100 Years of Nuclear Isomers

The nuclear structure of neutron-rich actinide ${}^{248}$Cf was investigated at the Tokai Tandem Accelerator Laboratory of the Japan Atomic Energy Agency. This isotopes lies two neutrons and two protons below the generally accepted $Z$=100 and $N$=152 deformed shell gaps, but recently the location of these gaps has been heavily debated and $Z$=98 has also been suggested. ${}^{248}$Cf was produced using the ${}^{249}$Cf(${}^{18}$O,${}^{19}$O)${}^{248}$Cf neutron-removal reaction. The $\gamma$ rays emitted by ${}^{248}$Cf were detected using a composite array of Ge detectors and LaBr${}_3$ scintillators. Two isomeric states with half-lives in the nanosecond range were found among low-lying excited states. The first, with $t_{1/2}$$\sim$5 ns, is the previously known band-head of the $K^{\pi }$=2${}^{-}$ octuple vibrational band at 592 keV. The second, with $t_{1/2}$$\sim$11 ns, is a new state which decays via a low-energy $E1$ transition to a much longer-lived state lying below 1 MeV of excitation. It will be shown how the observation of these low-lying isomeric states favors $Z$=100 over $Z$=98 for the location of the deformed proton shell gap also in ${}^{248}$Cf.