Temperature dependence of x-ray induced luminescence from medical x-ray screens

Texts in diagnostic radiography give rules for adjustment of exposure parameters to compensate for the effect of temperature on the efficiency of film-screen combinations. These rules are appropriate for traditional phosphors, such as calcium tungstate (CaWO4) and lead barium sulfate (BaSO4:Pb) but may not be applicable to newer screens in current use such as those of the rare-earth type. The effect of temperature on x-ray induced luminescence from modern and traditional screens has been compared over a temperature range of approximately-130 to approximately 60 degrees C and has been found to differ markedly. Typically, the luminescent emission from rare-earth phosphors varies by less than 10% over this temperature range compared to the 60% change in efficiency for conventional screens over the same temperature range. One type of modern screen, however, shows a continuous decrease in luminescent efficiency as the temperature is lowered. Qualitative physical explanations for these effects are presented.