Isotopes of dubnium (element 105) with mass numbers A = 256, 257, and 258 w
ere produced by the reaction Bi-209 (Ti-50,xn)(259-x) Db (x = 1, 2, 3) at p
rojectile energies of (4.59 5.08) AMeV. Excitation functions were measured
for the 1n, 2n and 3n evaporation channels. The same position of the excita
tion function was observed for the 1n channel as for the previously measure
d 1n channel of the reaction Pb-208 (Ti-50,1n)(257)Rf. The measured alpha -
decay data of (257)Db and its daughter products resulted in the identificat
ion of alpha -decaying isomeric states in (257)Db and Lr-253. Two new isoto
pes, (256)Db and Lr-252, were produced at the highest bombarding energies o
f 4.97 AMeV and 5.08 AMeV. They were identified by delayed alpha-alpha coin
cidences. The measured half-lives are (1.6(-0.3)(+0.5)) s for (256)Db and (
0.36(-0.07)(+0.11)) s for Lr-252. Besides alpha -decay, a spontaneous fissi
on activity of T-1/2 = (2.3(-0.6)(+1.1)) s was observed and attributed to a
n electron-capture branch of (256)Db, which feeds the fissioning nucleus (2
56)Rf. A branching ratio of 0.36 +/- 0.12 was obtained. The isotope (255)Rf
was produced by the reaction Pb-207 (Ti-50,2n)(255)Rf. Improved decay data
have been obtained by means of alpha- and alpha-gamma spectroscopy.