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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.

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.

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 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.

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.

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 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%.

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 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.

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.

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.

During the last years, several Cm and Cf isotopes have been studied by our research group in the frame of a systematic investigation of gas emission characteristics in ternary fission. Here we report on new results on the energy distribution and the emission probability of 3H, 4He and He particles emitted in the spontaneous ternary fission of 250Cf (Eexc = 0 MeV) and in the neutron induced ternary fission of 249Cf (Eexc = 6.625 MeV). Both measurements were performed using suited and well‐calibrated ΔE−E telescope detectors, at the IRMM (Geel, Belgium) for the spontaneous fission and at the very intense neutron beam PF1b at the Institute Laue‐Langevin (Grenoble, France) for the neutron induced fission measurement. In this way, the existing database can be enlarged with new results for Z = 98 isotopes, which is important for the systematic investigation. Moreover, the investigation of the ‘isotope couple’ 249Cf(n,f)–250Cf(SF), together with corresponding data for other isotopes, will yield valuable informat...

Los Alamos conducted a dual fission‐chamber experiment in the 1970s in the Bigten critical assembly to determine fission product data in a fast (fission neutron spectrum) environment, and this defined the Laboratory’s fission basis today. We describe how the data from this experiment are consistent with other benchmark fission product yield measurements for 95,97Zr, 140Ba, 143,144Ce, 137Cs from the NIST‐led ILRR fission chamber experiments, and from Maeck’s mass‐spectrometry data. We perform a new evaluation of the fission product yields that is planned for ENDF/B‐VII.1. Because the measurement database for some of the FPs is small—especially for 147Nd and 99Mo—we use a meta‐analysis that incorporates insights from other accurately‐measured benchmark FP data. The %‐relative changes compared to ENDF/B‐VI are small for some FPs (less than 1% for 95Zr, 140Ba, 144Ce), but are larger for 99Mo (3%) and 147Nd (5%). We suggest an incident neutron energy dependence to the 147Nd fission product yield that accounts ...

International audience; The even-odd staggering observed in the experimental fission-fragment nuclear-charge yields is investigated over a wide systematics of fission fragments measured at Lohengrin in direct kinematics and at GSI in inverse kinematics. The general increase of the even-odd staggering in the fission-fragment charge yields towards asymmetric charge splits is explained by the absorption of the unpaired nucleons by the heavy fragment. As a consequence, the well established trend of evenodd staggering in the fission fragment charge yields to decrease with the fissility is attributed in part to the asymmetry evolution of the charge distribution. This interpretation is strongly supported by the data measured at GSI, which cover the complete charge distribution and include precise yields at symmetry. They reveal that the even-odd effect around symmetry remains constant over a large range of fissility.