γ-ray spectroscopy of neutron rich Ag isotopes: πg9/2n × νh11/2m structure and triaxiality

International audienceThe fission process produces neutron-rich nuclides far from stability withcomparable excitation energy and relatively high angular momentum. Therefore it isused as an important tool to study the variety of different nuclear structures . Thecollective band around neutron-rich Ag isotopes have configuration of two intruderorbitals πg9/2n ×νh11/2m which presents triaxiality . The interpretations of thesenuclei towards the mid-shell were usually carried out by the deformed basis collectiveshell models as- suming deformation in the core and few quasi particles. The viewfrom large-scale shell model with spherical basis could bring new perspective in thisregion, such as unexpected breaking of natural seniority in In arising from protonneutron interaction.The 113-121Ag isotopes which lie at the borderline of spherical and deformedbasis shell model calculations were studied by exploiting the prompt γ-rayspectroscopy of the isotopically identified fission fragments using the VAMOS++spectrometer and the EXOGAM Ge-array at GANIL [5]. Low-lying high-spin states ofneutron-rich 113,118-121Ag have been established for the first time [6]. The excitedstates of both odd-A and even-A isotopes followed the smooth systematics of thosein Cd core. A large signature splitting in odd-A isotopes and a signature inversion ineven-A isotopes, which is often used as a finger print of triaxial deformation, throughout long chain of isotopes in 50 < N < 82 region, is striking. The large-scale shellmodelcalculations reproduce the experimental spectra and the signature splitting,revealing their microscopic nature. The natural seniority ordering was broken moreseverely than In isotopes [4] inducing strong mixed wave functions.The main features of signature splitting are reproduced by crude shell modelincluding only πg9/2 and νh11/2, indicating the yrast band in the Ag isotopes can beessentially understood based on πg9/2n×νh11/2m multiplet, without any particularassumption about the deformation. This simplicity behind the complex structures inthe wave function could be owing to the strong contribution from both proton andneutron intruder orbitals with unique parity, which makes the configuration of theband fairly pure. Perspectives of the recent experiment with VAMOS++ and AGATAγ-ray tracking detectors will be discussed