Particle accelerator
A particle accelerator is a device that uses electromagnetic fields to accelerate charged particles to high speeds, causing them to collide with other particles. In this way, a multitude of new particles are generated which -generally- are very unstable and last less than a second, this allows a more in-depth study of the particles that were disintegrated by means of which they were generated. There are two basic types of particle accelerators: linear and circular. The cathode ray tube in a television is a simple form of particle accelerator.
Particle accelerators somewhat resemble the action of cosmic rays on Earth's atmosphere, which randomly produces a shower of exotic and unstable particles. However, accelerators provide a much more controlled environment in which to study these generated particles, and their decay process.
This study of particles, both unstable and stable, may be useful in the future for the development of medicine, space exploration, electronic technology, etc.
High energy accelerators
Linear Accelerators
High-energy linear accelerators (often the acronym linac) use a set of plates or tubes placed in a line to which an alternating electric field is applied. When the particles approach a plate, they accelerate towards it when applying a polarity opposite to its own. Just as they pass through it, through a hole in the plate, the polarity is reversed, so that at that moment the plate repels the particle, thus accelerating it towards the next plate. Generally, not a single particle is accelerated, but a continuum of particle beams, so that a carefully controlled alternating potential is applied to each plate so that the process is continuously repeated for each beam.
As particles approach the speed of light, the reversal velocity of electric fields becomes so high that they must operate at microwave frequencies, and for this reason, at very high energies, resonant cavities are used of radio frequencies instead of plates.
The types of direct current accelerators capable of accelerating particles to speeds high enough to cause nuclear reactions are Cockcroft-Walton generators or potential multipliers, which convert an alternating current to a high voltage direct current, or Van de Graaf generators that use static electricity carried by belts.
These accelerators are often used as the first stage before introducing the particles into the circular accelerators. The world's longest linear accelerator is the Stanford Linear Accelerator (SLAC) electron-positron collider, 3 km in length.
These accelerators are used in radiotherapy and radiosurgery. They use klystron valves and a certain configuration of magnetic fields, producing electron beams with an energy of 6 to 30 million electron volts (MeV). In certain techniques these electrons are used directly, while in others they are collided with a high atomic number target to produce X-ray beams. The safety and reliability of these devices is pushing back the old cobalt therapy units.
Two important technological applications in which this type of accelerator is used is Spallation for the generation of neutrons applicable to power amplifiers for the transmutation of the most dangerous radioactive isotopes generated in fission.
Circular Accelerators
These types of accelerators have an added advantage over linear accelerators by using magnetic fields in combination with electric ones, being able to achieve higher accelerations in smaller spaces. In addition, the particles can remain confined in certain configurations theoretically indefinitely.
However, they have a limit to the energy that can be reached due to the synchrotron radiation that the charged particles emit when they are accelerated. The emission of this radiation involves a loss of energy, which is greater the greater the acceleration imparted to the particle. By forcing the particle to describe a circular trajectory, what is really done is accelerate the particle, since the speed changes its direction, and in this way it is inevitable that it loses energy until it equals the one that is given to it. supplied, reaching a maximum speed.
Some accelerators have special facilities that harness that radiation, sometimes called synchrotron light. This radiation is used as high energy X-ray sources, mainly in studies of materials or proteins by X-ray spectroscopy or X-ray absorption by fine structure (or XAS spectrometry).
This radiation is greater when the particles are lighter, so very light particles (mainly electrons) are used when it is intended to generate large amounts of this radiation, but usually heavy particles, protons or heavier ionized nuclei are accelerated, that make these accelerators can reach higher energies. This is the case of the large circular accelerator at CERN where the large electron-positron collider has been replaced by the large hadron collider.
The largest and most powerful particle accelerators, such as the relativistic heavy ion accelerator, the Large Hadron Collider, or the Tevatron, are used in particle physics experiments.
Cyclotron
The first cyclotron was developed by Ernest Orlando Lawrence in 1929 at the University of California. In them the particles are injected into the center of two pairs of magnets in the shape of a "D". Each pair forms a magnetic dipole and is further charged in such a way that there is an alternating potential difference between each pair of magnets. This combination causes acceleration.
These accelerators have a low speed limit compared to synchrotrons due to the effects. Even so, the speeds that are reached are quite high, called relativistic because they are close to the speed of light. For this reason, units of energy (electronvolts and their submultiples usually) are often used instead of units of speed. For example, for protons, the limit is about 10 MeV. For this reason, cyclotrons can only be used in low energy applications. There are some technical improvements such as the synchrocyclotron or the synchronous cyclotron, but the problem does not go away. Some machines use several coupled phases to use higher frequencies (for example the rhodotron).
These accelerators are used, for example, for the production of radioisotopes for medical use (such as the production of 18F for use in PET), for the sterilization of medical instruments or some foods, for some cancer treatments and in the investigation. They are also used for chemical analysis, forming part of the so-called mass spectrometers.
To reach higher energies, of the order of GeV and higher, it is necessary to use synchrotrons.
Synchrotron
One of the first synchrotrons, which accelerated protons, was the Bevatron built at the Brookhaven National Laboratory (New York), which began operating in 1952, reaching an energy of 3 GeV.
The synchrotron has some advantages over linear accelerators and cyclotrons. Mainly that they are capable of achieving higher energies in accelerated particles. However, they need much more complex electromagnetic field configurations, going from the simple electric and magnetic dipoles used by the rest of the accelerators to quadrupole, sextupole, octupole and larger configurations.
These accelerators are associated with the use of greater technological and industrial capacities, such as and among many others:
- the development of superconductors, able to create the necessary electromagnetic fields, without the need to raise electrical consumption to unthinkable peaks,
- vacuum systems, which allow to keep particles in the duct where particles are kept, without loss of the impermissible beam,
- supercomputers, able to calculate the trajectories of the particles in the different simulated configurations and, later, to assimilate the enormous amounts of data generated in the scientific analysis of the great accelerators as the great hadron collider.
As in other areas of cutting-edge technology, there are multiple developments that were made for its application in these accelerators that are part of people's daily lives. Perhaps the best known was the development of the World Wide Web (commonly called the web), developed for its application in the Large Electron-Positron Collider.
The only way to raise the energy of particles with these accelerators is to increase their size. Generally, the length of the perimeter of the circumference is taken as a reference (they do not really form a perfect circumference, but rather a polygon as close as possible to it). For example, we would have the large electron-positron collider with 26.6 km, capable of reaching 45 GeV (91 GeV for a collision of two beams in opposite directions), currently converted into the large hadron collider of which higher energies are expected. at 7 TeV.
Higher energy accelerators
There are several projects to overcome the energies reached by the new accelerators. These accelerators are expected to serve to confirm theories such as the Grand Unification Theory and even for the creation of black holes that would confirm the theory of superstrings.
By 2015-2020, the International Linear Collider is expected to be built, a huge 31 km long linac, initially of 500 GeV that would be expanded to 1 TeV. This accelerator will use a laser focused on a photocathode for the generation of electrons. In 2007 it had not yet been decided which nation would host it.
The Superconducting Super Collider (SSC) was a project for the construction of an 87 km long synchrotron in Texas that would reach 20 TeV. In 1993 the project was canceled after having built 23.5 km of the tunnel due to its extremely high cost caused by the large deviation from the planned budget. In 2006 the properties and facilities were sold to an investment group, and the site is currently in a state of abandonment.
It is believed that the acceleration of plasmas using lasers will achieve a dramatic increase in the efficiencies that are achieved. These techniques have already achieved accelerations of 200 GeV per meter, albeit over distances of a few centimeters, compared to 0. 1 GeV per meter that is achieved with radio frequencies.
Physical Fundamentals
Particle Generation
Charged particles (the only ones that can accelerate the electromagnetic fields present in accelerators) are generated in various ways. The easiest way is to use the movement itself that is generated when heating a material. This is usually done by heating a filament to its incandescence by passing an electric current through it, although it can also be done by focusing a laser on it. As the temperature increases, the probability that an electron will momentarily leave the atomic shell also increases. If there is no nearby electromagnetic field that accelerates it in the opposite direction, this electron (negatively charged) would soon return to the ionized atom (positively) by attracting opposite charges. However, if we place a second plate near the filament, creating a potential difference between the filament and it, we will be able to accelerate the electron.
If we make a small hole in that plate, and behind it a conduit from which the air has been extracted, we will be able to extract electrons. However, if that hole does not exist, the electron will hit the plate generating X-rays.
When trying to generate protons, however, it is necessary to ionize hydrogen atoms (composed only of 1 proton and 1 electron). For this purpose, the simple electron accelerator described above can be used as a first stage by impinging the electron or X-ray beam on a valve filled with hydrogen gas. If we again place a pair of plates on that valve on which we apply a potential, accelerated electrons will be obtained on the one hand and accelerated protons on the opposite. An example of this type of accelerator is LANSCE or if at the Los Alamos National Laboratory (United States).
Positrons are generated in a similar way, except that we will need to make photons with energies higher than 1.1 MeV hit a target (gold, tungsten or any other heavy material). That energy is the minimum needed to create an electron-positron pair. The efficiency of this generation is very low, which means that a large part of the energy consumed in this process is spent in electron-positron colliders.
Currently there is also interest in generating neutrons for use in transmuting machines. To do this, protons generated as described are used, which impact on targets whose effective section or probability of generating neutrons is high. Since neutrons cannot be accelerated any further (as said, only charged particles can be accelerated), their final speed (or energy) will depend exclusively on the initial energy of the proton.
Virtually all the particles described are used for medical treatments, either in diagnosis (X-rays, CT, PET), as well as in the treatment of solid tumors (the use of protons and neutrons is becoming more and more generalized for the treatment of difficult-to-treat tumors).
Lorentz Equations
All accelerators are governed by the basic equations of electromagnetism developed by Maxwell. However, there is a very simple equation that serves to define the forces that act on each type of accelerator. This is the Lorentz equation or equations (when used separately). The equation can be written in basic form as:
- F→ → =q⋅ ⋅ (E→ → +v→ → × × B→ → ){displaystyle {vec {F}}=qcdot ({vec {E}}}}{vec {v}times {vec {B}}}}}}
where F→ → {displaystyle {vec {F}}} is the strength that suffers the particle loaded into the electromagnetic field, q is the load of the charged particle (-1 for the electron, +1 for the positron or proton, and greater for heavy cores), E→ → {displaystyle {vec {E}} is the value of the electric field, B→ → {displaystyle {vec {B}}} the magnetic field v→ → {displaystyle {vec {v}}} particle speed.
The equation means that the particle receives an acceleration that is proportional to its charge and inversely proportional to its mass. In addition, electric fields push the particle in the direction of motion (the direction will depend on the sign of the charge and the direction of the electric field itself), while magnetic fields curve i> the trajectory of the particle (only when the magnetic field is perpendicular to the trajectory), pushing it towards the center of a circle whose radius will depend on the magnitude of the magnetic field, the speed the particle has at that moment and its charge and mass.
In short, electric fields bring about changes in the magnitude of the velocity of the particle, speeding it up or slowing it down, while magnetic fields make it describe curved trajectories without modifying its magnitude (this is not exactly the case, since particles will lose energy to synchrotron radiation, but it serves as a first approximation).
Accelerator Components
Accelerators have a few basic components which are:
Force generating components
- Electric dipolos. A potential difference is applied, generating an electric field E→ → {displaystyle {vec {E}} between two plates or tubes. This makes the particle accelerate, like between two phases of a lymphac.
- Magnetic dipolos. It creates a magnetic field B→ → {displaystyle {vec {B}}} (usually artificially by coils) perpendicular to the trajectory of the particle so that the curve. For example among the D of a cyclotron, describing a 180-degree arch to return to the separation between the two. Also to bend slightly (small arches) the particle beam in a synchrotron.
- Magnetic multipoles. Used for focus particle beams, so that the fields exercise their actions more efficiently and losses are avoided on the way.
Whites
To create the particles generated in the large accelerators, targets are needed, where the particles impact, generating an enormous amount of secondary particles.
Targets can be distinguished between fixed and mobile. The fixed ones include all those that make the accelerated particles impact against an immobile target, such as X-ray machines or those used in spallation. In mobiles there are those that make the particles impact each other, for example in colliders, thus easily doubling the energy that accelerators can achieve.
Detectors
In order to see the particles generated in the impact against the target, detectors are needed, which would act as the eyes of the scientists.
Two of the best-known detectors built to detect particles created in collisions are: CMS and ATLAS, installed at the Large Hadron Collider.
A simple version of the set accelerator-target-detector would be the television set. In this case, the cathode ray tube is the accelerator, which drives the electrons towards the internally coated phosphor screen that would act as a target, transforming the electrons into photons (with energy in the visible range) that, if we were watching television, would impact the cones and rods of our retinas (detectors), sending electrical signals to our brain (the supercomputer) which interprets the results.
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