ASNT

air or forced air, water, or other liquid cooling increases the heat delivery to atmospheric tempera- ture heat reservoirs but adds the complexity of a pump, filters, or special oil reservoirs.

TARGET ANGLE The angle the target surface presents to the incident electron beam can be used to decrease the target power density but maintain resolution for imaging. A stationary target at 45 º to the incident e-beam uses 40% more surface along one direction, the length, compared to a perpendicular angle of incidence (Figure 3). The window can be placed 90 º to the incident e-beam to allow the X-rays to emerge for imaging. The apparent focal spot size remains the same as the incident beam profile. High-power medical imaging tube anode designs use the rotating anode. Significantly higher electron beam power can be focused onto very slender zones (tens to hundreds of kilowatts onto 1 × 10 mm [0.04 to 0.39 in.] for short periods, 5 to 10 s) to produce high-brightness sources. Bearing life limits the use of these sources in the radiographic inspection field. Cost and complexity are other limiting factors. Running the bearing at a slower rotational speed might provide a solution for longer exposures, but the gain in brightness is an inverse function of angular rotation, ω , according

Cathode

Cathode

Thin tungsten coating

Tungsten plate

Thick copper anode

Anode

Anode

Copper anode cooling: air, liquids

Copper anode cooling: air, liquids

(a)

(b)

Figure 2 Stationary target using tungsten: (a) coated onto surface of high-heat conductivity anode; (b) incorporated into copper anode (casting, welding, brazing, or similar manufacture).

Cathode

Cathode

Apparent length

Apparent length

Anode

Detector

Detector

Anode

Thermal length

Thermal length

(a)

(b)

Figure 3 Target angle: (a) incident electron beam length strikes angled anode (45º) in typical inspection tube, heating the target over the thermal length. Seen at the detector, the apparent length of the focal spot is the same as the beam length emerging from the cathode. Apparent length = thermal length × sin 45º; (b) smaller target angles produce much smaller apparent spot lengths, realizing a big gain in resolution while minimizing focal spot loading, thus dissipating heat more effectively. Focal spot width dimension is largely unchanged by target angle compared to length.

CHAPTER 3

57

Part 1