Magnetic field does not cause deflection in
Magnetic force acts on a charged particle, due to which it deflects from its path. The magnitude of this force is measured as |`vec"F"`| = `|q(vec v xx vec B)|`.
Here, q is the charge on the particle that is moving with speed v in a uniform magnetic field B.
Since alpha, beta-plus and beta-minus are charged particles, they suffer deflection due to the field applied. On the other hand, gamma rays are photons and due to zero charge, they do not suffer any deflection.