Which statement about alpha particles is true?

• A. They have high LET and are easily stopped by a sheet of paper.
• B. They travel long distances in air.
• C. They are identical to gamma rays.
• D. They are unaffected by matter.

Answer: (A)

The key idea is how alpha particles interact with matter. Alpha particles are heavy, positively charged helium nuclei, so they ionize atoms very intensely as they pass through material. This high linear energy transfer means they deposit most of their energy over a very short distance, giving them a very limited range. Because of this, a sheet of paper is enough to stop them completely. In contrast, gamma rays are uncharged photons that penetrate much more deeply, so they are not easily stopped by paper. They also are not unchanged by matter; gamma rays interact through different processes and can travel longer distances in air. So the statement is true because alpha particles have high LET and a short range that makes light shielding like paper sufficient to stop them.