A proton and an \( \alpha -\)particle, having kinetic energies \( K_{p} \) and \( K_{\alpha} \) 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 \( K_{p} : K_{\alpha} \) is:
