Radiation Source Use and Replacement Abbreviated Version (2008) / Chapter Skim
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4 ACCELERATOR AND DETECTOR TECHNOLOGIES
4 ACCELERATOR AND DETECTOR TECHNOLOGIES
Pages 67-84
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From page 67... ...
This is because more efficient detectors could enable radiographers and well loggers to use lower activity radionuclide radiation sources or substitute machine sources that are unable to generate the flux achieved with high-activity radionuclide radiation sources. Detectors, too, are improving, but not primarily in response to desires to reduce the use of high-activity radiation sources.
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From page 68... ...
It should be noted that there are many other industrial applications that have been made possible because of the properties of electron accelerators or where the clear advantages of the accelerators have already made them, rather than radionuclide radiation sources, the technology of choice. These include applications in material processing such as cross-linking of polymers or curing of composites where the high energy density in an accelerator beam is required (see, e.g., Masefield, 2004)
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From page 69... ...
For example, the optimized conversion efficiency of a 1-MeV electron beam into xrays is only about 1–2 percent, whereas this increases to about 8 percent at 5 MeV and to about 12 percent at 7.5 MeV. The x-rays generated through bremsstrahlung have a continuous energy distribution extending down from the electron beam kinetic energy with an average energy that is much lower, typically 20–30 percent of the beam energy; an example of a bremsstrahlung spectrum produced by a 5-MeV electron beam striking a tungsten x-ray target is shown in Figure 4-3.
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From page 70... ...
for megavoltage electron beams in the range from 9 to 32 MeV and in (d) for heavy charged particle beams (187 MeV protons, 190 MeV deuterons, and 308 MeV carbon ions)
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From page 71... ...
Beam energies between 100 keV and about 30 MeV are potentially useful for most applications that are considered in this report. Typical medical accelerators operate with beam powers of roughly 1 kW; low-power accelerators for radiography may only have beam powers of a few watts; and high-power accelerators for irradiation operate with average beam powers of tens to hundreds of kilowatts.
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From page 72... ...
These accelerators operate at voltages between 100 keV and 10 MeV and are frequently used for electron-beam irradiation in materials processing, for example, to improve the physical properties of plastics, cables, and wires, or for materials analysis; similar sources are also used at a much lower energy for ion implantation in semiconductor development. In general, the electrodynamic accelerators produce greater beam powers and are used for materials processing applications.
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From page 73... ...
, which make them an attractive alternative in situations where small radiation sources are required. Low-energy (40 to 50 kVp)
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From page 74... ...
These are described below, except for circular rf accelerators, which are used in proton and carbon ion radiotherapy but are not typically used in the energy range of interest as potential radionuclide radiation source replacements. Radiofrequency Linacs Radiofrequency linacs were developed in the 1940s and are used for many applications ranging from the generation of x-rays in a hospital environment to injectors into higher energy synchrotrons at particle physics laboratories.
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From page 75... ...
The overall electrical efficiency from power source to emitted beam power of early rf linacs was only about 20 to 30 percent, compared with efficiencies of 60 to 80 percent for the dc accelerators. The efficiency can be improved by using more efficient rf power sources and maximizing the efficiency of the accelerator cavity by choosing the beam current so that the beam-induced voltage is comparable to the unloaded voltage -- the optimization depends on the detailed cavity design.
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From page 76... ...
voltage into the high-voltage dc power needed for the rf power sources. As noted earlier, the improved high power capability of semiconductor switches, in particular the IGBT and the IGCT, has led to improved power handling, efficiency, and reliability in both modulators and high-voltage power supplies.
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From page 77... ...
. Commercial Rhodotrons are produced by IBA Industrial and operate at energies up to 10 MeV with beam powers as high as 700 kW (IBA Industrial)
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From page 78... ...
. Because the high peak power is not necessary when considering radionuclide source replacement and induction linacs are relatively expensive for a given average power, they are not considered as potential radiation source replacements in this report, although additional induction linac research and development is mentioned below.
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From page 79... ...
These guns are designed to produce very high quality continuous wave beams with beam energies of a few MeV and beam powers of 100 to 1,000 kW. For example, one gun presently being fabricated by a collaboration between Jefferson Laboratory and Advanced Energy Systems is designed to produce a 7-MeV beam with a current of 100 milliamperes; this device is presently being commissioned at lower power levels.
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From page 80... ...
Setting aside spallation sources, which require much higher energies (GeV) and therefore large facilities, acceleratordriven neutron sources direct a beam of deuterium nuclei at a target loaded with deuterium or tritium,7 causing fusion reactions.
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From page 81... ...
RADIATION HAZARD AND RADIOACTIVE WASTE FROM ACCELERATORS It must be noted that switching from radionuclide radiation sources to machine sources of radiation does not obviate the need for radiation safety and radioactive waste management. Particle accelerators emit radiation primarily along their beamlines, but also to much lesser extent they emit x-rays in other directions.
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From page 82... ...
Radiation sterilization and radiotherapy use the deposited energy to kill bacteria, viruses, or cancer cells; blood irradiators use it to kill white blood cells. This can be accomplished with a radionuclide radiation source or radiation generator working alone.
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From page 83... ...
How useful an apparatus is depends on the performance of the overall system, not just the source, and so, improving the detector can relax the demands on the source. That is, a more sensitive detector may enable users to accomplish the same tasks with a lower intensity radiation source, including a small radiation generator.
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From page 84... ...
Concurrently, there have been dramatic improvements in CCD technology, computer software and compact x-ray sources dedicated to the development of the state-of-the-art CT scans commercially available today. Recognizing that improved detectors for well logging applications would be beneficial in decreasing the activity of the current radiation sources, several alternative materials, such as bismuth germanate (BGO)
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