The radius of the path is $r = p/(qB)$. Since momentum (p) and magnetic field (B) are equal, the radius is inversely proportional to charge ($r \propto 1/q$). A proton has charge +e, and an alpha particle has charge +2e. The ratio of radii (proton:alpha) is $(1/e) : (1/2e)$, which simplifies to 2:1.