Ch. Vieu

Yrast levels of 220Ra populated in the 208Pb(14C, 2n) reaction

Abstract Yrast states in the nucleus 220 Ra were studied by means of the 208 Pb( 14 C, 2n) reaction at 61 and 64 MeV. A staggering sequence of levels of positive and negative parity has been observed up to spin and parity I π = 16 + (18 + ) and from I π = 5 − to I π = 17 − , respectively. These states are connected by strong E1 transitions competing with the stretched E2 transitions, the B( E 1) B( E 2) ratio being ~ 10 −6 fm −2 . The ratio of the excitation energy of the 4 + state to that of the 2 + state is close to the vibrational limit. The moment of inertia associated with the negative-parity yrast states is slightly increasing with the rotational frequency ω. It is considerably higher than that of the positive-parity states at lower spins, the difference decreasing monotonically with increasing ω. The data are discussed with reference to the octupole vibrational picture as well as to the results of recent models predicting reflection-asymmetric shapes in the Ra-Th region.

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.

The particle asymmetric rotor descriptions of187?193Ir positive parity states

A detailed analysis of the positive parity levels in odd iridium isotopes is carried out with an extended version of the Hecht and Satchler model. A satisfactory description of187–193Ir positive parity states and their electromagnetic properties is obtained by coupling several Nilsson particle configurations to an asymmetric rotor.

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.

The recoil shadow anisotropy method

Abstract The Recoil Shadow Anisotropy Method (RSAM) is a new experimental method for identifying isomers in the nanosecond range and measuring their half-lives. This method can be applied to experiments performed with thin targets and γ-ray multidetector arrays including collimated composite detectors and does not require any additional device. It uses the shadow effect imposed by the collimators on the different elements of composite detectors for γ-rays emitted by recoiling nuclei. RSAM was developed for the clover detectors of the Eurogam-2 array and tested using several data sets obtained with this array. A number of known isomers with half-lives lying between 0.9 and 18 ns in 194Hg, 191Au, 148Gd, 149Gd, 193Pb and 194Pb have been successfully re-measured, proving the ability of RSAM for lifetime measurements.

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 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.

Spectroscopic study of short lived, high spin isomers through the recoil method

Abstract The γ and conversion electron de-excitation spectra of nuclei, produced through (HI, χn) reactions, are spoiled by various phenomena, most of them related to the reaction itself. When, in the isotopes studied, one or several isomers are present, the half-life of the higher energy one being about 15 ns or more, then the compound nucleus recoil method can provide much cleaner spectra. This method is extensively described and discussed here, together with others, more briefly treated, also meant to reduce the unwanted effects; these techniques may be used either separately or combined, depending on the experiments performed. Examples of the results obtained are shown to illustrate the substantial improvements reached.

High-spin states in 148Eu

Abstract High-spin states in the odd-odd nucleus 148 Eu, populated by a carbon-13 induced reaction on a lanthanum target, were investigated with several different tools of in-beam nuclear spectroscopy. The low-energy levels show collective excitations, interpreted as 3 − octupole-phonon couplings to multi-particle-hole states. A probable two-octupole-phonon state is identified. Otherwise most 148 Eu high-spin levels are interpreted in terms of fully aligned multi-particle-hole states rather well described both by a semi-empirical shell model and a deformed independent-particle model.