Observation of new neutron-rich micro-second isomers among fission products of $^{238}$U at $345$~MeV/u

In the production of the radioactive isotope (RI) beam using projectile fragment separators, $\gamma$ rays emitted from metastable states, isomers, of the reaction products can be used as a fingerprint of the isotope that is analyzed in the separator. In the operation of the superconducting in-flight RI beam separator BigRIPS [1] at RIKEN RI Beam Factory, the detection of such $\gamma$ rays plays important roles not only in the identification of the RI beam [2] but also in searching for new isomers, providing valuable spectroscopic information on the isotopes. In the recent new-isotope production experiment with BigRIPS using in-flight fission of $345 $~MeV/nucleon $^{238}$U at the beam intensity around $0.3$~pnA, we have observed a number of short-lived isomeric decays which include unknown decays from more than $10$ isotopes, using three clover-type Ge detectors with a beam stopper of aluminum. The primal result of the observations will be reported. [1] T. Kubo: Nucl. Instr. and Meth. B {\bf 204} (2003) 97. [2] T. Ohnishi et al.: J. Phys. Soc. Japan, {\bf 77} (2008) 083201.