A cap can comprise an aperture, and an attenuator may block the aperture during
at least one point in time. In one embodiment, the attenuator can include a cover
that may be displaced by a spring. In another embodiment, such as an electron gun,
may comprise a support cap with an aperture, a displaceable cover that may cover
the aperture, and a spring. A material attached to the spring and acting as a fuse
may release the spring and expose the aperture after an electrical current blows
the "fuse". In yet another embodiment, a method for using a tube may comprise evacuating
the tube while a cover covers an aperture in the support cap of a electron gun
that is at least partially in the tube and moving the cover to expose the aperture
after the tube is sealed.
