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