ASNT

large amount of energy gain. For example, in a 24 MeV betatron that is 38cm (15 in.) in diameter, the electrons circle the orbit about 350 000 times, traveling a distance of 418 km (260 in.). The average voltage gain per turn at the orbit is about 70 V, which gives about 24 MeV. As the electrons reach maximum energy, they are def lected by an electrical pulse and caused to spiral outward until they strike a target. In order to generate X-rays via bremsstrahlung, betatrons must be equipped with targets made of high Z materials, such as platinum or tungsten. The nature of the betatron geometry tends to provide highly asymmetric X-ray beams with aspect ratios as much as 1:10—such as 0.2 × 2.0 mm (0.008 × 0.08 in.) or 0.3 × 3.0 mm (0.012 × 0.12 in.) effective source size. Relative to linear accelerators, betatrons do utilize less active componentry to reach MeV energy levels; notably, a high-power radio frequency (RF) or microwave excitation source is not required. As discussed, relative to linear accelerators, betatron output dose levels are between 10 × and 100 × lower. In terms of weight, if shielding is not required and a large personnel exclusion zone is acceptable, an unshielded betatron can weigh less than a linear accelerator system of comparable energy. However, if shielding is required, the total X-ray

source weights of betatrons and linear accelerators become much closer to one another in many cases, especially if configured for comparable dose, as the shielding weight can dominate the total system weight in situations where significant shielding is required to minimize undesired leakage of X-rays. A commercially available betatron is shown in Figure 7. ELECTRON LINEAR ACCELERATORS Electron linear accelerators, similar to the one shown in Figure 8, accelerate electrons down a guide or collection of radio frequency cavities by means of RF voltages. These voltages are applied so that the electron reaches an acceleration point in the field at precisely the proper time. The accelerator guide consists of a series of resonant cavities that can provide electron acceleration when the radio frequency power is applied at proper frequency and power. The cavities are usually fabricated of copper with the highest possible purity and conductivity, and they are assembled in a linear fashion with holes in each end that allow electrons to pass from one cavity to the next cavity of the brazed-together vacuum device. When an electron is injected at the proper time with respect to the RF power with the

Figure 7 Portable 6 MeV betatron with total system weight of 190 kg (420 lbs), which facilitates transport in the field for certain nondestructive testing applications. Dose rate is 0.03 Gy/min (3 Rad/min), which is lower than linear accelerators, but portability is beneficial.

CHAPTER 3

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Part 2