  The Boltzmann-Uehling-Uhlenbeck (BUU) Model, which includes the Fermi motion,
the mean field, individual nucleon-nucleon (N-N) interactions and the Pauli
blocking effect etc., is used to calculate the total reaction cross section
$\sigma_R$ induced by $\alpha$-particles on different targets in the incident
energy range from 17.4 to 48.1 MeV/u. The calculation result can reproduce the
experimental data well. The nucleus-nucleus interaction radius parameter $r_0$
was extracted from experimental $\sigma_R$. It is found that $r_0$ becomes
constant with increasing the mass number of target.
