Luftkerma

Luftkerma, commonly known in English as air kerma, is a dosimetric quantity used in radiation protection and diagnostic radiology to quantify the energy transferred from ionizing photons to charged particles in air per unit mass at a reference point. It represents the initial kinetic energy imparted to electrons and other charged secondary particles produced by photon interactions in a small mass of air, before the energy is subsequently deposited or scattered. Luftkerma is typically reported as a rate or as a value per unit mass, and is measured in gray (Gy) for energy per kilogram, or in Gy per unit time when describing a dose rate.

Definition and measurement: Air kerma is defined as the sum of the kinetic energies of all charged particles liberated by ionization in a mass of air, divided by the mass of that air. It is experimentally determined with calibrated ionization chambers and serves as an operational proxy for characterizing radiation fields. In practical terms, K_air relates to exposure and to the energy-absorption properties of air through established conversion factors.

Applications and relation to dose: Luftkerma is used to characterize radiation fields, calibrate dosimeters, and estimate patient dose in radiographic procedures through energy- and geometry-dependent conversion factors. It is a convenient quantity when photons interact in air, but it is not identical to absorbed dose in tissue. While air kerma can approximate tissue dose under conditions of charged particle equilibrium and appropriate corrections, differences in mass energy-absorption coefficients between air and tissue mean that dose to tissue generally requires conversion factors.

Units and terminology: The SI unit is the gray (Gy). The quantity is often denoted K_air or Kair and may be given as a rate (Gy per unit time) for dose-rate measurements. The term Luftkerma is the German-language equivalent of air kerma.