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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.

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.

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...

In the framework of nuclear waste transmutation studies, the Mini‐INCA project has been initiated at CEA/DSM with objectives to determine optimal conditions for transmutation and incineration of Minor Actinides (MA) in high intensity neutron fluxes. Our experimental tools based on alpha‐ and gamma‐spectroscopy of irradiated samples and the development of fission micro‐chambers could gather both microscopic information on nuclear reactions (total and partial cross sections for neutron capture and/or fission reactions) and macroscopic information on transmutation and incineration potentials. Cross sections of selected actinides (241Am, 242Am, 242Pu, 237Np, 238Np) have already been measured at ILL, showing some discrepancies when compared to evaluated data libraries but in overall good agreement with recent experimental data.

Fission cross section of short‐lived minor actinides is of prime importance for the incineration of minor actinides in high and thermal neutron fluxes. But due to the shortness of their half‐lives, measurements are difficult to handle on these isotopes and the existing data present some large discrepancies. An original method has been developed, in the framework of the Mini‐INCA project at ILL, to measure the fission and capture cross sections of minor actinides with low error bars associated even for short‐lived isotopes. This method lies on a quasi on‐line alpha‐ and gamma‐spectroscopy of irradiated samples and on the use of fission micro‐chambers. Coupled to a very powerful Monte‐Carlo simulation, both microscopic information on nuclear reactions (total and partial cross sections for neutron capture and/or fission reactions) and macroscopic information on transmutation and incineration potentials could be gathered. In this paper, the method is explained in its originality and some recent results are given and compared with existing measurements and evaluated data libraries.

Nuclear fuel devices of Pressurised Water Reactors are composed of uranium oxide pellets which are enclosed in zircaloy cylinders. During reactor operation, major actinides are created among which plutonium 239. Further on, energy deposition of fission recoils leads to sputtering of uranium and plutonium onto the inner surface of the cladding material. Thus, sputtered Pu and U ions start to migrate outwards. Experiments to investigate the migration process were performed at the Institut Laue Langevin (Grenoble) on the Lohengrin mass spectrometer. A model is proposed to deduce an apparent uranium and plutonium diffusion coefficient in zirconia (ZrO2) from the energy distribution broadening of a selected fission product.

γ-spectroscopy is an important tool also for nuclear waste transmutation studies. An experiment has been performed at ILL Grenoble to measure 241Am and 242gsAm thermal capture cross sections that play a major role for the transmutation of 241Am in a high thermal neutron flux, coupling γ-spectroscopy with mass spectrometry methods. σc(242gsAm), for which the three main evaluation data libraries were giving very different values, has been measured to be 280±40 barns. That confirms the low cross section value making possible the 241Am transmutation by thermal neutrons. In the MINI-INCA project, we plan to widen the measurement to other minor actinides of interest using γ and α-spectroscopy of neutron irradiated samples for thermal and epi-thermal fluxes up to 2.1015 n/s/cm2.

Amongst the minor actinides issued from the spent nuclear fuel, 241Am is present in high concentration and contributes significantly to the long-term radiotoxicity of nuclear waste. A major uncertainty was present in the transmutation chain of 241Am when irradiated by a high intensity thermal neutron flux. This uncertainty was brought about by the poor knowledge of the 242gsAm neutron capture cross section. A dedicated experiment has been performed at the Institut Laue-Langevin in Grenoble, which gives a definitive experimental answer to this problem.

241Am goes by neutron capture to the two 242Am states: the ground and the meta-stable. We have measured both thermal cross sections, with a relative uncertainty of 7%, by γ-spectroscopy of neutron irradiated 241Am samples. The results are: σcg.s.=σc(241Am(nth,γ)242Amg.s.)=636±46 barns and σcm=σc(241Am(nth,γ)242Amm)=60±4 barns. These measurements lead to a ratio σcg.s./σctot=0.914±0.007 with a total capture cross section σctot=696±48 barns. While the ratio is compatible with previous measurements, the total capture cross section σctot is increased by 16% with respect to the JENDL-3.2 value.

Ternary events in the thermal neutron induced fission of 241Pu were studied with the recoil separator LOHENGRIN at the Institut Laue Langevin in Grenoble. Yields and energy distributions could be determined for most isotopes of the elements hydrogen to oxygen. Also several heavier nuclei up to 30Mg could be observed. Yields were measured for 42 isotopes, for further 17 isotopes upper limits could be deduced. For the first time the halo nuclei 11Li and 14Be were found in neutron induced fission with yields of some 10−10 per fission. This is orders of magnitude lower than predicted by most of the ternary fission models.

Several μs isomers have been observed in the mass range A=88–109. The detection is based on time correlation between reaction products selected by the LOHENGRIN spectrometer and the γ-rays depopulating the isomers. Three new isomers have been observed in 94Y, 96Rb, and 106Nb.

Energy distributions and yields of ternary particles (TP’s) emitted in the thermal neutron induced fission of 229Th, 233U and 239Pu have been determined. Special emphasis was put on the study of heavy ternary particles. The measurements were performed at the high flux reactor of the Institut Laue-Langevin (ILL). For identification of particles the mass separator Lohengrin and a ΔE-E ionisation chamber were used. In the case of 233U(nth,f) it was possible to detect 25 different TP’s, with 19 of them having never been detected before in this reaction. For 229Th(nth,f) all 17 TP’s that were observed were new. From the reaction 239Pu(nth,f) 21 out of 34 detected TP’s were new. The energy distributions and yields of the TP’s are given. The yields of TP’s as a function of mass number depend on the mass of the decaying compound nucleus. Heavy TP’s are increasingly favored in the decay of heavy mother nuclei.

At the mass separator LOHENGRIN of the Institut Laue-Langevin, fragment mass, charge and kinitic energies have been measured for the target nuclei {sup 245}Cm and {sup 241}Pu. The mass range for the Curium measurement was 76{

Statistical approaches have been widely used for describing the nuclear-fission process. They were quite successful in explaining several prominent features of the global nuclide distributions and many other aspects, while there are controversial conclusions on the origin of the even-odd effect in the nuclear-charge yields. We analyze the ingredients of the main statistical approaches to the even-odd effect in fission and show up that many of their deficiencies rely on unrealistic assumptions. Finally, we demonstrate that the large body of experimental results, obtained in the recent years to a great part at GSI, Darmstadt, is very successfully reproduced by a new stringent statistical approach.

Fission times have been measured by the blocking technique in single crystal for uranium nuclei and for super-heavy elements with Z = 120. The fission times measured for uranium nuclei can be reproduced by statistical calculations following the Bohr and Wheeler approach only if a friction coefficient increasing with temperature is considered. In the super-heavy element domain, a discrimination between the fast quasi-fission process and the slow fusion-fission one has been achieved from reaction time measurements. A minimum cross-section $\sigma$= 22 mb for formation of Z = 120 compound nuclei in $^{238}$U+Ni reactions at 6.62 MeV/A has been inferred.

At a certain finite neck radius during the descent of a fissioning nucleus from the saddle to the scission point, the attractive nuclear forces can no more withstand the repulsive Coulomb forces producing the neck rupture and the sudden absorption of the neck stubs by the fragments. At that moment the nucleons, although still characterized by their pre-scission wave functions, find themselves in the newly created potential of their interaction with the separated fragments. Their wave functions become wave packets with components in the continuum. The probability to populate such states gives evidently the emission probability of nucleons at scission. In this way we have studied scission neutrons and protons for the fissioning nucleus 236U, using two-dimensional realistic nuclear shapes.

NESTER; International audience; The neutron-rich nuclei $^{72}$Ni and $^{72}$Cu have been produced in the proton-induced fission of $^{238}$U at the LISOL and ISOLDE facilities of Louvain-La-Neuve and CERN. Partial beta-decay schemes are presented, giving some information about the nuclear structure of their daughter nuclei $^{72}$Cu and $^{72}$Zn. The lifetime of $^{72}$Cu was determined to be $T_1/2$ = 6.63(3) s, in line with previous measurements. No $\beta$-decaying isomeric states were identified in $^{72}$Cu, in contrast to $^{70}$Cu. The spin and the parity of several states in $^{72}$Cu are tentatively assigned and a comparison is made with different shell-model predictions.

Production cross sections and kinematical properties of the complete set of fission fragment residues from the reaction 238U (1 A.GeV) + p have been obtained. Isotopic distributions are measured for all elements from O (Z = 8) to W (Z = 74). Fission velocities and production cross sections are shown as a function of Z, the charge and N, the number of neutrons of the fragments. The very asymmetric pairs of fragments can be attributed to excited fissioning parent nuclei of charge Z, 88< Z < 92.