Cross sections of the (HI, αn) channel in the cold-fusion-type reactions 209Bi+ 40Ar and 208Pb+ 37Cl

By applying an off-line radiochemistry technique, cross sections for the production of the isotope ²⁴⁰Cm(T _{1}/{2}=27 d) in the cold-fusion-type reactions ²⁰⁹Bi+ ⁴⁰Ar and ²⁰⁸Pb+ ³⁷Cl at the bombarding energy E_lab⩽230 MeV were determined to be 0.5±0.2 nb and 0.6±0.3 nb, respectively. The production of ²⁴⁰Cm was attributed to the 1n-deexcitation channel of the composite systems ²⁴⁹Md and ²⁴⁵Es. At the same time, the measured ²⁴⁰Cm production cross sections represent upper cross-section limits for the (HI, αn) channel of the reactions under study. These upper limits are about 100 times lower than the cross-section values reported recently by Nomura et al. for the ( ⁴⁰Ar, αx n) channels with x=1,2 of the ²⁰⁹Bi+ ⁴⁰Ar reaction at E_lab=208 MeV. In the context of the above measurements, presented and discussed is the up-to-date summary of the available evidence on cross sections of the (HI, αxn) channels in the cold-fusion-type reactions induced by projectiles ranging from ³⁷Cl to ⁵⁰Ti on targets of ^203,205Tl, ²⁰⁸Pb, and ²⁰⁹Bi nuclei. Appreciable EC(β ⁺)-delayed or/and spontaneous fission effects were detected in the ²⁰⁹Bi+ ⁴⁰Ar, ²⁰⁸Pb+ ³⁷Cl, and ²⁰⁶Pb+ ³⁷Cl reactions. The obtained data point, in particular, to the EC(β ⁺)-delayed fission occuring in the decay chains ? and ?.