Observation of new microsecond isomers among fission products from in-flight fission of 345 MeV/nucleon<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msup><mml:mrow/><mml:mn>238</mml:mn></mml:msup></mml:math>U

A search for isomeric \ensuremath{\gamma} decays among fission fragments from 345 MeV/nucleon ${}^{238}$U has been performed at the RIKEN Nishina Center RI Beam Factory. Fission fragments were selected and identified using the superconducting in-flight separator BigRIPS and were implanted in an aluminum stopper. Delayed \ensuremath{\gamma} rays were detected using three clover-type high-purity germanium detectors located at the focal plane within a time window of 20 \ensuremath{\mu}s following the implantation. We identified a total of 54 microsecond isomers with half-lives of \ensuremath{\sim}0.1--10 \ensuremath{\mu}s, including the discovery of 18 new isomers in very neutron-rich nuclei: ${}^{59}$Ti${}^{m}$, ${}^{90}{\mathrm{As}}^{m}$, ${}^{92}$Se${}^{m}$, ${}^{93}$Se${}^{m}$, ${}^{94}$Br${}^{m}$, ${}^{95}$Br${}^{m}$, ${}^{96}$Br${}^{m}$, ${}^{97}$Rb${}^{m}$, ${}^{108}$Nb${}^{m}$, ${}^{109}$Mo${}^{m}$, ${}^{117}$Ru${}^{m}$, ${}^{119}$Ru${}^{m}$, ${}^{120}$Rh${}^{m}$, ${}^{122}$Rh${}^{m}$, ${}^{121}$Pd${}^{m}$, ${}^{124}$Pd${}^{m}$, ${}^{124}$Ag${}^{m}$, and ${}^{126}$Ag${}^{m}$, and obtained a wealth of spectroscopic information such as half-lives, \ensuremath{\gamma}-ray energies, \ensuremath{\gamma}-ray relative intensities, and \ensuremath{\gamma}\ensuremath{\gamma} coincidences over a wide range of neutron-rich exotic nuclei. Proposed level schemes are presented for ${}^{59}$Ti${}^{m}$, ${}^{82}$Ga${}^{m}$, ${}^{92}$Br${}^{m}$, ${}^{94}$Br${}^{m}$, ${}^{95}$Br${}^{m}$, ${}^{97}$Rb${}^{m}$, ${}^{98}$Rb${}^{m}$, ${}^{108}$Nb${}^{m}$, ${}^{108}$Zr${}^{m}$, ${}^{109}$Mo${}^{m}$, ${}^{117}$Ru${}^{m}$, ${}^{119}$Ru${}^{m}$, ${}^{120}$Rh${}^{m}$, ${}^{122}$Rh${}^{m}$, ${}^{121}$Pd${}^{m}$, ${}^{124}$Ag${}^{m}$, and ${}^{125}$Ag${}^{m}$, based on the obtained spectroscopic information and the systematics in neighboring nuclei. The nature of the nuclear isomerism is discussed in relation to the evolution of nuclear structure.