sentences:  The annihilation peak produced in a photon spectrum by annihilation radiation therefore has a higher full width at half maximum (FWHM) than decay-generated gamma rays in spectrum.The difference is more apparent with high resolution detectors, such as Germanium detectors, than with low resolution detectors such as Sodium iodide detectors.Because of their well-defined energy (511 keV) and characteristic, Doppler-broadened shape, annihilation radiation can often be useful in defining the energy calibration of a gamma ray spectrum.