The new isotopes 258 105, 257 105, 254 Lr and 253 Lr
1985
Evaporation residues from the heavy-ion fusion reaction50Ti on209Bi were investigated. They were separated from the projectile beam by the velocity filter SHIP and identified after implantation into an array of position-sensitive surface-barrier detectors by analyzing theirα-decay chains. Spontaneous fission was also observed. Four newα emitters,258105 (T1/2=4.4
−0.6
+0.9
s),257105 (T1/2= 1.4
−0.3
+0.6
s),254Lr (T
1/2= 13
−2
+3
s), and253Lr (T
1/2=1.3
−0.3
+0.6
s) could be identified. For the isotope257105 we obtained a spontaneous-fission branch of about 20%. A spontaneous-fission activity with a halflife comparable to that for theα decay of258105 was explained as fission of258104, formed by electron capture from258105. An excitation function for evaporation-residue production was measured for bombarding energies in the range ofE
CM=184.4 MeV toE
CM=196.6 MeV. Nearly all evaporation residues we observed, could be attributed to the 1n and 2n deexcitation channels. The maximum cross sections wereσ(1n)=c/2.9±0.3) nbarn, andσ(2n)=c/2.1±0.8) nbarn, respectively. We could measure the total kinetic energy of the fission fragments of258104 to be TKE=(220±15) MeV, a value that fits into empirical systematics based on aZ
2/A
1/3 dependence.
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