Abstract: A model for the fluorescence escape efficiency of CsI:Na(CsI:Tl) phosphor is proposed,and the converting factor of CsI:Na(CsI:Tl) to X-rays is discussed in detail.Calculated results show that the fluorescence escape efficiency,as well as the converting factor,strongly depends on the substrate reflectivity and 1/(σL)(σ is the absorption coefficient of CsI:Na(CsI:Tl) to fluorescence, and L is the phosphor layer thickness.).For a phosphor,1/(σL) should be above 10,and 30~40 is even better.In point of converting factor and spatial resolution,the reflection model is superior to the transmission model.The maximum converting factor can be obtained by optimizing the phosphor layer thickness.

Key words: CsI:Na (CsI:Tl) phosphor, X-ray imaging, converting factor, fluorescence escape efficiency