ELSEZVlER Nuclear Physics A731 (2004) 291-298 www.elsevier.comllocateinpe From ripples to tidal waves: Low lying vibrational motion in nuclei A. Aprahamian” “Department of Physics, University of Notre Dame, Notre Dame, IN 46556, USA Low lying K” = O+ bands are one of the most fundamental excitation modes in the spectra of deformed nuclei, however, very little is known about the nature of these exci- tations, We report on new lifetime measurements using the GRID technique at ILL, and high-precision (p,t) reactions to elucidate the character of low-lying K” = O+ bands in deformed nuclei. 1. INTRODUCTION Dynamics of various quantum mechanical systems such as molecules, atomic clusters and nuclei are crucial to our most fundamental understanding of motion. In nuclei, these dynamics can be understood in terms of rotational and vibrational degrees of freedom. Rotational degrees of freedom are well understood in nuclei while vibrational degrees of freedom remain elusive. The existence of low-lying collective vibrational motion in nuclei is one of the most fundamental questions in nuclear structure physics today. The answer involves deciphering between the roles of single-particle effects which are relevant in the nuclear fermionic system and collective motion described in terms of phonons or linear superpositions of particle-hole excitations. Vibrational degrees of freedom in both spherical and deformed nuclei are described by phonon excitations resulting from the oscillations around an equilibrium shape. In spherical nuclei, harmonic vibrational motion results in an excitation spectrum consisting of equally spaced degenerate multiplets. Although exact harmonic phonon excitations have never been observed, there are numerous examples of nuclei exhibiting near-harmonic or anharmonic vibrational mo tion. In fact, one- and two-phonon excitations have been observed in tens of nuclei as well as isolated cases of three-phonon excitations[l,2] and in one case up to five quadrupole phonons[2]. In deformed nuclei, there are two types of quadrupole vibrations superimposed on the rotational states: p and y. The ,8 vibration has its angular momentum aligned along the symmetry axis, whereas the y vibration breaks axial symmetry and has a projection of K” = 2+ on the symmetry axis. Extending this description to the spectra of deformed nuclei, the first excited K” = 2+ and K” = O+ bands have traditionally been labelled as single-phonon “7” and “/3” vibrational excitations. The “7” (K” = 2+) excitations show typical B(E2:2j -+ O&) transition probabilities of l-10 Weisskopf units (W.U.) that vary smoothly across a given isotopic chain and can be theoretically understood whereas the single K” = O+ “p” types of excitations have remained an enigma in nuclear structure physics. Figure 1 contains a compilation of all the known absolute B(E2:2gmT0+ + O&) 2 0375-9474/$ - see front matter 0 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nuc1physa.2003.11.040