, Volume 322, Issue 4, pp 557-566

The new isotopes258105,257105,254Lr and253Lr

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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.

This work is part of the PHD thesis of F.P. Heßberger
The authors thank N. Angert and the UNILAC staff for the excellent performance of the accelerator. We are indebted to M. Müller and W. Jacoby for the development of the highly efficient sources for isotopically enriched material and H. Folger, W. Hartmann, J. Klemm and W. Thalheimer for the skilful preparation of the large area Bi-targets. Further we are indebted to the GSI experimental electronics group, namely M. Richter, for their helpful assistance. We also thank H.G. Burkhard who is in charge of the mechanical set-ups for the SHIP experiments.