J.M. Lagrange

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 .

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.

Case studies of target properties in nuclear spectroscopy

Abstract A review of in-beam electron-gamma-ray coincidence measurements in nuclear spectroscopy particularly influenced by target properties is outlined. To illustrate such an influence, aspects of the recoil ion catcher method for the study of short lived nuclear isomers performed at the Orsay MP-tandem accelerator, are reported. This method is operated with a pulsed heavy-ion beam bombarding a thin self-supported target but avoiding to hit the catcher foil. The required characteristics of the targets and recoil ion stopper foils needed for these measurements are analyzed in detail for a few typical experimental arrangements.

Electron Measurements in the 194,195,196Pb Isotopes

The 184W(16O, 4n, 5n, 6n) reactions are used to obtain the 196,195,194Pb isotopes and study their de-excitation. The conversion coefficients are measured for the three nuclei and the transition multipolarities deduced. For 194Pb and 195Pb other new data come from multidimensional coincidences, life-time and angular distribution measurements. The 195Pb level scheme is rather similar to the 197Pb one, the same thing being true of the two even isotopes. The data are compared to the results of microscopic calculations, performed in the two or three quasi-particle approximation, with a surface delta interaction (corrected to reduce the pairing component intensity).

High spin states in lead isotopes with 193<=A<=199

Theγ de-excitation of odd neutron deficient lead isotopes with mass numbers 193–199, produced by (heavy ion,x n) reactions, was studied with the particular aim of searching for high spin states of the (i13/2)−3 configuration. The properties of levels in199, 197, 195Pb are compared with the predictions of a three quasi-particle calculation, forJ=29/2−15/2, and with the corresponding results in the neighbouring even lead isotopes.

High-spin levels in the 199Pb and 200Pb isotopes

Abstract The de-excitation of the 200 Pb and 199 Pb isotopes, produced through the 186 W( 18 O,4n, 5n) reactions, is investigated using γ-ray and conversion electron spectroscopy. The conversion coefficients are measured and the multipolarities deduced for a great number of transitions. The e − γ- and γγ-coincidences and the half-life measurements lead to several new levels in both level schemes; the 200 Pb one is extended up to nearly 7 MeV, where an isomer is found. The experimental results are compared to those of microscopic calculations, performed in a two or three quasi-particle approximation, using a surface delta interaction with a reduced pairing component.