The presently known most neutron-rich isotopes of element 106 (seaborgium, Sg), ${}^{265}$Sg and ${}^{266}$Sg, were produced in the fusion reaction ${}^{22}$Ne+${}^{248}$Cm at beam energies of 121 and 123 MeV. Using the On-Line Gas chemistry Apparatus OLGA, a continuous separation of Sg was achieved within a few seconds. Final products were assayed by $\alpha$-particle and spontaneous fission (SF) spectrometry. ${}^{265}$Sg and ${}^{266}$Sg were identified by observing time correlated $\alpha$-$\alpha$-$\left(\alpha \right)$ and $\alpha$-SF decay chains. A total of 13 correlated decay chains of ${}^{265}$Sg (with an estimated number of 2.8 random correlations) and 3 decay chains of ${}^{266}$Sg (0.6 random correlations) were identified. Deduced decay properties were $T_{1/2}$=${7.4}_{-2.7}^{+3.3\mathrm{}}$ s (68% c.i.) and $E_{\alpha }$=8.69 MeV (8%), 8.76 MeV (23%), 8.84 MeV (46%), and 8.94 MeV (23%) for ${}^{265}$Sg; and $T_{1/2}$=${21}_{-12}^{+20\mathrm{}}$ s (68% c.i.) and $E_{\alpha }$=8.52 MeV (33%) and 8.77 MeV (66%) for ${}^{266}$Sg. The resolution of the detectors was between 50–100 keV (full width at half maximum). Upper limits for SF of $<~$35% and $<~$82% were established for ${}^{265}$Sg and ${}^{266}$Sg, respectively. The upper limits for SF are given with a 16% error probability. Using the lower error limits of the half-lives of ${}^{265}$Sg and ${}^{266}$Sg, the resulting lower limits for the partial SF half-lives are $T_{1/2}^{\mathrm{SF}}$${(}^{265}$Sg)$>~$13 s and $T_{1/2}^{\mathrm{SF}}$${(}^{266}$Sg)$>~$11 s. Correspondingly, the partial $\alpha$-decay half-lives are between $T_{1/2}^{\alpha }$${(}^{265}$Sg)=4.7–16.5 s (68% c.i.) and $T_{1/2}^{\alpha }$${(}^{266}$Sg)=9–228 s (68% c.i.), using the upper and lower error limits of the half-lives of ${}^{265}$Sg and ${}^{266}$Sg. The lower limit on the partial SF half-life of ${}^{266}$Sg is in good agreement with theoretical predictions. Production cross sections of about 240 pb and 25 pb for the $\alpha$-decay branch in ${}^{265}$Sg and ${}^{266}$Sg were estimated, respectively.

• Received 25 November 1997

DOI:https://doi.org/10.1103/PhysRevC.57.1648