External exposure is best described as energy deposited by which radiation?

• A. Gamma & X-ray
• B. Alpha & Beta
• C. Neutron
• D. All of the above

Answer: (A)

External exposure means energy deposited in tissue by radiation that comes from outside the body. The radiations that penetrate to deposit energy from an external source are photons, specifically gamma rays and X-rays. Their penetrating power allows them to traverse air and skin and deposit dose throughout tissues along their path. In contrast, alpha and beta particles deposit most of their energy very close to the entry point and have limited penetration, so they contribute less to external exposure from outside the body. Neutrons can also contribute in some settings, but gamma and X-ray photons are the primary descriptors of external exposure in typical scenarios.