An alpha particle $(4$He nucleus with twice the elementary charge e and roughly

four times the mass of the proton mp$)$ enters a region of uniform magnetic field of

strength B$.$ Its velocity is perpendicular to both magnetic field lines and the border of

the region. Upon leaving the region, the particle is deflected from its original direction

of motion by a small angle $\text{'}.$ If the size of the magnetic field region is L$,$ what is the

speed of the alpha particle?