragment mass distributions in the reactions of 30Si36,34S+238U were investigated. In the reactions for 30Si+238U, the mass distributions showed a Gaussian shape in the above barrier region, having a character of fusion‐fission. At the sub‐barrier energies, the asymmetric fission component around AL/AH≈90/178 appears. The asymmetric fission is attributed to quasifission. Strong variation of the mass distributions with energy is observed for the reactions of 36,34S+238U. The results suggested the competition between fusion and quasifission, resulting from the orientation effects of the deformed target nuclei on the reaction.

A new measurement program was set up at SCK⋅CEN to determine the thermal neutron‐induced fission cross section of a number of Cm isotopes. These transuranium isotopes are produced in nuclear reactors and are candidates for transmutation. This paper presents preliminary results of our 245Cm(n,f) cross‐section measurement.

Nucleon transfer is studied with focusing on the charge equilibration. The proposed classification of nucleon transfer in deep inelastic collisions at intermediate energies is verified by three‐dimensional TDHF calculations.

For experiments aimed at the study of spontaneous fission of transfermium nuclei improvements in the focal plane detector system of recoil separa tor VASSILISSA have been made. The neutron detector consisting of 54 3He ‐filled counters has been mounted around the focal plane detector chamber. The multiplicity of prompt neutrons emitted in spontaneous fission of 246 Fm was measured.

Two‐dimensional thermal potential energy surfaces have been investigated for 232Th using the finite temperature HF+BCS approach with Skyrme energy density functional SkM*. At low excitation energy, the calculated static fission path goes through families of triaxial and reflection‐asymmetric shapes. With increasing excitation energy, the shallow third minimum associated with a nonzero octupole moment becomes washed out, and the static fission valley moves towards the axial and reflection‐symmetric limit.

Recent results from an experiment at GANIL, performed to investigate the main prop- erties of fission-fragment yields and energy distributions in different fissioning nuclei as a function of the excitation energy, in a neutron-rich region of actini des, are presented. Transfer reactions in inverse kinematics between a 238 U beam and a 12 C target produced different actinides, within a range of excitation energy below 30 MeV. These fissioning nuc lei are identified by detecting the target-like recoil, and their kinetic and excitation energ y are determined from the reconstruction of the transfer reaction. The large-acceptance spectrometer VAMOS was used to identify the mass, atomic number and charge state of the fission fragments in flig ht. As a result, the characteristics of the fission-fragment isotopic distributions of a variety of neutron-rich actinides are observed for the first time over the complete range of fission fragments.