Device that accelerates a beam of fast-moving, electrically charged atoms (ions) or subatomic particles. Accelerators are used to study the structure of atomic nuclei (see atom) and the nature of subatomic particles and their fundamental interactions. At speeds close to that of light, particles collide with and disrupt atomic nuclei and subatomic particles, allowing physicists to study nuclear components and to make new kinds of subatomic particles. The cyclotron accelerates positively charged particles, while the betatron accelerates negatively charged electrons. Synchrotrons and linear accelerators are used either with positively charged particles or electrons. Accelerators are also used for radioisotope production, cancer therapy, biological sterilization, and one form of radiocarbon dating.
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A linear particle accelerator (also called a linac) is an electrical device for the acceleration of subatomic particles. This sort of particle accelerator has many applications, from the generation of X-rays in a hospital environment, to an injector into a higher energy synchrotron at a dedicated experimental particle physics laboratory. The design of a linac depends on the type of particle that is being accelerated: electron, proton or ion. They range in size from a cathode ray tube to the 2-mile long Stanford Linear Accelerator Center in California.
As the particle bunch passes through the tube it is unaffected (the tube acts as a Faraday cage), while the frequency of the driving signal and the spacing of the gaps between electrodes are designed so that the maximum voltage differential appears as the particle crosses the gap. This accelerates the particle, imparting energy to it in the form of increased velocity. At speeds near the speed of light, the incremental velocity increase will be small, with the energy appearing as an increase in the mass of the particles. In portions of the accelerator where this occurs, the tubular electrode lengths will be almost constant.
The acceleration of the particles can be made with three general methods:
Linacs are also capable of prodigious output, producing a nearly continuous stream of particles, whereas a synchrotron will only periodically raise the particles to sufficient energy to merit a "shot" at the target. (The burst can be held or stored in the ring at energy to give the experimental electronics time to work, but the average output current is still limited.) The high density of the output makes the linac particularly attractive for use in loading storage ring facilities with particles in preparation for particle to particle collisions. The high mass output also makes the device practical for the production of antimatter particles, which are generally difficult to obtain, being only a small fraction of a target's collision products. These may then be stored and further used to study matter-antimatter annihilation.
As there are no primary bending magnets, this cost of an accelerator is reduced.
Medical grade linacs accelerate electrons using tuned-cavity waveguide in which the RF power creates a standing wave. Some linacs have short, vertically mounted waveguides, while higher energy machines tend to have a horizontal, longer waveguide and a bending magnet to turn the beam vertically towards the patient. Medical linacs utilise monoenergetic electron beams between 4 and 25 MeV, giving an x-ray output with a spectrum of energies up to and including the electron energy when the electrons are directed at a high-density (such as tungsten) target. The electrons or x-rays can be used to treat both benign and malignant disease. The reliability, flexibility and accuracy of the radiation beam produced has largely supplanted cobalt therapy as a treatment tool. In addition, the device can simply be powered off when not in use; there is no source requiring heavy shielding.
At the end all fields are absorbed by a dummy load or cavity losses.
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