A proton is at rest at the plane vertical boundary of a region containing a uniform vertical magnetic field B. An alpha particle moving horizontally makes a head-on elastic collision with the proton. Immediately after the collision, both particles enter the magnetic field, moving perpendicular to the direction of the field. The radius of the proton’s trajectory is R. Find the radius of the alpha particle’s trajectory. The mass of the alpha particle is four times that of the proton, and its charge is twice that of the proton.