Question:
This video features three different radioactive nuclides: Plutonium-239 (alpha source), Strontium-90 (beta source) and Radium-226 (gamma radiation). Why do you think their penetrating ability differs?

Answer:

(Plutonium-239) Alpha particles are highly ionising. Ironically, exactly because they cause so much ionisation per unit distance, they lose energy rapidly. This is why all it takes is a few cm of air or a piece of tissue paper to completely bring alpha particles to a stop.

(Strontium-90) Beta particles are less ionising than alpha. They cause less ionisation per unit distance, and therefore lose less energy per unit distance. Depending on how energetic the beta particles are, a few mm of aluminium is usually sufficient to soak up the energy of beta particles.

(Radium-226) Gamma radiation are really EM radiation. Being uncharged, they cause ionisation only indirectly. Ironically, it is exactly because they are so weakly ionising, that they have very high penetrative power. Even with a thick piece of lead, gamma radiation cannot be stopped completely.

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