# Darmstadt 2016 – wissenschaftliches Programm

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# HK: Fachverband Physik der Hadronen und Kerne

## HK 68: Structure and Dynamics of Nuclei XII

### HK 68.1: Gruppenbericht

### Freitag, 18. März 2016, 14:00–14:30, S1/01 A03

**Axial asymmetry of excited heavy nuclei as essential feature for the prediction of level densities** — •Eckart Grosse^{1}, Arnd R. Junghans^{2}, and Ralph Massarczyk^{3} — ^{1}Institute of Nuclear and Particle Physics, Technische Universität Dresden — ^{2}Institute of Radiation Physics, Helmholtz-Zentrum Dresden-Rossendorf — ^{3}Los Alamos National Laboratory, New Mexico, USA

In previous studies a considerable improvement of predictions for neutron resonance spacings by a modified back-shifted Fermi-gas model (BSFM) was found. The modifications closely follow the basic priciples for a gas of weakly bound Fermions as given in text books of statistical physics: (1) Phase transition at a temperature defined by theory, (2) pairing condensation independent of A, and (3) proportionality of entropy to temperature (and thus the level density parameter) fixed by the Fermi energy.

For finite nuclei we add: (4) the back-shift energy is defined by shell correction and (5) the collective enhancement is enlarged by allowing the axial symmetry to be broken. Nearly no parameter fitting is needed to arrive at a good reproduction of level density information obtained by various methods for a number of nuclei in a wide range of A and E. To that end the modified BSFM is complemented by a constant temperature approximation below the phase transition point. The axial symmetry breaking (5), which is an evidently essential feature, will also be regarded with respect to other observables for heavy nuclei.