Exotic Modes of Collective Excitations: Nuclear Tidal Waves and Chirality

Two exotic modes of collective excitations of nuclei have been investigated in this work: the multiphonon excitations in the vibrational nucleus, ¹⁰²Pd and the phenomenon of chirality in the ¹³³Ce nucleus. The vibrational yrast states in ¹⁰²Pd are described semiclassically as quadrupole running ('tidal') waves on the surface of the nucleus, and the propagating tidal wave interpreted as a rotating condensate of interacting, spin-aligned d bosons. The tidal wave concept has been investigated experimentally by measuring lifetimes of levels in the yrast band of the ¹⁰²Pd nucleus using the Doppler shift attenuation method (DSAM). The extracted reduced transition probabilities, B(E2), for the yrast band display a monotonic increase with spin, in agreement with the interpretation based on rotation-induced condensation of aligned d-bosons, and the observed constant B(E2)/J ratios imply that the gain in angular momentum originates from the increase of the wave amplitude (deformation). In the second investigation, two distinct sets of chiral-doublet bands based on the three quasi-particle configurations π(1h_11/2)²⊗ ν (1h_11/2)^-1 (higher-energy, negative parity) and π(1g_7/2)^-1(1h_11/2)¹ ⊗ν(1h_11/2)^-1 (lower-energy, positive parity) were identified in the nucleus ¹³³Ce. The properties of these bands were observed to satisfy the established fingerprints of nuclear chirality and were found to agree with results of calculations based on a combination of the constrained triaxial relativistic mean field (RMF) theory and the particle-rotor model. They constitute a multiple chiral doublet (MχD), a phenomenon first predicted by RMF calculations. This study has provided the first experimental evidence for the existence of the MχD phenomenon, that represents, in general, a confirmation of triaxial shape coexistence.