... conservation of energy and momentum.density of modes The number of modes of the radiation field in an energy range dE. Thedensity of modes is a function of the boundaryconditions ofthe spaceunder ... represents a degree of freedom of the distribution.(2) The number of independent coordinatesneeded for the description of the microscopicstate of a system is called the number of degrees of freedom. ... masses. To determinethe energy of the original photon, all the de-posited energy must be measured, and this in-cludes the capture of the two annihilation pho-tonsof0.511MeVeach, emittedwhenapositronat...