[en] The nature and evolution of collectivity and coherence in nuclei is one of the most fundamental issues in nuclear structure and its evolution with N and Z. Despite many experiments, the nature of nuclear vibrational modes in deformed nuclei and the nature of nuclear phase/shape transitions are not at all understood. The authors discuss new experiments on phonon and multi-phonon states in the rare earth nuclei and on new evidence for phase coexistence in Sm that relates to the possible existence of phase transitional behavior in finite nuclei

[en] The energy ratio of the first excited states of even-even nuclei R_4/2 triple-bond E(4+₁)/E(2+₁), is an excellent signature of structure, varying smoothly from <2 in open-quotes precollectiveclose quotes nuclei to 3.33 for good rotors. We will discuss this phenomenology, in terms of the extremely simple idea that a high R_4/2 results when the wave function has large amplitudes for pairs of orbits, differing by Δj=2, each orbit occupied by an odd numbers of nucleons. In such a case, the angular momenta must be co-planar leading to a large attractive interaction energy. This explains why light 2-valence nucleon nuclei such as ¹⁸O and ⁴²Ce have R_4/2∼1.7 (light shells have Δj=2 pairs) while ¹³⁴Te, ²¹⁰Pb, ²¹⁰Po have R_4/2∼1.3 (in heavy nuclei the 1 circ s force lowers the open-quotes unique parityclose quotes state, leaving no Δj=2 pairs early in the shell). It also seems to account for the systematics of R_4/2 in singly magic nuclei, and links the structure of pre-collective nuclei naturally onto the particle-hole RPA structure of vibrational modes

[en] Topics discussed in this International Summer School of Nuclear Physics include time dependent and Hartree-Fock methods in quantum mechanics and nuclear models; angular momentum, high spin states, giant resonance, nuclear deformation, collective model and excitations; nuclear molecules; heavy ion reactions; fission reactions; and inertial confinement

[en] An analysis of yrast energies in Collective medium and heavy even-even nuclei leads to two remarkable results. First, nuclei from Z=38-82 and with E(4₁+)/E(2₁+) ranging from 2.05 to 3.15 can be described by a generic anharmonic vibrator equation with nearly constant anharmonicity, independent of internal phonon structure. Secondly, the transition from anharmonic vibrator behavior to rotor can be cast as a critical phase transition with a critical point and a nuclear order parameter. These results suggest a need to reconsider the nature and evolution of nuclear structure, the universality of phonon and rotation-vibration interactions, and the manifestation of phase transitional behavior in finite-body nuclear systems

[en] The evolution of nuclear structure along the Xe and Ba isotopes with N<82 is studied in comparison with that in the Pt-Os isotopes and with interacting boson approximation calculations. As is well known, the Pt isotopes from ^188-196Pt exhibit a stable structure very close to O(6), the lighter Pt isotopes and the Os nuclei evolve towards a rotor, and the heaviest Pt nuclei, ^198,200Pt, show a tendency towards vibrational character. In contrast, we show that the Ba nuclei are best described by a trajectory from U(5) towards SU(3) that passes through an intermediate structure that resembles O(6) for N∼72 endash 76. The Xe appears to evolve from U(5) to O(6)-like. copyright 1998 The American Physical Society

[en] Cross sections for the (p,t) reaction to ground states of even-even nuclei from Zr to Hg are interpreted in a simple approach. They are shown to be sensitive both to the degree of shell filling (which is related to the number of L=0 pairs in the ground state) and to the difference in the number of these pairs between target and final nuclei (that is, to the overlap of the initial and final systems). The cross sections separate according to the two broad classes of nuclei vibrational and rotational. The analysis is carried out in both a model-independent way and within the framework of the interacting boson approximation. copyright 1998 The American Physical Society

[en] The properties of K=0⁺ excitations in deformed and transitional nuclei have recently been of intense interest. We present results of a study of the deexcitations of the lowest excited 0⁺ state in ¹⁵²Sm from the ε-decay of ¹⁵²Eu, yielding one of the few precisely known values of the branching ratio R_0g^γ=B(E2;2_γ⁺→0₂⁺)/B(E2;2_γ⁺→0₁⁺) =0.048(4), which is extraordinarily small. From T_1/2(2_γ⁺) we also obtain B(E2;2_γ⁺→0₂⁺)=0.17 W.u. Values of R_0g^γ calculated in the interacting boson model (IBA) go to zero extremely rapidly, changing by orders of magnitude for a narrow range of parameter values. ¹⁵²Sm is a rare case of a transitional nucleus that lands almost at the minimum. ¹⁵²Sm and ¹⁵⁴Gd are the only nuclei from 90≤N≤114 where the B(E2) values for all four transitions 2_γ⁺→0₂⁺, 2_γ⁺→0₁⁺, 0₂⁺→2₁⁺, and 2₁⁺→0₁⁺ are now known. In ¹⁵²Sm these B(E2) values span three orders of magnitude, from 144 to 0.17 W.u. and are reproduced to within a factor of 2 endash 3 by the IBA. The rather strong B(E2;0₂⁺→2₁⁺) value of 33 W.u. suggest that the 0₂⁺ level is an example of a good low energy β-vibration. copyright 1998 The American Physical Society

[en] Short communication

No abstract available

[en] It was recently demonstrated that the empirical linear correlation of E(4₁⁺) with E(2₁⁺) is reproduced by the interacting boson model 1 (IBA-1) if a constant value for the coefficient κ of the Q·Q term is used. This constancy motivates the present investigation of whether it is possible to find sets of IBA-1 parameters, with fixed κ, that reproduce the data for a more general set of observables and a wide range of nuclei. A consistent procedure, based on contour plots of key observables, was used to extract ε and χ values for 145 nuclei spanning the Z = 50 endash 82 shell, which give reasonable overall agreement with the data with smoothly varying parameters. The constant κ constraint sometimes leads to parameter values different than normal: finite ε values (∼ 0.1 MeV) even for rotational nuclei and large χ values for transitional and vibrational nuclei that even increase towards the vibrational region. Finally, a systematic correlation of ε values with E(2₁⁺) was found. copyright 1997 The American Physical Society