Heidelberg 2015 – scientific programme

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HK: Fachverband Physik der Hadronen und Kerne

HK 45: Structure and Dynamics of Nuclei 9

HK 45.2: Group Report

Wednesday, March 25, 2015, 15:00–15:30, T/HS2

Axial asymmetry of excited heavy nuclei as essential feature for the prediction of compound nuclear cross sections and decay rates — •Eckart Grosse¹, Arnd R. Junghans², and Ralph Massarczyk^2,3 — ¹IKTP, Technische Universität Dresden — ²ISP, Helmholtz-Zentrum Dresden-Rossendorf — ³LANL, New Mexico, USA

Nuclear level densities ρ(Ex,J) are a very important input for the prediction of compound nuclear cross sections and decay rates. Different experimental data enter and theory based parameterizations are needed. Fermi energy and pairing gap fix the level density parameter â and the pair condensation energy, but in finite nuclei the energy backshift changes with surface and shell effects controlled by comparing mass formulae to data. A (surprisingly small) surface term added to â is used as the only free global fit parameter and, using a CTM at low Ex, we describe well neutron capture resonance spacings as observed for 146 even-even target nuclei with 51<A<253. This modelling of collectively enhanced level density avoids any ad hoc assumption about an axial symmetry of excited nuclei.

A combination of the photon strength from the global description of IVGDR shapes and their tails by the sum of three Lorentzians (TLO), summed up to obey the TRK sum rule, to the new parameterization for ρ(Ex,J) compares well to cross section data for radiative neutron capture - including Maxwellian averages. Photon scattering and other experiments show that additional minor dipole strength exists; it weakly increases capture cross sections.