title=VCE Physics/Unit 2/AoS 2/2.8 : How do particle accelerators work?

Description from the Study Design

In this option students explore the function and use of particle accelerators to produce radiation and to collide particles. The use of particle accelerators has allowed observations to be made of particles that may once have existed in nature but are no longer present. Investigation of these particles allows theories of the early Universe to be developed and challenged. Students investigate the development of, and comparisons between, various accelerator technologies. Particle accelerators and colliders include the Australian Synchrotron and the Large Hadron Collider.

Introduction

There are several physics concepts you'll need in order to understand what particle accelerators are and how they work:

• You can use an electric field to accelerate a charged particle.

You can create an electric field by generating a potential difference with a battery or other electrical source. For example, the potential difference generated by a battery in an electric circuit creates an electric field in the circuit, which causes electrons to move through the wires of the circuit (Unit 1, Area of Study 2 : How do electric circuits work?).

• You can use a magnetic field to bend the path of a charged particle.

You might have come across this before, but in this course you don't study the details of this interaction until Unit 3, Area of Study 1 : How do things move without contact?. For now, you just need to know it's possible to do this.

• If you accelerate a charged particle it emits radiation in the form of light (an electromagnetic wave). The wavelength of the light emitted depends on the motion of the particle. The light carries away energy so the particle will decelerate due to this emission

You studied the spectrum of light as electromagnetic waves of different wavelengths in Unit 1, AoS 1 : Applying thermal physics to climate science. You encountered the phenomena of accelerated charged particles emitting radiation in Unit 1, Area of Study 3 : What is matter and how is it formed?.

What is a particle accelerator?

A particle accelerator uses electric fields to accelerate charged particles (such as electrons and protons), and magnetic fields to bend the path of the particles.

What are particle accelerators used for?

Particle accelerators are used for a variety of purposes, for example:

• In medicine: beams of protons are sometimes fired at cancer cells to kill a tumour – this is known as proton therapy.
• In industry: boron ions, or other ions, may be fired at wafers of silicon to produce semi-conductors – this is known as ion implantation.
• In research: archaeologists often determine the age of organic finds, such as bones, using radiocarbon dating – this will often be done with a particle accelerator known as an accelerator mass spectrometer.

The particle accelerators in these examples work at relatively low energies. Here, by energy, we mean the kinetic energy given to the particles, so low energy = low particle speed. However, for this option, you'll mainly explore the application of particle accelerators which accelerate particles to the highest energies (highest speeds) we can reach. These are particle accelerators which:

• produce synchrotron radiation; and
• collide particles (smash them into each other).

Particle accelerator energies (electronvolts)

You can characterise particle accelerators by how much kinetic energy they give to the particles they use i.e. to what speeds they accelerate the particles. The SI unit for energy is the Joule (symbol "J"), however, you'll usually find the kinetic energy related to a particle accelerator given in electronvolts (symbol "eV"). This is partly because it can be simpler to work with electronvolts. For example, a particle accelerated through an electric field generated by a potential difference of 100 V has a kinetic energy of 100 eV. Electronvolts have also just become the convention when dealing with particle energies.

Is 1 eV a large amount of energy? How many joules is 1 eV (look it up)?

It might seem you could use the equation $E_{k}={\tfrac {1}{2}}mv^{2}$ from Unit 2, Area of Study 1 : How can motion be described and explained? to calculate the speed of a particle from its kinetic energy (after first converting from eV to J). However, as you'll find out if you study Unit 3, Area of Study 3 : How fast can things go? this equation is only an approximation for objects moving at speeds that are small compared to the speed of light. Even in low energy particle accelerators, such as those used for proton therapy, the particles move at significant fractions of the speed of light and so this equation is no longer a good approximation.

For this option you don't need to be able to calculate the speed of a particle with a given energy in eV. You just need to understand that for the same type of particle a greater kinetic energy in eV means the particle will be moving faster. For example, look up the typical energy for the protons used in proton therapy and compare this to the final energy of the protons at the Large Hadron Collider. Which are travelling faster? However, be careful, if you have two particles of different masses moving at the same speed (for example, a proton and an electron) the particle with the greater mass has the greater energy, for example a proton has more energy than an electron moving at the same speed. You can understand this from $E_{k}={\tfrac {1}{2}}mv^{2}$ even though this is not quite the correct equation.

Types of particle accelerators

There are many types of accelerators but they can generally be divided into:

• linear particle accelerators (linac, for short), which accelerate particles in a straight line (linear means "a straight line"); and
• those that accelerate particles in an approximately circular path – often made up of straight sections combined with curved sections.

Linear accelerators

Old TVs generated images by firing electrons at a glass screen coated in different coloured phosphors (red, green and blue) that would glow when hit. The path of the electrons were bent by magnetic fields to reach the different parts of the screen. This was essentially a very low-energy linac. However, the components which accelerated electrons in an old TV are known as an "electron gun" (because they fire electrons). They are really only the components of a linac. The term linac is generally used for an accelerator that has more components which accelerate the charged particles to much higher speeds - but the principals involved are the same.

For all accelerators, charged particles are generated in a similar fashion to an "electron gun" and then accelerated in a linac. For circular accelerators they are then "injected" into a circular path. Sometimes there is more than one circle - with the particles first being accelerated in a small circle before being transferred to a larger circle when they get too fast to be contained in the smaller circle.

Circular accelerators

Circular accelerators are more complicated than linacs because of the need to significantly bend the path of the particles while, at the same time, accelerating them. This is why they often have straight sections, in which the particles are accelerated, connected by curved sections to create a roughly circular path. However, they can generally produce higher energy particles than a linac because the particles can travel around the circular path many times providing a greater distance, and more time, in which to accelerate the particles.

A cyclotron is a circular particle accelerator which is relatively simple because it has a fixed magnetic field. The particles move in a circular spiral – the radius of the circular path increases as their speed increases, because they travel further while being deflected the same amount by the fixed magnetic field.

A synchrotron is a circular particle accelerator which uses varying magnetic fields. The strength of the magnetic fields, which bend the path of the particles, increases as the speed of the charge particles increases, so the particles stay on the same circular path. Synchrotrons are more complicated because the increase in the magnetic fields has to be synchronised (hence the name) with the increase in the particles' speed.

Even though many particle colliders are "synchrotrons", the term synchrotron is generally only used to refer to accelerators which use the radiation the particles produce. Synchrotrons which are used to collide particles are referred to as "colliders".

Synchrotrons

As already mentioned, the term synchrotron is now generally only used to refer to accelerators which use the radiation the particles produce such as the Australian synchrontron.

For details of how a synchtron works, how the radiation it produces differs from other sources of radiation, and the applications of synchrotron radiation, start by checking out the old Australian Synchrotron website.

Particle colliders

This part of the option links most closely to Unit 1, Area of Study 3 : What is matter and how is formed?.

A particle collider is designed to collide particles rather than use the radiation they can generate. In a particle collider the radiation emitted by charged particles becomes a problem rather than part of the design. As particles are accelerated in a circle to raise them to the speed / energies required for collision, they emit radiation and loose energy, slowing down. The faster the particles circulate, the greater their acceleration, and the more energy they loose. So there is a battle to pump more energy into the circulating particles than they are loosing, with the requirement to pump more energy into the particles the faster they circulate (to overcome the increasing losses).

The aim of colliding the particles is to investigate the constituents of the universe at the smallest possible scales. In this sense a particle collider is the most powerful microscope on earth. While there are microscopes that can help us to "see" molecules and atoms (such as Scanning Tunnelling Microscopes (STM) and Atomic Forc Microscopes (AFM)) we can use particle colliders to see inside the atom, inside the nucleus, to examine the nature of electrons, protons, neutrons and more.

The main particle collider currently running is the Large Hadron Collider at Cern. While there are other particle colliders, there are none that reach the same energies as the LHC and few that even get close. The expertise, effort and money required to run the LHC, testing the limits of our knowledge of fundamental particles and their interactions, is such that it is only possible with international cooperation between countries. The LHC is a multi-national effort with a multi-billion dollar budget - it's an incredible feat of engineering as well as an amazing science experiment.

The LHC, in particular, can also be considered a "time-machine". Not in the sense that we can use it to travel back in time, but that using it, we can recreate conditions that existed in the early universe and so help us to understand the origins of the universe, such as the Big Bang.

Relevant dot points from the Study Design

Particle accelerators and the production of light

• distinguish between the use of particle accelerators to produce synchrotron light and to collide particles
• distinguish between the capabilities of a particle collider and the capabilities of the Australian Synchrotron
• explain the general purpose of the electron linac, circular booster, storage ring and beamlines in the Australian Synchrotron
• explain, using the characteristics of brightness, spectrum and divergence, why for some experiments synchrotron radiation is preferable to laser light and radiation from X-ray tubes.

Accelerator technology and the development of modern particle physics

• explain the evolution of collider technology including:
  • particles involved in the collision event
  • the increasing energies attained since the 1950s
• evaluate the role of colliders in the development of the Standard Model of particle physics, including reference to subatomic structure and processes
• describe the products of collisions with reference to symbol, charge, rest energy and lifespan
• compare the physical designs and purposes of particle detectors at the Large Hadron Collider including ATLAS,CMS, ALICE and LHCb.

Current and future applications of accelerator technology for society

• explain how the immense amount of data collected by the Large Hadron Collider is stored and analysed, and the associated role particle detectors have had in the development of information processing technologies
• describe at least one application of particle accelerators selected from:
  • materials analysis and modification which results in the improvement of consumer products such as heat-shrinkable film and chocolate
  • implanting of ions in silicon chips to make them more effective in electronic products such as computers and smart phones–nuclear energy applications such as the use of thorium as an alternative fuel for the production of nuclear energy or the treatment of nuclear waste
  • pharmaceutical research involving the analysis of protein structure leading to the development of new pharmaceuticals to treat major diseases
  • DNA research involving the analysis of protein metabolism leading to the development of new antibiotics
  • medical applications such as the production of a range of radioisotopes for medical diagnostics and treatments or cancer therapy through the use of particle beams
  • use of spectrometry in environmental monitoring or the use of blasts of electrons in the treatment of pollution such as contaminated water, sewage sludge and gases from smokestacks
  • use of particle accelerators in a selected experiment or scientific endeavour
• investigate current and proposed future directions of collider technologies.