ASNT

Electron emitter

Electron emitter

Grid cup

Grid cup

Electron trajectories

Electron trajectories

Deflection magents

Deflection magents

Objective coils

Objective coils

Reflection mode target

Transmission mode target

(a)

(b)

Figure 4 Micro- and nanofocus X-ray tubes; (a) in reflection mode, use small emitter surfaces, apertures, and an objective lens to produce a reduced image of the emitting surface; (b) in transmission mode, targets serve as both vessel walls and X-ray production target.

has a density of 19.3 g/cm 3 (1204.85 lb m /ft 3 ) yielding 86% X-ray transmission at 200 keV. At lower X-ray energies, values for µ/ r increase, giving reason for careful control of thicknesses in transmission targets. Current micro- and nanofocus sources are equipped with 180 to 450 kV tube high-voltage generators. Some designs for tubes are not permanently sealed structures—one design includes a hinged door that allows access to the target chamber, the cathode chamber, an active pump, and a vacuum gauge for automatic operation with modern process controls.

structure so that targets have sufficient life for imaging tasks. Finally, the X-rays must emerge from the focal spot zone where the radiation is generated, some 5 to 10 µm (2 × 10 –4 to 4 × 10 –4 in.) deep at 200 keV, through the remainder of the tungsten, and through the support so that the object under inspection can be illuminated. At 200 keV µ/ r for iron is 0.146 cm 2 /g. Steel’s density is 7.8 g/ cm 3 (486.93 lb m /ft 3 ) so the attenuation for a stainless steel support 0.01 cm (0.0036 in.) thick using I = I 0 × e –(µ/ ρ ) × ρ × t shows 98.9% transmission. Beryllium window/support at 200 keV is much more transpar- ent since µ/ r is smaller and the density is 1.85 g/cm 3 (115.49 lb m /ft 3 ). Tungsten with µ/ r of 0.785 cm 2 /g ,

CHAPTER 3

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Part 1