ASNT

PART 1 X-ray Tubes

100 kV. Oil, water, or forced air cooling can be easily and safely designed at ground potential. The anode is usually coated or made entirely of tungsten ( Z = 74) since X-radiation production increases as the target atomic number increases. Frame . The frame provides mechanical support for the cathode and anode. The electrostatic fields established by the high-voltage generator guide the electrons across the anode-cathode acceleration gap in the vacuum. The two parts must remain fixed and reliably positioned with respect to one another during operation over a range of temperatures from cold (room tempera- ture) to hot (approximately 70 ° C [158 ° F]) for the walls of the frame, and much hotter—hundreds of degrees celsius—for the cathode and anode structures themselves). The frame must also provide insulation for the high-voltage potentials applied and maintained by the generator. The frame is a vacuum vessel, so all joints and fixturing must be vacuum tight, have a slow overall leak rate, and a low outgassing rate. Finally, the frame must allow cooling of the parts through conduction. There is little residual gas at pressures below 10 –3 kPa (10 –5 mbar), so cooling is all radiative or conductive. Casing. The casing envelops the insert (vacuum-tight frame containing cathode and anode) and provides for attachment of the high-voltage connections. The frame supports shielding from X-rays emitted from the focal spot where the electron beam strikes the X-ray production target. At energies typically used for radiographic applications—

OVERVIEW All high-voltage X-ray tubes used for inspection tasks consist of a closed evacuated vessel or frame containing a cathode and anode surrounded by an enclosure or casing. Inspection X-ray tubes are designed for relatively long exposures (seconds to minutes) and so must be equipped with anode and frame cooling. Shielding (layers of lead metal) is incorporated in the casing, and a window is provided for relatively low absorption path to the object under inspection. Cathode . The cathode is a metal structure designed to support an electron emitter and provide nearby electrically biased electrodes that help shape the e-fields as they focus and shut off the electron beam. The electron emitter is a heated metal (thermionic) structure—coiled wire, thin f lat plate, or other device. Most often this is a cylindrically symmetric coiled tungsten wire. A power supply at high-negative voltage provides heating current to the tungsten wire filament; once sufficiently high temperature is reached, and an electric field is imposed, electrons leave the emitter and move into the vacuum toward the anode. Anode . The anode serves as a target for the electron beam to strike and produce X-radiation. The anode is biased positively with respect to the cathode and therefore attracts the negatively charged electrons. A grounded anode is the simplest to cool; cooling is necessary to manage the heat delivered by the electron beam since X-ray production is only 1% of total incident power at

CHAPTER 4

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Part 1