An apparatus includes an electron collector includes a body having an internal bore formed therethrough along a first direction and a window side having an aperture formed in a first portion thereof along a second direction different from the first direction. The apparatus also includes a cover plate having a bottom surface coupled to a second portion of the first surface of the electron collector, and an x-ray transmission window coupled to the cover plate and aligned with the aperture along the second direction, wherein a recess is formed along the second direction in one of the first portion of the first surface of the electron collector and a portion of the bottom surface of the cover plate, and wherein a gap is formed between the bottom surface of the cover plate and the first surface of the electron collector.