student activity
12.1 The Australian synchrotron – reading comprehension
The $206 million Australian Synchrotron has recently been built in Melbourne. It has a circumference
of 216 m and is the size of a football field. There are only about 50 synchrotrons in the world.
Aerial photo of the Australian Synchrotron
Image courtesy: Australian Synchrotron, State of Victoria
What is a synchrotron and why has Australia
invested in one?
A synchrotron is a particle accelerator that
accelerates electrons to a speed very close to the
speed of light. The fast electrons are then stored
for many hours in a large diameter storage ring.
The storage ring is in the shape of a polygon and is formed from repeated curved and straight sections
all connected together to form a roughly circular ring path for the electrons. When fast electrons are
forced to follow a curved path the acceleration makes them give off electromagnetic waves. These
waves are emitted in a tangential beam, similar in shape to the headlight beam from a train as it goes
around a bend. The very strong electromagnetic waves produced by synchrotrons are why they are
useful. For example, the X-rays emitted from a synchrotron are approximately 100 million times more
intense than the X-rays emitted from a hospital X-ray machine.
The powerful X-rays, UV light, visible light, and infrared light produced by the synchrotron will be
used by over 1200 scientists per year. These scientists will use it for a range of research including
forming detailed medical and biological images, and experimenting
to determine the structure of various atoms, molecules and novel
materials.
Development of synchrotrons began in the late 1920’s. Ernest
Lawrence produced one of the first circular particle accelerators
called a cyclotron (which can be seen in the photo left). This
cyclotron had a diameter of 12.7 cm and for this invention he was
awarded the 1939 Nobel Prize in Physics. In a cyclotron electric
fields are used to increase the speed of the particles and magnetic
fields are used to make the particles follow a curved path. A
synchrotron is a further development of a cyclotron.
Ernest Lawrence holding his cyclotron
Courtesy of: American Institute of Physics
The electromagnetic radiation emitted by electrons in the first
particle accelerators was regarded as a nuisance because it caused the
electrons to lose energy. Later it was realised that the
electromagnetic radiation could be put to good use and this led to the development of synchrotrons
built for the sole purpose of generating intense electromagnetic radiation.
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The Australian Synchrotron has three key elements which result in the production of electromagnetic
radiation; a linear accelerator, a booster synchrotron and a large storage ring. Electrons from the linear
accelerator are transferred into the booster synchrotron where they are accelerated until they are close
to the speed of light. In the booster synchrotron the magnetic field strength is ‘synchronised’ to
increase as the speed of the electrons increases. The magnetic force applied to the electrons by the
‘bending’ magnets provides the centripetal force for the curved paths that have to be followed. The
centripetal force required increases as the speed of the particles increases, therefore the magnetic force
must also increase.
The storage ring is designed to store electrons with kinetic energy of three gigaelectron volts (3 x 10 9
eV). Electrons stay in the booster synchrotron until they reach this energy and are then injected into
the storage ring. In the storage ring the stored electrons are manipulated so that electromagnetic
radiation with specific characteristics is produced. This manipulation in completed in various ways,
including applying magnetic fields of specific strength and direction. This magnetic force is used to
constrain the paths of the electrons in a precise way. This occurs in both the curved and straight
sections of the ring.
Albert Einstein
Image courtesy: The Albert Einstein Archives,
The Hebrew University of Jerusalem, Israel
Albert Einstein, as part of his work on relativity, showed that there is an upper speed limit that can be
reached for particles. He derived an equation to show that the mass of a particle increases with its
mo
speed: m 
2
1  v2
c
where

m = mass of a moving particle (kg)
mo = rest mass of particle (kg)
v = speed of particle (m s-1)
c = speed of light (3.0 x 108 m s-1)
This equation shows that the mass of a particle is not a constant, as Newton’s laws state, but increases
as the speed (v) of the particle increases. It also illustrates that the speed of the particle cannot exceed
the speed of light c. If v = c then the denominator is zero and the mass is infinite.
Einstein also showed that the total energy of a particle is given by E  mc 2
The kinetic energy Ek is defined as the difference between the total energy and the inherent ‘rest
energy’.

Ek  mc 2 mo c 2

Section 12.1

Page 256

mo
1  v2
2
 mo c 2
c

Electrons in the Australian Synchrotron have a kinetic energy of three gigaelectron volts (3 x 109 eV†)
and using Einstein’s equation we can calculate that the electrons travel at a speed of 99.9997% of the
speed of light.
†
1 eV = 1.6 x 10-19 Joules; m0 for an electron is 9.11x10-31kg
Questions
1. What is a synchrotron? [1 mark]
2. An electron moving in a circular path at constant speed is accelerating. Why? [2 marks]
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3. The centripetal force required for the circular trajectories of particles in a circular storage ring
would be supplied by the magnetic force from large magnets. Derive an expression for the radius
of curvature (r) of the path of an electron of mass (m), charge (q), in a magnetic field (B), while
moving with speed (v). (Assume the direction of the B field is perpendicular to v)
[3 marks]
4. The name ‘synchrotron’ derives from the fact that the created magnetic field must be synchronised
to increase as the speed of a particle increases while it is moving in a curved path of fixed radius
(r). Explain why, with reference to your answer in Question 3 and also the information from the
passage.
[3 marks]
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5. Calculate the speed of the 3 gigaelectron volt kinetic energy electrons in the Australian
Synchrotron. Express your answer to two decimal places. [5 marks]
6. The electrons in a synchrotron are continually supplied with energy by electric fields. Without
continuous boosts in energy from the electric fields the electrons would slow down. Why? [2
marks]
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Section 12.1
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