# The LINAC

A linac - linear accelerator - is a type of particle accelerator in which charged particles are accelerated in a straight line.

This is done either by a steady electrical field or by means of radio-frequency alternating electric fields (typically in the range 12 - 24 MHz, in the middle of the shortwave band). Using a steady electric field would require a massive voltage over a large distance. This would have implications for insulation and safety. The more common aproach is to use radio-frequency alternating electric fields that are applied over short distances to 'kick' the particles into a higher energy.

The charged particles are accelerated by an electric field that is applied between the drift tubes (across the gap between them). The particles travel in 'drift tubes' between 'kicks' by the electric field. As the particles leave a tube section there must be an accelerating field across the gap so the tube it is in must be of the same potential as the particle's charge (repelling it away from where it has come from) and the one it is about to enter be of the opposite potential (attracting it towards the next tube).When it enters the drift tube it must experience NO electric field so that it 'drifts' on at the same velocity until the half of the cycle when it can be accelerated again.The drift tubes are made of conducting material and therefore form 'Faraday cages'.

The Faraday Cage Effect (named after its discoverer) means that the electric charge on a conductor sits on the outer surface of it. Therefore, no electrostatic field is present within the conductor. This is done so that the alternating potential does not affect the particles when it is in opposition to the direction they are travelling. The passage of the particle between drift tubes is synchronized with the phase of the accelerating field - the particle is only subjected to the field when it is in the part of the cycle that accelerates it (not the other half which would decelerate it!).

The tube sections need to get larger as the particles travel faster as they have to be inside the tube for the full half cycle

The equation that allows you to work out the acceleration is:

# F = qE

where

F is the force vector,

q is the charge and

E is the electric field intensity vector.

Once you know the force you can woork out the acceleration by using;

# F = ma

where

F is the force vector,

m is the mass and

a is the particle's acceleration vector.

The SLAC Linear Accelerator (linac) is a two-mile long accelerator, consisting of a cylindrical, disc-loaded, copper waveguide placed on concrete girders in a tunnel about 25 feet underground.

Experimental research began at SLAC (Stanford Linear Accelerator Center) in 1966 with the completion of the two-mile-long linear electron accelerator or linac, a machine capable of producing an electron beam with an energy up to 20 GeV (giga-electron volts) or 20 billion electron volts. Initial experiments directed these electrons onto stationary targets to study the structure of matter. The maximum energy of the linac was increased over the years to 50 GeV. The two-mile accelerator continues to generate high-intensity beams of electrons with the highest energy available in the world.

Linac use in medical facilities

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. The very high speed electrons from it produce a uniform dose of high-energy x-rays when fired at a metal target. These can then be directed to the region of the patient's tumorr. These x-rays can destroy the cancer cells while sparing the surrounding normal tissue.

### How does it work?

The medical use of 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 tumour. 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.

### Who operates this equipment?

The patient's radiation oncologist prescribes the appropriate treatment volume and dosage. The medical radiation physicist and the dosimetrist determine how to deliver the prescribed dose and calculate the amount of time it will take the accelerator to deliver that dose. Radiation therapists operate the linear accelerator and give patients their daily radiation treatments.

### How is safety ensured?

Patient safety is very important. During treatment the radiation therapist continuously watches the patient through a closed-circuit television monitor. There is also a microphone in the treatment room so that the patient can speak to the therapist if needed. Port films (x-rays taken with the treatment beam) are checked regularly to make sure that the beam position doesn't vary from the original plan.

The linear accelerator sits in a room with lead and concrete walls so that the high-energy x-rays are absorbed and do not increase the risk of cancer for Staff. The radiation therapist must turn on the accelerator from outside the treatment room. Because the accelerator only gives off radiation when it is actually turned on, the risk of accidental exposure is extremely low. Indeed, pregnant women are allowed to operate linear accelerators because the safety levels are so high.

Modern radiation machines have internal checking systems to provide further safety so that the machine will not turn on until all the treatment requirements prescribed by your physician are perfect. When all the checks match and are perfect, the machine will turn on to give your treatment.

Quality control of the linear accelerator is also very important. There are several systems built into the accelerator so that it won't deliver a higher dose than the radiation oncologist prescribed. Each morning before any patients are treated, the radiation therapist uses a piece of equipment called a "tracker" to make sure that the radiation intensity is uniform across the beam. In addition, the radiation physicist makes more detailed weekly and monthly checks of the accelerator beam.

LOJ November 2001- amended December 2006 to include medical applications- information taken from www.radiologyinfo.org