  The differential cross sections of two-nucleon transfer reactions
238U(18O,16O)240U around 10 MeV per nucleon are calculated by one-step
Born-approximation with a 16O+2n+238U three-body model. The three-body wave
function in the initial channel is obtained with the continuum-discretized
coupled-channels method, and that in the final channel is evaluated with
adiabatic approximation. The resulting cross sections have a peak around the
grazing angle, and the spin distribution, i.e., the cross section at the peak
as a function of the transferred spin, is investigated. The shape of the spin
distribution is found not sensitive to the incident energies, optical
potentials, and treatment of the breakup channels both in the initial and final
states, while it depends on the excitation energy of the residual nucleus 240U.
The peak of the spin distribution moves to the large-spin direction as the
excitation energy increases. To fulfill the condition that the peak position
should not exceeds 10 hbar, which is necessary for the surrogate ratio method
to work, it is concluded that the excitation energy of 240U must be less than
10 MeV.
