  How to extract an electric dipole (E1) breakup cross section \sigma(E1) from
one- neutron removal cross sections measured by using 12C and 208Pb targets,
\sigma_(-1n)^C and \sigma_(-1n)^Pb, respectively, is discussed. It is shown
that within about 5% error, \sigma(E1) can be obtained by subtracting \Gamma
\sigma_(-1n)^C from \sigma_(- 1n)^Pb, as assumed in preceding studies. However,
for the reaction of weakly-bound projectiles, the scaling factor \Gamma is
found to be two times as large as that usually adopted. As a result, we obtain
13-20% smaller \sigma(E1) of 31Ne at 250 MeV/nucleon than extracted in a
previous analysis of experimental data. By compiling the values of \Gamma
obtained for several projectiles, \Gamma=(2.30 +/- 0.41)\exp(- S_n)+(2.43 +/-
0.21) is obtained, where S_n is the neutron separation energy. The target mass
number dependence of the nuclear parts of the one-neutron removal cross section
and the elastic breakup cross section is also investigated.
