What measurement unit is used to express air kerma?

Air kerma is a measurement used in radiation physics to quantify the energy transferred from ionizing radiation to air. It specifically measures the kinetic energy of charged particles produced by X-rays and gamma photons in a unit mass of air. The unit of air kerma is the gray (Gy), which is defined as one joule of energy absorbed per kilogram of matter.

Thus, using the gray as the unit for air kerma is appropriate because it directly correlates to the energy delivered to the air due to radiation exposure. This measurement is crucial in fields like radiology and radiation protection, where understanding the dose of radiation that patients or environments receive is essential. In contrast, other measurement units provided in the options refer to different aspects of radiation or energy, such as the sievert, which measures biological effect or dose equivalent, the becquerel, which measures radioactivity, and kilojoules, which are a unit of energy not typically associated with radiation dose measurements.