Which of the following is a source of secondary radiation?

Secondary radiation refers to the radiation that is generated when primary radiation interacts with matter, causing the matter to emit its own form of radiation. Among the provided choices, leakage radiation serves as a source of secondary radiation because it is radiation that escapes from the protective housing of a radiographic machine after initial interactions have occurred. This leakage happens when the primary radiation interacts with materials surrounding the equipment, such as the walls or other structures, which can then emit radiation as a result of that interaction.

In contrast, direct radiation typically refers to the original, unmodified radiation emitted directly from a source, which does not involve any interaction with materials that would alter its characteristics. Primary radiation specifically describes the initial radiation emitted from a source before it interacts with any other materials, while ionizing radiation describes a broader category of radiation capable of displacing electrons from atoms, but not specifically indicative of secondary radiation sources.