New K isomers in Cf248

Abstract

The nuclear structure of $^{248}\mathrm{Cf}$ was investigated at the Tokai Tandem Accelerator Laboratory of the Japan Atomic Energy Agency. $^{248}\mathrm{Cf}$ was one of the nuclei produced in the $^{18}\mathrm{O}+^{249}\mathrm{Cf}$ multinucleon transfer reaction. An array of Ge and ${\mathrm{LaBr}}_{3}$ detectors was used to detect the $\ensuremath{\gamma}$ rays emitted. More than ten new $\ensuremath{\gamma}$-ray transitions from $^{248}\mathrm{Cf}$ were observed and, for the first time, lifetimes of excited states in the nanosecond range were measured. For the previously known bandhead of the ${2}^{\ensuremath{-}}$ octupole vibrational band at 592 keV, a lifetime of 6.7(2) ns was found. Another, new isomeric state, with a lifetime of 16.8(5) ns, decays via a 48-keV $E1$ transition to a much longer-lived state lying at 0.9(3) MeV, for which a lifetime larger than $\ensuremath{\approx}200$ ns is estimated. For this latter state, the unobserved decay points to $K\ensuremath{\geqslant}5$. Considering available theoretical models, possible spin-parity assignments of new states are discussed.