We discuss how to extract an electric dipole (E1) breakup cross section $sigma ({mathrm {E1}})$ from one-neutron removal cross sections measured at 250?MeVucleon by using ${}^{12}$C and ${}^{208}$Pb targets, $sigma _{-1n}^{mathrm {C}}$ and $sigma _{-1n}^{mathrm {Pb}}$, respectively. It is shown that, within about 5% error, $sigma ({mathrm {E1}})$ can be obtained by subtracting $Gamma sigma _{-1n}^{mathrm {C}}$ from $sigma _{-1n}^{mathrm {Pb}}$, as assumed in preceding studies. However, for the reaction of weakly bound projectiles, the scaling factor $Gamma $ is found to be about twice as large as that usually adopted. As a result, we obtain a 13–20% smaller $sigma ({mathrm {E1}})$ of ${}^{31}$Ne at 250?MeVucleon than extracted in a previous analysis of experimental data. By compiling the values of $Gamma $ obtained for several projectiles, $Gamma =(2.30pm 0.41)exp (-S_n)+(2.43pm 0.21)$ 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.
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