  A method is proposed to determine the one-neutron $S_n$ or two-neutron
$S_{2n}$ separation energy of neutron-rich isotopes. Relationships between
$S_n$ ($S_{2n}$) and isotopic cross sections have been deduced from an
empirical formula, i.e., the cross section of an isotope exponentially depends
on the average binding energy per nucleon $B/A$. The proposed relationships
have been verified using the neutron-rich copper isotopes measured in the 64$A$
MeV $^{86}$Kr + $^{9}$Be reaction. $S_n$, $S_{2n}$, and $B/A$ for the very
neutron-rich $^{77, 78, 79}$Cu isotopes are determined from the proposed
correlations. It is also proposed that the correlations between $S_n$, $S_{2n}$
and isotopic cross sections can be used to find the location of neutron drip
line isotopes.
