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.