Study of shape evolution in very neutron-rich Cs isotopes

We have searched for the isomers in neutron-rich Cs isotopes of $^{144-148}$Cs in one of the EURICA campaign experiments at RIBF in RIKEN, in order to investigate the shape evolution in the very neutron-rich Cs isotopes. The isomers were produced using the in-flight fission reaction of the 345 MeV/u $^{238}$U$^{86+}$ beam. The fragment separator system consisting of BigRIPS and Zero Degree Spectrometer (ZDS) was tuned for the neutron-rich Sb, Te, I, Xe and Cs isotopes with $A=$140-150. The isotopes were implanted into a stack of 5 double-sided Si strip detectors (WAS3ABi). The beta ray and gamma ray emitted from the stopped isotopes were detected by WAS3ABi and EURICA, respectively. Event-by-event unambiguous particle identification was achieved by careful analyses using the ion-optics matrices as well as the Delta E information from the ion chambers and the timing information from the plastic scintillators. The isomers in $^{145}$Cs, $^{146}$Cs, $^{147}$Cs, $^{148}$Cs were observed for the first time. Isomers in odd-odd $^{146}$Cs, $^{148}$Cs could be explained to be caused by the low-energy transition. On the other hand, isomers in odd $^{145}$Cs, $^{147}$Cs are supposed to be candidate of $K$ isomer, which indicate that these nuclei have prolate deformation.