A transmission-target x-ray tube can include an x-ray window mounted on a window-housing. The window-housing can be made of a high density material (e.g. ≥12 g/cm3) with a high atomic number (e.g. ≥45), and can include an aperture with an increasing-inner-diameter region, with a smaller diameter closer to the electron-emitter and a larger diameter closer to the window-mount, for blocking x-rays and electrons. An example angle in the increasing-inner-diameter region is between 15 degrees and 35 degrees. The x-ray tube can further comprise a window-cap. The x-ray window can be sandwiched between the window-housing and the window-cap. The window-cap can be made of a high density material (e.g. ≥12 g/cm3) with a high atomic number (e.g. ≥45) for blocking x-rays in undesirable directions, and can include an aperture for allowing x-rays to transmit in desirable directions.