Maximum Efficiency for High Energy Excitation

In thi.s. section we will consider those types of exciting radiation which create many electrons and holes in the luminescent material under consideration. This type of excitation is usually indicated as high energy ionizing radiation. The most well-known examples are cathode rays and X rays, but one can also think of y rays and a particles. It is usually assumed that the excitation process proceeds as follows  

More generally we can write for the average energy required to create an electron-hole pair 

Here is the high-frequency dielectric constant, the static dielectric constant, and Lo the frequency of the longitudinal optical vibration mode. The values of P range from about 3 (GaP, ZnS, Csl, Nal), via 4(La202S), 5.6 (Y3AI5O12), to 7 (CaW04, YVO4). 

The total expression for the radiant effiency looks as follows T) = (1 -r)., S.q. 

Here r is the amount of radiation which is not absorbed, Ve is the (averaged) frequency of the emitted radiation, E is defined by Eq. (4.4), S is the efficiency of transfer of electron-hole pair energy to the luminescent centre, and q is the quantum efficiency of the luminescent centre. 

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