[en] Properties of hot nuclei have been studied in the reactions of ⁴⁰Ar with ^natAg at 7, 17, 27 and 34 MeV with AMPHORA, a 4π multidetector for charged particles. For central collisions, the results support the classical picture of hot (E^* as great as 5 MeV/Nucleon), heavy (M ∼ 100-130 amu) nuclear systems, with high spin (J_max ≥100h/2π) that emit a long chain of particles and fragments. Out-of-plane fragment production increases strongly with incident energy. This suggests that symmetric fission gradually disappears, as three or more body breakup processes become preferred modes of decay. Furthermore, the fragment-fragment correlations seem to indicate that Li fragments are emitted rapidly (τ≤10^-21s) and often before the protons and α particles. This implies that thermalization of the composite nucleus is so fast that it clouds a clear signal for the hypothesis of 'instantaneous' multifragmentation

[en] An experimental programme to study the properties of fission induced by intermediate energy neutrons (i.e. several hundreds of keV to several hundreds of MeV) is in progress at the Los Alamos Neutron Science Center, USA. Experiments using the germanium array for neutron induced excitations (GEANIE) high resolution gamma and X ray spectrometer have been performed with ²³⁸U targets. Information on the fragment yields (i.e. excitation functions and isotopic distributions) were extracted from the germanium array for neutron induced excitations inclusive gamma and gamma-gamma data. Charge yields were extracted by measuring prompt X rays using a specially designed thin fission sensitive target. The experimental results are presented and compared with the systematics of Wahl. (author)

[en] A fission-fragment-sensitive detector built for low-energy photon spectroscopy applications at the WNR 'white' neutron source at Los Alamos is described. The detector consists of eight layers of thin photovoltaic cells, onto which 1 mg/cm² of pure ²³⁸U is deposited. The detector serves as an active target to select fission events from background and other reaction channels. The fairly small thickness of the detector with respect to transmission of 20-50 keV photons permits the measurement of prompt fission-fragment X-rays. Results with the GEANIE photon spectrometer are presented

[en] Neutron emission spectra from inelastic neutron scattering on natural nickel at the FIGARO facility have been measured by a double time-of-flight technique. The incident neutrons are produced from the spallation source of the Weapons Neutron Research facility, and their energies are determined by time of flight. The emitted neutrons and gamma rays are detected by 16 liquid scintillators and one high-resolution germanium or one barium-fluoride detector, respectively. The results for incident neutron energies from 2 to 10 MeV are compared with predictions of nuclear model calculations performed with the code EMPIRE-II. Finally, the level density parameters 'a' and 'ΔE' are extracted.

[en] Energy distributions of prompt neutrons in coincidence with fission induced on ²³⁸U were measured for incident neutron energies up to 200 MeV. The double time-of-flight technique was used to deduce incident and emitted neutron energies. The experimental average and standard deviations of the fission neutron spectra (FNS), for emitted neutron energies from 0.65 to 7.5 MeV, are reported. The results compare well to predictive calculations with the improved Los Alamos model below 20 MeV incident neutron energy. The observed dip at the opening of the second chance fission channel at 6 MeV is confirmed and analyzed. Above 20 MeV, the experimental results of the FNS are smaller than the calculated ones. At 50 MeV and higher, the data suggest a slight increase of the temperature and the kinetic energy of the fission fragments

[en] Prompt-fission-neutron multiplicities were measured at a 'white' neutron source for the fission of ²³⁸U and ²³⁵U up to 200 MeV. The data are of great importance in the connection of accelerator-coupled nuclear reactor systems incinerating actinides, with uranium considered as a prototype actinide. We report that the fission induced by 200 MeV neutrons produces ∼10 more prompt neutrons than the fission induced by reactor neutrons. In conclusion, new data on (bar ν)_p are reported with errors ≤ 15%. (bar ν)_p grows steadily in the fission of ^238,235U induced by neutrons up to 200 MeV. At this energy, it exceeds by ∼10 the number of neutrons from fission induced by reactor neutrons. Uranium 238 shows enhanced neutron emission compared to uranium 235 due to the higher mass and lower binding energy of the neutrons. In future experiments, we plan to investigate on a much broader range the energy and the angular distributions of the emitted neutrons to improve the precision of the data.

[en] Taking advantage of the neutron source of the LANCSE, it has been possible to obtain a measure of the velocity distribution and the number of prompt-neutrons emitted in the neutron-induced fission of ²³⁸U and ²³⁷Np over a broad incident neutron energy range. The mean kinetic energy was extracted and is shown as the function of the incident-neutron energy. We confirm here the observation, for both reactions, of a dip around the second chance fission which is explained by the lower kinetic energy of the pre-fission neutrons. Such a observation is reproduced by Los Alamos model as implemented at Bruyeres le Chatel and by the Maslov model. As far as the neutron multiplicity is concerned, a similar dip is observed. However, such a behavior is not present in data measured by other groups

[en] The availability of the last generation of heavy ion accelerators and their sophisticated equipments opens new ways to produce and study Radioactive Nuclear Beams (RNBs). In particular, secondary beams of nuclei in an isomeric state can be produced. Such isomeric beams are of great interest in order to investigate the nuclear structure of nuclei in an excited state. We have produced a pure isomeric beam of ⁴² Sc^m at GANIL with the ¹² C(⁴⁰ Ca, ⁴² Sc^m)¹⁰ B transfer reaction at 30 MeV/A. The secondary beam has been separated with the LISE3 on line spectrometer. We will present results obtained with the same isomer by other transfer reactions and by fragmentation at GANIL (Caen). A total reaction cross section measurement using an isomeric beam has been performed at GANIL in December 1995. The method used as well as preliminary results will be discussed.(author)

[en] Prompt gamma-ray and x-ray spectroscopy techniques are being employed to study fast-neutron-induced fission of actinides to determine independent (pre-beta-decay) yields for a wide range of product nuclides. Data are acquired using the GEANIE high-resolution gamma-ray spectrometer at the LANSCE/WNR unmoderated spallation neutron source providing neutrons with energies from below 1 MeV to over 400 MeV. Three different techniques (identification by characteristic gamma rays, by gamma-gamma coincidences, and by fission-gamma coincidences) are being used to gather complementary data sets from which detailed fission yields can be extracted. From these data, mass and charge distributions are determined over a wide incident-neutron energy range. The phenomena of interest include the transition from asymmetric to symmetric fission, the competition between neutron and gamma-ray emission, nuclear structure effects in fission and the angular momentum imparted to the fission products. Results for 238U and 236U are presented.

[en] In heavy-ion induced nuclear reactions one can produce transient systems with excitation energies up to 5 MeV per nucleon and spins up to ≅100 ℎ. The equilibrium statistical model can predict the mean lifetime for particle emission from moderately hot nuclei provided they are completely thermalized. However, as the excitation energy is increased, one expects to reach a situation of incomplete equilibration and hence a breakdown of the simplest equilibrium model. Determinations of the lifetime for (or intervals between) particle or fragment emissions can be useful both for testing the equilibrium model at low temperatures as well as for characterizing pre-equilibrium emission from partially thermalized nuclei. The net effect is best demonstrated by means of a correlation function, which can be interpreted by comparison to a reaction simulation. By such comparisons one can characterize the mean time intervals between emissions. The simulation programs MENEKA and COULGAN have been written for this purpose; they are Monte Carlo programs based on the following elements: a) Particles are emitted from the surface of an excited nucleus with a distribution of orbital angular momenta. b) Emission energies of the particles are chosen to reproduce experimental measurements or theoretical calculations. c) The distribution of time delays between particle emissions is given by exponential decay laws. d) A three-body trajectory is followed for the two particles and for the recoil nucleus. e) An event is accepted as a valid coincidence if the particle pair satisfies experimental requirements of detector thresholds and geometry. Particle trajectories are calculated numerically using time steps that are controlled by the requirement for energy conservation. An ancillary program SHOWTRAJ can be used to display and study trajectories event by event. (orig.)