  Fusion-evaporation in the $^{124}$Sn+$^{136}$Xe system is studied using a
high intensity xenon beam provided by the Ganil accelerator and the LISE3 wien
filter for the selection of the products. Due to the mass symmetry of the
entrance system, the rejection of the beam by the spectrometer was of the order
of $5times10^8$. We have thus performed a detailed statistical analysis to
estimate random events and to infer the fusion-evaporation cross sections. No
signicant decay events were detected and upper limit cross sections of 172 pb,
87 pb and 235 pb were deduced for the synthesis of $^{257}$Rf, $^{258}$Rf and
$^{259}$Rf, respectively.
