PARTICLE ACCELERATORS
PARTICLE ACCELERATORS
Introduction
In the early twentieth century, discovered the structure of the atom. Found that the atom was made of very small pieces called subatomic particles: most notably the proton, neutron and electron.
However, experiments conducted in the second half of the twentieth century with "atom smashers" or particle accelerators, revealed that the subatomic structure of the atom was much more complex. Particle accelerators can take a particle, such as an electron, speed it up to nearly the speed of light, I hit it with an atom and thereby discover its internal parts.
Smashing Atoms
In the 30s, scientists investigated the cosmic rays. When these highly energetic particles (protons) from outer space hit atoms of lead (for example, the nuclei of atoms), many smaller particles were sprayed.
These particles were protons or neutrons, but much smaller. Therefore, the scientists concluded that the nucleus must be made of smaller and more elementary. The search began for these particles.
At that time, the only way to highly energetic particles collide with atoms was on top of a mountain where the rays were more common and where the experiments were conducted. However, physicists soon built devices called particle accelerators, or atom smashers. In these devices, you accelerate particles to high speeds (high kinetic energies) and collide with the target atoms.
The fragments resulting from the collision, and the emitted radiation, are detected and analyzed. The information tells us about the particles that make up the atom and the forces that hold the atom together. An experiment in a particle accelerator has been described as determining the structure of a television by looking at the pieces of it after being dropped from the Empire State Building in New York.
A particle accelerator
Did you have a type of particle accelerator at home? In fact, you are probably reading this article with a! The cathode ray tube (CRT) of any TV or computer monitor is, in fact, a particle accelerator.
The CRT takes particles (electrons) from the cathode, speeds them up and changes its direction using electromagnets in a vacuum. Then do the molecules collide in phosphorus screen. The result of the collision is a point of light, or pixel, on your TV or computer monitor.
A particle accelerator works the same way, except that the accelerators are much larger, the particles move much faster (at almost the speed of light) and the collision results in more subatomic particles and various types of nuclear radiation. The particles are accelerated by electromagnetic waves inside the unit, much the same way that a surfer is pushed by the wave. The more energetic particles, most visible is the structure of matter. It's like breaking the rack in the ball pool game. When the cue ball (energized particle) increases the speed, it gets more energy and so can spread the ball better (releasing more particles).
There are two basic types of particle accelerators:
- Linear - Particles travel down a long, straight and collide with the target
- Circular - Particles travel around in a circle until they collide with the target
In linear accelerators, particles travel in a vacuum over a copper tube. The electrons on the waves created by the generators called klystrons. Electromagnets keep the particles confined in a narrow beam. When the particle beam strikes a target at the end of the tunnel, several detectors record the events: the subatomic particles and radiation released. These accelerators are huge, and are kept underground. An example of a linear accelerator is the linac at the Laboratory of the Stanford Linear Accelerator (SLAC) in California, which is about 3 km long.
Circular accelerators do essentially the same jobs as linacs. However, instead of using a long linear, driving the particles, often around a circular path. At each step, the magnetic field is strengthened so that the particle beam accelerates with each consecutive pass. When the particles are at their highest energy or desired, a target is placed in the path of the beam, the detector or close to them. Circular accelerators were the first type of accelerator invented in 1929. In fact, the first cyclotron was only 10 cm in diameter.
Lawrence's cyclotron magnets used in the form of D (called Dee) separated by a small gap. The magnets produced a circular magnetic field. An oscillating voltage created an electric field across the gap to accelerate the particles (ions) at every turn. As the particles moved faster, the radius of its circular path became bigger until they hit the target in the outermost circle. Lawrence's cyclotron was effective, but could not reach the energies of modern circular accelerators.
Modern circular accelerators klystrons and electromagnets placed around a circular copper tube to accelerate the particles. Many circular accelerators also have a short linear accelerator to accelerate the particles initially before entering the ring. An example of a circular accelerator is the modern National Accelerator Laboratory Fermi (in English - Fermilab) in Illinois, which holds nearly 25.6 kilometers?.
Within a particle accelerator
All particle accelerators, both linear and circular, have the following basic parts.
- Source of particles - provides the particles will be accelerated;
- Copper tube - the particle beam travels through the vacuum inside of the tube;
- Klystrons - microwave generators that make waves in which the particles "go";
- Electromagnets (conventional superconductors) - keep the particles confined in a narrow beam as they travel through a vacuum and also mix the radius where necessary;
- Targets - with the accelerated particles collide;
- Detectors - devices that look for fragments and for the radiation that was released from the collision;
- Vacuum systems - remove the air and dust from the accelerator tube;
- Cooling systems - remove the heat generated by the magnets;
- Computer / electronic systems - control the operation of the accelerator and analyze data from experiments;
- Shield - protects operators, technicians and the public from radiation generated by experiments;
- Monitoring systems - closed circuit television and detectors of radiation to see what happens inside the accelerator (for the purpose of security);
- Power systems - provides electricity to the entire device;
- Storage rings - store the beams temporarily when not in use.
Particle Source, Copper Tube and klystrons
Particle Source
The particle source provides the particles will be accelerated. The particles can be electrons, protons, positrons (the first antimatter particle - like an electron but positively charged), ions and nuclei of heavy atoms like gold. At SLAC, an electron gun uses a laser to knock electrons from the surface of a semiconductor. The electrons then enter the part of the linac accelerator.
At SLAC, the positrons can be made when we shoot an electron beam at tungsten. In the collision, are formed pairs of electrons and positrons. The positrons can be accelerated by reversing the directions of electric and magnetic fields inside the accelerator.
Copper Tube
The largest structure in the particle accelerator is the copper pipe. The copper tube has a strong vacuum inside which the particles travel. The tubes are made of copper because it conducts well as electricity and magnetism. In the SLAC linac, the copper tube is made of more than 80 thousand cylinders coated with copper and imprisoned for more than 3.2 km.
The copper tube is arranged to form a series of cells called cavities. The space of the cavities is combined with the wavelength of microwaves. The spaces allow electric and magnetic fields repeat their pattern every three cavities. The electrons or positrons in the beam pass through holes in small groups. The arrival of each group has a certain time so as to obtain a pulse electric field to the wells.
Klystrons
Klystrons produce the microwave as a microwave oven except that the microwave klystrons are about 1 million times more powerful. The microwave klystrons produce by way of an electron gun. The electrons travel through the clístron cavity, where their speed is regulated. As the electrons change speed in clístron, they release radiation in the form of microwaves. The microwaves are conducted through waveguides of copper to the copper tube of the accelerator. The waveguide carrying the waves effectively without losing the intensity. The clístron and waveguides are kept under high vacuum to facilitate the flow of waves.
Magnets
Magnets, both conventional electromagnets or superconducting magnets are placed along the accelerator tube at regular intervals. These magnets keep the beam of particles confined and focused.
Imagine that the particle beam is like lead balls shot from a shotgun pellet. Typically, the balls (electrons) tend to spread. If the balls are scattered, so they do not cause many collisions in a small area of the target. However, if the balls are confined by an external force (magnetism) to a narrow path, then they will cause many collisions in a narrow area of the target. The more collisions, more events are observed in any experiment.
The magnets generate a field within its core. There is no magnetic force in the center where the electrons travel. If the electrons go astray from the center, they will feel a magnetic repulsion to the middle. Arranging the magnets in a series of alternating poles, the electrons can remain confined for the length of the tube.
Targets
The targets vary with the type of experience. Some targets can be thin metal sheets. In some experiments, the beams of different particles (electrons, positrons) collide with each other inside the detectors.
Detectors
The detectors are one of the most important pieces of equipment in the accelerator. They see the particles and radiation after the collision. There are several types of detectors, since bubble chambers and electronic detectors to fog and ice. A lab collider may have several types of detectors located in various parts of the accelerator. For example, a bubble chamber contains a liquid gas, such as liquid hydrogen. As the particles are released from the collision with the camera, they vaporize a little of the liquid, leaving a trail of bubbles.
A detector spray chamber has a saturated vapor inside. As an energetic particle passes through the steam, it is ionized, producing a trail like a jet going through a cloud.
A detector at SLAC is the SLAC Large Detector (SLD - SLAC Large Detector). The SLD is a large detector solid barrel-shaped, equivalent to six stories high, and weighs over 4 tons!
The SLD is a multi-detector and each layer sees a different event:
- Vertex detector - detects the position of the tracks of particles;
- Board of displacement - detects the positions of charged particles at various points along their tracks. The curved tracks reveal the time of the particle (related to its mass and speed);
- Cerenkov detector - see the radiation released by particles moving (quickly) and determines the speed of the particles;
- Liquid argon calorimeter - stops most of the particles and measures their energies;
- Calorimeter hot iron - detects muons (a subatomic particle);
- Magnet coil - separates the two calorimeters.
Vacuum and cooling
Vacuum Systems
The vacuum should be maintained in accelerators for two reasons:
- To prevent discharge of sparks caused by microwaves in the air, which may damage the accelerator structures and waveguides;
- To avoid the loss of energy that would occur if the beam collide with air molecules.
A combination of rotary pumps and cold traps are used to maintain the low vacuum (one millionth of an atmosphere). Rotary pumps work like fans to remove the air. The cold trap using liquid gases (usually nitrogen) to cool the surface of the trap. Every molecule of air or dust will be attracted to the cold surface and removed from the tube. Cold traps should remain cold, otherwise release the air molecules and dust collected.
Cooling Systems
The electric currents passing through the copper pipe, the gas, produce a large amount of heat. This heat must be removed for two reasons:
- To prevent the copper tubing to melt - that would destroy the structure
- To prevent the copper tubing to expand - it would break the seals of the vacuum
The SLAC linac to have water pipes to cool the copper pipe structure of the accelerator and the magnets. The circulating cooling water for cooling towers above the ground, removing the heat. Any superconducting magnet is cold with liquid nitrogen or liquid helium. Because the linac room in the basement, there is less chance of heating and cooling seasons.
Computers and Electronics
Computers and electronic systems have several tasks in the operation of a particle accelerator:
- Control the source of particles, klystrons and magnets used in the acceleration of particles;
- Monitor the beam;
- Collect and record data from experiments;
- Analyze the data;
- Monitor the security systems;
- Turn off the system in case of emergency;
Particle accelerators have many computers that control the system. These computers often have high-speed microprocessors, large memory and data storage. They usually are networked. In some cases, the analysis of computer data can be done by supercomputers (on site or not).
Shielding, monitoring, and energy storage
Shielding
As the accelerated particles are forced to change speed, change direction or hit targets, they end up losing energy. This energy is usually in the form of ionizing radiation such as X-rays or gamma rays. In addition to radiation, the energized particles themselves represent a hazard to human health. To prevent leakage of radiation while the accelerators are operating, they are shielded.
The structure of the accelerator are generally located in concrete tunnels, underground. The concrete and earth protect the environment.
The coaches are not in the tunnels as accelerators operate, and control rooms are shielded with concrete. In addition, employees use radiation dosimeters and are monitored constantly. Particle accelerators in the United States are under the jurisdiction of the Nuclear Regulatory Commission, which allows its use and inspect regularly as a security measure.
If the accelerator is affiliated to a university, the agency's radiation safety of the university also participates in the process.
Monitoring
The tunnels are usually equipped with closed circuit television to monitor the equipment and gauges inside the accelerator. The radiation detectors are located throughout the structure of the accelerator to monitor leaks in the shield and protect workers.
Power system
As you can guess, the description of the equipment, particle accelerators use much electricity. In some places, it is provided through the local power company. Some accelerators have their own electric generators on site.
Storage rings
Because they need a lot of force to accelerate the particles in an experiment, many accelerators have storage rings. Storage rings maintain a beam that has been accelerated. For example, if colliding a beam of electrons with a beam of positrons, you may have to maintain a stored beam while accelerating the other.
A storage ring has the same components as the main accelerator, but with less klystrons. The particles travel around the ring at the accelerated rate, needing only one or two klystrons to offset any loss of energy as the beam changes direction.
Subatomic
With all this technology, we learned about the structure of matter? When physicists began to use the accelerators in the 50 and 60, they discovered hundreds of particles smaller than the three well-known subatomic particles: protons, neutrons and electrons.
As the largest accelerators were built (those who could provide beams with higher energies), more particles were discovered. Most of these particles exist for only fractions (less than a billionth) of a second, and some of them combine to form more stable composite particles. Some particles are involved in the forces that hold the nucleus of an atom together and others do not.
According to this model, the matter can be divided into the following groups:
- Fermions - subatomic particles that make known matter and antimatter
* Subject:
- Leptons - elementary particles that do not help keep the nucleus together (examples: electron, neutrino)
- Quarks - elementary particles that help keep the nucleus together
- Antimatter - anti-particles of quarks and leptons (antiquarks, antiléptons)
- Hadrons - composite particles (eg proton, neutron)
- Bosons - particles that carry forces (four types).
Observations on Interactions
There are four fundamental forces or interactions:
- Strong - holds the nucleus of an atom together
- Weak - involved in radioactive decay
- Electromagnetism - interactions between charged particles (electricity and magnetism)
- Gravity - the attractive force based on mass and distance.
Fermions: Matter and antimatter
Fermions are distinguished between matter (leptons and quarks) and antimatter.
Leptons
Leptons are extremely small particles (less than 10-15 m radius) that have no known size or internal structure. They have tiny masses, they travel very fast and are described in a better way for the wave functions. The best known examples of leptons are the electron and neutrino. Leptons were classified as:
- Electron-electron neutrino
- Muon-muon neutrino
- Tau-tau neutrino
Quarks
The quarks are extremely small particles (less than 10-15 m radius) that participate in the strong nuclear force. Quarks (unique) isolates were never found, probably because they combine very quickly. Quarks also have fractional electric charges, they are classified as follows:
- Down (d) - charge = -1 / 3
- Up (u) - charge = +2 / 3
- Strange (strange) (s) - charge = -1 / 3
- Charm (charm) (c) - charge = +2 / 3
- Bottom (b) - charge = -1 / 3
- Top (t) - charge = +2 / 3 (more massive, discovered in 1995)
From now on, the quarks are regarded as the most fundamental particles.
Antimatter
Not much is known about the antimatter. The first particle of antimatter discovered was the positron, which has a mass similar to that of an electron, but with a positive charge. This area of particle physics is currently being investigated.
Hadrons, bosons and the Big Bang
Collider
These particles are combinations of quarks, they have mass and reside in the nucleus. The two most common examples of hadrons are protons and neutrons, and each one is a combination of three quarks:
- Proton = 2 up quarks + 1 down quark [+1 charge of proton = (+2 / 3) + (+2 / 3) + (-1 / 3)]
- Neutron = 2 quarks + 1 down quark up [0 neutron load = (-1 / 3) + (-1 / 3) + (+2 / 3)]
Bosons
It is considered that these particles change when the interactions occur. An interaction is defined as a push or pull. But that does not tell us what it really is or how it is mediated. Richard Feynman suggested that the interactions occur when two particles exchange a boson, or particle size.
Think of two people on roller skates: If a person throws a ball and the other picks, they are being pushed in opposite directions. In this analogy, the skaters are the fundamental particles, the ball carrier force and repulsion force. In the case of particles, we see the strength that is the effect, but not the exchange.
There are four known bosons:
- Gluon - mediator of the strong interaction, but only operates over distances of 10-13 cm;
- W and Z - mediator of the weak interaction (1/10.000 of the strong interaction), but only operates over distances of 10-15 cm;
- Photon - mediator of the electromagnetic interaction (1 / 137 of the strong interaction) and operates on an infinite distance;
A fifth particle size (graviton) has been proposed, but has not been found. The graviton is considered as a mediator of gravity, which is 10-39 of the strong interaction and operates on an infinite distance.
Historically, James Clerk Maxwell unified electricity and magnetism in the nineteenth century. As physicists had built more powerful accelerators with temperatures and higher energies, they realized that certain interactions came together, or unified.
Experiments with particle accelerators have shown that the electromagnetic interaction and weak interaction can be grouped in the electroweak interaction. Many physicists believe that all forces were from a single force that has existed for a long time.
Theories that attempt to unify the forçassão called unified theories or large unified theories (GUT). It is expected that the GTUs tell us that the universe might have been like in its infancy. Because experiments with accelerators simulate (which is considered to be) the conditions that existed a split second after the Big Bang, they can provide evidence to support or contradict various GTUs.
According to the Big Bang theory:
- Before the Big Bang, the universe was extremely hot and small and matter existed only as free quarks;
- Since the explosion happened:
- Rapid swelling occurred and the universe cooled;
- Quarks are combined into hadrons;
- The interactions were cut off;
- Matter (atoms) is formed;
- Condensed matter in galaxies, stars, etc..
Increasing more and more particle accelerators, physicists can simulate the conditions that existed within 10-43 seconds of the Big Bang!
Future Directions in Particle Physics
Several questions still remain unanswered about the standard model.
- Why there are three pairs of quarks when it seems that only one is needed to create matter?
- What gives mass to particles (also to atoms and matter)?
- Why is the top quark (which is 35 times larger than the bottom quark) is so massive compared to others?
These are just some of the issues that remain in the world of particle physics.
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