A proton and an a-particle, having kinetic energies $\mathrm{K}_{\mathrm{a}}$ and $\mathrm{K}_{\mathrm{a}}$, respectively, enter into a magnetic field at right angles.
The ratio of the radii of trajectory of proton to that of $\alpha$-particle is $2: 1$. The ratio of $\mathrm{K}_2: \mathrm{K}_\alpha$ is :