Let us assume that we wish to design an X-Ray tube that has a high power rating and a small effective focal spot, such as 2.0 x 2.0 mm. Let us also assume that the maximum field covered by the emergent beam is 12" x 12" at 24 FFD (focal spot to film distance, measured in inches). At a 24 FFD, 12" subtends an angle of 2 x tan-1 (6/24) at the focus, and consequently, half this (1 x tan-1 (6/24)) between the central ray and the anode side of the field. Therefore the target angle must be less than or equal to 1 x tan-1 (6/24) = 14°.
Having calculated the target angle of the tube, we need to now concentrate on the electron source (the cathode). The tube operates with High Voltage between the cathode and anode (the target). The two electrodes are separated as far apart as possible within the vacuum gap. The anode/ cathode spacing (A/C spacing) varies between (0.200" - 0.750"). The field created between the cathode and the anode must influence the electrons to be accelerated towards the target. When these electrons bombard the target, X-Rays are generated at varying depths within the target material. The maximum intensity of the X-Ray radiation decreases as this penetration into the target material increases.
The cathode should be designed to produce an electron beam that, after travelling through the cathode-anode vacuum gap, strikes the target with a rectangular shape. The actual focal length (L) is calculated in our example as L= 2 / sin14° = 8.26 mm.
The electron beam emitter consists of a helix (filament) that has been carefully annealed to ensure that no distortion occurs over its working temperature range. The length and diameter of the tungsten wire in the helix are designed to provide the surface area required to produce the desired emission, at a temperature consistent with adequate filament life.
This filament is set centrally in a slot machined in a metal focusing cup (cathode cup). Cathode cups are typically manufactured from molybdenum, nickel or an iron alloy. This design is called an Electronic Lens System with a resulting focal spot width that depends upon:
The diameter of the filament helix.
The width of the cathode cup slot.
The depth of the filament in the cathode cup slot.
The above parameters are adjusted until a combination is reached which produces sharp focusing of the electrons at the desired anode/cathode spacing, and adequate electron emission from the cathode.
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