We use the approximation of the mean nuclear radius $R = R_0A^{1/3}$, where $R_0$ is a constant equal to $1.2 \times 10^{-15}$ m and $A$ is the atomic mass number.
For the alpha particle, $A = 4$, so the radius $r_{\alpha} = R_0 \times 4^{1/3} = 1.2 \times
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