Particle Accelerators: The Tools Behind Modern Particle Physics
A particle accelerator is a machine used by physicists to accelerate particles up to high velocities.
Why do we accelerate particles?
How can we test particle physics theories? We need a way to probe the inside of matter. This will then let us observe the particles that are predicted by our theories or discover unexpected new particles that can be used to modify the theory.
Ironically, we have to probe these particles by using other particles. This actually isn't too unusual, it is how we probe our everyday environment. When we see an object it is because photons, particles of light, scatter off the object and are then absorbed by our eyes (which then sends a signal to our brain).
When using waves for an observation, wavelength limits the detail that can be resolved (the resolution). A smaller wavelength allows smaller details to be observed. Visible light, the light that our eyes can see, has a wavelength of around 10-7 metres. The size of an atom is roughly 10-10 metres, therefore the examination of atomic substructure and fundamental particles is impossible through everyday methods.
From the quantum mechanical principle of wave-particle duality, we know that particles have wave-like properties. The wavelength associated with a particle is called the de Broglie wavelength and it is inversely proportional to the particle's momentum.
When a particle is accelerated, its momentum increases. A particle accelerator can therefore be used by physicists to reach a particle momentum that is large enough to allow the probing of atomic substructures and to 'see' elementary particles.
If the accelerator then collides the accelerated particle, the resulting release of kinetic energy can be transferred into creating new particles. This is possible because mass and energy are equivalent, as famously shown by Einstein in his theory of special relativity. Therefore, a large enough release of kinetic energy can be converted into unusually high mass particles. These new particles are rare, unstable and not typically observed in everyday life.
How do particle accelerators work?
Although there are many types of accelerator they all share two underlying basic principles:
- Electric fields are used to accelerate the particles.
- Magnetic fields are used to steer the particles.
The first principle is a requirement for all accelerators. The second principle is only required if the accelerator steers the particles in a non-linear path. The specifics of how these principles are implemented gives us the different types of particle accelerator.
The first particle accelerators utilised a simple setup: a single, static high voltage was generated and then applied across a vacuum. The electric field generated from this voltage would then accelerate any charged particles along the tube, due to the electrostatic force. This type of accelerator is only suitable to accelerate particles up to low energies (around a few MeV). However, they are still commonly used to initially accelerate particles before sending them into a modern, larger accelerator.
Linear accelerators (known as LINACs) improve upon the electrostatic accelerators by using a changing electric field. In a LINAC the particles pass through a series of drift tubes that are connected to an alternating current. This is arranged so that a particle is initially attracted to the next drift tube but when it has passed through the current flips, meaning the tube now repels the particle away towards the next tube. This pattern repeated over multiple tubes, rapidly accelerates the particle. However, the particle getting faster causes it to travel further in a set period of time and the drift tubes need to keep getting longer to compensate. This means that reaching high energies will require very long LINACs. For example, the Stanford linear accelerator (SLAC), which accelerates electrons to 50 GeV, is over 2 miles long. Linacs are still commonly used in research but not for the highest energy experiments.
The idea of using magnetic fields to steer particles around circular paths was introduced to reduce the amount of space taken up by high energy accelerators. There are two main types of circular design: cyclotrons and synchrotrons.
A cyclotron consists of two hollow D shaped plates and a large magnet. A voltage is applied to the plates and alternated in such a way that it accelerates particles across the gap between the two plates. When travelling within the plates, the magnetic field causes the particle's path to bend. Faster particles bend around a larger radius, leading to a path that spirals outward. Cyclotrons eventually reach an energy limit, due to relativistic effects affecting the particle's mass.
Within a synchrotron the particles are continuously accelerated around a ring of constant radius. This is achieved by a synchronised increasing of the magnetic field. Synchrotrons are much more convenient for constructing large scale accelerators and allow us to reach much higher energies, due to particles being accelerated multiple times around around the same loop. The current highest energy accelerators are based around synchrotron designs.
Both circular designs utilise the same principle of a magnetic field bending the path of a particle but in different ways:
- A cyclotron has a constant magnetic field strength, maintained by allowing the radius of the particle's motion to change.
- A synchrotron maintains a constant radius by changing the magnetic field strength.
After the acceleration, there is then the choice of how to collide the accelerated particles. The beam of particles can be directed onto a fixed target or it can be collided head on with another accelerated beam. Head on collisions produce a much greater energy than fixed target collisions but a fixed target collision ensures a much greater rate of individual particle collisions. Therefore, a head on collision is great for producing new, heavy particles but a fixed target collision is better for observing a large number of events.
Which particles are accelerated?
When choosing a particle to accelerate, three requirements need to be met:
- The particle needs to carry an electrical charge. This is necessary so it can be accelerated by electric fields and steered by magnetic fields.
- The particle needs to be relatively stable. If the particle's lifetime is too short then it could disintegrate before being accelerated and collided.
- The particle needs to be relatively easy to obtain. We need to be able to generate the particles (and possibly store them) before then feeding them into the accelerator.
These three requirements lead to electrons and protons being the typical choice. Sometimes, ions are used and the possibility of creating accelerators for muons is a current field of research.
The Large Hadron Collider (LHC)
The LHC is the most powerful particle accelerator that has ever been built. It is a complex facility, built upon a synchrotron, that accelerates beams of protons or lead ions around a 27 kilometre ring and then collides the beams in a head on collision, producing an enormous 13 TeV of energy. The LHC has been running since 2008, with the aim of investigating multiple particle physics theories. Its biggest achievement, so far, was the discovery of the Higgs boson in 2012. Multiples searches are still ongoing, alongside future plans to upgrade the accelerator.
The LHC is a phenomenal scientific and engineering achievement. The electromagnets used to steer the particles are so strong that they require supercooling, through the use of liquid helium, to a temperature even colder than outer space. The huge amount of data from the particle collisions requires an extreme computing network, analysing petabytes (1,000,000 gigabytes) of data per year. Costs of the project lie within the region of billions and thousands of scientists and engineers from across the world work on it.
Detection of particles is intrinsically linked to the topic of particle accelerators. Once, particles have been collided the resulting picture of collision products needs to be detected so particle events can be identified and studied. Modern particle detectors are formed by layering multiple specialised detectors.
The innermost section is called a tracker (or tracking devices). The tracker is used to record the trajectory of electrically charged particles. The interaction of a particle with the substance within the tracker produce an electrical signal. A computer, using these signals, reconstructs the path travelled by a particle. A magnetic field is present throughout the tracker, causing the particle's path to curve. The extent of this curvature allows the particle's momentum to be determined.
The tracker is followed by two calorimeters. A calorimeter measures a particle's energy by stopping it and absorbing the energy. When a particle interacts with the matter inside the calorimeter, a particle shower is initiated. The particles resulting from this shower then deposit their energy into the calorimeter, which leads to an energy measurement.
The electromagnetic calorimeter measures particles that primarily interact via the electromagnetic interaction and produce electromagnetic showers. A hadronic calorimeter measures particles that primarily interact via the strong interaction and produce hadronic showers. An electromagnetic shower consists of photons and electron-positron pairs. A hadronic shower is much more complex, with a greater number of possible particle interactions and products. Hadronic showers also take longer to develop and require deeper calorimeters than electromagnetic showers.
The only particles that manage to pass through the calorimeters are muons and neutrinos. Neutrinos are almost impossible to directly detect and typically identified through noticing a missing momentum (as total momentum must be conserved in particle interactions). Therefore, muons are the last particles to be detected and the outermost section is comprised of muon detectors. Muon detectors are trackers specifically designed for muons.
For fixed target collisions, the particles will tend to fly forwards. Therefore, the layered particle detector will be arranged in a cone shape behind the target. In head on collisions, the direction of collision products isn't as predictable and they can fly outwards in any direction from the collision point. Therefore, the layered particle detector is arranged cylindrically around the beam pipe.
Studying particle physics is only one of many uses for particle accelerators. Some other applications include:
- Materials science - Particle accelerators can be used to produce intense particle beams which are used for diffraction to study and develop new materials. For example, there are synchrotrons primarily designed to harness their synchrotron radiation (a by-product of the accelerated particles) as light sources for experimental studies.
- Biological science - The aforementioned beams can also be used to study the structure of biological samples, such as proteins, and help in the development of new drugs.
- Cancer therapy - One of the methods of killing cancer cells is the use of targeted radiation. Traditionally, high energy x-rays produced by linear accelerators would have been used. A new treatment utilises synchrotrons or cyclotrons to produce high energy beams of protons. A proton beam has been shown to produce more damage to the cancer cells as well as reducing the damage to surrounding healthy tissue.
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© 2018 Sam Brind