ASNT

to Δ T ~ ω –1/2 , where Δ T is the target track tempera- ture increase above target cap temperature due to rotation of the target at a given power and track radius. Oosterkamp’s 1948 analysis of heat dissipa- tion in rotating anodes subject to electron beam bombardment gives Equation 3, where P is the beam power, t is the time subject to bombardment, and k and c are heat conductivity and specific heat capacity, respectively, of the target track material (Oosterkamp 1948). Time under the beam, t , is just the focal spot width divided by the linear speed of the target in motion at diameter d , v = d × ω . stationary targets. Stationary targets can be rotated incrementally to new angular positions by vacuum break, target rotation, and re-evacuation to recover operating pressure. These operations require time and come with some risks. This also requires active pumping and reusable vacuum seals on the target support. MICRO- AND NANOFOCUS TUBES Radiographic inspection of small objects can be carried out using microfocus or nanofocus tubes. Computed tomography data sets can be collected using these sources with precision rotation tables, stable mechanical bases, and small pixel-pitch solid-state detectors at large magnification configuration to produce very high-resolution three-dimensional reconstructions of small parts. These tubes are designed in reflection mode (electron beam incident on the same side of the target that serves as exit of X-rays) or transmission mode (X-rays emerge through the target to illuminate the object of inspection), as shown in Figure 4. The tubes use electrostatic focusing, electrostatic electron acceleration, and magnetic focusing to produce very small beam spots—micrometer to nanometer scales. Cathode The simplest source of electrons is a bent wire (hairpin) emitter whose area is very small compared to 1 × 10 mm 2 (0.04 × 0.39 in. 2 ) wound tungsten filaments used in the majority of 225 to (Eq. 3) Δ T = 2 P t π kc Most radiographic inspection tubes use

450 kV radiographic inspection sources. A single bent wire 10 or 20 µm (4 × 10 –4 to 8 × 10 –4 in.) in diameter acts as a source of electrons approxi- mately 0.02 × 0.02 mm 2 (0.001 × 0.001 in. 2 ) in area (Goldstein 1992). The entitlement in e-beam current from this area is 4 × 10 –5 the total current of a standard tube. A grid is called out in Figure 4 showing that shutting off the beam is necessary for tube operation. The grid can also serve to produce a measured current and focal spot size due to the electrostatic effect of the electric field shape The magnetic coils are arranged to perform as lenses that focus the beam through apertures and onto the target. The focus coils are typically wound and behave as axially symmetric windings or solenoids. There are coils that produce dipole fields, which act as def lectors in either direction perpendicular to the beam direction. These designs resemble scanning surface characterization apparatuses, where small focal spot and low beam current characterize the beam on target. Anode A transmission target is different from standard massive targets used in ref lection mode in two ways. First, cooling options are fewer since it is difficult to run cooling oil or water on the back side of such a thin target and stay out of the way of the emerging X-ray beam. Thick copper is not an option. Edges or target disk perimeter anchoring means become the primary cooling avenue. Second, the X-rays emerge through the target material itself. Target material, typically tungsten, is deposited on a support. The support must have relatively low attenuation for X-rays, suggesting a metal such as beryllium ( Z = 4). The support must also be able to sustain atmospheric pressure across the entire face of the support since it acts as the tube’s window. The beam spot is only microns or ten of microns in diameter; a 50 µm (2 × 10 –3 in.) diameter thin target support sustains total force, F = P × A , of 50 µN. The disadvantage to a very small window is the target cannot be rotated to bring fresh target material into the focal spot. Various designs optimize tungsten thickness, support thickness, and target support produced by the grid aperture. Electron Beam Focusing

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Part 1