M. Montoya

Deeply inelastic collisions as a source of intermediate mass fragments at E/A=27 MeV☆

Abstract Intermediate-mass fragments detected in coincidence with heavy residues were measured in 40 Ar induced reactions on Ag at E / A =27 MeV. From the observed characteristics, it is inferred that intermediate-mass fragments associated with the so-called intermediate-velocity source come mainly from deeply inelastic collisions occuring after or at the same time as preequilibrium particle emission.

Dynamics and thermalization in violent collisions between40Ar and Ag at 27 MeV/nucleon

For the violent collisions of 27 MeV/nucleon40Ar with Ag, coincidence measurements have been made between heavy residues and intermediate mass fragments (3≦Z≦14) or light charged particles. From the analysis of the correlation between heavy residues (mass and velocity) and intermediate mass fragments, the main characteristics of the dominant mechanisms, fusion and partially damped collisions preceded or accompanied by a preequilibrium emission, are presented. Balances concerning mean values of parallel linear momentum, mass and atomic number, are established and confirm that a complete description of violent collisions was obtained. Then thermalization is discussed, first in terms of excitation energies derived from kinematics between heavy residues and intermediate mass fragments, and secondly in terms of initial temperature estimates derived from light charged particle spectra. Very hot nuclei (T⋍5.7–6.6 MeV) are produced over a large impact parameter range from very central collisions to medium peripheral ones. Various experimental results are compared to predictions obtained with semi-classical calculations (Landau-Vlasov equation). From their good agreement one may conclude that, depending on the impact parameter, thermal equilibrium is achieved within 4–10×10−22 s.

Mass and kinetic energy distribution in cold fission of233U,235U and239Pu induced by thermal neutrons

The time-of-flight technique was used to measure the mass and kinetic energy distribution of fragments from fission of233U,235U and239Pu, induced by thermal neutrons at the Grenoble High Flux Reactor. The data array is presented as equal probability lines in the high kinetic energy regions. The fluctuations observed in those experimental lines are explained by a static scission configuration model, in which the most important influence comes from the Coulomb interaction energy between the two fragments. The highest values of total kinetic energy are obtained for fragmentations with heavy fragmentsZ=50–52,N=80–82 and light fragmentsZ=40–42,N=60–64.

Search for ternary fragmentation in the reaction 856 MeV 98Mo+51V: kinematic probing of intermediate-mass-fragment emissions

Abstract A new technique has been applied to coincidence measurements between fission fragments (FF) and intermediate mass fragments (IMF) emitted from the composite system 149 65 Tb at an excitation energy of 224 MeV. The method permits simultaneous observation of IMF emissions along and normal to the FF separation axes. For the integrated total of 0.10± 0.02 IMF emitted per fission, we find no significant correlation with FF direction, suggesting that IMFs associated with fission reactions are predominantly emitted from the system prior to fission.

Excitation energy partition in deeply inelastic collisions between40Ar and Ag at 27 MeV per nucleon

The dynamics of the two partners produced in dissipative collisions has been experimentally studied for the system40Ar + Ag at 27 MeV per nucleon. Primary masses of the fragments can then be calculated; the excitation energy partition between the two fragments is derived from the number of particles evaporated by each fragment. We found that this division evolves from equipartition to a repartition close to thermal equilibrium in the excitation energy range 300–350 MeV or interaction times 5-10×l0−22 s.

Decay of the compound nucleus179Au formed in the cold fusion reaction90Zr+89Y

For the compound nucleus179Au formed at an excitation energy of 26 MeV in the fusion reaction90Zr+89Y, the energy spectra of promptly emitted protons,α particles andγ rays were measured in concidence with the evaporation residues. On the basis of the measured total decay energy, the 1p and 1α decay channels were separated from all other evaporation-residue channels. The energy spectra and absolute cross sections, together with previously measured excitation functions for various decay channels, are successfully described by statisticalmodel calculations with the Monte Carlo code CODEX.

Coulomb effects in low energy fission

The structures in the total kinetic energy (TKE) distributions in cold fission of234U and236U are interpreted in terms of the Coulomb interaction energy (C) between fragments at the scission point. The maximal value ofC, Cmax, corresponding to the most compact scission configuration, is calculated for several mass fragmentations. It is shown that withQ being constant,Cmax increases if one increases the charge asymmetry for a given primary fragmentation. This condition produces oscillations with a period of approximately 5 amu ofCmax as a function of the light fragment mass which are correlated with the observed oscillations of the maximal value of TKE. Moreover, the enhancement of the yields of the more asymmetric charge fragmentation for a given fragmentation is explained.

Observation of enhanced low-energy charged-particle emission in the cold fusion reaction 90Zr+89Y

Abstract Protons and α-particles emitted from the compound nucleus 179 Auexcited to 26 MeV were detected in coincidence with the corresponding evaporation residues. The charged-particle spectra are shifted to lower energies than spectra from statistical-model calculations based on a spherical compound nucleus.

The influence of the surprising decay properties of element 108 on search experiments for new elements

Abstract Results of experiments to synthesize the heaviest elements are reported. Surprising is the high stability against fission not only of the odd and odd-odd nuclei but also of even isotopes of even elements. Alpha decay data gave an increasing stability of nuclei by shell effects up to 266 109, the heaviest known element. Theoretically, the high stability is explained by an island of nuclei with big quadrupole and hexadecapole deformations around Z = 109 and N = 162. Future experiments will be planned to prove the island character of these heavy nuclei.

Effects of Neutron Emission on Fragment Mass and Kinetic Energy Distribution from Thermal Neutron‐Induced Fission of 235U

The mass and kinetic energy distribution of nuclear fragments from thermal neutron‐induced fission of 235U(nth,f) have been studied using a Monte‐Carlo simulation. Besides reproducing the pronounced broadening in the standard deviation of the kinetic energy at the final fragment mass number around m = 109, our simulation also produces a second broadening around m = 125. These results are in good agreement with the experimental data obtained by Belhafaf et al. and other results on yield of mass. We conclude that the obtained results are a consequence of the characteristics of the neutron emission, the sharp variation in the primary fragment kinetic energy and mass yield curves. We show that because neutron emission is hazardous to make any conclusion on primary quantities distribution of fragments from experimental results on final quantities distributions.