Van de Graaff, accelerator

The Cockroft-Walton and Van de Graaff accelerators are linear that is, they accelerate particles in a straight line. A short time later Ernest Lawrence got the idea to build a circular accelerator, called a cyclotron, and with the help of M. Stanley Livingston he constructed it in 1932. This first cyclotron accelerated protons to about 4 MeV. Since then, many other cyclotrons have been built, and they have been used to accelerate particles to more than 50 times as much kinetic energy as the original one. Also, other kinds of circular accelerators, such as synchrotrons, have been constructed. 

The development of mass spectrometric techniques for nuclide identification using a tandem Van de Graaff accelerator at the University of Rochester Nuclear Structure Laboratory by H. Gove, K. Purser, A. Litherland, and numerous associates has provided an excellent means for the precise measurement of 36C1 concentrations in natural water [43]. Thus far, about 40 groundwater related samples which have been collected and purified chemically by H. Bentley have been analyzed for 36C1 by D. Elmore, H. Bentley, and others using the University of Rochester machine. Some of these samples are listed in Table 2. 

Protons can also be used instead of X-rays or electrons to create the initial vacancies in the inner electron shells, giving rise to a method known as proton induced X-ray emission (PIXE). In these instruments a high intensity, highly focused beam of protons is produced by a van de Graaff accelerator,... 

A radiation dose of 1 kw. could irradiate about 795 pounds per hour of any material to a dose level of 1 Mrad at 100% efficiency. The power efficiency of the different machines varies from a low of 15% for a Van de Graaff accelerator or resonant transformer to 90% for an insulating core transformer. On the other hand, the Van de Graaff accelerator, being a d.c. machine, can be precisely controlled and adjusted, which is important for research and development. The resonant transformer, Dynamitron, and ICT are less precise, but they have lower operating costs and higher output power, making them more suitable for production purposes. 

Van de Graaff Irradiations. The vertically mounted Van de Graaff accelerator (High Voltage Engineering Co.), used for the electron irradiations, was operated at 2 m.e.v. at all times. 

The electron source was a 2-m.e.v. van de Graaff accelerator equipped with a 15-inch beam scanner. Samples of foam, 2 X 3% X 5% inches, were placed on a shuttle and passed 10 times under the electron beam of the machine operating at 60 / a. and 2 m.e.v. Each pass delivered about 1,000,000 rads, so the total dose to the foam was about 10 megarads. All of the irradiations were carried out in air at room temperature. 

The 7Li(p, n) reaction is used commonly to produce approximately monoener-getic fast neutrons. The protons are accelerated to an energy of a few MeV by a small van de Graaff accelerator and strike a cooled rotating Li target. Thick target neutron yields are > 109 n/s-pA. The energy of the neutrons can be obtained from the Q value equation (Chapter 10), which can be expressed (for 0° neutrons) as... 

Figure 14.5 A highly schematic view of the important components in a Van de Graaff accelerator. Positive ions created by a corona discharge near ground potential are swept by a moving belt to a similar corona contact attached to the inside of the high-voltage terminal. The positive ions then evenly distribute themselves on the surface of the terminal (Rrane, 1988).

An important improvement of the single-ended Van de Graaff accelerators came through the replacement of the positive ion source inside the high-voltage terminal with an external negative ion source (Fig 14.6). 

Figure 14.6 Two-stage tandem Van de Graaff accelerator. [From R. J. Van de Graaff, Nucl. Instr. Meth. 8, 195 (I960).]...

Describe the duty cycle of a typical Van de Graaff accelerator, a linac, and a synchrotron. 

That free radicals are produced in the radiolysis of liquid systems was first shown conclusively by the esr experiments of Fessenden and Schuler71. The esr spectra were observed during the continuous irradiation of the liquid samples by an electron beam from a Van de Graaff accelerator. The spectrum obtained... 

In the work to be described, the University of Kentucky 6.5 MV Van de Graaff accelerator was used with a proton beam incident on a % gas target. 

The films were then irradiated in air to the desired dose by passing under a 2.5-Mev. Van de Graaff accelerator. The delay time between irradiation and the start of the grafting varied between 5 and 10 min. 

Conversion of several percent of low molecular weight materials by nonchain reactions requires radiation doses of the order of 100 megarads. A 3-m.e.v. Van de Graaff accelerator with a gold target supplies this dose to a small sample in less than 1 hour. The sample can be held at any desired temperature in a Dewar flask. The products, many of them highly reactive, are detected by direct distillation at low temperatures and very low pressure into a time-of-flight mass spectrometer. With this basically simple technique, a survey of radiolysis of many systems,... 

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