A Microfocus X-ray Tube Used with Focusing Collimators

The design and performance are described of a low-power copper-target X-ray tube with an elongated focus foreshortened to a diameter between 10 and 20 µm. The tube is designed for use with a specularly reflecting ellipsoidal mirror placed between 10 and 20 mm from the source and focusing the X-rays onto the sample, which may be 600 mm distant. The design maximizes the solid angle of collection of the emitted X-rays and thus achieves a high intensity at the sample. The electron beam, of circular cross section, from the gun is focused by an axial magnetic lens and then drawn out by a quadrupole lens to form an elongated spot on the target which is viewed at a small take-off angle. The limits imposed on the maximum intensity at the specimen by aberrations in the lens and by the permissible power dissipation in the target are discussed. The observed intensity is in reasonable agreement with the calculated intensity; it is comparable with that achieved with nonfocusing X-ray optics used with conventional X-ray tubes operated at a power more than 100 times as great.