Which of the following is an example of densely ionizing radiation that is commonly contrasted with X-rays?

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

Answer: (B)

Densely ionizing radiation produces a lot of ionizations in a very short distance, which comes from a high linear energy transfer along its path. Alpha particles fit this description because they are heavy, highly charged particles that deposit a large amount of energy over a short track in matter, creating many ionizations in a small region.

X-rays and gamma rays are electromagnetic photons and interact with matter in a more spread-out way, leading to ionizations along much longer paths and thus lower ionization density. Ultraviolet radiation, while energetic, behaves similarly as an electromagnetic wave and does not produce the dense ionization pattern characteristic of alpha particles. That contrast—high ionization density for alpha particles versus the lower density for X-rays—makes alpha particles the classic example of densely ionizing radiation.