For α particle, mα=4 a.m.u and charge qα=2e.
For proton, mp=1 a.m.u and charge qp=e.
As the charged particle are moving through same potential, the kinetic energy acquired by the particle will be equal to electric potential energy i.e., 21mv2=qV, where, V is potential.
Using the relation between momentum (p) and kinetic energy, we have
p=2mE=2mqV
Then, the ratio of linear momenta of two particles is pppα=mpqpmαqα=14×12
=122=22:1