Analysis Of A Linear Accelerator

Analysis Of A Linear Accelerator Nowadays patient with cancer are treated by radiation, surgery, chemotherapy or with a combination of these options. The radiotherapy treatment unit used to deliver radiation to cancerous cells and tissues is the linear accelerator, also known as linac. The linear accelerator has been defined by Khan F. M. (2003) as a device that uses high-frequency electromagnetic waves to accelerate electrons, to high energies through a linear tube. The electron beam itself can be used for treating superficial toumors, or it can strike a target to produce x-rays for treating deep-seated toumors. The energy used for the radiotherapy treatment of deep situated tissues varies from 6-15 MV (photons) and the treatment of superficial toumors (less than 5cm deep) is between 6-20MeV. (Khan, 2003) The purpose of this essay is to describe a linear accelerator, analyse its components in the stand and the gantry of the linac, and explain the principles of operation and then discuss why it is best situated to the task for which it was designed. Some advantages and disadvantages of the linac will also be included in the discussion part of the essay. Main body: Figure 1:http://www.cerebromente.org.br/ As you can see from the schematic picture above, the major components of a linac are: Klystron: source of microwave power Electron gun: source of electrons. Waveguide (feed and accelerating waveguide): microwaves travel through the feed waveguide and then to the accelerating waveguide, where electrons are accelerated from the electron gun. Circulator: a device that prevents microwaves of being reflected back from the accelerator. Cooling water system: cools the components of the linac. Bending magnet: â€Å"A bending magnet is used to change the direction of the accelerated electron beam from horizontal to vertical.† (Hendee et al, 2005) X-ray target: electrons hit the target and produce x-rays. Flattening filter: even out the intensity of the beam. Ionisation chambers: they control th e dose leaving the head of the linac. Beam collimation: shape the radiation beam to a certain size Klystron: There are two types of microwave power. The klystron and the magnetron. Magnetrons are used for lower energy linacs. In the high energy linear accelerator klystron is used. All modern linacs have klystrons. Both klystron and magnetron are special types of evacuated tubes that are used to produce microwave power to accelerate electrons. (Karzmark and Morton, 1998). â€Å"The tube requires a low-power radiofrequency oscillator to supply radiofrequency power to the first cavity called the buncher.† (Hendee et al, 2005) In the bunching cavity, electrons produced from the electron gun, are bunched together to regulate their speed. The microwave frequency is thousands times higher than ordinary radio wave frequency. For a linac to work, the microwave frequency needed is 3 billion cycles per second. (3000MHz) (Karzmark and Morton, 1998) Electron gun: The electron gun is part of the klystron. Here, electrons are produced and then accelerated to radiofrequency cavities. The source of electrons is a directly heated filament made from tungsten, which will release electrons by thermionic emission. (Bomford, 2003) Tungsten is used because it is a good thermionic emitter with high atomic number, providing a good source of electrons. Klystrons usually have 3-5 cavities, used to bunch electrons together and increase microwave power amplification.