ASNT

The majority of fielded linear accelerator systems are used in medicine and security, and are based upon RF sources operating at S-band radio frequencies close to 2.998 GHz. Designs that incorporate increased operating frequencies, such as X-band near 9.3 GHz, can permit smaller, lighter X-ray heads for special applications. However, the weight of radiation shielding generally reduces this relative benefit for a given X-ray output. Generally, the highest energy (15 MeV) and highest dose systems for NDT use high-power klystrons for RF power, whereas lower energy (1MeV to 9MeV) linear accelerator systems for NDT utilize more economical magnetrons to provide the RF power that accelerates electrons in the linear accelerator. Some linear accelerator system configurations have been developed that allow for the operation of the accelerator and beam collimator at a distance from the drive electronics and even the radio frequency source (Figure 10). The source supplies power through high-voltage cables and, in some special cases, an extended and sometimes f lexible wave guide. The total weight of the X-ray head can be reduced in that special case, especially if shielding is not required. This permits easy positioning for

field inspection of pipelines, valves, and other test objects of limited accessibility, such as in nuclear power plants. In most cases, it is beneficial to use a linear accelerator system with all of its major components integrated into a single cabinet, such as that depicted in Figure 8. This configuration eliminates any exposed high-voltage cables or exposed waveguides. The single cabinet configu- ration generally reduces the overall footprint and overall weight of the complete MeV X-ray-generating linear accelerator subsystem. One special consideration when using electron linear accelerators is activation of the object. If the photon energy is higher than 10 MeV, nuclear reactions are activated in the material/object being inspected, which could result in the generation of fast neutrons. This is a radiation safety issue that should be evaluated prior to the inspection to ensure the correct procedures are in place. The amount of activation in the material is a function of the incident X-ray peak energy, X-ray f lux, exposure time, and the material composition. The object may be active for hours after the inspection so it should be monitored and not handled until the activity is at safe levels.

CHAPTER 3

65

Part 2