Acronym for kinetic energy released in materials. Kerma is the sum of the initial kinetic energies of all the charged particles liberated by uncharged ionizing radiation (neutrons and photons) in a sample of matter, divided by the mass of the sample. Kerma is expressed in gray (or its submultiples), and, unless otherwise specified, refers to the energy liberated per unit mass in a small sample of tissue. Free-in-air kerma refers to the amount of radiation at a location before adjustment for any external shielding from structures or terrain.