M. Lefort

Réactions entre protons de 30 à 155 MeV et noyaux lourds complexes

Abstract Excitation functions have been established between 20 and 156 MeV for twelve (p, x n) reactions and nine (p, p x n) reactions induced on Bi by protons . In order to determine the cross sections for light polonium isotopes, new identifications were made for the disintegration schemes on 197–202 Po. New isomeric states were found for 203 Po, 201 Po, 199 Po and 197 Po. The experimental excitation functions have been compared to theoretical values which were deduced from the two-step mechanism (Serber model). It has been shown that the experimental data are in agreement with a direct-interaction model in which prompt cascades are calculated in a diffusive-edge nucleus. However a model in which reflections and refractions are included for the penetration and the exit of the prompt nucleons, yields much too high proportions of compound nucleus, and therefore a simple model in which the impulse approximation is strictly applied for all collisions is a better description of the direct-interaction process than more elaborate calculations. The de-excitation process is described with great success by the statistical theory when at each step of the neutron evaporation chain, a competition is introduced for charged-particle evaporation and for fission. The best fit is obtained with level density parameters a = 13 MeV −1 for the particle emission and a f = 13.5 MeV − for fission. The agreement with experimental data obtained for all excitation functions along such a large energy range (20–155 MeV) seems to be a great success of the Serber-Goldberger model for the description of medium-energy, nuclear reactions.

Evidence for the formation of highly excited compound-like nuclei (T⋍5 MeV) in collisions of 720 MeV C projectiles with 238U, 232Th and 197Au targets

Abstract In collisions of 720 MeV C projectiles with U, Th and Au targets the study of low energy light charged particles detected in coincidence with fission fragments gives strong support for the formation of highly excited compound-like nuclei having reached a large degree of relaxation before decaying.

Reactions of 40Ar with 159Tb, 142Nd and 144Sm; New α-activities from 189Bi, 173Pt and 177Au

Abstract Reactions of 40 Ar ions with targets of 159 Tb, 142 Nd, and 144 Sm have been studied at energies below 300 MeV with a helium gas-jet system. Excitation functions for (Ar, x n) reactions, where x = 5–10, were obtained for the radioactive products that decay by α-emission. Based on the characteristics of these excitation functions and on the systematics of α-decay, evidence is presented for the existence of the nuclides 189 Bi with α-particle energy E α = 6.67±0.01 MeV and half-life 173 Pt with E α = 6.19±0.01 MeV; and for the emission from 177 Au of an α-particle with E α = 6.15±0.01 MeV.

Recherche de fission provenant de l'lnteraction 84Kr+209Bi: La fusion entre ions krypton et noyaux lourds est-elle possible?

Abstract Fragments emitted in binary fission from complete fusion nuclei have been investigated for krypton induced reactions on heavy nuclei. Cross sections are between 25 and 5% of the total reaction cross section. It is deduced that complete fusion between krypton projectiles and heavy nuclei is a very improbable process. Most of the reaction products seem to result from a very inelastic interaction which looks like very asymmetric fission.

Ca induced reactions on 141Pr and 150Sm: New gold and lead isotopes 176Au, 175Au, 185Pb

Abstract Reactions induced by a beam of 40 Ca ions on 141 Pr and 150 Sm have been studied between 180 and 290 MeV. From the characteristics of the ( 40 Ca, xn) excitation functions and from the systematics of α-decay, we deduce the existence of the following new neutron deficient nuclides: 176 Au ( E α = 6.26 ± 0.01 MeV and T α 1 2 = 1.25 ± 0.30 sec and E α = 6.29 ± 0.01 MeV), 175 Au ( E α = 6.44 ± 0.01 MeV ) and 185 Pb ( E α = 6.40 ± 0.01 MeV and 6.48 ± 0.02 MeV). New information is obtained on the α-decay of 177 Au and 187 Pb.

Nuclear reactions induced by 12C and 16O on 205Tl and 209Bi; 214Fr isomeric ratio; compound nucleus and 8Be transfer process

Abstract Targets of 205Tl have been bombarded by 12C (60–90 MeV) and by 16O (60–100 MeV), and 209Bi has been bombarded by 12C (60–90 MeV). Excitation functions were determined for 211Fr, 212Fr, 213Fr and for the two isomers 214Fr. Since 214mFr cannot be produced as a daughter of 218Ac, it was possible to study the de-excitation of 221 Ac into 218Ac by the measurement of 214Fr. Inversely, 214mFr is a prompt product of nuclear reactions. The thresholds of the reactions have been determined and correspond to low Coulomb barriers ( τ o = 1.52 fm for 12 C and 1.45 fm for 1 O ). The effect of high angular momenta on the feeding of the isomeric state, 214mFr (9kh), in the reaction 205Tl(C, 3n)214Fr has been clearly demonstrated. The results on the excitation function of 214mFr in the high-energy region show that a partial fusion nucleus is formed after the transfer of a 8Be cluster (209Bi + 8Be).

Réactions de transfert d'un grand nombre de nucléons induites par 84Kr sur 232Th: étude des seuils de réaction

Abstract Excitation functions have been measured for the formation of 225 Th, 226 Th, 223 Ac and 224 Ac, when 232 Th was bombarded by 84 Kr ions at energies ranging between 450 and 494 MeV. The thresholds of all the reactions appear at 450 MeV (330 MeV c.m.), a higher energy than expected. Cross sections of the order of 1 mb are reached for these transfer reactions corresponding to a large number of neutrons. The results are compared with the same reactions induced with lighter projectiles.

Evaporation residue cross sections from reactions with argon ions

Abstract Evaporation residue cross sections have been measured in argon bombardments of Ni, Ge, Ag, and Sb at energies ranging from 167 to 296 MeV. Earlier Sb results appear to be in error in the energy range above 200 MeV. Results are compared with statistical model calculations which include rotating liquid drop fission barriers.

Competition fission-evaporation de neutrons dans les noyaux issus du bombardement de Bi par des protons de 100 à 150 MeV

Abstract The cross sections have been measured of polonium isotopes with mass numbers 197 to 203 produced in the bombardment of a bismuth target by 100 to 150 MeV protons. Since a competition fission evaporation occurs at each step of the de-excitation process, the fission width for the de-excitation of every polonium nucleus from mass 198 to mass 210 and its dependence on the excitation energy can be deduced. It has been shown that the total fission cross section of 120 mb is due to fission events occurring mainly on light polonium isotopes excited to excitation energies lower than 50 MeV. The neutron spectra at 0° and 90° have beencalculated, assuming that neutrons are emitted from excited nuclei before fission or from excited fragments.

Correlations between fission fragments and light charged-particles in the reaction20Ne+197Au at 13 MeV/nucleon

Abstractrgy of light charged-particles has been measured in coincidence with one or two fission fragments in the reaction20Ne+197Au at a bombarding energy of 13 MeV/u. The fission cross section was found equal to 1,340±260 mb. Assuming that it represents the totality of the fusion cross section, the critical valuelc is deduced equal to 93±9ħ, higher thanlBf=0=74 in the rotating liquid drop model. The main emission source for protons and alpha particles seems to be a thermally equilibrated composite system. The competition between fission and charged particle emission is unexpected in the frame of the statistical treatment. A high energy component is observed in the forward direction. These results correspond to the first step of a study programme on the evolution of nucleus-nucleus collisions between 10 and 100 MeV per nucleon.