arXiv:0805.1912v1 [nucl-ex] 13 May 2008 Search for Intrinsic Collective Excitations in 152 Sm W. D. Kulp, 1 J. L. Wood, 1 P. E. Garrett, 2, 3 C. Y. Wu, 4, ∗ D. Cline, 4 J. M. Allmond, 1, † D. Bandyopadhyay, 5, ‡ D. Dashdorj, 6, ∗ S. N. Choudry, 5 A. B. Hayes, 4 H. Hua, 4 M. G. Mynk, 7 M. T. McEllistrem, 5 C. J. McKay, 5 J. N. Orce, 5 R. Teng, 4 and S. W. Yates 5, 7 1 School of Physics, Georgia Institute of Technology, Atlanta, Georgia 30332-0430, USA 2 Department of Physics, University of Guelph, Guelph, Ontario N0B 1S0, Canada 3 TRIUMF, 4004 Wesbrook Mall, Vancouver, British Columbia V6T 2A3, Canada 4 Nuclear Structure Research Laboratory, Department of Physics, University of Rochester, Rochester, New York, 14627, USA 5 Department of Physics and Astronomy, University of Kentucky, Lexington, Kentucky 40506-0055, USA 6 Department of Physics, North Carolina State University, Raleigh, North Carolina, 27695-8202, USA 7 Department of Chemistry, University of Kentucky, Lexington, Kentucky 40506-0055, USA The 685 keV excitation energy of the first excited 0 + state in 152 Sm makes it an attractive candi- date to explore expected two-phonon excitations at low energy. Multiple-step Coulomb excitation and inelastic neutron scattering studies of 152 Sm are used to probe the E2 collectivity of excited 0 + states in this “soft” nucleus and the results are compared with model predictions. No candidates for two-phonon K π =0 + quadrupole vibrational states are found. A 2 + , K = 2 state with strong E2 decay to the first excited K π =0 + band and a probable 3 + band member are established. Accepted for publication as a Rapid Communication in Physical Review C, http://prc.aps.org/. Copyright (2008) by the American Physical Society. Low-energy collective structure in nuclei is a funda- mental manifestation of simple behavior in finite many- body quantum systems. Nuclear collectivity is divided into two basic types: rotational (exhibited in the band structures of deformed nuclei) and vibrational (suggested to be dominant in spherical nuclei)[1]. Deformed nuclei also are suggested to be capable of vibrations about an equilibrium deformed shape. The emergence of predom- inantly prolate spheroidal shape moments suggested two vibrational modes in deformed nuclei: “gamma” vibra- tions (Y 22 + Y 2−2 multipole mode) and “beta” vibrations (Y 20 multipole mode). There is a voluminous literature that discusses one- phonon β-vibrational and γ -vibrational states in de- formed nuclei; and the lowest-lying excited K π =0 + and 2 + states, respectively, are generally identified with these modes. However, vibrations in quantum systems should exhibit multi-phonon eigenstates. Evidence for multiple (two) phonon excitations in deformed nuclei is sparse and has been difficult to obtain. The best examples for multiple phonon excitations are limited to evidence for two-phonon γ vibrations, but con- troversy over this structural interpretation persists (see, e.g., [2, 3, 4, 5, 6, 7, 8]). That the double gamma vi- bration is hard to identify is connected to the high level density in well-deformed nuclei and the low spin of the states of interest. In contrast to the gamma vibration, whose excitation energy generally decreases with mass, ∗ Present address: Lawrence Livermore National Laboratory, Liv- ermore, California 94551, USA † Present address: Department of Physics, University of Richmond, Richmond, Virginia 23173, USA ‡ Present address: TRIUMF, 4004 Wesbrook Mall, Vancouver, British Columbia V6T 2A3, Canada the beta vibration is expected to have the lowest energy in lighter nuclei, which have a lower level density. There is no unequivocal evidence for two-phonon β vibrations. The nucleus 152 Sm is particularly well-suited as a case study for the existence of multi-phonon β and γ vibra- tions in a deformed nucleus. It has one of the lowest- energy candidate β vibrations in any deformed nucleus (and a fairly low-energy candidate γ vibration), such that 2- and even 3-phonon excitations should be below the pairing gap, if they exist. Indeed, recently it has been suggested [9] that 152 Sm and its neighboring iso- tone, 154 Gd, are the best candidates for establishing the β-vibrational mode in deformed nuclei. It has long been regarded as a “soft” nucleus [10]. To address the expectation that these simple multi- quantum excitation modes should exist in 152 Sm, we have carried out a very-high-statistics study using multiple- step Coulomb excitation (multi-Coulex). This study was made using the Gammasphere array [11] of Compton- suppressed Ge detectors in conjunction with the CHICO charged-particle detector array [12]. The experiment used a beam of 152 Sm (E = 652 MeV, an energy in- sufficient to surmount the Coulomb barrier) incident on a thin 208 Pb target (400 μg/cm 2 , 99.86% enrichment) at the Lawrence Berkeley National Laboratory’s 88-Inch Cyclotron. Signals from two ions detected by CHICO in coincidence with at least one “clean” γ ray signal in Gam- masphere (i.e., a p-p-γ coincidence) triggered an event. The CHICO array provided kinematic characterization of scattered ions and recoiling target nuclei for Doppler cor- rections to the γ rays emitted from the Coulomb-excited beam nuclei. High angular resolution was provided both by CHICO, which has 1 ◦ angular resolution, and by Gam- masphere, which was operated with 104 Ge detectors. A total running time of 62 hours provided 7 × 10 8 p-p-γ ,8 × 10 7 p-p-γ -γ , and 1 × 10 7 p-p-γ -γ -γ events. If a two-phonon excitation exists at low energy in