reaction ratios reads R S$_{1}$ i ( P + Q ) R S$_{2}$ i ( P + Q ) = ∑$_{J}$$_{π}$ ˆ Qσ S 1 ( U,J $^{π}$) · B S 1 i ( U,J $^{π}$) ( ˆ P + ˆ Q ) σ S 1 ( U,J $^{π}$) ∑$_{J}$$_{π}$ ˆ Qσ S 2 ( U,J $^{π}$) · B S 2 i ( U,J $^{π}$) ( ˆ P + ˆ Q ) σ S 2 ( U,J $^{π}$) = R n$_{1}$ i ( U ) R n$_{2}$ i ( U ) · ∑$_{J}$ π ˆ Qσ $^{S$_{2}$}$( U, J $^{π}$) · B S$_{2}$ i ( U, J $^{π}$) ∑$_{J}$ π ˆ Qσ $^{S$_{2}$}$( U, J $^{π}$) · B S$_{2}$ i ( U, J $^{π}$) = R n$_{1}$ i ( U ) R n$_{2}$ i ( U ) , (12) where the weak Weisskopf-Ewing condition (Eq. (7)) and proportionality of σ $^{S$_{1}$}$( U, J $^{π}$) and σ $^{S$_{2}$}$( U, J $^{π}$) were employed. Therefore, the surrogate ratio method has a capability to work even when breakup (or preequilibrium) reaction occurs. Even though the derivation here is qualitative, it was enough to assume that the ratios of ˆ P and ˆ Q to be the same for the 2 surrogate reactions used in the ratio method. This can be satisfied if the breakup mechanisms are the same, which is a reasonable assumption. Again, it must be noted that we do not need to understand the breakup reaction mechanism itself, which is a formidable task, but just require them to be the same for the 2 reactions employed in the ratio method. It can be easily verified experimentally by observing the spectra of emitted particles. This may explain the reason why the ratio method worked to measure the $^{236}$U( n, f ) cross section for energies above several MeV as reported by Lyles et al. [8] where the 2nd and 3rd chance fission occur, which corresponds to the condition that the breakup reaction can occur in the surrogate method. We have investigated the condition that the surrogate reaction should work. It was found that the surrogate absolute method will give a marginal result for fission cross sections but it seems to be hopeless to apply it for the capture cross section measurements. On the contrary, it was shown that, without assuming any specific (schematic) spin parity distributions, the surrogate ratio method has a high potential to determine neutron fission and capture cross sections. The achievable accuracy would be around 3 ∼ 5 % for the fission and 10 % for the capture cross sections under the condition investigated in this work (up to difference of spin values of between neutron-induced and surrogate reactions of around 10 ¯ h ) for nuclei in Uranium region at around 2.5 to 5 MeV. The success is brought by the weak Weisskopf-Ewing V. CONCLUDING REMARKS 9