Which of the following are small particles that move at high speed and are moderately penetrating of tissue?

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

Answer: (B)

Understanding how different types of radiation interact with tissue helps identify beta particles. Beta particles are fast-moving electrons emitted from unstable nuclei, so they are small charged particles that travel quickly. Their penetration in tissue is moderate: they can pass through skin and a short distance into tissue but are usually stopped by a few millimeters of plastic or aluminum.

In contrast, alpha particles are heavy and highly ionizing but have very limited penetration, being stopped by a sheet of paper or outer skin. Gamma rays and X-rays are not particles at all; they are high-energy photons with far greater penetrating power and require dense shielding to reduce exposure.

So the description of small, fast-moving particles with moderate tissue penetration best fits beta radiation.