Linear Accelerator- Definition, Working Principle and Advantages

Linear Accelerator

What is a linear accelerator?

Also known as a linear particle accelerator, a linear accelerator has many applications, including the generation of X-rays and high-energy electrons for use for radiation therapy in the medical field, serving as particle injectors for accelerators with high energy, and are directly employed in particle physics to attain the highest amount of light particle energy of the kinetic type.

A particle accelerator of the linear type or linac increases the kinetic energy of charged ions or subatomic particles to a great extent by subjecting them to a set of oscillating electrostatic potentials along a linear beamline. Leó Szilárd invented this method of particle acceleration. In medicine, ionizing radiation functions by damaging the cells’ DNA, including that of cancer cells.

How does a linear accelerator work?

A bunch of electrons are given by the ion source when they are taken and directed to accelerate toward the first drift tube as they have a negative potential, while the drift tube has a positive potential. As soon as the electrons enter the tube, the RF source changes its polarity. Then, the charge of the first drift tube changes to negative while that of the second drift tube changes to positive. Because the tube has inertia, the electrons come out of it when they are pushed with the charge of the first drift tube, while the second drift tube attracts them in the same direction.

As the electrons accelerate, their velocity increases and they cover a longer distance at the same time. This is why we need longer drift tubes as the electrons move closer to the target as their velocity is higher. If we require a very high velocity, because of a big number of drift tubes and longer drift tubes, linac has to be very long.

How is safety ensured?

The safety of the patient when using these devices is of utmost importance and needs to be ensured at all costs. Following are some of the safety measures of a linear accelerator:

A treatment plan is designed before the treatment is started, which is approved in collaboration with the radiation dosimetrist and physicist by the radiation oncologist. Before starting the treatment, the plan is thoroughly checked, and quality procedures are checklisted to ensure that the treatment is delivered in a planned way. Many systems are built into the accelerator that does not allow it to project higher doses of radiation than the one prescribed.

Checks are performed by the radiation therapist on the machine every morning before a patient is treated. The equipment used to perform this is called a “tracker”. It ensures that the radiation intensity is maintained at a uniform level across the beam and that it functions optimally. The radiation physicist also conducts weekly and monthly checks of the linear accelerator.

To deliver radiation therapy in a customized way, a linear accelerator is programmed to deliver high-energy x-rays before each session that confirms the tumor’s specific type, size, and shape. This way, the cancer cells can be targeted and destroyed in an area of the patient’s body precisely, with the surrounding healthy tissue receiving minimal exposure from the rays. A linear accelerator has several built-in safety features to ensure patient safety. These features prevent the delivery of radiation in a higher dose than required. Each machine is routinely checked at regular intervals before operation. By using a device called a tracker, the radiation therapist can confirm if the intensity of the beam of radiation is consistent.

A linear accelerator can be employed to treat cancer in any body part by executing a variety of radiation delivery techniques such as:

• Conventional external beam radiation therapy
• Image-guided radiation therapy
• Stereotactic body radiation therapy
• Intensity-modulated radiation therapy

Delivery of radiation therapy by targeted delivery techniques is vital to improve the outcome of the procedures and improve the quality of life of the patient. However, even the most advanced linear accelerators require a skilful operation to get the best results.

Discovery

A Swedish physicist named Gustaf Ising 1924 proposed a technique of accelerating particles with the help of electric fields that are alternating and drift tubes placed at appropriate intervals to protect the particles in the half-cycle if the direction of the field is wrong for the acceleration process. Rolf Wideröe, the Norwegian engineer, designed the first machine of this type four years later, carrying out a successful acceleration of potassium ions to an energy of 50 kiloelectron volts.

Linear machines that help in the acceleration of lighter particles, including electrons and protons, awaited the discovery of radio-frequency oscillators, which were much more powerful and were developed during world war 2 for radars. The operation of proton linacs is typically at frequencies of around 200 MHz, while in electron linacs, the accelerating force is provided by an electromagnetic field that has a microwave frequency of around 3,000 megahertz.

Luis Alvarez, an American physicist, designed the proton linac in 1946. It is a more efficient version of the structure designed by Wideröe. The electric fields are set up in this accelerator as standing waves that have a cylindrical metal “resonant cavity” when the central axis has the drift tubes suspended in it. Los Alamos Clinton P. Anderson Meson Physics Facility is the largest proton linac. Its length is around 875 meters, and it can generate an acceleration of 800 milli electron volts in protons. Structural variation is utilized by this machine for much of its length, called the side-coupled cavity accelerator, where acceleration is generated in cells present on the axis that are coupled together by cavities at their sides, which stabilize the accelerator performance against alterations in the resonant frequencies of the cells undergoing acceleration.

Traveling waves instead of standing waves are utilized by electron linacs. Because their mass is small, navigation of electrons occurs at speed close to the speed of light at very low energies like five mega electron volts. Therefore, they can navigate along with the wave under acceleration, riding the wave crest in effect and therefore always experiencing a field under acceleration. The 3.2-kilometre (2-mile) machine inside the Linear Accelerator Center of Stanford University is the longest electron linac in the world. There are important applications of much smaller linacs of both the electron and protons variants in the industry as well as medicine formulations.

What are the advantages of a linear accelerator?

• The linear accelerator has the ability to generate higher particle energies than the electrostatic particle accelerators that were available previously. In the conventional accelerators that were in use before their invention, the particles would be accelerated only once because of the voltage applied. Therefore, the particle energy is equal to the machine’s accelerating voltage in electron volts, which insulation breakdown would limit to a few million volts. The particles in linac are accelerated many times due to the voltage applied, so the accelerating voltage does not limit its energy.
• There has also been the development of high-power linacs to produce electrons at relativistic velocities. These are required because fast traveling electrons in an arc lose energy via synchrotron radiation. This imposes a limitation on the maximum amount of power that we can impart to the electrons in a particular-sized synchrotron.
• Linacs can also produce a near-constant stream of particles, boasting its prodigious output. On the other hand, a synchrotron can raise the particles to adequate energy only periodically. Because the density of output of the linac is so high, it is good for use in loading storage ring facilities. There are particles prepared for collisions of the particle to particle type. The excellent output of the device with a high mass also makes it practical for producing antimatter particles that are usually challenging to obtain, as they are only a minute fraction of the collision products of the target.

Conclusion

We hope that all your doubts regarding the linear accelerators were cleared by this discussion as we have discussed most aspects related to it. As these devices have wide applications, knowing about them is important from a physics perspective.

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