7D10.70 - Alpha Particles with Electroscope
Observe a nearby alpha source with an electroscope.
Alpha particles are He nuclei (2 protons and 2 neutrons). They are highly ionizing, which is to say that they are very ready to assimilate free electrons.
When we charge the electroscope negatively, we produce a net excess of electrons. However, (on a dry day) they have no where to go from the electroscope.
In the presence of incident alpha particles, these excess electrons readily join the alpha particle to become helium.