ASNT

120 to 450 keV—X-radiation is emitted nearly isotropically. Shielding is needed to stop all radiation that is not used for imaging to avoid unnecessary human exposure and damage to electronics. The insert frame is subject to radiative cooling of the cathode and anode. A 3 kW-rated X-ray tube means that a 3 kW electron beam is incident upon the stationary target; approximately 30 W of X-rays emerge. The remainder is waste heat deposited in the target and nearby frame, and some 10 or more watts are radiated to the cathode assembly. Cooling is critical to maintain manageable temperatures for the parts of the tube. Uncontrolled increase in temperature leads to failure. The space between the casing and the insert frame can contribute to cooling by circulating high dielectric strength oil and providing heat exchange apparatus. The casing must allow for mechanical support for orientation and protection for the window.

CATHODE Conventional radiographic X-ray tube cathodes have a tungsten wire emitter that is wound in the shape of a right-circular cylinder (Figure 1). The supporting legs of the filament serve as electrical connection; they are simply the ends of the wire clipped to desired shape. The filament is set into a metal structure, a cathode cup, by attachment of the legs to insulating supports which are set into the cup’s shaped features. The filament wire is an alloy of tungsten chosen for mechanical properties related to stiffness when heat treated. The diameter of the coils, spacing of the coils, and total length of the coil are related to the total power needed in the electron beam, the shape of the focal spot desired, and to the angle of the target face with respect to the electron beam. The richardson-dushman equation relates achievable current density J ( A /cm 2 ) to a material-dependent constant A (of order 60 A /cm 2 × K 2 ), the emitter temperature T (K),

Anode

Large fillament

Small fillament

Electron trajectories

Cathode

(a)

(b)

Figure 1 Conventional cathode cup: (a) side view shows simplified channels cut in cup material. Width and depth of channels, placement of coil filament, and width and height of features determine shape of focal spot on X-ray production target (anode). Metal cup features at the ends of filaments determine length/direction focusing. Focusing and placement of emitters guarantee coincidence of the small and large spots; (b) top view shows two filaments and two apertures provided for small and large focal spots. Small spot affords better resolution in image. Large spot can sustain more current (large area emitter), but resolution is limited.

CHAPTER 3

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Part 1