Decay properties of neutron-deficient isotopes 256,257Db^{256, 257}Db, 255Rf^{255}Rf, 252,253Lr^{252, 253}Lr

Isotopes of dubnium (element 105) with mass numbers $A = 256$, 257, and 258 were produced by the reaction $^{209}$Bi($^{50}$Ti,xn)$^{259-{x}}$Db ($x=1,2,3$) at projectile energies of (4.59-5.08) AMeV. Excitation functions were measured for the 1n, 2n and 3n evaporation channels. The same position of the excitation function was observed for the 1n channel as for the previously measured 1n channel of the reaction $^{208}$Pb($^{50}$Ti,1n)$^{257}$Rf. The measured $\alpha$-decay data of $^{257}$Db and its daughter products resulted in the identification of $\alpha$-decaying isomeric states in $^{257}$Db and $^{253}$Lr. Two new isotopes, $^{256}$Db and $^{252}$Lr, were produced at the highest bombarding energies of 4.97 AMeV and 5.08 AMeV. They were identified by delayed $\alpha$-$\alpha$ coincidences. The measured half-lives are $(1.6^{+0.5}_{-0.3})$ s for $^{256}$Db and $(0.36^{+0.11}_{-0.07})$ s for $^{252}$Lr. Besides $\alpha$-decay, a spontaneous fission activity of $T_{1/2}=(2.3^{+1.1}_{-0.6})$ s was observed and attributed to an electron-capture branch of $^{256}$Db, which feeds the fissioning nucleus $^{256}$Rf. A branching ratio of $0.36 \pm 0.12$ was obtained. The isotope $^{255}$Rf was produced by the reaction $^{207}$Pb($^{50}$Ti,2n)$^{255}$Rf. Improved decay data have been obtained by means of $\alpha$- and $\alpha$-$\gamma$ spectroscopy