ASNT

PART 2 Accelerator Sources

In the twenty-first century, betatrons are still used in NDT applications requiring a high-energy (1 MeV to 10 MeV) portable source, but tolerant of low dose levels (on the order of 10 rad/min or 0.1 Gy/min, which can lead to long exposure times). When higher dose levels at MeV energies are required, linear accelerators are almost always employed, allowing even greater penetration and shorter exposure durations. Like betatrons, linear accelerators also provide MeV electrons for X-ray generation, but at X-ray dose levels that are 10 × to 100 × higher X-ray doses than available from betatrons. Linear accelerators are commercially available in NDT configurations with energy levels between 1 MeV and 15 MeV and with dose rates as high as 100 Gy/min at the highest energy range, which allows an increase in penetration and a decrease in exposure time. However, 50 rad/min to 1500 rad/min (0.5 Gy/min to 15 Gy/min, respectively) are more typical dose rates. Two other significant applications of MeV linear accelerators are in radiation therapy for the treatment of cancer and in the scanning of cargo for security purposes; these two applications are responsible for the majority of the installed base of linear accelerator systems around the world, which is on the order of 10 000 systems. While the number of MeV accelerator-based systems in use worldwide for NDT may be closer to 100, the NDT application has benefited greatly from the technology developments associated with the cancer treatment and security markets.

BACKGROUND Commercially available X-ray tubes and high- voltage direct current (HVDC) power supplies routinely provide X-rays associated with electron energies up to 200 kV, with some specially configured products allowing up to 800 kV. Applications requiring X-ray photon energies higher than 1000 kV, or 1 MeV, require some form of additional electron acceleration beyond that achieved with HVDC power supplies and conventional X-ray tubes. In NDT, higher energy is generally required when greater penetra- tion of a thick or dense object under inspection is desired, while higher dose may be required to reduce exposure time necessary to generate an acceptable image. The earliest machines to reach megaelec- tronvolt electron energies included resotrons (resonant transformer X-ray machines), van de graaff generators (electrostatic generators invented in the 1930s and later extended to the MeV energy range), and betatrons (invented in 1941 producing 2.3 MeV at useful dose levels). Very high voltages can be achieved with van de graaff generators and betatrons, but the low currents of such devices lead to low-dose X-ray output upon the physics of bremsstrahlung generation in a high- Z target. The later development of practical MeV linear acceler- ators for electrons solved this problem, providing much higher dose rates.

CHAPTER 3

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Part 2