Ionizing Events in Small Device Structures

A theoretical model is described for calculating the number and average energy of ionizing events produced by x-rays in regions with characteristic dimensions ranging from mm to fractions of a ¿m. Microdosimetry theory is applied to estimate the variance of the event distribution and it is found that when device dimensions are in the ¿m range or less the distribution approaches lognormality. The model does not involve extensive calculations and uses only readily available data tabulations. It indicates that dose fluctuations could become significant if present trends towards smaller device elements continue.