Dipole excitations in open shell nuclides near the neutron threshold energy from (g,g') experiments: The case of Ge isotopes

The dipole response of the open-shell nuclide 70Ge has been investigated in high-resolution (g,g') experiments using bremsstrahlung produced with electron beams of energies of 8.5 and 14.7 MeV at the linear accelerator ELBE. A resonance-like structure of levels mostly with spin J = 1 has been identified, distributed between 5 MeV up to neutron separation energy Sn as in the case of 76Ge and in contast to 74Ge where the level density is lower and ceases abruptly at about 1 MeV below Sn . The distibution strength was complemented by the unresolved levels using simulations of statistical gamma-ray cascades, corrected by estimations of branching transitions. The summed strength in 70 Ge, completed by the data from 74,76Ge do not fit with a linear trend as function
of the neutron excess. Such unexpected behaviour might be related to the nuclear deformation which seems to play the major role in the moderately deformed germanium isotopic chain.