The emission produced by mutual annihilation of a pair of triplet excitons in single crystalline anthracene was observed to be an indicator of low level gamma-ray dose. Under steady red light illumination the blue emission intensiv Fris related to gamm-ray dose D by F−1/2= a + bD in the range 300R to 104R. Independent measurements show the effect cannot be due to changes in absorption in the red region, changes in quantum yield of fluorescence, nor to changes in the specific rate of triplet interaction. An interpretation based on dose induced quenchers and the long range of triplet excitons is given. By use of mulitple techniques an individual single crystal of anthracene can be used as a wide range dosimeter; triplet-triplet annihilatin in the range 102to 104R, space charge limited photo-current in the range 103to 106R, fluorescence quenching in the range 105to 108R, and absorption spectroscopy above 107R.