Wait a minute! Your checking for leakage from an electron microscope, 10 Kev to ?, and you've been using an instrument that you pressume to be completely unreliable and unproven, for the purpose you are using it?
The manufacture should be able to give you a valid spread of the spectrum of X-ray, low to high. Stick to a scintilation detector and forget about the ion chamber! Looks like a good NRRPT question!