What form of ionizing radiation can penetrate deeply into body tissue?

• A. Alpha
• B. Beta
• C. Gamma
• D. Photon

Answer: (D)

Understanding penetration comes down to how the radiation interacts with matter. Alpha particles are heavy and carry a +2 charge, so they deposit energy rapidly and are stopped by outer layers like skin or a thin sheet. Beta particles are lighter and carry charge too; they can penetrate a little further but are still stopped by a small amount of tissue or shielding. Photons, which are particles of electromagnetic radiation, include X-rays and gamma rays when they are highly energetic. They have no mass or electric charge and travel through tissue with less immediate interaction, so their penetration is much deeper. High-energy photons can pass through significant thicknesses of body tissue before being absorbed or scattered, which is why gamma rays are known for deep penetration. Thus, the form of ionizing radiation that can penetrate deeply is a photon.