Alpha, Beta, and Gamma Radiation: Penetration Power

Radioactive nuclei can emit three main types of radiation, each with dramatically different properties:

Alpha (α): A helium-4 nucleus ($^{4}_{2}\text{He}$, 2 protons + 2 neutrons). Massive, +2 charge. Travels ~3–5 cm in air. Stopped by a sheet of paper or skin. Highly ionizing (knocks electrons off every atom it passes). Dangerous if ingested/inhaled (internal exposure), harmless from outside the body.

Beta (β): A high-speed electron (β⁻) or positron (β⁺). Much lighter, ±1 charge. Travels ~30 cm to several meters in air. Stopped by a few mm of aluminum. Moderately ionizing.

Gamma (γ): A high-energy photon (electromagnetic radiation). Zero mass, zero charge. Can travel many meters in air. Reduced (not fully stopped) by thick lead or concrete. Weakly ionizing per interaction but highly penetrating — the most dangerous for external exposure.

Why the difference?

Alpha particles are large and heavily charged — they interact strongly with every atom they encounter, depositing all their energy in a short distance. Gamma rays are uncharged photons that pass through most atoms without interacting, only occasionally being absorbed or scattered. Beta particles are intermediate.

Ionizing power (ability to damage tissue per cm): α > β > γ

Penetrating power (ability to pass through material): γ > β > α

These are inversely related: the more a radiation type interacts, the less far it gets.

Try it in the visualization

Watch three beams (α, β, γ) hit successive barriers: paper, aluminum, and lead. Alpha stops at the paper. Beta passes through paper but stops at aluminum. Gamma passes through both and is only attenuated by the lead. Toggle the ionization trail to see how each type damages tissue differently.