  A previous study of symmetric collisions of massive nuclei has shown that
current models of multi-nucleon transfer (MNT) reactions do not adequately
describe the transfer product yields. To gain further insight into this
problem, we have measured the yields of MNT products in the interaction of 977
(E/A = 4.79 MeV) and 1143 MeV (E/A = 5.60 MeV) $^{204}$Hg with $^{208}$Pb. We
find that the yield of multi-nucleon transfer products are similar in these two
reactions and are substantially lower than those observed in the reaction of
1257 MeV (E/A = 6.16 MeV) $^{204}$Hg + $^{198}$Pt. We compare our measurements
with the predictions of the GRAZING-F, di-nuclear systems (DNS) and improved
quantum molecular dynamics (ImQMD) models. For the observed isotopes of the
elements Au, Hg, Tl, Pb and Bi, the measured values of the MNT cross sections
are orders of magnitude larger than the predicted values. Furthermore, the
various models predict the formation of nuclides near the N=126 shell, which
are not observed.
