Air KERMA is defined as which of the following?

• A. radiation energy deposited in tissue
• B. absolute number of ionization events in air
• C. speed of radiation in air
• D. kinetic energy absorbed by ions in air

Answer: (D)

The main idea is that Air KERMA measures the energy actually transferred to charged particles in a small amount of air, per unit mass, right after the photons interact. When photons collide with air molecules, they impart kinetic energy to secondary charged particles (mostly electrons). KERMA is that kinetic energy carried by those particles per unit mass of air, before that energy is further redistributed as heat or radiation later on. So the statement that Air KERMA is the kinetic energy absorbed by ions in air captures this immediate transfer of energy to charged particles in the air.

Why the other ideas don’t fit: energy deposited in tissue describes absorbed dose (energy actually deposited as heat in tissue), not the initial energy transfer to charged particles in air. The absolute number of ionization events in air is about how many ion pairs are created, not how much energy is carried by the liberated charged particles. The speed of radiation in air is not related to the energy transfer concept described by KERMA.