SEC4Accelerators8_09_000

SEC4Accelerators8_09_000 - Section 4 : Accelerators In...

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1 Section 4 : Accelerators In addition to their critical role in the evolution of nuclear science, nuclear particle accelerators have become an essential tool in both industry and medicine. Table 4.1 summarizes the number of nuclear particle accelerators world-wide and the types of applications in which they are used. Table 4.1 World wide inventory of accelerators, in total 15,000. The da ta have been co l lec ted by W . Sca r f and W . Wiesczycka (See U. Amaldi Europhysics News, June 31, 2000) Category Number Ion implanters and surface modifications 7,000 Accelerators in industry 1,500 Accelerators in non-nuclear research 1,000 Radiotherapy 5,000 Medical isotopes production 200 Hadron therapy 20 Synchrotron radiation sources 70 Nuclear and particle physics research 110 For a non-technical (no equations) discussion of accelerators, a good reference is http://nobelprize.org/physics/articles/kullander/ . The principles of accelerator design fall into several categories, as discussed below. Electrostatic Accelerators Electrostatic devices operate on the principle of a constant electric field E, and are usually referred to as Van de Graaf or Cockroft-Walton accelerators, in acknowledgement of their developers. These are high voltage accelerators that depend on accelerating ions through a large potential difference, Δ E = qe Δ V, (Eq. 4.1) where q is the atomic charge state of the ion, e is the electric charge and Δ V is the potential difference in Volts. Schematically, the operation is illustrated below:
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2 The energy that can be achieved in an electrostatic device is limited by the maximum voltage that can be sustained by the electrostatic field gap before electric discharge occurs. A simple direct-current accelerator is diagrammed in Fig. 4.1. Fig. 4.1 Principle of operation of an electrostatic accelerator. Positive charge is stored on a spherical terminal and positive ions are repelled to ground through an evacuated tube. In order to reach very high voltages, it is critical to insulate the positive and negative terminals from one another to prevent sparking. The largest terminal voltage that has been reached on any existing electrostatic accelerator is 25 MeV at the Oak Ridge National Laboratory Holifield facility, using SF 6 gas (which is very stable toward ionization) as an insulator.
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3 In order to obtain higher ion energies, the concept of the Tandem Van de Graaf has been employed. The basic idea of the Tandem is to employ a two –step process to accelerate negative ions. Negative ions with charge state -q are accelerated into a high voltage terminal with positive potential Δ V, reaching an eneergy Δ E 1 = q e Δ V . In the center of the terminal the ions are passed through a very thin stripping medium to create positive ions of charge +Z, which are then repelled by the positive potential to ground. In this way the ions receive two energy kicks from the same accelerator potential.
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SEC4Accelerators8_09_000 - Section 4 : Accelerators In...

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