Chapter 1 Linear Accelerator PDF

Chapter 1 Linear Accelerator The linear accelerator is also known as a linear particle accelerator. It comes in use in the field of medicine and therapeutic applications as it generates electrons and X- rays of high energy. They can generate high kinetic energy. It is ideal for protons and electron...

Chapter 1 Linear Accelerator The linear accelerator is also known as a linear particle accelerator. It comes in use in the field of medicine and therapeutic applications as it generates electrons and X- rays of high energy. They can generate high kinetic energy. It is ideal for protons and electrons in particle physics for getting high kinetic energy. 1.1 Introduction to Linear Accelerator It is a kind of particle accelerator that enhances the kinetic energy of charged ions or subatomic particles. The charged particles are subjected to oscillating electric potential with a linear beam line. The ionizing radiation functions by damaging DNA cells like cancer cells. Though Linear Accelerator was invented in 1930, it took over 4 decades for the Linear particle accelerator to come into existence. Created by Stanford professor Dr Peter Fessenden in 1972, the main purpose of the accelerator was to be able to combine X ray and electron treatment under one specific instrumentation. His core subject of study was to understand the usability of two different type of radiations in the treatment of Tumors. Working Principle of a Linear Accelerator Due to the applied electric field between positive and negative terminal the charge particle moves from positive to negative direction (1) 1 And also the electric field is force on moving charge (2) From equations 1 and 2 Then, the acceleration is , Since, Then, 0 Linear accelerator works using microwave technology to initiate the electron response. The electrons collide with metal iodes to cause a chain reaction that results in the formation of high energy x-rays. Further, the source of ion provides an electron bunch that accelerates to either drift tube of positive and negative potential. The RF source shifts the polarity when 2 electrons enter the tube. Moreover, the first tube negatively charges while the second drift tube gets a positive charge. After that, the electron comes outside due to inertia. At this instance, the electrons move with the first drift tube and attract by another in the same direction. The accelerated electrons have high velocity and travel at long distances. Hence, the drift tube should be long when the electron comes close to the target due to high velocity. Further, the main reason for the development of the linear accelerator is to deliver high energy rays at a target-specific location. Thus the high velocity electron emerging from the drift tube and be easily directed towards the specific source or tumour using the gantry of the drift tube. 1.2 Application of a Linear Accelerator Many oncologists recommend volume and dosage to patients. Further, the medical physicist decides the time and amount of dosage and calculates acceleration time. However, different industries have wide usage of Linear accelerator with a distinct purpose.  LINAC synchrotron injector- It is the initial stage for a high energy accelerator.  Boron Neutron Capture Therapy- Nuclear reactors come in use traditionally as a neutron source. LINAC sources offer a controlled spectrum of the neutron. It generates less radioactive waste.  Isotope production- It is perfect for generating PET isotopes. 1.3 Safety Assurance while using Linear Accelerator LINAC machines come in use for treating cancer and generate high radioactive energy. Moreover, this device can harm patients. Radio oncologists plan proper treatment as per the medical report. Further, the plan is made with the radiation physicist. Tracker is a device that checks the proper working of the machine, radiation intensity, and velocity. 3 FAQ on Linear Accelerator Question 1: What is the working principle behind a Linear Accelerator? Answer 1: We refer to the Linear Particle Accelerator as LINAC also. It is one of the variants of Particle Accelerator that uses oscillating electric potential through a linear beam to accelerate the charge of a subatomic particle. Question 2: What does linear Acceleration mean? Answer 2: In a specified given time frame if there is a change in the velocity of any object moving in a straight line, then the acceleration will also increase. Thus, on the basis of this, the increasing or decreasing velocity, the rate of acceleration will also increase or decrease accordingly. Thus, we refer to this as Linear Acceleration. Question 3: What are the different types of Particle accelerators? Answer 3: Particle Accelerator are of 2 types 1. Linear Particle accelerator directs the movement of a particle in the direction of the straight or linear beam line. 2. Circular Particle Accelerator directs the movement of particles along a circular track line. 4 1.4 Clinical Linear Accelerators The underlying principle of a LINAC is remarkably simple, implementation of that principle to produce a consistent stable beam requires a precise and sophisticated design. Understanding the basics of that design is essential to ensure patient safety and machine up-time. In the basic accelerator design, a heated filament releases a cloud of electrons. These electrons are then accelerated by an electric field applied between the filament (cathode) and a thin metal window (anode). Clinical LINACs operating in the MeV region require an Accelerating Waveguide to achieve the required acceleration over a reasonable distance. The electrons then hit a target (where they produce Bremsstrahlung X-rays) or a scattering foil (to spatially distribute the electron beam). Finally, the beam may be further shaped in the treatment head. Components of a Clinical Linear Accelerator External Components 1- Couch (Patient Positioning System): The couch supports and positions the patient during treatment. Modern couches facilitate precise patient positioning by moving along the x, y, and z axes. Advanced couches may also include the ability to adjust patient roll, pitch, and yaw. 2- Electronic Portal Imaging Device (EPID): The electronic portal imaging device forms an image using the MV treatment beam. EPIDs are valuable tools for monitoring patient setup and quality assurance. 5  3- Gantry: The LINAC is mounted on a rotating gantry which treatment from multiple angles. 4- kV Imaging System: The kilovoltage imaging system consists of a kV X-ray generator and an electronic imaging device. The lower energy of this imaging system improves contrast, especially when used to generate a cone-beam CT. 5- Stand: The stand connects the gantry to the treatment room floor and contains electronics and other systems required for linac operation. Internal Components 6- Accelerating Waveguide: A series of microwave resonance cavities used to accelerate the electron beam to high energies. 6 7- Bending Magnet: The bending magnet is a magnetic lens used to focus and position the beam to intercept the target (for photon treatments) or scattering foil (for electron treatments). The angle of bending varies by manufacturer but may be either 90°, 112.5°, or 270°. Magnetic focus attempts to be achromatic (does not separate by energy at point of focus). 8- Circulator: A device in the waveguide that is used to prevent microwave energy from reflecting backwards to the Klystron/Magnetron. 9- Cooling System: Production of a clinical treatment beam is an energy inefficient process due to losses in microwave generation and acceleration. A water or air cooling system is required to maintain a stable operation temperature necessary for consistent beam energy production. 10- Electron Gun: An electron gun produces the electrons which are accelerated in the accelerating waveguide. Electron guns consist of a heated filament (~800°C – 1100°C) which “boils off” a cloud of electrons. These electrons are immediately accelerated by a low E field (~40kV).2 Electron guns may be either of the diode or triode types. Diode electron guns consist simply of the heated cathode and an anode which set the accelerating voltage. Triode electron guns add a control grid between the cathode and anode which serves to recollect a portion of the liberated electrons. Thus, the triode design allows for variable beam current by preventing a variable fraction of electrons from reaching the accelerator. 11- Energy Selector: An energy selector may be placed within the bending magnet array to narrow the allowed electron energy range incident on the target/scattering foil. Typical energy band pass range in of the order of 6% (97%- 103% of desired energy). 7 12- Klystron/Magnetron: Klystrons and Magnetrons produce the microwave used to power the accelerating waveguide. 13- Treatment Head: The treatment head contains components required for beam production and shaping including targets, scattering foils, beam shaping collimators and the optical distance indicator. 14- Waveguide: The waveguide is a channel directing the microwave power from the Klystron/Magnetron to the Accelerating Waveguide. The waveguide is filled with an insulating gas (typically Sulfur Hexaflouride, SF6) to prevent electrical arcing. Microwave transparent ceramic barriers prevent the SF6 from leaking into the vacuum spaces filling the Klystron/Magnetron and the Accelerating Waveguide. 8