O. Möller

Test of the critical point symmetry X(5) in the mass A = 180 region

Based on the energy spectra and relative transition probabilities 176,178,180 Os turned out to be very promising candidates for testing the features of the critical point symmetry X(5). In order to also test absolute transition probabilities predicted in the framework of the X(5) symmetry lifetime measurements were performed at the Cologne FN tandem accelerator. The lifetimes of the first 2 + states in 176,178,180 Os were measured using electronic timing and in the case of 178 Os with the recoil distance Doppler shift (RDDS) technique using an 16 O-induced fusion evaporation reaction. In addition, a coincidence RDDS measurement was performed at the Laboratori Nazionali di Legnaro with the GASP spectrometer using the 154 Sm( 29 Si,5n) 178 Os reaction. The deduced transition quadrupole moments agree well with the X(5) predictions, thus further supporting an X(5) structure for 178 Os. The experimental data for 178 Os are compared to calculations in the framework of the interaction boson model (IBM) and of the general collective model (GCM).

Octupole and hexadecapole bands in 152Sm

The nucleus {sup 152}Sm is characterized by a variety of low-energy collective modes, conventionally described as rotations, {beta} vibrations, and {gamma} vibrations. Recently, it has been suggested that {sup 152}Sm is at a critical point between spherical and deformed collective phases. Consequently, {sup 152}Sm is being studied by a variety of techniques, including radioactive decay, multi-step Coulomb excitation, in-beam ({alpha},2n{gamma}) {gamma}-ray spectroscopy, and (n,n{gamma}) spectroscopy. The present work focuses on the latter two reactions; these have been used to investigate the low-lying bands associated with the octupole degree of freedom, including one built on the first excited 0{sup +} band. In addition, the K{sup {pi}} = 4{sup +} hexadecapole vibrational band has been identified.

Systematic exploration of 0 + states in structurally diverse nuclei

The puzzling nature of 0 + excitations, especially in transitional and deformed nuclei, has attracted new attention. Following a recent experiment studying 158 Gd, we investigated 15 additional nuclei in and near the rare earth region. Our aim was to map out the energies of as many 0 + states as possible. A series of (p, t) experiments was performed at the Q3D magnetic spectrograph at the University of Munich MP tandem accelerator laboratory. These reactions are particularly sensitive to 0 + states. Outgoing tritons were recorded at various lab angles, and their relative cross sections are compared to those calculated using the distorted wave Bom approximation (DWBA). The L = 0 angular distribution is easily discerned. New 0 + states are found in each of these nuclei, and their systematics are discussed.

Lifetime determination in delayed-coincidence experiments using the differential decay-curve approach

Abstract A new method for the analysis of delayed-coincidence lifetime experiments is proposed following the approach of the Differential decay-curve method. Examples of application of the procedure using simulated and experimental data reveal its reliability for lifetimes in the nanosecond and sub-nanosecond range. The procedure is expected to improve the treatment of systematic errors and scarce data. Possible further expansions and practical aspects of the procedure are also discussed.

Lifetimes and electromagnetic transition strength in 157Dy

Excited states in 157Dy have been studied by γ-γ coincidence measurements via the reaction 124Sn(36S,3n) at a beam energy of 155MeV. Lifetimes of the relatively lower-spin states in 157Dy were measured by means of the Recoil Distance Dopplershift technique in the coincidence mode. The experiment was performed at the Laboratori Nazionali di Legnaro with the GASP array and the Cologne plunger device. With the same setup a Doppler- shift attenuation (DSA) lifetime measurement was performed for the higher spin states. The Differential decay-curve method was applied for the lifetime determination.

Electromagnetic transition strengths in 155Dy

Lifetimes of excited states in 155Dy were measured by means of the Recoil Distance Doppler-shift technique in the coincidence mode. The experiment was performed at the Laboratori Nazionali di Legnaro with the GASP array and the Cologne plunger using the reaction 124Sn(36S,5n)155Dy at a beam energy of 155 MeV. The Differential decay-curve method was applied for the lifetime determination. The measured transition probabilities in 155Dy and the energy spectrum are compared to the predictions of the Particle plus rotor model. The comparison indicates slightly different quadrupole deformations characterizing the low-lying one-quasineutron bands which may point to a shape coexistence.