B. Saha

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).

Supersymmetry of identical bands in 171,172Yb supported by lifetime data

Abstract Precision lifetimes of six states in the ground-state bands of 171Yb have been measured using the recoil distance method following Coulomb excitation with a detector of the Euroball Cluster type at the FN-Tandem accelerator in Cologne. From these and previous data on branching and mixing ratios from Canberra, we found, that the transition quadrupole moments for corresponding E2 transitions in 171,172Yb are almost equal. This gives strong support for a weakly broken supersymmetry in 171,172Yb. On the other hand, the neighbor pair of nuclei 171Tm and 172Yb shows no supersymmetry demonstrating the importance of an even pseudoorbital angular momentum as condition for supersymmetry.

Test of the critical point symmetry X(5) in neutron deficient osmium isotopes at A≈180

The energy spectra and relative transition rates of 176,178,180Os were found to be very similar to the values calculated in the framework of the critical point symmetry X(5). In order to enable more stringent tests, including also absolute transition probabilities, two coincidence RDDS lifetime measurements were performed at the Laboratori Nazionali di Legnaro with the GASP spectrometer using the 152,154Sm(29Si,5n)176,178Os reactions. The deduced transition quadrupole moments in 178Os agree well with the X(5) predictions. For 176Os only preliminary results were obtained so far. These preliminary data support an X(5) like structure also in the case of 176Os. The experimental data of both nuclei are compared to calculations in the framework of the interacting boson model (IBM) and of the general collective model (GCM).

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.

Measurement of lifetimes in {sup 46}V with the EUROBALL {gamma}-ray spectrometer

In {sup 46}V picosecond lifetimes were determined using the recoil distance Doppler-shift technique with the Cologne plunger device coupled to the EUROBALL IV spectrometer. The experiment was carried out using the {sup 24}Mg({sup 28}Si, {alpha}pn) reaction at 110 MeV at the Strasbourg VIVITRON accelerator. Subsequently the differential decay curve method in coincidence mode was employed to derive lifetimes for four excited states in the K{sup {pi}}=0{sup -} band. The resulting transition probabilities give a comparison of isospin allowed and forbidden E1 transitions, which clarifies the decay properties of the 2{sup -},T=0 state. Furthermore the B(E2) values within the K{sup {pi}}=0{sup -} band are discussed.

Plunger Lifetime Measurements in 102Pd

Recently, an intense experimental effort has been devoted to the search of empirical proofs of critical‐point symmetries in nuclear structure. These symmetries describe shape‐phase transitions and provide parameter‐free predictions (up to over‐all scale factors) for excitation spectra and B(E2) values. This contribution reports on recent plunger‐lifetime measurements ON 102Pd carried out at LNL, Legnaro, with the Cologne plunger apparatus coupled to the GASP spectrometer and using the 92Zr(13C,3n)102Pd reaction at 48 MeV. According to the results of our measurements, 102Pd is so far the best known paradigm of the E(5) critical‐point symmetry.