Which type of ionizing radiation has the lowest penetration and is blocked by skin or a sheet of paper?

• A. Alpha radiation has the lowest penetration and is blocked by skin or a sheet of paper.
• B. Beta radiation penetrates more and is blocked by plastic or light shielding
• C. Gamma radiation has high penetration and requires dense shielding (lead/concrete) or maximum distance.
• D. X-ray radiation is easily shielded by ordinary barriers

Answer: (A)

Alpha radiation has the lowest penetration because its particles are heavy, doubly charged helium nuclei that interact strongly with matter and lose energy quickly. This short-range nature means they can be stopped by just a few centimeters of air, a sheet of paper, or the outer dead layer of skin. That’s why external exposure from alpha sources is largely blocked by the skin, though if alpha-emitting material is ingested or inhaled it can cause significant internal damage due to concentrated ionization.

In comparison, beta particles are lighter electrons that penetrate a bit farther and can be stopped by plastic or lightweight shielding. Gamma rays are high-energy photons with no mass or charge, so they penetrate widely and require dense shielding like lead or thick concrete or substantial distance to reduce exposure. X-rays behave similarly in terms of needing appropriate shielding to limit exposure.