The radius of the circular path is given by $r = p/(qB)$. Since momentum (p) and magnetic field (B) are the same, the radius is inversely proportional to the charge ($r \propto 1/q$). A proton has charge e, and an alpha particle has charge 2e. The ratio of radii $r_p/r_\alpha = q_\alpha/q_p = 2e/e = 2:1$.