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A long lasting phosphor (LLP), which emits bright green light for a long period after irradiation has been stopped, is a promising candidate as a simple and easy-to-use radiation detection element for digital radiography. Luminescence from the LLP, SrAl 2 O 4 :Eu 2+ ,Dy 3+ , exposed to various ionizing radiations has been measured to evaluate the dependence of luminescence efficiency on quality of ionizing radiation. The following items were confirmed. (1) Total luminous intensity from the LLP was proportional to total absorbed dose on the LLP element. (2) The average energy to produce one photon, W p , of the LLP exposed to either beta or gamma rays at 300 K was 18±2.7 eV, while that for the exposure to alpha rays was 480±72 eV. (3) The difference in W p for beta and gamma rays and alpha rays arose from the difference in linear energy transfer, LET, in the LLP. (4) The W p values were determined by the short-term recombination rate of electron and hole pairs in the LLP as well as their generation rate. (5) A model to evaluate the W p based on behaviors, e.g., generation, recombination and trapping, of electron and hole pairs values was proposed to explain the W p dependence on the quality of ionizing radiation.

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