[Ifeffit] Hephaestus ion chamber absorption

Hello, Can anyone explain why the values given by the ion chamber utility in Hephaestus differ from those given for a 10 cm He filled ion chamber in figure 3.14 of Grant Bunker's Intro to XAFS book at high energy? For instance, figure 3.14 indicates an absorption fraction of ~3E-7 at 40 keV, whereas Hephaestus indicates 2E-4 at the same energy. Thank you, George