Kerma

Kerma stands for kinetic energy released per unit mass. It is a dosimetric quantity used in radiation physics to describe the amount of energy transferred from ionizing radiation to charged particles, primarily electrons, within a small mass of material at a point. Kerma can be considered as the initial energy imparted to secondary charged particles, before any further transport or energy loss processes occur.

In practice, kerma is related to but distinct from absorbed dose. Absorbed dose measures the energy actually deposited in the material per unit mass, while kerma accounts for the energy transferred to charged particles, which may later scatter, slow down, or escape the region. Kerma can be broken down into collision kerma (energy transferred by collisions to charged particles) and radiative kerma (energy carried away by photons produced by these charged particles, such as bremsstrahlung). Under charged particle equilibrium, kerma and absorbed dose are equal.

The SI unit of kerma is the joule per kilogram, the same as the gray. In practice, the term air kerma is widely used, referring to the kinetic energy released in air per unit mass of air and serving as a standard reference quantity in diagnostic radiology and radiation protection. Air kerma measurements are commonly made with calibrated ionization chambers and expressed in grays or milligrays.

Kerma is foundational in dosimetry, calibration, and radiation protection, informing exposure levels in medical imaging and industrial radiography. While useful, kerma is not itself a direct measure of dose in heterogeneous media; understanding the conditions of charged particle equilibrium and energy transport is essential for relating kerma to absorbed dose in a given situation. See also: absorbed dose, radiation dosimetry, air kerma.