Hello. Not sure if this is the right board to post. This is not homework. I am just self-teaching quantum physics as I read a book on the topic of X-ray Physics. It has about 30 chapters in total and so far I find myself having about 3 questions per chapter. Hope you guys don't mind me asking here. Why does attenuation coefficient decrease (general trend of decrease) with increased energy? Diagram at the very end of this link shows attenuation coefficient (y axis) and energy in keV on X-axis. http://www.ndt-ed.org/EducationResources/CommunityCollege/Radiography/Physics/attenuationCoef.htm Also diagrams here: http://imageshack.us/photo/my-images/259/p13922459f57a1064518b4c.png/ My understanding is attenuation coefficient be it linear or mass, is a description of how good a material is at attenuating incidence beam. For example linear attenuation coefficient is the log of the ratio of incidence intensity against exit intensity, per unit length of the material. Mass attenuation coefficient is that divided by density, and is the same regardless of what state the substance is in (eg. vapour, liquid, ice all have same mass attenuation coefficient). What I don't understand therefore is, how does increasing the intensity of incidence beam, decrease attenuation coefficient? I thought the coefficient is more like a constant for a given material, and should therefore be independent of the intensity of the incidence beam. The question I am asking is: why does attenuation coefficient go down with increased energy? why isn't the coefficient a constant for that material regardless of energy intensity of the incidence beam? Thanks.