  The gamma-ray decay of excited states of the one-valence-proton nucleus 133Sb
has been studied using cold-neutron induced fission of 235U and 241Pu targets,
during the EXILL campaign at the ILL reactor in Grenoble. By using a highly
efficient HPGe array, coincidences between gamma-rays prompt with the fission
event and those delayed up to several tens of microseconds were investigated,
allowing to observe, for the first time, high-spin excited states above the
16.6 micros isomer. Lifetimes analysis, performed by fast-timing techniques
with LaBr3(Ce) scintillators, reveals a difference of almost two orders of
magnitude in B(M1) strength for transitions between positive-parity medium-spin
yrast states. The data are interpreted by a newly developed microscopic model
which takes into account couplings between core excitations (both collective
and non-collective) of the doubly magic nucleus 132Sn and the valence proton,
using the Skyrme effective interaction in a consistent way. The results point
to a fast change in the nature of particle-core excitations with increasing
spin.
