The linear accelerator
The linear accelerator
The late 19th century discovery of X-rays led to revolution in medical science that would culminate in the medical advancements we have today. The capability of X-rays to penetrate the human skin became a very exciting adventure to most scientists at the time they were being initially explored. The excitement was however short-lived because soon after, it was discovered that X-ray penetrations carried potential health risks. Consequently, focus started shifting to the safety of the rays with the formation of various standard committees around the world.
The health scare did not however dampen the interest that the scientists had in X-rays as more studies and advancements continued to be made in the field. With time, radiotherapy technology has improved, and this has led to it finding more applications because it has resulted in better control of the rays produced. Consequently, application of radiotherapy has gained more significance, the most telling evidence of this being the emergence of medical physics as a profession in the mid 20th century.
Although X-rays remain the most prominent in medical radiotherapy, other forms such as use of proton therapy are starting to gain prominence as well. On the technology front, there are two technologies of particle acceleration used in radiotherapy. The two technologies are linear and circular acceleration technologies. The linear technology commonly referred to as Linac technology is the more popular and established of the two. The concept of linear technology was the first to be used in the acceleration of electrons across a charged field in the production of X-rays.
Its use has since dominated the world of medical radiotherapy as compared to the cyclotron, which has a lesser presence. The aim of this paper is to explore linear technology starting with its mechanism and after that look at some of its design improvements and enhancements. Earlier forms of linear acceleration Linear acceleration is the default technology in particle acceleration because it was through it that the earliest forms of X-ray production were discovered. An early prototype design for the purposes of linear acceleration is shown in appendix 1.
The basic components to note from that design are the evacuated chamber, the anode (the positively charged electrode) and the cathode (the negatively charged electron). In this unit, the electrons are produced from the cathode after which they are accelerated to the anode by the potential difference existing between the two. The accelerated electrons are then made to hit a target, an action that leads to the production of energy in the form of X-rays. Current form of the linear accelerator
From studying the earlier form of linac, one can clearly see that its basic structure did not allow for most of the factors to be controlled. Given the potential high energy that can be emitted from an X-ray production procedure, it is important that one is able to control the amount and nature of energy produced. Subsequent advancements in particle acceleration have to some extent taken care of this requirement. An outstanding difference between the traditional linac shown above and the current one is the multiple acceleration mechanism.
An illustration of this is contained in figure 2. The idea behind particle acceleration is to get particles to attain maximum speeds before allowing them to hit the target. To get the maximum acceleration, the particles should be subjected to multiple acceleration points, and for linear acceleration this is achieved by placing accelerators strategically on the path of the electrons. On the other hand, the circular accelerators use repetitive circular motion to achieve this objective. Basic components of the linac
Construction of the linac and any other particle accelerators for that matter is dependent on the manufacturer. Apart from ensuring that the equipment conforms to requirements needed to realize the basic mechanisms of linear acceleration, the manufacturer retains a freehand on the other aspects of the construction. Of importance is that the linac has to have a unit that generates the particles, another to accelerate and a target on which the particles will hit to generate the energy needed to carry out the radiotherapy procedure. Generation of particles
Initially, electrons were the only particles used in radiation therapy, but advancements in the field have resulted in mechanisms that have enabled even heavier particles such as protons and neutrons to be used. However, electrons remain the most widely used particles because of the relative maturity of the technology involve in the use of electrons. To generate electricity, a metal filament is heated by means of an electrical current flowing through it. The heat is generally a function of current I and resistance R and is governed but the equation given below:
H=I2R Where: H is the heat generated I is the current flowing through the filament R is the resistance of the filament The heat in the filament excites the electrons, something that leads to them moving closer to the surface of the metal with some even leaking out to the surrounding area. The extent of excitement will mainly depend on the amount of heat generated in the heating action. To supplement the heating action, an electric field with an opposite charge is applied near the filament. This action will extract electrons and further streamline their direction.
An alternative method of generating electrons is to use a laser light to knock off electrons from the surface of a semi conductor. The action accomplished this far is the generation of electron particles with minimal acceleration. It thus means that the electrons have just been presented to the Linac’s structure, but are relatively stationary. The next action is to accelerate them to high speeds. Acceleration Acceleration is the next stage in the process. To accelerate the particles, the principle of combining like and unlike poles suitably so as to achieve attraction or repulsion is used.
The first important step in the acceleration chamber is evacuation. It is important to ensure that the chamber is free of any matter that may reduce the electron energy through actions such as friction. Therefore, acceleration has to take place in a vacuum. A simple structure of acceleration consists of two plates of opposite charge arranged in series with respect to the electron path. As the electron emerges from the filament it enters the between the two plates propelled by its own momentum.
The first action it receives is repelling action from the negatively charged plate and an attractive action from the positive plate. This leads to the electron being propelled across the two plates by a combination of attraction-repulsion force combination. However, as the electron approaches the positive terminal, the charge in the terminal is changed to negative. It is important to note that timing of the change should be so strategic because poor judgement will lead to the repulsion force created by the change overcoming the energy created by the initial attraction-repulsion force between the plates.
The idea is to switch the charge in such a manner that the repulsion force applies minimally on the electron’s approach, but is maximum as the electron leaves the plate. Hence, the acceleration is achieved by repelling the electron from the now negatively charged plate towards a positively charged plate. Again, as the electron reaches the positive plate, its charge is changed to negative to repel the electron towards another positive plate. This action is repeated several times along the linac’s profile with the resulting electron acquiring super speeds. Particle deceleration
After electrons have reached their optimum velocities, they need to be stopped appropriately to produce the desired effect. The deceleration action is responsible for producing relevant particles (for example neutrons from uranium) or X-rays in the case of a radiotherapy application. At the end of the linac, a tungsten target is placed. Once the electrons hit the target, there are four possible ways the energy so generated will be utilised. This may be an ionisation or excitation action of the outer electrons of the target, emission of X-rays from the ionisation action and production of bremmsstrhlung radiation or braking radiation.
The two significant processes for purposes of radiotherapy are the latter two, that is, production of the braking radiation and generation of X-rays. Of importance here is that X-rays are produced either through ionisation or through the impact of electrons hitting the target. Focusing the X-rays Once the X-rays have been produced, they need to be focused so that they only end up in acting on places they were meant to. This is especially important in the field of radiotherapy because of the health risk associated with X-rays.
The focusing function is achieved by using a combination of concave lenses, which provide a reflecting surface that ends confining the X-rays to a narrow path. Design variations Each of the manufacturers decides on how to achieve the above mechanisms. As is to be expected therefore, each of them will have a different approach in achieving these actions. Even with all the variations in design approaches, there is a common denominator in that each of them has an aim of optimising every process in the cycle. Some of the design issues are looked at below. Cathode
Factors given prominence in the design of the cathode are its life and the energy needed to extract the electrons. In terms of lifetime, the cathode is judged on its ability to be used for a certain length of time without losing its quantum efficiency. Degradation of quantum efficiency can result from entry of foreign materials through adsorption or dissipation of the cathode’s material through desorption. Although the idea is to get a cathode with robust quantum efficiency, a balance has to be struck because those with high quantum efficiencies tend to degrade faster while those with lower efficiencies are slow at degrading (NAP, 2010).
In addition to the lifetime issue, a good cathode is one that needs the least energy for its electrons to be excited. Lower energy means that the working temperature of the cathode can severely be reduced because the electron excitement will occur at lower temperatures. Some of the enhancements that can be used in the electron generation stage insulation of the cathode to reduce leakage of the electron and placement of a magnetic lens behind the gun for purposes of focusing the electron beam generated (Wei-Ling, Quan-feng, & Yun-Kai, 2001). Acceleration
The most crucial component in the production of X-rays is the acceleration action. The linear profile of a linac is its most glaring weakness because linearity points to a limit in terms of acceleration that can be achieved. Acceleration is given by the increase in velocity over time. Assuming that velocity is increasing at a constant rate, then for maximum acceleration to be achieved there is need for the particle to be given the maximum time possible. That can be achieved by having the particle accelerated for the longest distance possible, hence the need for the linac to have the maximum possible length.
This is one of the weaknesses of the linac; it needs a substantial hardware and space for its particles to achieve maximum energy. For example, the Stanford linac is the most powerful in the world, but that power comes at a price because even though it is capable of achieving energies in the order 32Gev (gigaelectron volts), the linac is 3km long. A 3km long linac in a medical facility setting may be too much because it will take up too much space. This may not however be necessary because electron-generated X-rays may need to use all that energy generated in the Stanford linac.
Such a linac may only be necessary for medical facilities that want to use heavier particles such as neutrons, protons and carbon ions. Therefore, linacs still serve the purpose although they may soon need more optimization as proton and other heavier ion therapies continue to gain importance. An alternative to the long linacs would be the circular accelerators, but they too, are expensive and have limited expertise. Optimum acceleration is achieved by ensuring that the electrons get in to the electron guns at maximum attainable speeds. For instance a good linac ensures that the electrons are at least 60% in proportion to the speed of light.
On exiting from the acceleration system, they should have attained a speed that is as close to the speed of light as it is possible. One of the strategies that can be used to optimise electron speeds is ensuring that they exit the linear accelerator on to the target at the most optimum velocity. This can be achieved by varying the distance between the accelerating plates so that each of the pairs has more spacing between them than the previous ones. This way, the acceleration distance is gradually increased so as to take in to account the changing energies of the electrons as they move between the pairs.
Optimising electron flow Divergence and dispersal of electrons ought to be minimised during flow to maximise on energy conservation. Avoidance of divergence requires an action aimed at ensuring the electrons flow with maximum density. To achieve this, magnetic poles can be used to push the electrons towards a definite centre. A suitable magnetic pole arrangement that can focus the electrons is shown in figure 3. From the figure, one can notice that the arrangement involves alternating the north and south poles so that electrons straying in any direction can be taken care of.
Another important aspect in optimising electron flow is the grouping them in to bunches. This is achieved using radiation along the linac profile provided by klystron. On appendix 2, the microwave radiation shown is provided by the klystron. The radiation’s role is to provide a force on the electron in such a manner that the electrons end up being packed in to clusters. Clustering of the particles leads to the electrons developing more momentum (thus conserving energy) as they move across the linac’s profile. Conclusion
Linear accelerators are yet to achieve electron speeds similar to those of circular accelerators, but the fact that electrons are still the most widely used particles gives them an edge over the circular accelerators. Circular accelerators may be capable of higher speeds but come with higher costs. In the long term however, linacs will face stiff competition from circular accelerators as heavier particles continue to gain prominence because protons have generally shown that they have higher and more predictable energy compared to the electrons.
The future of linacs, therefore, lies in their particle acceleration abilities being increased for them to handle heavier particles without the accompanying huge hardware and space demands as it is now. Bibliography e-radiography. (n. d. ). A Brief History of X-rays. Retrieved Jul 22, 2010, from e-radiography: http://www. e-radiography. net/history/general. htm factstaff. (n. d. ). How “Atom Smashers” Work. Retrieved Aug 03, 2010, from factstaff: http://www. facstaff. bucknell. edu/mvigeant/univ_270_03/Jaime/index. html lbl. (n. d. ). Linac. Retrieved Aug 08, 2010, from lbl: http://www. lbl. gov/MicroWorlds/ALSTool/ALS_Components/Linac/ NAP. (2010).
Acronyms and Glossary. Retrieved Aug 03, 2010, from The National Academies Press: http://www. nap. edu/openbook. php? record_id=12484&page=50 Wei-Ling, H. , Quan-feng, L. , & Yun-Kai, Z. (2001). Electron gun used in the accelerator for customs inspection systems. Proceedings of the second Asian Particle Accelerator conference (pp. 657-659). Beijing : Tshingua University. Womersley, j. (2001). What next in the search for the Higgs. Beam line , pp. 13-20. Appendix Figure 1: traditional prototype Source: (e-radiography) Figure 2 modern prototype Source: (lbl)
University/College: University of Chicago
Type of paper: Thesis/Dissertation Chapter
Date: 21 September 2016
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