### Conveners

#### Empirical aspects of quantum phase transitions in nuclei

- Norbert Pietralla
- Rick Casten

#### Empirical aspects of quantum phase transitions in nuclei

- Norbert Pietralla
- Rick Casten

The 10th edition of the International Workshop on Quantum Phase Transitions in Nuclei and Atomic Systems and other anniversaries call for some remarks on the development of this topic. I will point out crucial milestones for determining empirical aspects of quantum phase transitions and critical-point phenomena in nuclei and give an introduction to this session and to the rationale of...

It is now well understood that shape coexistence appears in many regions throughout the nuclear chart. Until very recently, there were only a few candidates suggested for nuclei possessing multiple, i.e., more than two, distinct shapes. Within the past several years, however, this has changed dramatically with detailed spectroscopy revealing structures not previously observed, and results...

The tin isotopes, being proton-magic with a long chain of experimentally accessible nuclei, are an important testing ground for nuclear structure models.

Present data show systematic deviations between measured electric quadrupole (E2) ground-state excitation strengths depending on the used techniques.

Also, various nuclear structure models come to different predictions on the systematics...

Two of the main observables which have emerged as model-independent probes of the shape-coexistence phenomenon are the transition strengths, in particular B(E2) and $\rho^2$(E0) [1]. From them, the matrix elements can be extracted, which are particularly sensitive to the wavefunctions of the states they connect and to the degree of mixing between the different nuclear shapes. Therefore, they...

The one-phonon mixed-symmetry $2^+$ of $^{132}$Te is of high interest due to the specific structure of this nucleus, with two valence-proton particles and two valence-neutron holes with respect to the doubly-magic nucleus $^{132}$Sn. In recent experiments, the second excited $2^+$ state has been assigned as the one-phonon mixed-symmetry $2^+$ state [1], due to the high B(M1) transition...

Recent work on the neutron-rich 148Ce showed that it marks the boundary of the N=90 shape-phase transitional region. However, a model analysis within the IBM shows its proximity to the QPT, however, involving the gamma-degree of freedom. This is brought into context with the evolution of trajectories in the symmetry space (Casten triangle) and the NpNn formalism, in which larger numbers of...

For certain combinations of protons and neutrons it is expected that the shape of atomic nuclei can undergo octupole deformation, which would give rise to reflection asymmetry or a "pear shape". In this talk I will review the experimental evidence for octupole instability in medium-mass and heavy nuclei, including the results of recent experiments carried out at CERN using REX-ISOLDE [1] and...

The Coriolis interaction is known to induce perturbations to a Hamiltonian which is diagonal in projection quantum numbers $K$. Precision values for the branching ratios between the scissors mode and the $2^+_1$ state of $^{164}$Dy unveil such a scenario in first order. Employing a two-state mixing calculation, the $K$-mixing matrix element along with first information on $\Delta K = 0$ $M1$...

Nuclear level lifetimes, together with branching ratios and multipole-mixing ratios, give access to transitions strengths between excited states. Such information can be used to examine the nature of these states and underlying common structures and can be indicative of nuclear-structure phenomena such as shape coexistence.

A method targeting lifetimes of excited low-spin states in the order...

The A≈100 region is an interesting region of the nuclear chart with the occurrence of different nuclear structure phenomena. For example, the well known sudden onset of collectivity in the neutron-rich Sr and Zr isotopes [1, 2], the multiple shape coexistence in the neutron-rich stable Cd isotopes [3, 4] or the evidences for 𝛾-softness in the Mo, Ru and Pd isotopes. Lifetimes of excited states...

Many examples of shape coexistence are found, when one type of nucleon is at or close to a shell closure and the other type approaches midshell. This holds also for the Z=50 shell gap at the tin isotopes [1]. Shape coexistence in the region is attributed to 2p-2h intruder states in Sn, or 2p-4h states in Cd. This discovery led to a change in the perception of many nuclei especially in the Cd...