What phenomenon occurs when an original ionizing event happens directly on a macromolecule?

The phenomenon that occurs when an original ionizing event happens directly on a macromolecule is known as direct interaction. In this scenario, the ionizing radiation interacts with the macromolecule itself, leading to immediate effects such as damage to the molecular structure, including ionization or excitation of the macromolecule's atoms. This can result in the alteration of the macromolecule's biological function, which may lead to significant biological effects, including cell death or mutation.

Direct interaction is distinct from other types of interactions. Indirect interaction refers to situations where ionizing events occur on surrounding materials, such as water, which then produce free radicals that can subsequently damage macromolecules. Secondary interaction encompasses indirect effects that are not related to primary ionization, while radiant interaction does not specifically refer to ionizing radiation effects on macromolecules. Thus, the distinction lies in the nature and location of the initial ionizing event relative to the macromolecule itself.