Which particles have little penetrating power and are a risk only through internal exposure?

The correct choice is alpha particles because they consist of two protons and two neutrons, making them relatively heavy and positively charged. Due to their mass and charge, alpha particles have low penetration power and cannot easily pass through materials. For example, they can be stopped by a sheet of paper or even the outer layer of human skin, which prevents external exposure from being a significant risk.

However, if alpha-emitting materials are ingested, inhaled, or enter the body through wounds, they can cause significant internal damage due to their high mass and energy, which can lead to cellular damage and increased cancer risk. This characteristic highlights the distinct hazard posed by alpha particles mainly when they are internally located, rather than from external exposure.

Other options like X-rays, beta particles, and gamma rays possess much higher penetrating abilities. X-rays and gamma rays, being forms of electromagnetic radiation, can pass through the body and various materials, posing a risk even with external exposure. Beta particles, while lighter than alpha particles and able to penetrate the skin to a certain extent, can still present an external threat as they can penetrate tissues more effectively than alpha particles. This is why they do not share the same profile of posing risks primarily through internal exposure.