(a) The orbit or a particle in an isochronous cyclotron alternating strong and weak azimuthal regions are created (“sectors” or “hills and valleys”) to provide axial focusing to offset the defocusing effect owing to radially increasing magnetic field. Eventually, the radius will reach a maximum value, determined by the radius of the magnet pole faces, and the particle will escape. The frequency with which the accelerating voltage must be changed is the cyclotron frequency defined in Equation 8 for practical cyclotrons this frequency is in the megahertz range, and is usually referred to as radiofrequency (RF). During the time that the particle is inside the D, the polarity of the voltage is changed, so that it is continually accelerated. A potential difference is established between the two Ds, so that each time the particle crosses from one D to the other, it sees a small accelerating voltage and moves outwards, following a new orbit at a slightly larger radius. in a Faraday cage), so that only the magnetic force applies, and the particle moves along a circular path. While the particle is inside the D, it is shielded from the electric field ( i.e. A particle is injected into the magnetic field with an initial velocity, somewhere near the centre of the magnet, inside two “dee” (D) shaped hollow electrodes, which are placed between the poles of the magnet. A schematic diagram of a cyclotron is shown in Figure 2. A glossary of terms frequently used by accelerator physicists is included as an appendix.Ī simple cyclotron consists of a large circular magnet providing a constant magnetic field across the gap between the pole-faces. This article attempts to explain some of these aspects for the benefit of multidisciplinary hospital staff. Consequently there is a need for medical physicists and oncologists to be aware of the basic principles of CPT accelerator systems, how the equipment operates and the potential impact of new technologies in this area. The commercial manufacture of cyclotrons and synchrotrons for radiotherapy by several companies over the past decade has allowed turnkey centres within hospital cancer centres to be installed. Cyclotrons have been used in medicine for many years, especially for radioisotope production. The recent expansion, in several countries, in charged particle therapy (CPT), which uses protons and light ions to treat cancer has been achieved using relatively conventional, cyclotron or synchrotron accelerator technology. 5–15 MeV electrons are used to generate megavoltage X-rays or deliver these electrons, without prior collision, from a linear-accelerator to treat cancer. The general public is largely unaware that the greatest use of particle accelerators worldwide is within hospitals. However, there are many other applications of lower particle energies, especially in medical physics. There is growing public awareness of particle accelerators in science owing, in part, to the publicity surrounding the Large Hadron Collider at CERN in Geneva, although protons can be accelerated to high energies at several accelerator laboratories around the world.
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