Apparatus with a cap and cover assembly, an electron gun with a cap assembly, and a method of using a tube

An electron gun comprising:

• a cap assembly comprising a cap, a cap aperture, a cover, and a spring, wherein:
• the cover overlies the cap aperture during at least one point in time;
• and
• the spring comprises a first end attached to the cover and a second end attached to the cap;
• and
• a focus electrode spaced apart and electrically insulated from the cap assembly and comprising a focus aperture in alignment with the cap aperture.

A method of using a tube comprising:

• placing at least a portion of an electron gun within a first end of the tube, wherein the electron gun comprises:
• a cap including an aperture;
• and
• an attenuator assembly including an attenuator adjacent to the aperture, wherein the attenuator lies along a path for an electron beam within the electron gun when the electron is activated;
• flowing a gas at least near a portion of the electron gun while the attenuator blocks the aperture;
• and
• sealing the tube.