ASNT

on a support that conducts heat through the support’s cross section to the cathode insulator and finally to the receptacle high-voltage connector. Air-cooled cathodes cannot sustain as much power as oil-cooled ones. The cathode insulation is shaped to match the high-voltage cable plug and provide a sealed two-surface interface. Standardized plug-and-receptacle equipment for high voltages to 225 kV allow for multiple conductors for emitter currents and control voltages to be isolated from the grounded outside cable surface for safety. The plug-to-receptacle interface is sealed by high-dielectric strength grease against moisture, air, and contaminants encountered in the field and in the lab. ANODE The electron beam is generated, focused, and accelerated across the cathode-anode vacuum gap to strike the X-ray production target at the focal spot. l The anode is designed to produce the maximum X-ray f lux per unit beam current emerging from the smallest focal spot possible. l The anode is also designed to sustain electron bombardment at this spot over long periods of time in radiographic exposure modes. The first design goal maximizes the value of the X-ray tube as a component in an imaging system. A small focal spot area that produces a large X-ray f lux is said to have high brightness. The imaging system resolution and speed at imaging tasks increase as brightness of the source increases. However, increasing the electron beam power while reducing the focal spot size leads to high-power density at the focal spot (high focal spot loading) and unsustainably high temperatures at the target surface. The second design goal provides for long exposures necessary for radiographic inspection of large, thick, dense objects. A large object requires a similar distance from X-ray source to the object so that practical detection means can be employed and image distortion is kept to a minimum. The intensity of radiation (X-rays per unit area) decreases as the source-to-object distance squared. This results from the near-isotropic distribution of X-rays when created by electron-solid interaction at

low energies (below 1 MeV). Objects absorb X-radia- tion according to the thickness × density of material in exponential function (Hubble and Seltzer 2004). This absorption law, using material-dependent mass attenuation coefficients, is simply stated in Equation 2, relating the incident X-ray intensity, I 0 , to the transmitted intensity, I , after passing through material thickness, t , of mass attenuation coefficient, µ/ r , where r is the mass density of the material. These two aspects of radiation interaction with matter provide design constraints for practical X-ray sources. The goal is highest achievable power over long periods of time for many practical imaging tasks. HIGH BRIGHTNESS High brightness can be achieved by using a target material that produces large X-ray f lux for each incident electron. The fundamental equation describing this X-ray production method easily guides choice of target material; high Z materials are best (Bethe 1934). Expensive, radioactive, and low-melting point materials cannot be used. Pure tungsten is one good choice, but its heat conductiv- ity is poor and it is expensive and hard to machine. Tungsten coatings on molybdenum substrate have emerged as the solution for the highest brightness electron-impact X-ray sources; these are used in demanding high-power medical imaging applica- tions. Tungsten has a high melting point, low vapor pressure, and can be alloyed with a few percent (by weight) of rhenium for ductility. HIGH HEAT DISSIPATION Stationary targets used in radiographic inspection tubes have similar design, but the high heat dissipation requirement leads to the following design options shown in Figure 2: tungsten plate or coatings deposited onto copper or other materials with high-heat conductivity. Tungsten has superior X-ray production at 100s keV electron energy, and copper has superior heat conductivity. Passive cooling options are the most reliable. Circulating (Eq. 2) I = I 0 e – ρ ⎛ ⎝ ⎜ ⎞ ⎠ ⎟×ρ× t

CHAPTER 3

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Part 1