I am intending to perform extended absorption fine structure (EXAFS) experiments on a thin film in order to investigate the near order around Europium ions. The thin film is going to be deposited on a silicon substrate, and the question is in reality related to how thick the substrate should be in order to be suitable for doing EXAFS in transmission. My supervisor asked me to calculate the transmission of a 1 mm thick silicon substrate at the wavelengths used for investigating Eu. This would be at 48.5 keV (i.e. wavelength of 2.56E-11 m), or possibly 8.05 keV (1.54E-10 m). Now I would assume I can just use Beer's law, but I don't know the absorption coefficient of silicon at these wavelengths, and can't seem to find it anywhere. Don't know if they have to be determined experimentally or if I'm just missing something, or if there is some other way to go about it? If anyone think they can somehow help me with this problem it would be greatly appreciated.