When the nuclear radiation is subjected to a magnetic field, the direction of deflection is governed by:

Flemming's Left Hand Rule

Reason — The α-particles and β-particles are charged particles, so a beam of these radiations is equivalent to a current flowing in a conductor. When such a beam passes through a magnetic field, it experiences a force whose direction is obtained by Fleming's left hand rule, which gives the direction of the force acting on a current carrying conductor placed in a magnetic field.