Background: Lifetimes of nuclear excited states in fission fragments have been studied in the past following isotope separation, thus giving access mainly to the fragments' daughters and only to long-lived isomeric states in the primary fragments. For the first time now, short-lived excited states in the primary fragments, produced in neutron-induced prompt fission of ${}^{235}\mathrm{U}$ and ${}^{241}\mathrm{Pu}$, were studied within the EXILL&FATIMA campaign at the intense neutron-beam facility of the Institute Laue-Langevin in Grenoble.

Purpose: We aim to investigate the quadrupole collective properties of neutron-rich even-even ${}^{138,140,142}\mathrm{Xe}$ isotopes lying between the double shell closure $N=82$ and $Z=50$ and a deformed region with octupole collectivity.

Method: The $\gamma$ rays emitted from the excited fragments were detected with a mixed array consisting of 8 HPGe EXOGAM Clover detectors (EXILL) and 16 ${\mathrm{LaBr}}_3$(Ce) fast scintillators (FATIMA). The detector system has the unique ability to select the interesting fragment making use of the high resolution of the HPGe detectors and determine subnanosecond lifetimes using the fast scintillators. For the analysis the generalized centroid difference method was used.

Results: We show that quadrupole collectivity increases smoothly with increasing neutron number above the closed $N=82$ neutron shell. Our measurements are complemented by state-of-the-art theory calculations based on shell-model descriptions.

Conclusions: The observed smooth increase in quadrupole collectivity is similar to the evolution seen in the measured masses of the xenon isotopic chain and is well reproduced by theory. This behavior is in contrast to higher $Z$ even-even nuclei where abrupt change in deformation occurs around $N=90$.

• Received 25 July 2016

DOI:https://doi.org/10.1103/PhysRevC.94.034302