The radius of a charge particle q moving in a magnetic field is given by,
r=qBmv=qBp; where m is mass, v is velocity and p is momentum of the charge particle.
For constant momentum and magnetic field,
r∝q1
So, the ratio of the radii of Hydrogen ion to α-particle will be,
rαrH=qHqα=12
rH:rα=2:1.