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Linear Accelerator

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.

Various types of devices collectively known as particle accelerators are of major importance in nuclear research. The particles accelerated to velocities corresponding to many millions of volts, and used as high-energy projectiles for bombarding atoms and for other purposes, are usually the nuclei of light atoms such as the proton from hydrogen or the alpha particle from helium; heavier nuclei may also be used. In the accelerator the particles acquires a kinetic energy equal to its electric charge multiplied by the difference in electric potential through which it falls.

The corresponding unit of energy is the electron-volt (ev), based on the electronic charge and the volt as a unit of potential; the unit Mev represents 1,000,000 electron-volts.The principle of multiple acceleration whereby these extremely high kinetic energies are attained is illustrated by the mechanical model in Fig.1, where the particle is speeded on its way at an increasingly high velocity by a succession of rotating hammers.

The class of apparatus comprising what may be termed “circular accelerators” has been dealt. In a linear accelerator the particles travel in a straight line and are accelerated by a rapidly alternating potential. The earlier linear accelerators were so-called resonance accelerators (Fig.2a), in which the particles are accelerated in steps by the repeated application of a relatively small voltage.

The accelerator consists of a series of tubular units which are alternately connected to the poles of a high-frequency generator. Acceleration of the particles e.g., low-energy ions occurs at the gaps between the units, the frequency of the accelerating voltage being so adjusted that the correct polarity to speed the particle on its way is applied at the correct instant when the particle arrives at the gap (Figs 2b and 2c).

In simple terms: when a positive particle enters a gap just when the next tubular unit is negative and the preceding unit is positive, it will be attracted by the former and repelled by the latter and thus accelerated; by the time the particle reaches the next gap, the polarity has reversed, and it is again similarly attracted and repelled.

To satisfy this condition there must be the following relation between the frequency f, length l of a tubular unit, and the velocity v of the particle: fl = v. Since v increases as the particle proceeds along the accelerator, while f remains unchanged, this means that I must progressively increase i.e., the gaps must be spaced farther apart the tubular units must be longer. The ions to be accelerated are produced by an ion source and injected into the accelerator at a suitable high initial velocity with the aid of an appropriately applied voltage.

In its present-day condition the linear accelerator makes use of an electromagnetic (radio) wave, with a frequency of around 3000 megacycles/second, traveling along an evacuated waveguide i.e., a hollow metal conductor through which high-frequency microwaves more particularly, very short radio waves are propagated (Fig.4). This type of apparatus is used for the acceleration of electrons. These are injected into the waveguide and travel in the same direction as the wave.

The apparatus is so designed that the wave and the electron have the same velocity at all points along the guide; the electron thus travels synchronously with the wave. The latter has an electric field component which is directed along the axis of the waveguide. An electron which enters the guide at the correct time (or phase) is propelled along by a force arising from the interaction of its charge and the electric field. Since the wave and the electron travel at the same velocity, the electron is subjected to this force all along the waveguide and thus travels faster and faster.

To achieve the desired synchronization, the phase velocity of the high-frequency wave is adjusted to the electron velocity by means of suitably dimensioned diaphragms with circular openings in them spaced at intervals equal to one-quarter of the wavelength (Fig.3).

The world’s largest linear accelerator is at Stanford University, in Palo Alto, California. It is nearly 2 miles long and can, in its present stage of development, accelerate electrons to energies of 20 million electron-volts (20,000 Mev). The electron beam can easily be brought out from the accelerator, so that certain precision experiments can be performed which are not possible with circular accelerators.

A linear accelerator (LINAC) is the device most commonly used for external beam radiation treatments for patients with cancer. The linear accelerator can also be used in stereotactic radiosurgery similar to that achieved using the gamma knife on targets within the brain. The linear accelerator can also be used to treat areas outside of the brain. It delivers a uniform dose of high-energy x-ray to the region of the patient's tumor. These x-rays can destroy the cancer cells while sparing the surrounding normal tissue.A linear accelerator is also used for Intensity-Modulated Radiation Therapy (IMRT).

The linear accelerator uses microwave technology (similar to that used for radar) to accelerate electrons in a part of the accelerator called the wave guide, then allows these electrons to collide with a heavy metal target. As a result of the collisions, high-energy x-rays are scattered from the target. A portion of these x-rays is collected and then shaped to form a beam that matches the patient's tumor.

The beam comes out of a part of the accelerator called a gantry, which rotates around the patient. The patient lies on a moveable treatment couch and lasers are used to make sure the patient is in the proper position. Radiation can be delivered to the tumor from any angle by rotating the gantry and moving the treatment couch.