Linear electron accelerators

Miller, C.W., Power sourees for irradiation processing the linear accelerator. In Charlesby, A. (Ed.), Radiation Sources. The MacMillan Company, New York, 1964, pp. 197-219. Sehonberg, R.G., Radiation considerations for operation of a portable 6-MeV Electron Linear Accelerator. Proceedings 20th Midyear Topical Symposium, Health Physics Society, Reno, NV, February 8-12, 1987, p. 297. 

Linear accelerator A device that uses microwave technology to accelerate electrons in a highly focused beam to deliver targeted radiation to tumor sites. 

Photo-excitation and de-excitation are basic processes in nuclear reactions. A Japanese-Hungarian cooperation investigating these processes has yielded good results during the past few years [21-26], These studies used weighable amounts of "Tc to look at the (y, y ) reaction that leads to the production of the nuclear isomer "mTc by electron linear accelerator irradiation. 

The major steps involved were first the move from 200-kV x-rays to cobalt-60 and then to modern electron linear accelerators providing x-ray beams of about 20 MV (and electrons when needed for some patients). Improvements in the beam penetration were, in general, combined with improvements in beam delivery systems, e.g., isocentric gantries, variable collimators, later on multileaf collimators, etc. 

More recent technical developments of the electron linear accelerators are impressive. The reliability, mechanical stability, beam delivery, and collimation systems of the new generation of accelerators have reached a high level of quality [52]. These machines now allow irradiation of nearly any target volume with reduced irradiation of the surrounding organs at risk (OAR). 

Fig. 3. Principle of linear accelerator (linac). Partially accelerated electrons from a source, such as a Cockcroft-Walton generator, arc further accelerated by stages as rhe electrons pass through radio-frequency cavities, powered by if oscillators. Each paiticle receives a small push as it passes from one cavity to the next until the final desired accelerated beam is produced Tile machine must be carefully synchronized CSG = Cockcroft-Walton generator, RFO = radio-frequency oscillator RFC = radio-frequency cavity...

T n 1962 the U. S. Army opened at its Natick Laboratories in Natick, Mass., the world s largest irradiation laboratory (2) for preserving foods by ionizing energy (Figure 1). This laboratory is unique in that, in addition to having two radiation sources, a 24-m.e.v., 18-kw. electron linear accelerator and a 1,250,000-curie cobalt-60 isotope source, it includes a food development-preparation laboratory and an experimental development kitchen (Figure 2). 

Figure 3. The electron linear accelerator with energy analyzing and scanning systems...

Almost since the earliest attempts to produce well-defined beams of low energy positrons, various types of accelerator have been used for this purpose, e.g. electron linear accelerators, microtrons and cyclotrons (see e.g. Dahm et al., 1988 Itoh et ai, 1995). Positron beams have also been developed at nuclear reactors (Lynn et ai, 1987). 

The apparatus for this experiment is shown schematically in Figure 9a. The positrons are produced by bremsstrahlung and pair-production at the beam dump of a 36 MeV electron linear accelerator. The positrons are moderated in tungsten vanes and transported to a Molybdenum n = 2 formation foil on the... 

Fig. 1. Block diagram of the nanosecond pulse radiolysis system using the Hokkaido University 45 MeV electron linear accelerator...

So far the microwave electron linear accelerator is the most suitable for this purpose. In this accelerator electrons are injected into an evacuated cylindrical waveguide in which pulsed radiofrequency of several megawatts from a klystron oscillator travels. Electrons enter the radiofrequency field at the correct phase are accelerated to a velocity close to that of light. By means of gun control, electrons are injected only during the radiofrequency pulse, and thus the electron pulses of several nanosecond duration, useful for conventional nanosecond or microsecond pulse radiolysis, are produced. 

Pulse radiolysis systems capable of picosecond time resolution use the fine structure of the output from the electron linear accelerator. Electrons in the accelerating tube respond to positive or negative electric field of the radiofrequency, and they are eventually bunched at the correct phase of the radiofrequency. Thus the electron pulse contains a train of bunches or fine structures with their repetition rate being dependent on the frequency of the radiofrequency (350 ps for the S-band and 770 ps for the L-band). 

With the aid of a subharmonic prebunching cavity operated at a few tenths of the main accelerating microwave frequency, the electron linear accelerator is... 

A direct synthesis of N2F2 in low yield and admixed with other nitrogen fluorides has been reported from the irradiation of N2-F2 mixtures with ra-y-radiation from a nuclear reactor admixed with other high-energy radiation from uranium fission products (85). There is also a radiochemical synthesis of N2F2 (1.5%) and NF3 (42%) when an N2-F2 mixture is irradiated with 30-MeV electrons in an electron linear accelerator (86). Reaction of fluorine diluted with N2 and NH3 also gives some N2F2 (159,213). 

The synchrotron was developed by elementary particle physics in order to accelerate electrons, positrons, protons and other particles. It consists of a ring with a diameter of about a few meters up to more than 100 m in which a vacuum of 10 mbar can be sustained and to which strong electric and magnetic fields can be applied (see Fig. 1). A bunch of electrons or positrons is first accelerated in a linear accelerator to an energy usually lying between 40 MeV and 380 MeV. 

Sisman and Bopp, Charlesby, Turner and Petrov and Karpov studied the yield of total gas evolution from natural mbber, poly butadiene and various GR-S type copolymers subjected to ionizing radiation (reactor, Co or electron accelerator). Most of the gas is H2 + CH4 (100% in the case of polybutadiene), however for some rubbers a small amount of CO2 + C3H6 was found also. Turner found that under bombardment with accelerated electrons, the evolution of hydrogen from purified natural mbber was linear with dose, up to ISO megarads, and corresponded to G(H2) = 0.64. This radiolytic yield is noticeably smaller than those found in low-molecular-weight olefins. 

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