الأربعاء، 14 مايو، 2014

" Comparison between linear accelerator and Cyclotron"

Chapter One
linear accelerator

(1-1)Introduction:
While kilovoltage machines are suitable for superficial treatments and diagnostic imaging, they are limited by the high skin dose arising from their low energy photons. Photons with energies over 1 MeV are desirable in the setting of deeply seated tumours, as they allow significant skin sparing. Unfortunately, the voltages required to accelerate the electrons to the desired speeds are unrealistic for application in an x-ray tube, and therefore another method of megavoltage photon generation was required.(1)
This was first accomplished on a worldwide basis by using cobalt-60, a radioactive isotope that decayed over several years. It releases two photons of energies 1.17 and 1.33 MeV. Techniques using isotopes were limited by the energies released by isotopes, and there were also radiation issues associated with their use.(1)
The linear accelerator is a device that uses an electromagnetic field to accelerate electrons close to the speed of light. By bombarding a target with these electrons, photons with energies of 18 MeV (and above) are realisable. Like the conventional x-ray tube, the electrons generate bremsstrahlung radiation in a tungsten target; however their highly increased energies means that the design of the machine is significantly different(1).








linear accelerator, also called Linac,  type of particle accelerator that imparts a series of relatively small increases in energy to subatomic particles as they pass through a sequence of alternating electric fields set up in a linear structure. The small accelerations add together to give the particles a greater energy than could be achieved by the voltage used in one section alone.(4)
In 1924 Gustaf Ising, a Swedish physicist, proposed accelerating particles using alternating electric fields, with “drift tubes” positioned at appropriate intervals to shield the particles during the half-cycle when the field is in the wrong direction for acceleration. Four years later, the Norwegian engineer Rolf Wideröe built the first machine of this kind, successfully accelerating potassium ions to an energy of 50,000 electron volts (50 kiloelectron volts). (4)
Linear machines for accelerating lighter particles, such as protons and electrons, awaited the advent of powerful radio-frequency oscillators, which were developed for radar during World War II. Proton linacs typically operate at frequencies of about 200 megahertz (MHz), while the accelerating force in electron linacs is provided by an electromagnetic field with a microwave frequency of about 3,000 MHz.(4)
The proton linac, designed by the American physicist Luis Alvarez in 1946, is a more efficient variant of Wideröe’s structure. In this accelerator, electric fields are set up as standing waves within a cylindrical metal “resonant cavity,” with drift tubes suspended along the central axis. The largest proton linac is at the Clinton P. Anderson Meson Physics Facility in Los Alamos, N.M., U.S.; it is 875 m (2,870 feet) long and accelerates protons to 800 million electron volts (800 megaelectron volts). For much of its length, this machine utilizes a structural variation, known as the side-coupled cavity accelerator, in which acceleration occurs in on-axis cells that are coupled together by cavities mounted to their sides. These coupling cavities serve to stabilize the performance of the accelerator against changes in the resonant frequencies of the accelerating cells.(4)
Electron linacs utilize traveling waves rather than standing waves. Because of their small mass, electrons travel at close to the speed of light at energies as low as 5 megaelectron volts. They can therefore travel along the linac with the accelerating wave, in effect riding the crest of the wave and thus always experiencing an accelerating field. The world’s longest electron linac is the 3.2-kilometre (2-mile) machine at the Stanford (University) Linear Accelerator Center, Menlo Park, Calif., U.S.; it can accelerate electrons to 50 billion electron volts (50 gigaelectron volts). Much smaller linacs, both proton and electron types, have important practical applications in medicine and in industry.(4)
(1-2)Components of a Linear Accelerator:
The linear accelerator generates photons in several steps:
·         Electrons must be generated and guided into the accelerating waveguide - the electron generation component
·         Electrons must be accelerated close to the speed of light - the electron acceleration component
·         Electrons must be transported to the x-ray target - this is beam transport
·         Bremsstrahlung occurs in the target, generating a photon beam
·          The beam must be modified for clinical use in the treatment head. This includes collimation, flattening filters and ionisation chambers. (1)
Several accessories are required for the linear accelerator to work:
·         A radiofrequency generator that produces the electromagnetic wave in the acceleration component
·         A pulse modulator which generates timed pulses of energy to the electron gun and the RF generator
·         A control panel to operate the linear accelerator .(1)





(1-3)The Linear Accelerator work:

In a linear accelerator, particles pass through a series of tubes. At either end of each tube are electrodes. An alternating current is used. This means that, when particles pass an electrode to which they are being attracted, the electrode switches charge, and starts to repel the particle. The distances between electrodes increase as you go along the accelerator, since, as the particles accelerate, they travel further per. oscillation of the current. (2)





Fig(1) Aerial photo of the Stanford Linear Accelerator Center, with detector complex at the right (east) side.(2)

The difficulties of maintaining high voltages led several physicists to propose accelerating particles by using a lower voltage more than once. Lawrence learned of one such scheme in the spring of 1929, while browsing through an issue of Archiv für Elektrotechnik, a German journal for electrical engineers. Lawrence read German only with great difficulty, but he was rewarded for his diligence: he found an article by a Norwegian engineer, Rolf Wideröe, the title of which he could translate as "On a new principle for the production of higher voltages." The diagrams explained the principle and Lawrence skipped the text. (3)




Fig(1-2) Linear Accelerator(3)
Particles with a positive electric charge are drawn into the first cylindrical electrode by a negative potential; by the time they emerge from the tube the potential has switched to positive, which propels them away from the electrode with a second boost. Adding gaps and electrodes can extend the scheme to higher energies. (3)

(1-4)linear accelerator uses:


linear accelerator (LINAC) is the device most commonly used for external beam radiation treatments for patients with cancer. The linear accelerator can also be used in stereotactic radiosurgery similar to that achieved using the gamma knife on targets within the brain. The linear accelerator is used to treat all parts/organs of the body. It delivers a uniform dose of high-energy x-ray to the region of the patient's tumor. These x-rays can destroy the cancer cells while sparing the surrounding normal tissue. The LINAC is used to treat all body sites with cancer and used in not only external beam radiation therapy, but also for Stereotactic Radiosurgery and Stereotactic Body Radiotherapy.(5)
A linear accelerator is also used for Intensity-Modulated Radiation Therapy (IMRT)(5)







Fig(1-3) Radiotherapy

 procedure(5)

(1-4-1) How does the equipment work:
The linear accelerator uses microwave technology (similar to that used for radar) to accelerate electrons in a part of the accelerator called the "wave guide", then allows these electrons to collide with a heavy metal target. As a result of the collisions, high-energy x-rays are produced from the target. These high energy x-rays will be directed to the patient’s tumor and shaped as they exit the machine to conform to the shape of the patient’s tumor. The beam may be shaped either by blocks that are placed in the head of the machine or by a multileaf collimator that is incorporated into the head of the machine. The beam comes out of a part of the accelerator called a gantry, which rotates around the patient. The patient lies on a moveable treatment couch and lasers are used to make sure the patient is in the proper position. The treatment couch can move in many directions including up, down, right, left, in and out. Radiation can be delivered to the tumor from any angle by rotating the gantry and moving the treatment couch.(5)
(1-4-2) How is safety ensured:
Patient safety is very important. During treatment the radiation therapist continuously watches the patient through a closed-circuit television monitor. There is also a microphone in the treatment room so that the patient can speak to the therapist if needed. Port films (x-rays taken with the treatment beam) or other imaging tools are checked regularly to make sure that the beam position doesn't vary from the original plan.(5)
The linear accelerator sits in a room with lead and concrete walls so that the high-energy x-rays are shielded. The radiation therapist must turn on the accelerator from outside the treatment room. Because the accelerator only gives off radiation when it is actually turned on, the risk of accidental exposure is extremely low. Indeed, pregnant women are allowed to operate linear accelerators.(5)
Modern radiation machines have internal checking systems to provide further safety so that the machine will not turn on until all the treatment requirements prescribed by your physician are perfect. When all the checks match and are perfect, the machine will turn on to give your treatment.(5)
Quality control of the linear accelerator is also very important. There are several systems built into the accelerator so that it won't deliver a higher dose than the radiation oncologist prescribed. Each morning before any patients are treated, the radiation therapist performs checks on the machine to ensure that it is working properly using a piece of equipment called a "tracker" to make sure that the radiation intensity is uniform across the beam. In addition, the radiation physicist makes more detailed weekly and monthly checks of the linear accelerator.(5)

Beams of high-energy particles are useful for both fundamental and applied research in the sciences, and also in many technical and industrial fields unrelated to fundamental research. It has been estimated that there are approximately 26,000 accelerators worldwide. Of these, only about 1% are the research machines with energies above 1 GeV (that are the main focus of this article), about 44% are for radiotherapy, about 41% for ion implantation, about 9% for industrial processing and research, and about 4% for biomedical and other low-energy research.(6)
For the most basic inquiries into the dynamics and structure of matter, space, and time, physicists seek the simplest kinds of interactions at the highest possible energies. These typically entail particle energies of many GeV, and the interactions of the simplest kinds of particles: leptons (e.g. electrons and positrons) and quarks for the matter, or photons and gluons for the field quanta. Since isolated quarks are experimentally unavailable due to color confinement, the simplest available experiments involve the interactions of, first, leptons with each other, and second, of leptons with nucleons, which are composed of quarks and gluons. To study the collisions of quarks with each other, scientists resort to collisions of nucleons, which at high energy may be usefully considered as essentially 2-body interactions of the quarks and gluons of which they are composed. Thus elementary particle physicists tend to use machines creating beams of electrons, positrons, protons, and anti-protons, interacting with each other or with the simplest nuclei (e.g., hydrogen or deuterium) at the highest possible energies, generally hundreds of GeV or more. Nuclear physicists and cosmologists may use beams of bare atomic nuclei, stripped of electrons, to investigate the structure, interactions, and properties of the nuclei themselves, and of condensed matter at extremely high temperatures and densities, such as might have occurred in the first moments of the Big Bang. These investigations often involve collisions of heavy nuclei – of atoms like iron or gold – at energies of several GeV per nucleon. At lower energies, beams of accelerated nuclei are also used in medicine, as for the treatment of cancer.(6)
Besides being of fundamental interest, high energy electrons may be coaxed into emitting extremely bright and coherent beams of high energy photons – ultraviolet and X ray – via synchrotron radiation, which photons have numerous uses in the study of atomic structure, chemistry, condensed matter physics, biology, and technology. Examples include the ESRF in Europe, which has recently been used to extract detailed 3-dimensional images of insects trapped in amber. Thus there is a great demand for electron accelerators of moderate (GeV) energy and high intensity.(6)








Fig(1-4) Beamlines 
leading 


from the Van de Graaf accelerator to various experiments, in the basement of the Jussieu Campus in Paris.(6)

(1-5) Construction and operation


A linear particle accelerator consists of the following elements:
  • The particle source. The design of the source depends on the particle that is being moved. Electrons are generated by a cold cathode, a hot cathode, a photocathode, or radio frequency (RF) ion sources. Protons are generated in an ion source, which can have many different designs. If heavier particles are to be accelerated, (e.g. uranium ions), a specialized ion source is needed.(6)
  • A high voltage source for the initial injection of particles.
  • A hollow pipe vacuum chamber. The length will vary with the application. If the device is used for the production of X-rays for inspection or therapy the pipe may be only 0.5 to 1.5 meters long. If the device is to be an injector for a synchrotron it may be about ten meters long.(6)
  • Within the chamber, electrically isolated cylindrical electrodes are placed, whose length varies with the distance along the pipe. The length of each electrode is determined by the frequency and power of the driving power source and the nature of the particle to be accelerated, with shorter segments near the source and longer segments near the target. The mass of the particle has a large effect on the length of the cylindrical electrodes; for example an electron is considerably lighter than a proton and so will generally require a much smaller section of cylindrical electrodes as it accelerates very quickly. Likewise, because its mass is so small, electrons have much less kinetic energy than protons at the same speed. Because of the possibility of electron emissions from highly charged surfaces, the voltages used in the accelerator have an upper limit, so this can't be as simple as just increasing voltage to match increased mass.(6)
  • One or more sources of radio frequency energy, used to energize the cylindrical electrodes. A very high power accelerator will use one source for each electrode. The sources must operate at precise power, frequency and phase appropriate to the particle type to be accelerated to obtain maximum device power.(6)


Quadrupole magnets surrounding the linac of the Australian Synchrotron are used to help focus the electron beam
  • An appropriate target. If electrons are accelerated to produce X-rays then a water cooled tungsten target is used. Various target materials are used when protons or other nuclei are accelerated, depending upon the specific investigation. For particle-to-particle collision investigations the beam may be directed to a pair of storage rings, with the particles kept within the ring by magnetic fields. The beams may then be extracted from the storage rings to create head on particle collisions.(6)






Fig1-5 



Schema of an linear accelerator.(6)

(1-6)Design of Particle Accelerators

There are many types of accelerator designs, although all have certain features in common. Only charged particles (most commonly protons and electrons, and their antiparticles; less often deuterons, alpha particles, and heavy ions) can be artificially accelerated; therefore, the first stage of any accelerator is an ion source to produce the charged particles from a neutral gas. All accelerators use electric fields (steady, alternating, or induced) to speed up particles; most use magnetic fields to contain and focus the beam. Meson factories (the largest of which is at the Los Alamos, N.Mex., Scientific Laboratory), so called because of their copious pion production by high-current proton beams, operate at conventional energies but produce much more intense beams than previous accelerators; this makes it possible to repeat early experiments much more accurately. In linear accelerators the particle path is a straight line; in other machines, of which the cyclotron is the prototype, a magnetic field is used to bend the particles in a circular or spiral path.(7)





















Chapter two

Cyclotrons


(2-1)iNtroduction:
A cyclotron is like a linear accelerator, except that, instead of using lots of different electrodes, it uses the same two over and over again. The particles move around in a circle due to a magnetic field. The radius of this circle depends on the velocity of the particles. The orbits of the particles are enclosed by two semi-cylindrical electrodes. An alternating current is used to accelerate the particles. When the particles enter one half of the cyclotron, they are pulled back to the other half. When they reach the other half, the current switches over, and they are pulled back to the first half. All the time, the magnetic field keeps them moving in circles. As they gain energy from the electric field, the radii of their orbits increase, and their velocities increase, until the radius is as large as the cyclotron.(2)



Fig(2-1) The original patent for a cyclotron.(2)
The cyclotron is a particle accelerator conceived by Ernest O. Lawrence in 1929, and developed, with this colleagues and students at the University of California in the 1930s. (For a short pictorial history, see The Development of the Cyclotron at LBNL.)
A cyclotron consisted of two large dipole magnets designed to produce a semi-circular region of uniform magnetic field, pointing uniformly downward.(8)
These were called Ds because of their D-shape. The two D's were placed back-to-back with their straight sides parallel but slightly separated.(8)
An oscillating voltage was applied to produce an electric field across this gap. Particles injected into the magnetic field region of a D trace
out a semicircular path until they reach the gap. The electric field in the gap then accelerates the particles as they pass across it.(8)
The particles now have higher energy so they follow a semi-circular path in the next D with larger radius and so reach the gap again. The electric field frequency must be just right so that the direction of the field has reversed by their time of arrival at the gap. The field in the gap accelerates them and they enter the first D again. Thus the particles gain energy as they spiral around. The trick is that as they speed up, they trace a larger arc and so they always take the same time to reach the gap. This way a constant frequency electric field oscillation continues to always accelerate them across the gap. The limitation on the energy that can be reached in such a device depends on the size of the magnets that form the D's and the strength of their magnetic fields.(8)












(Fig(2-2

 Circular Accelerators


(2-2)History:
E. O. Lawrence and his graduate students at the University of California, Berkley tried many different configurations of the cyclotron before they met with success in 1929. The earliest cyclotron was very small, using electrodes, a radio frequency oscillator producing 10 watts, a vacuum, hydrogen ions, and a 4 in (10 cm) electromagnet. The accelerating chamber of the first cyclotron measured 5 in (12.7 cm) in diameter and boosted hydrogen ions to energy of 5-45 MeV depending on the settings. One mega electron volt (MeV) is 1.602 × 1013 J. (J stands for Joule, the standard unit for energy.) The design, construction, and operation of increasingly larger cyclotrons involved a growing number of physicists, engineers, and chemists. Lawrence was never certain as to whether his research should be classified as nuclear physics or nuclear chemistry.(8)

(2-3)Cyclotron Contents:
Cyclotrons accelerate charged particles using a high-frequency, alternating voltage (potential difference). A perpendicular magnetic field causes the particles to spiral almost in a circle so that they re-encounter the accelerating voltage many times.(6)
The first cyclotron was manufactured by Ernest Lawrence, of the University of California, Berkeley who started operating it in 1932, though others had been working along similar lines at the time The first European cyclotron was founded in Leningrad in the physics department of the Radium Institute (Head Vitali Khlopin). In 1932 George Gamow and Lev Mysovskii presented a draft for consideration by the Scientific Council of the Radium Institute, and the approval of it, under the guidance and direct participation of the Igor Kurchatov and Lev Mysovskii cyclotron was installed and running by 1937.(6)



TRIUMF, Canada's national laboratory for nuclear and particle physics, houses the world's largest cyclotron. The 18m diameter, 4,000 tonne main magnet produces a field of 0.46 T while a 23 MHz 94 kV electric field is used to accelerate the 300 μA beam. TRIUMF is run by a consortium of sixteen Canadian universities and is located at the University of British Columbia, Vancouver, Canada.(6)
cyclotron consists of two D-shaped cavities sandwiched between two electromagnets. A radioactive source is placed in the center of the cyclotron and the electromagnets are turned on. The radioactive source emits charged particles. It just so happens that a magnetic field can bend the path of a charged particle so, if everything is just right, the charged particle will circle around inside the D-shaped cavities. However, this doesn't accelerate the particle. In order to do that, the two D-shaped cavities have to be hooked up to a radio wave generator. This generator gives one cavity a positive charge and the other cavity a negative charge. After a moment, the radio wave generator switches the charges on the cavities. The charges keep switching back and forth as long as the radio wave generator is on. It is this switching of charges that accelerates the particle.(9)














(Fig(2-3) Cyclotron Contents.(9


The design of the cyclotron varies according to the specifications of the purchaser. Ebco Technologies Inc. builds two different types of negative ion cyclotrons, one capable of accelerating protons to a maximum energy level of 19 MeV (TR19) and the other capable of accelerating protons to 32 MeV (TR32). The standard configuration of the TR19 cyclotron is with two external beamlines but there is a scaled down version with an option of one beamline. The TR19 standard target configuration is with two external beamlines and eight targets. There is a design option of two to four targets on one beamline, with the upgrade to up to eight targets at a later date. The TR19 is also available in a self-shielded or unshielded configuration. The self-shielded feature eliminates the need for a cyclotron vault or major upgrades to existing facilities. Additionally, the magnet gap in the TR19 is vertical to minimize space.(8)

(2-4)cyclotron works:
The electrodes shown at the right would be in the vacuum chamber, which is flat, in a narrow gap between the two poles of a large magnet.(6)
In the cyclotron, a high-frequency alternating voltage applied across the "D" electrodes (also called "dees") alternately attracts and repels charged particles. The particles, injected near the center of the magnetic field, accelerate only when passing through the gap between the electrodes. The perpendicular magnetic field (passing vertically through the "D" electrodes), combined with the increasing energy of the particles forces the particles to travel in a spiral path.(6)
With no change in energy the charged particles in a magnetic field will follow a circular path. In the cyclotron, energy is applied to the particles as they cross the gap between the dees and so they are accelerated (at the typical sub-relativistic speeds used) and will increase in mass as they approach the speed of light. Either of these effects (increased velocity or increased mass) will increase the radius of the circle and so the path will be a spiral.(6)
(The particles move in a spiral, because a current of electrons or ions, flowing perpendicular to a magnetic field, experiences a force perpendicular to its direction of motion. The charged particles move freely in a vacuum, so the particles follow a spiral path.) (6)


The radius will increase until the particles hit a target at the perimeter of the vacuum chamber. Various materials may be used for the target, and the collisions will create secondary particles which may be guided outside of the cyclotron and into instruments for analysis. The results will enable the calculation of various properties, such as the mean spacing between atoms and the creation of various collision products. Subsequent chemical and particle analysis of the target material may give insight into nuclear transmutation of the elements used in the target.(6)
(2-5)Uses of the cyclotron:
For several decades, cyclotrons were the best source of high-energy beams for nuclear physics experiments; several cyclotrons are still in use for this type of research.(6)
Cyclotrons can be used to treat cancer. Ion beams from cyclotrons can be used, as in proton therapy, to penetrate the body and kill tumors by radiation damage, while minimizing damage to healthy tissue along their path.(6)
Cyclotron beams can be used to bombard other atoms to produce short-lived positron-emitting isotopes suitable for PET imaging.(6)

The modern cyclotron uses two hollow D-shaped electrodes held in a vacuum between poles of an electromagnet. A high frequency AC voltage is then applied to each electrode. In the space between the electrodes an ion source produces either positive or negative ions depending on the configuration. These ions are accelerated into one of the electrodes by an electrostatic attraction, and when the alternating current shifts from positive to negative, the ions accelerate into the other electrode. Because of the strong electromagnetic field, the ions travel in a circular path. Each time the ions move from one electrode to another they gain energy, their rotational radius increases, and they produce a spiral orbit. This acceleration continues until they escape from the electrode. The accelerated particles are extracted from the cyclotron when they reach the end of the spiral acceleration path. This beam of accelerated subatomic particles can be used to bombard a variety of target materials to produce radioactive isotopes.(8)

Various isotopes are used in medicine as tracers that are injected into the body and in radiation treatments for certain types of cancers. Cyclotrons are also used for research purposes in academic and industrial settings, and for positron emission tomography (PET). Positron emission tomography (PET) is a technique for measuring the concentrations of positron-emitting radioisotopes within the tissue of living subjects. The usefulness of PET is that, within limits, it has the ability to assess biochemical changes in the body. Any region of the body that is experiencing abnormal biochemical changes can be seen through PET. PET has had a huge impact on the clinical applications of neurological diseases, including cerebral vascular disease, epilepsy, and cerebral tumors.(8)

(2-6)Mathematics of the cyclotron

The centripetal force is provided by the transverse magnetic field B, and the force on a particle travelling in a magnetic field (which causes it to be angularly displaced, i.e. spiral) is equal to Bqv. So,(8)

(Where m is the mass of the particle, q is its charge, B the magnetic field strength, v is its velocity and r is the radius of its path.(8)



  The speed at which the particles enter the cyclotron due to a potential difference, V.
Therefore,
v/r is equal to angular velocity, ω, so
And since the angular frequency is
ω = 2πf
Therefore,
The frequency of the driving voltage is simply the inverse of this frequency so that the particle crosses between the dees at the same point in the voltage cycle.(8)


References
1)
2)
www.wikibooks.org
3)
4)
http://www.britannica.com
5)
6)
7)
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