A singly charged ion of mass m is accelerated from rest by a potential difference ?V. It is then deflected by a uniform magnetic field (perpendicular to the ion's velocity) into a semicircle of radius R. Now a doubly charged ion of mass m' is accelerated through the same potential difference and deflected by the same magnetic field into a semicircle of radius R' = 2R. What is the ratio of the masses of the ions?