Computed tomographic method and apparatus includes an electron or ion beam having a beam axis, a refractory metal disk； at least one slit in the refractory metal disk that receive the beam, wherein the slit is at an angle to the beam axis； a beam entrance opening in the slit that allows the beam to enter； an effective beam exit opening in the slit that allow the beam to exit, wherein the beam effective exit opening is smaller than the beam entrance opening； and a system for moving the beam across the refractory metal disk, wherein the beam enters the slit through the beam entrance opening and exits the slit through the effective beam exit opening； and a computed tomographic device for measuring the beam that enters and exits the slit for analyzing the beam.
