The isotope ${}^{257}\mathrm{Rf}$ was produced in the fusion-evaporation reaction ${}^{208}\mathrm{Pb}({}^{50}\mathrm{Ti},n){}^{257}\mathrm{Rf}$. Reaction products were separated and identified by mass. Delayed spectroscopy of ${}^{257}\mathrm{Rf}$ and its decay products was performed. A partial decay scheme with configuration assignments is proposed based on $\alpha$ hindrance factors. The excitation energy of the $1/2^{+}[620]$ configuration in ${}^{253}\mathrm{No}$ is proposed. The energy of this $1/2^{+}$ state in a series of $N=151$ isotones increases with nuclear charge, reflecting an increase in the $N=152$ gap. This gap is deduced to grow substantially from 850 to 1400 keV between $Z=94$ and 102. An isomeric state in ${}^{257}\mathrm{Rf}$, with a half-life of ${160}_{-31}^{+42}\hspace{0.5em}\mu$s, was discovered by detecting internal conversion electrons followed by $\alpha$ decay. It is interpreted as a three-quasiparticle high-$K$ isomer. A second group of internal conversion electrons, with a half-life of $4.1_{-1.3}^{+2.4}$ s, followed by $\alpha$ decay, was also observed. These events might originate from the decay of excited states in ${}^{257}\mathrm{Lr}$, populated by electron-capture decay of ${}^{257}\mathrm{Rf}$. Fission of ${}^{257}\mathrm{Rf}$ was unambiguously detected, with a branching ratio of $b_{\mathrm{Rf}}^{\mathrm{SF}}=0.02\pm 0.01$.

• Received 21 March 2009

DOI:https://doi.org/10.1103/PhysRevC.79.064319