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What is a Linear Accelerator?

A linear accelerator, also known as a linear particle accelerator, has many applications such as generating X-rays and high-energy electrons for medicinal purposes in radiation therapy and particle injectors for high-energy accelerators as well as used directly to obtain the highest kinetic energy for light particles (electrons and positrons) for particle physics.

It increases the kinetic energy of charged particles or ions by subjecting them to a wide range of oscillating electric potentials along a linear beamline. The ionizing radiation in this medicine works by damaging the DNA of cells, including cancer cells.

Linear accelerators, sometimes called linacs, are a type of particle accelerator that have the ability to accelerate charged subatomic particles or we can call them ions where the charged particles are subject to a wide range of electric potentials. This method of charged particle acceleration was first used by Leo Szilard. In this, cancer cells can be cured with drugs in which ionizing radiation damages the DNA of the cells.

How does linear accelerator work?

In this the ion source gives a bunch of electrons which are then accelerated towards the first drift tube due to their negative potential and positive potential of the drift tube and when the electrons move inside the tube, at that time the RF source changes its polarity. In this the first downstream tube then becomes negatively charged and the second downstream tube becomes positively charged. And due to the inertia of the tube the electrons move out and at that point, they are pushed by the first drift tube and attracted by the second in the same direction.

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As the electrons keep on moving, their velocity increases and they cover long distances at the same time. For the same reason the drift tube should be longer as the electrons move closer to the target due to the greater velocity. If it requires a large speed, the linac must be very long, due to the long downstream tubes and the large number of downstream tubes.

Linear accelerator wave guides use microwave technology (similar to those used for radar) to accelerate electrons to a part of the accelerator, then then spin these electrons into a heavy metal to generate high-energy X-rays. Allows hitting the target. And these high energy X-rays are shaped as needed as they exit the machine to suit the size of the patient’s tumor and the customized beam is directed to the patient’s tumor. The beam is shaped by a multileaf collimator which is incorporated into the machine head.

The patient lies on an ambulatory treatment couch and a laser is used to ensure that the patient is in the proper position. The treatment sofa can move in many directions, including down, up, left, right, out and in. The beam originates from a part of the accelerator called the gantry, which can be rotated around the patient. Radiation can be delivered to the tumor from multiple angles by rotating the gantry and moving the treatment couch.

What is linear accelerator working principle

In a linear particle accelerator, the drift tube has a positive potential while the electrons have a negative potential difference. As the particles enter the first drift tube, the RF source changes its polarity. Therefore, the first downstream tube becomes negatively charged, but the second downstream tube becomes positively charged. And the inertia of the electrons brings them out of the drainage tube, the electrons are pushed into the first tube, and at the same time, they are attracted to the second tube in the same direction.

The velocity of electrons becomes larger due to their acceleration which is why drift tubes are longer. As the electrons get closer to the target, the speed of the electrons increases. LINACs are longer due to the longer drift tubes, and also the greater number of tubes.

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The positive ions generated at S are made to move into the arrangement of the electrodes as shown in the figure. The ions are accelerated towards the E1 point and after entering this electrode they move with a constant velocity as there is no field inside it.

If the length of electrode E1 is such that the iron is released from the moment the phase of the alternating voltage has reversed, the iron is further accelerated towards electrode E2 by the potential between E1 and E2.

The ions then enter the electrode E2 and move at a constant velocity that is greater than the iron velocity in E1. In this the length of each electrode is adjusted in such a way that the time taken by the ions within the electrode should be equal to half of the period of the oscillator and apparently successive electrode length increases. Due to this reason ions gain additional energy at each interval. By using multiple electrodes, the final energy of the iron is made several times the potential difference that is used for an acceleration.

Who operates this equipment?

Radiation oncologists prescribe the dosage and amount of treatment to patients. Apart from this, dosimetrists and medical physicists also decide the dosage amount and timing, and calculate the time for acceleration. This radiation therapy also involves linear acceleration, and physicians may give radiation treatments to patients on a regular basis.

What is the application of linear accelerator

It encompasses the various uses of this highly efficient accelerator for a different purpose in different industries and research and medicine.

LINAC Synchrotron Injector: This is the first stage, for any high energy accelerator.

Semiconductor Processing: It is used to process various semiconductors.

Boron Neutron Capture Therapy (BNCT): In this therapy, primarily nuclear reactors are traditionally used as a neutron source. A LINAC based source of neutrons can provide a better controlled energy spectrum of neutrons. It is a low cost machine that produces less reactor associated radioactive waste

Isotope Production: These mainly linear particle accelerators are ideal for producing isotopes such as PET isotopes.

Main components of a linear accelerator

Here are the steps and some essential components to generate high energy photons.

Electron Generation: To guide and generate electrons in an accelerated waveguide.

Electron Acceleration: Accelerating electrons to nearly the speed of light.

Beam Transport: Transporting electrons to the target.

Ionization chambers, collimation and arcing filters: modifying the beam before use.

Several accessories to make linear acceleration work

Radiofrequency Generator: It generates electromagnetic waves in the component.

Pulse Modulator: It produces timed energy pulses for the RF generator and electron gun.

Control Panel: Its main function is the operating panel to control the function of the linear accelerator.

What is the advantage of linear accelerator

1) It gives a streamlined beam of ions.
2) It is quite economical to obtain very high energy particles.
3) Particles are able to reach very high energies without the need for extremely high voltages.

What are the drawbacks of Linear Accelerator

1) It is uncomfortably tall in size.
2) In this the intensity of the ionic current is less.
3) In this the particles travel in a straight line, each accelerated segment is used only once this is its main drawback

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How is protection against linear acceleration ensured?

Patient safety is very important in this and it is ensured in many ways.

Before this treatment is given to the patient, a treatment plan is developed and approved by the radiation oncologist in collaboration with the radiation dosimetrist and medical physicist. Before this treatment is given the plan is re-examined and quality assurance procedures are put in place to ensure that the treatment will be delivered as planned.

The quality assurance of linear accelerators is very important. The accelerator has several systems built in so that it does not deliver a higher dose than prescribed by a radiation oncologist. Every morning before any patient is treated, the Radiation Therapist is examined on the machine to ensure that the radiation intensity on the beam is uniform and that it is working properly. In addition, the medical physicist also conducts more detailed monthly and yearly checks of the linear accelerator.

Today’s modern linear accelerators also have an internal check system that does not allow the machine to start until all prescribed treatment requirements are met.

During treatment, the radiation therapist continuously examines the patient using a closed-circuit television monitor. There is also a microphone in the treatment room so that the patient can talk to the doctor whenever needed. Port film (X-rays taken with the treatment beam) or other imaging equipment such as cone beam CTs are regularly checked to ensure that the position of the beam does not differ from the original plan.

The safety of the employees operating this linear accelerator is also important. The linear accelerator sits in a room with moving lead and concrete walls so that the high-energy X-rays are shielded and no one outside the room is exposed to the X-rays. The radiation therapist must turn on the accelerator from outside the treatment room. Since the accelerator only emits radiation when it is actually turned on, the risk of accidental exposure is very low.

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