Which of the following is an example of densely ionizing radiation?

• A. X-rays
• B. Alpha particles
• C. Gamma rays
• D. Electrons

Answer: (B)

Densely ionizing radiation deposits a lot of energy in a very short distance, meaning it creates a dense cluster of ionizations along its track. This is described by high linear energy transfer (LET). Heavy charged particles fit this well because their large mass and +2 charge make them interact very strongly with atoms, producing many ionizations in a very short path. Alpha particles are a classic example for this reason. In contrast, X-rays and gamma rays are photons that interact sparsely, giving low LET and longer, more spread-out tracks. Electrons can ionize efficiently too, but their tracks are not as densely ionizing as alpha particles. So the example of densely ionizing radiation is alpha particles.