In high resolution, a radium hand in the cloud chamber with a strong magnet beneath the surface. The strength of the magnet was purposely reduced by setting it distant of the surface, because the curving of electrons was too much which reduced the visiblity of alpha tracks. The intensity is about 50 mT. This is a radium hands of "low activity". Other hands can have much more concentration of Radium isotope, it depend on the manufacturer.

About the slow motion at the end :
This is a bit complicated to resume here, but the short answer is that the electric field separate the ions (+ and -) left by the passage of the particle into two distinct tracks. The magnetic field have no incidence on this. This is not observable on all the alpha tracks, it depend how in height the particle traveled in the supersaturated layer. Remember that the camera is not perpendicular to the active surface : the angle is about 20°, so we can see sometimes, the separation of ions from a single track.

Thanks for watching,
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