M. Pautrat

Étude des isotopes du plomb de masse 200, 198, 196 et 194 formés par réaction (ions lourds, xn)

Abstract The low-lying excited states of 194, 196, 198, 200 Pb have been studied through the de-excitation γ-rays following the reactions Os( 12 C, 4n), W( 16 O, 4n) and Re( 14 N, 5n). Prompt and delayed spectra, γ-γ coincidences, γ-ray angular distributions and γ-ray time distributions have been obtained. Half-lives were measured for the 12 + level in the four isotopes, for the 9 − level in 194, 196, 200 Pb, for the 7− level in 200 Pb and for the 5− level in 196, 198 Pb. The results of microscopic calculations, performed with either a Gaussian interaction or a surface delta interaction, are compared to the experimental level schemes and transition probabilities, and discussed.

COLLISIONS OF FAST CLUSTERS WITH SOLIDS AND RELATED PHENOMENA

Abstract Fast ions, impinging on solid targets at a few MeV per atom, deposit energy in the material through electronic excitation processes. The relaxation of this energy induces emissions of photons, electrons, ions and neutral species from the target. The detailed study of these ejecta can give insight on the energy deposition and relaxation steps of the ion-solid interaction. Fast polyatomic projectiles allow to deposit very high energy densities (at the surface and in the bulk) well beyond that being possible with any single ion impact. As a result, new and unexpected phenomena are oberved which will be discussed: large non linear emission of ions and cluster ions, very high sputtering yields, production of giant tracks and craters in various irradiated materials including metals. When entering the solid, the atomic constituents of the projectile remain in a close proximity for a certain distance, and the energy density deposited in the overlapping trajectories region is high enough to induce collective effects. Secondary ion emission experiments, performed with various carbon cluster beams delivered by the Tandem accelerator in Orsay, aim to determine this depth of spatial correlation by probing these collective effects inside the solid. The influence of the proximity of the cluster constituents on some of their properties such as their charge state distributions inside the solid has been studied and it has been shown that the mean charge state of the constituents is significantly smaller than when the atoms penetrate the target independently.

Sputtering of large size clusters from solids bombarded by high energy cluster ions and fullerenes

Abstract It is shown that polyatomic ions at high energy (MeV) are able to emit large size cluster ions with a high yield when they bombard solid surfaces. Results on sputtering of clusters ejected from inorganic and organic targets as well as sputtering of C x − clusters from carbon foils are presented. Gold cluster Au n , carbon cluster C n and C 60 in the energy range from 2 to 20 MeV were used as projectiles.

Low-energy transitions in the 197Pb and 198Pb isotopes

Abstract The 198Pb and 197Pb isotopes are produced through the 186W(16O, 4n, 5n) reactions. Conversion-electron, γ- and X-ray spectra are measured using the compound-nucleus recoil method. Conversion coefficients and multipolarities are deduced for a large number of transitions. Together with angular distribution measurements and the results of γγt multidimensional coincidences they lead to decay schemes for the two isotopes. Microscopic calculations, performed in the two- or three-quasiparticle approximation with a surface delta interaction, fail to reproduce completely the observed properties, showing similar defects for the odd and even isotope.

Magnetic moments of the 12+ isomeric states in 194, 196, 198Pb

Abstract The g -factor of the 12 + isomeric state in lead isotopes with A = 194, 196, 198 was measured using the time-differential perturbed angular distribution method (TDPAD). The values obtained are respectively g (194) = −0.158(6); g (196) = −0.157(7); g (198) = −0.144(11). A more precise determination of the 12 + level half-life is also made. The g -factprs of these nuclear states, which are described with v (i 13 2 ) −2 as the main configuration, are surprisingly constant over a large mass range (between A = 206 and 194). A core polarization analysis explains this trend: the polarization induced on neutrons in i 13 2 orbit decreases with the mass number A (blocking effect), but a compensation is provided by the other spin-orbit partners f 7 2 -f 5 2 and P 3 2 -P 1 2 .

HIGH DESORPTION-IONIZATION YIELDS OF LARGE BIOMOLECULES INDUCED BY FAST C60 PROJECTILES

Abstract C60 molecules accelerated to MeV energies (20 MeV) have been used to induce the desorption-ionization of large biomolecules from solid samples. For a sample consisting of trypsin molecules, the secondary molecular ion emission yield is about two orders of magnitude larger than with MeV atomic ions. This is a consequence of the very high energy density deposited in solids by 20 MeV C60 projectiles that gives rise to a large amount of matter ejected after each impact. Although time-of-flight mass spectra can be recorded within a few seconds, it is more the acquisition of knowledge on mechanistic aspects, in comparison with other particle-induced desorption methods, which is the objective of obtaining these first results with energetic fullerenes.

Properties of 192, 190Pb: Behaviour of the very neutron-deficient even lead isotopes

Abstract The two very neutron-deficient isotopes of lead A = 190, 192 have been produced and studied through the reaction 156, 154Gd(40Ar, 4nγ)192, 190Pb. The identification of those nuclei was also checked by cross bombardment using the two additional reactions 182W(16O, 6γ)192Pb and 156Gd(40Ar, 6nγ)190Pb. The pulsed 40Ar beam available at ALICE (Orsay) enabled us to record singles γ-spectra as well as four-dimensional (energy-time) spectra corresponding to the correlation of two Ge(Li) detectors for different time windows. Tentative level schemes are proposed. No significant deviation from the systematic behaviour previously reported for the heavier isotopes is found. The data are consistently reproduced within a two-quasiparticle formalism using a surface δ-interaction. However, a slow decrease of the 21+ level energy with decreasing A is observed, which could be an indication of a weakly increasing collective trend when the number of neutron holes increases. These experimental conclusions are in fair agreement with the theoretical results obtained by Dickman and Dietrich who predict a spherical shape for the ground states of the lead isotopes with A ≧ 184 .

The E2 transition probability and effective charge for the neutron i132 subshell in lead isotopes

In (heavy ion, xn) reactions, X-rays and low energy γ-rays emitted by the compound nuclei are investigated using a catcher system for recoiling nuclei within the 10–100 keV energy range. The nucleide 198Pb is studied by means of this technique. A 90 keV E2 (or E2 + M1) transition is identified together with information related to the 12+ → 10+ E2 transition energy. The B(E2; 12+ → 10+) value is deduced. The effective charge extracted from the analysis of the reduced transition probability between two members of the (vi132)−2 multiplet is compared to the corresponding values for other lead isotopes.

High-spin states in the 192Pb and 193Pb isotopes

Abstract The 193 Pb and 192 Pb isotopes are produced through the 182 W( 16 O, 5n, 6n) reactions. The de-excitation γ-ray and conversion electron spectra lead to the conversion coefficients for most transitions. With the results of the γγ and e − γ coincidences, the half-lives measured for several states, the angular distribution coefficients for the odd isotope and the transition multipolarities, the data on the 192 Pb level scheme has been much enhanced and the 193 Pb one studied for the first time. The experimental schemes are compared to those given by microscopic calculations, in a two or three quasi-particle approximation using a surface delta interaction with a reduced pairing component. The discrepancies between theory and experiment are attributed to the increasing influence of proton configurations.

États excités des isotopes de plomb de masse 202, 200 et 198

Abstract The low-lying excited levels of 198, 200, 202Pb have been studied by observing γ- and β-rays and γ−γ coincidences in bismuth decay. Decay schemes have been proposed for these nuclei. Half-lives were determined for the 9−, 7−, 5− and 4+ levels in 200Pb, and for the 5− level in 198Pb. Microscopic calculations have been made with Gaussian and surface delta interactions to account for level schemes. The results are discussed.