Synchrotrons – making the light fantastic
Box 1 | The first particle accelerators and synchrotrons
The first particle accelerators were built by particle physicists – researchers who were interested in finding out about the particles that make up atoms. By particles we mean sub-atomic particles – protons, neutrons and electrons and the elementary particles from which they are made.
Particle physics
In the 1930s, scientists observed that when cosmic rays (highly energetic protons) from outer space hit atoms of lead, many smaller particles were sprayed out – particles that were much smaller than neutrons, protons and electrons. These were the elementary particles from which all matter is ultimately constructed. The hunt was on to find out what they were and how they worked. To study them in a controllable way, scientists needed to collide highly energetic particles with atoms. To do this, they built machines called particle accelerators, sometimes referred to as atom smashers.
In these devices, particles – usually electrons – are accelerated to tremendous speeds using electromagnetic force, and then slammed into target atoms. The resulting collision produces a shower of tiny particles and radiation. By analysing the results of these collisions, particle physicists are able to build models of what these elementary particles are and how they interact. The greater they can accelerate the initial speeding electron, the more violent the collision, and the more they can learn about the elementary particles that get thrown out.
The size of accelerators increased
The first particle accelerators were small affairs that could comfortably sit on the lab bench. However, as the technology for accelerating electrons progressed, the size of the machines grew. They quickly became great metal monsters consisting of vast arrays of magnets, conductors and sensors strung together in ever increasing complexity. And because the speeding electron has to travel in a vacuum – if it travels through air it will interact with the atoms in the surrounding gas – keeping a long tube evacuated of air for long periods takes a lot of machinery, further adding to the size of the apparatus.
Circular accelerators
In the early 1960s a new type of accelerator proved to be quite popular with researchers. These circular accelerators send electrons whizzing around in a giant vacuum tube in the shape of an enormous circular ring, which is usually over 50 metres in diameter. The electrons are held in this circular path by powerful magnets. Once or twice in each orbit the electrons are hit with synchronised pulses of microwaves to accelerate them. In a cyclotron, as the electrons approach the speed of light they are released from their circular orbit and allowed to fly off at a tangent to strike a target. In a synchrotron, the electrons continue to orbit within the ring, passing through magnetic fields and releasing beams of synchrotron radiation.
Accelerating the electrons
To understand how the electrons are accelerated, think of the biblical story of David and Goliath. David killed Goliath with a stone thrown from his sling. The sling was a leather strap with a stone at one end. David whirled the sling around his head, accelerating the stone to an enormous speed. He then released the stone with precision timing. It flew off at a tangent from its circular path striking Goliath in the head, killing him instantly. The stone is like the electrons in the cyclotron. The sling is like the magnets that hold the electrons on a circular path. When David synchronised his movements with the cycle of the stone, each orbit of the sling caused the stone to move faster and faster.
So synchrotrons are like enormous slings that swing electrons around in a circular path giving them more and more energy with each orbit. But there’s a major limitation with this process. When electrons (or any charged particles) are accelerated to keep them in a circular path they will emit electromagnetic radiation in a narrow beam in front of them (which is at a tangent to the circular path along which they’re moving). What’s more, the more energy you try to put into the electrons, the more electromagnetic radiation they emit.
This proved to be a real nuisance for particle physicists because it meant that they could only accelerate an electron to a certain level inside a synchrotron. Beyond this point it didn’t matter how much energy they pumped into the electron, it was simply emitted as intense electromagnetic radiation. So, it seemed, synchrotrons may have had their day because they could only achieve so much in particle physics.
Using the radiation from synchrotrons
But then scientists started wondering if the electromagnetic radiation being emitted by the accelerating electrons might actually serve some useful purpose. This radiation was known as synchrotron radiation – or synchrotron light – and it was quickly realised that the intensity of the radiation and the fact that it could be precisely controlled made it extremely valuable.
And so it was that through the 1960s and 70s, the synchrotrons that had been built to study sub-atomic particle physics began to be used for the synchrotron radiation that was produced as a by-product. These are known as first-generation synchrotrons.
Related sites
How atom smashers work (How Stuff Works, USA)
Accelerator: Circular accelerators (Stanford Linear Accelerator Center, USA)
Accelerators (Lawrence Berkeley National Laboratory, USA)
Updated October 2005.