Phase Transitions in a Symmetry-Conserving Framework

Phase transitions are often associated with the breaking of a symmetry in the low-temperature phase described by non-vanishing values of certain order parameters. However, in finite-size systems the correlated equilibrium configuration preserves the symmetries of the underlying Hamiltonian. We discuss a method to calculate the statistical distribution of the order parameters without breaking the corresponding symmetries. The maxima of these statistical distributions mimic the phase transitions that are found in a mean-field approximation. We demonstrate the method for the case of shape transitions in atomic nuclei

Limits on assigning a shape to a nucleus

International audienceThe interpretation of nuclear observables in the laboratory frame in terms of the intrinsic deformation parameters β and γ is a classical theme in nuclear structure. Here we use the quadrupole invariants, calculated within the framework of the configuration-interaction shell model, to clarify the meaning and limitations of nuclear shapes. We introduce a novel method that enables us to calculate accurately higher-order invariants and, therefore, the fluctuations in both β and γ. We find that the shape parameter β often has a non-negligible degree of softness and that the angle γ is usually characterized by large fluctuations, rendering its effective value not meaningful. Contrary to common belief, we conclude that doubly magic nuclei are not spherical, because the notion of a well-defined shape does not apply to them

6th International Workshop on Compound-Nuclear Reactions and Related Topics CNR*18

The Compound-Nuclear Reaction and Related Topics (CNR*) international workshop series was initiated in 2007 with a meeting near Yosemite National Park. It has since been held in Bordeaux (2009), Prague (2011), Sao Paulo (2013), Tokyo (2015), and Berkeley, California (2018). The workshop series brings together experts in nuclear theory, experiment, data evaluations, and applications, and fosters interactions among these groups. Topics of interest include: nuclear reaction mechanisms, optical model, direct reactions and the compound nucleus, pre-equilibrium reactions, fusion and fission, cross section measurements (direct and indirect methods), Hauser-Feshbach theory (limits and extensions), compound-nuclear decays, particle and gamma emission, level densities, strength functions, nuclear structure for compound-nuclear reactions, nuclear energy, nuclear astrophysics, and other topics. This peer-reviewed proceedings volume presents papers and poster summaries from the 6th International Workshop on Compound-Nuclear Reactions and Related Topics CNR*18, held on September 24-28, 2018, at Lawrence Berkeley National Lab, Berkeley, CA

Strong enhancement of level densities in the crossover from spherical to deformed neodymium isotopes

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Strong enhancement of level densities in the crossover from spherical to deformed neodymium isotopes

Understanding the evolution of level densities in the crossover from spherical to well-deformed nuclei has been a long-standing problem in nuclear physics. We measure nuclear level densities for a chain of neodymium isotopes 142,144−151Nd which exhibit such a crossover. These results represent the most complete data set of nuclear level densities to date for an isotopic chain between neutron shell-closure and towards mid-shell. We observe a strong increase of the level densities along the chain with an overall increase by a factor of ≈150 at an excitation energy of 6 MeV and saturation around mass 150. Level densities calculated by the shell model Monte Carlo (SMMC) are in excellent agreement with these experimental results. Based on our experimental and theoretical findings, we offer an explanation of the observed mass dependence of the level densities in terms of the intrinsic single-particle level density and the collective enhancement