Don’t break down
Don’t show any internal structure
No measurable size
Antimatter was first postulated by the English
◦ Paul A. M. Dirac,
◦ in 1928.
His postulate was verified by Anderson in
◦ who experimentally discovered the positron,
◦ in a cloud chamber.
It has been shown that a very high energy
photon can actually produce matter.
The most common process;
◦ is the production of an electron and positron.
A positron has the same mass as an electron,
◦ but the opposite charge.
The positron is said to be the antiparticle of
This is called pair production;
◦ the photon disappears in the process.
This is an example of rest mass being
produced from pure energy;
◦ as suggested by the formula,
◦ E = mc2.
The photon cannot just produce an electron;
◦ the Law of Conservation of Charge will be broken,
◦ and so a positron is produced.
◦ - + + (NB occurs in presence of a nucleus)
◦ 1.02 MeV 0.511 MeV each
The reverse can also occur.
The electron and positron can annihilate,
◦ and their energy create a photon of very high
◦ a gamma ray
Because of this process,
◦ positrons do not last long in nature.
The energy produced is given by E = mc2
E = (2 x 9.11 x 10-31) x (3 x 108)2
E = 1.64 x 10-13 J
E = (1.64 x 10-13)/(1.6 x 10-19)
E = 1.03 x 106 eV
E = 1.03 MeV
The photon produced has a frequency of:
E = hf
f = E/h
f = (1.64 x 10-13)/(6.63 x 10-34)
f = 2.47 x 1020 Hz
Pair production cannot,
◦ occur in empty space.
In the above example,
◦ energy is conserved but,
◦ the pair has no momentum to carry away,
◦ the initial momentum of the photon.
No matter what energy, a massive object such
as a nucleus must be involved,
◦ to conserve the momentum.
We have already seen that a positron can be
produced in + decay;
◦ along with a neutrino.
Other anti particles have also been
In 1955 the antiproton;
the bar representing an antiparticle.
was discovered by Emilio Segré,
◦ and Owen Chamberlain
◦ at the University of California in Berkeley.
◦ the antineutron was also discovered.
There are some particles,
◦ such as the photon,
◦ which do not have antiparticles.
They are considered to be their own
Through the study of collisions in
◦ we have discovered a number of
◦ some reactions do not take place.
One example is:
This is even though charge and energy are
To explain this,
◦ a new conservation law was hypothesised,
◦ the conservation of Baryon Number.
The Baryon number is the same as nucleon
◦ all nucleons have a number B = +1 while,
◦ all antinucleons have a number B = -1.
From the above example, if we add Baryon
+1+1 = +1+1-1
Another conservation law relates to
other subatomic particles being
Another conservation law relates to other
subatomic particles being discovered.
These particles include;
◦ Pions ( ),
◦ Kaons (K),
◦ Lambdas ( ).
It seemed that some of these particles
reacted with high probability,
◦ while some other reactions did not,
◦ even though there were no obvious violations of the
These particles behaved strangely and so a
new quantum number,
and a new conservation law,
◦ the conservation of strangeness,
There are other particles that have been
◦ and other characteristics,
◦ that help distinguish between them.
To identify these particles,
◦ a cloud chamber is used.
It was invented by a physicist;
◦ called Wilson in 1910.
A cloud chamber uses a gas;
◦ cooled to a temperature,
◦ slightly below its usual condensation point.
As the charged particle moves through the
◦ the gas molecules condense on the particles,
◦ and leave a trail.
This shows an antiproton
colliding with a proton
This produces a Xi-anti Xi
antiproton + proton - +
They decay into other
Neutral particles are shown
with dashed lines;
◦ as no bubbles are
Two or more electrons cannot occupy the
same quantum state at the same time.
Two electrons cannot occupy the same space
at the same time
If in same orbital, they have the opposite spin
Applies to electrons, protons & neutrons
◦ Not photons (bosons)
Explains why collisions in atoms can occur
with no loss of energy
Low energy states fill up first
High energy states cannot drop down as
lower energy states already occupied
In the mid 1930’s;
◦ atoms were considered to be made up of:
◦ protons, neutrons and electrons
◦ Called elementary/fundamental particles
Others were also known;
◦ positron, neutrino and photon
In the next few decades;
◦ hundreds of other particles were discovered
Many of these particles are now considered to
be made up of;
◦ two or three quarks
Some particles discovered were considered
Elementary particle physics begun in 1935
Hideki Yukawa (1907 – 1981)
◦ A Japanese physicist
Predicted a new particle that;
◦ would somehow mediate the strong nuclear force
To explain his idea, consider the
Electric force acts over a distance without
The force one charged particle exerts on a
second is due to;
◦ The electric field set up by the first
Similarly, the magnetic field can be said to
carry the magnetic force
E/M fields can travel through space as a wave
E/M radiation can be considered a wave or
As there is the wave/particle duality the e/m
force between particles is due to:
e/m field set up by one charged particle and
felt by other
The exchange of photons between them
How does this work?
An analogy is used
Imagine Harry & Julius throwing pillows at
Each catch results in the child being thrown
A repulsive force
If they grab the pillow out of the other’s hand
◦ Exchange pillows
They are pulled towards each other
An attractive force
For the electromagnetic force between 2
a photon is exchanged between 2 particles
gives rise to the force
Can be shown using a Feynman diagram
Named after the inventor
◦ Richard Feynman (1918 – 1988)
◦ An American physicist
Based on quantum electrodynamics (QED)
Simplest example is two electrons
approaching each other
It is a type of position vs time graph
Time is on the y axis
One electron emits a
photon and recoils
The second electron
absorbs the photon
In this collision
◦ Or interaction
energy and momentum is
As the photon is absorbed;
◦ very shortly after being emitted by the first electron
The photon is not observable and so is called
◦ Virtual Photon
The photon mediates or carries the;
◦ Electromagnetic Force
Back to Hideki Yukawa’s prediction that
a new particle;
◦ would somehow mediate the strong nuclear force
If a photon can mediate the electromagnetic field
◦ or force
There should be a particle that mediates;
◦ the strong nuclear force
Yukawa predicted the new particle would
have a mass between;
◦ An electron and a proton
Hence it was called a meson
◦ Meaning ‘in the middle’
If photons can be found as free particles
Why can’t Mesons?
A search began in the cosmic radiation from
the Sun and other radiation sources
It was found in 1937
There were problems with the particle called
It didn’t react strongly with matter
This meant it could not mediate the strong
It can be found as a positive or negative
Seemed to be nothing more than a massive
It had a mass 207 x that of an electron
The Yukawa particle was finally found in 1947
◦ By Cecil F Powell (1905 – 1969)
It is called the pi meson ( )
◦ Or pion
Comes in 3 charge states;
◦ +, - or 0
Mass of + and - = 139.6 MeV/c2 and;
o = 135.0 MeV/c2
All react strongly with matter
Reactions seen using particle accelerators
p + p p + p + o
p + p p + n + +
In 1949 Yukawa was awarded the Nobel
Prize for his prediction
In 1950 Powell was awarded the Nobel
Prize for discovering mesons
Others mesons were found over the following
The recent theory of quantum
◦ Has replaced mesons with gluons,
◦ as the basic carriers of the strong nuclear force
◦ More about quantum chromodynamics later
We have discussed particles that
mediate e/m & the strong nuclear force
What about the other forces?
There is a weak nuclear force
We know of its existence only because;
◦ It shows itself in certain types of radioactive decay
Particles were found to mediate this force in
They are called the exchange bosons;
◦ W+, W- and the Zo
They were discovered by:
Carlo Rubbia (1934 - )
◦ An Italian
Simon Van Der Meer (1924 - )
They shared the Nobel Prize in 1984 for their
The electromagnetic and weak nuclear
◦ instances of a single electroweak force with,
◦ two different types of exchange particles
The carrier for the gravitation force has not
It has been given a name, however;
The Four Forces in Nature by relative strength
(2 p in
1 Gluons (was
10-6 W and Zo
Gravitational 10-38 Graviton (?)
Since the discovery of the meson in the late
Many other sub nuclear particles have been
◦ Through the use of particle accelerators
They have been arranged in groups
◦ Based on their interactions
All have antiparticles
GAUGE (exchange) BOSONS
Named after the theory that describes them
◦ Gauge Theory
Include those particles which carry the e/m
and weak interactions
Photons, W & Z
Some of these particles have charge
And interact by the e/m force
Electron, muon ( ) & tau ( )
◦ was discovered in 1976
◦ 3000 x heavier than an electron
Particles which interact via the weak nuclear
◦ And gravitation force
◦ Electron neutrino ( e)
◦ Muon neutrino ( )
◦ Tau neutrino ( )
Interact via the strong nuclear force
◦ Can interact by other forces but the strong force
dominates at short distances
Divided into two groups
◦ Particles with Baryon number of +1
Or –1 for antiparticles
◦ Protons, Neutrons, Lamdas ( ), Sigmas( ), Xis ( )
and Omegas ( ) and others
Baryon number of 0
◦ Pions ( ), Kaons ( ) and Eta ( )
A selected summary of the particles are
shown on page 528 of your text book
In the early 1950s, the newly found particles
◦ , and
◦ behaved strangely in 2 ways
They were produced in pairs
- + p o + o
◦ Occurred with high probability
- + p o + n
◦ Never occurred
This was strange as it violated;
◦ no known conservation law
◦ and there was plenty of energy
◦ although they were clearly produced by the strong
◦ they did not decay at a rate characteristic of the
To explain this, a new quantum number and
conservation law was developed
The strangeness numbers were allocated as
shown on p 528 (column S)
Antiparticles were assigned negative numbers
Strangeness is conserved;
◦ In strong interactions
◦ But not weak
This is the first example of a ‘partially
In the 1960’s physicists knew;
◦ hadrons interact via the strong interaction,
◦ while leptons do not
A problem they had was that there only 4
◦ e-, -, e, :
the and were yet to be discovered
But there were hundreds of Hadrons
The leptons are considered to be truly
◦ Don’t break down
◦ Don’t show any internal structure
◦ No measurable size (upper limit of 10-18 m)
Hadrons are more complex and so can’t be
Murray Gell-Mann (1929 - ) and G Zweig in 1963;
◦ independently proposed no hadron was elementary
They proposed that hadrons are made up of;
◦ Combinations of 3 point like entities called quarks
◦ after the novel Finnegan’s Wake by James Joyce.
◦ The line is ‘Three quarks for Master Mark’
◦ Quark should rhyme with Mark
◦ In German, quark means ‘slime’!
The first quark is called the ‘up’ quark.
◦ It has a positive charge of 2/3 of the electronic
charge (+ 2/3e).
The second quark is the ‘down’ quark
◦ with a charge of -1/3 e.
The 3rd quark is the ‘strange’ quark;
◦ also with a charge of -1/3 e.
The strange quark also has the
◦ which helps elementary particles that contain
it to live longer than it should.
Using this theory, a proton is made up
of 2 up quarks and one down quark.
A meson consist of a quark,
+ Q = 2/3 e + 1/3 e = +1e
- Q = -2/3 e + -1/3 e = -1e
Once proposed, physicists went looking for
No direct detection has been possible
Perhaps they are so tightly bound;
◦ They might not exist in a free state
In 1964, several physicists proposed a 4th
◦ As at this stage, there were 4 leptons and nature is
This quark was said to be charmed
◦ As it solved some theoretical problems
◦ Has a new quantum number of charm
A charmed quark has a charge of +2/3 e
In 1974, the first charmed particle was
◦ J/ meson
Charm is conserved in strong interactions but
Partial conservation law
In the 1970s, 2 other leptons were
This means there would be 6 leptons and
only 4 quarks.
Two more quarks were postulated;
◦ Top and bottom quarks
◦ Some prefer the names Truth & Beauty
In 1977, using linear accelerators, they found
the bottom (beauty) quark
Strong evidence for the top (truth) quark has
only been found since 1995
It is hard to find as it has a high mass
◦ 200 GeV/c2
◦ and requires huge energy to produce it
A summary of the quark properties can be
found on page 531 of your text book.
The truly elementary particles are now
considered to be:
They can be arranged in 3 groups
◦ Or generations
Quarks u, d s, c b, t
Leptons E, e , ,
Bosons (photon) W , Zo gluons
The distinction between the 6 quarks was
referred to as;
Soon after the quark theory was proposed,
another property was suggested;
Each flavour quark can have 1 of 3 colours
◦ Red, Green & Blue
◦ The primary colours
Antiquarks are coloured
◦ Antired, antigreen & antiblue
Colour has nothing to do with our senses
Baryons are made up of 3 colours
◦ One of each colour
◦ They are said to be white or colourless
The colours of quarks seem to neutralise
◦ Like electrons and protons form neutrons
◦ Allows protons to remain close to each other
Each quark carries a colour charge
◦ Like electric charge
The strong force between quarks is referred
◦ as the colour force
This new theory is called;
◦ Quantum Chromodynamics (QCD)
The strong nuclear force arises out of
the colour force
This makes the colour force a
The particles that transmit the colour
force are called gluons
Particles have a wave nature
◦ Recall diffraction pattern of electrons through
If ball hits stationary ball and we know
velocity, we can calculate when they hit
If two electrons hit, we can’t exactly say when
they will hit
◦ Can say when most likely to hit
Cannot say where electron is exactly is
◦ Can say where it is most likely to be
This means position and time are probability
To know where a particle is, you must shine a
light on it
Shine a light and you give it energy
◦ Change momentum of particle
Can also be expressed in another form in
terms of energy
This is not just about measurement
It is about how things are defined
Energy of a particle is not an exact value
The small the uncertainty in time
◦ greater the range of E
Does not match classical theory
Virtual photon can exist for short time
without violating cons of E
Last for approx 10-20 s
Yet to be discovered
LHC aims to find it
When looking at collisions particles formed
are based on conservation of charge, lepton
Mass can change without following any rules
◦ Excepet cons of E & m
Can be explained if mass not part of particle
but of space
Mass makes something difficult to accelerate.
Does this something have to be of the
Analogy for Higgs Mechanism
Aaron Russo enters crowded party
Immediately surrounded by girlfriends
making it difficult to move.
OK if stationary but needs to get a drink.
Finds it hard to move.
Josh G arrives and tells a group of friends
some gossip about Owen
Gossip makes it around room
People cluster to hear it
Cluster makes it hard to move across the
Cluster can exist even if no-one wants to
move across room
For mechanism to work
All space must be covered by field
◦ Higgs Field
When particles move through field
◦ Exhibit properties we attribute to mass
e/m field has particle associated with it
Higgs field has particle
◦ Higgs Boson
In analogy, it is the cluster of people
The simplest example of a linear accelerator
◦ the Van de Graaf,
◦ as the ions move in a linear path.
The T.V. is also a linear particle accelerator.
In a TV, electrons escape from a hot metal
◦ same way as water molecules escape from the
surface of a hot pot of water.
The electrons are accelerated to a small
speed by the effect of an electric field,
◦ due to the potential difference between the
◦ and an anode a few centimetres in front of it.
They are then steered by magnetic or electric
◦ transverse to their motion,
◦ to hit a particular spot on the screen.
The screen is covered by a material;
◦ which gives off light when struck by an electron,
◦ thus producing the picture.
Most TVs use electric potentials between
parallel plates for steering,
◦ rather than magnetic fields from electromagnets.
Magnets are generally used in linear particle
◦ to steer electron beams.
Generally, the name is used for more
◦ that accelerate particles many times along a
There are other devices that accelerate
◦ which move in a circular path.
The linear accelerator shown above works by
passing ions through a series of tubes,
◦ each one a different polarity.
It is only when they arrive at a gap,
◦ they are accelerated.
This is because the tube ahead of will be of
opposite charge to the particle.
The power supply is a.c.
◦ so that this occurs every time the particle moves
from one tube to the next.
As the particle accelerates at each gap,
◦ their speed must increase,
◦ and cover more distance,
◦ in the same amount of time.
This means that each consecutive tube must
◦ if the a.c. current is to be kept in phase,
◦ with the particle movement.
The largest particle accelerator is 3 km long;
◦ can accelerate electrons to 50 GeV,
◦ located at Stanford
called the SLAC, Stanford Linear Accelerator Centre.
The way in which their accelerator works is
To obtain a "polarised" electron beam a
different electron source is used:
Polarised light from a laser is aimed;
◦ at a carefully-prepared gallium arsenide surface.
Electrons are knocked out of the surface by
the laser light;
◦ collected into the accelerator,
◦ by accelerating them across a small potential
Polarised electrons have their spins;
◦ internal angular momentum,
◦ aligned in a particular direction,
◦ in the same way that polarised light,
◦ corresponds to an alignment of the angular
momentum of the photons,
◦ in the travelling electromagnetic wave we call light.
Electric Fields Increase the Energy of the
The electrons at SLAC are produced in much
the same way as in a TV.
After the first acceleration by the electric
◦ due to the potential difference between the cathode
and the anode,
They enter a long copper pipe-like structure
which is the accelerator itself.
This accelerator pipe is pumped out to
produce a very good vacuum,
◦ so that there is nothing inside it,
◦ to deflect electrons travelling down the accelerator.
It contains discs every 3.6 cm with a small
hole in the centre;
◦ 2 cm in diameter.
The beam travels through these holes.
The discs make the pipe into a series of
◦ each about the size and shape of a baked bean can.
By feeding microwave power into this
◦ we make pulses of electric field,
◦ which effectively travel along the structure,
◦ and provide further acceleration,
◦ for the electrons travelling with them.
The electrons ride the microwaves;
◦ like surfers on a water wave,
◦ gaining energy from the force on them due to the
The electron "beam" is actually a series of
bunches of electrons;
◦ timed to enter each cavity just when the electric
field in that cavity,
◦ is close to its maximum,
◦ and pointing in the right direction,
◦ to accelerate the electrons down the accelerator.
The bunches sit a little ahead of the peak of
Any electron that falls a bit behind the bunch;
◦ feels a greater accelerating field,
◦ catches up with the bunch.
Conversely any that get a bit ahead;
◦ feel a weaker field,
◦ are less accelerated,
◦ so the bunch catches up with them.
To describe the same thing in a slightly more
The cavities respond to the microwave input;
◦ by resonating in a particular mode of microwave
◦ that has an electric field component along the axial
i.e. along the accelerator
◦ and a cylindrical magnetic field around the inside
of the pipe.
This electric field is just what is needed;
◦ to accelerate electrons,
◦ or positrons down the accelerator.
The magnetic field provides an inward force
towards the centre of the accelerator;
◦ that compensates for the electrical repulsion,
◦ among the many electrons in a bunch.
The relative phase of the wave is opposite in
◦ but the spacing of the cavities and the frequency of
◦ is chosen so that the electrons travelling at close to
the speed of light,
◦ are only in a cavity when the electric field,
◦ is pointing in the right direction,
◦ to accelerate them down the pipe.
Positrons can be accelerated too.
The positron is the antiparticle of the
It has the same mass and the same
magnitude of electric charge,
◦ but positive instead of negative charge.
When positrons are accelerated the positron
bunches are timed to enter the cavities;
◦ when the electric and magnetic fields,
◦ are reversed compared to the electron case,
◦ so they too are accelerated down the pipe by the
◦ and bunched together by the magnetic field.
Extracting accelerated electrons from part
way down the accelerator,
◦ and colliding them with a target,
◦ produce the positrons.
One of the possible processes that can occur;
◦ is that the incoming electron is deflected by
collision with a target atom,
◦ and then radiates an energetic (virtual) photon,
◦ which produces an electron and a positron.
The positrons are collected using electric and
magnetic fields to control their motion,
◦ and are sent back to the beginning of the
Magnetic Fields are used to steer electrons.
At the end of the accelerator they use large
◦ to produce a magnetic field perpendicular to the
direction of the beam.
An electron moving in a magnetic field is
subject to a force;
◦ that is perpendicular to its motion,
◦ hence will be deflected.
The magnets are used to steer the electrons,
◦ to the various experimental areas.
For some types of experiments, electrons and
◦ are fed into storage rings,
◦ in which they circulate many times.
In the storage rings dipole magnets;
◦ i.e. magnets with the two opposite pole tips
arranged above and below the beam,
Are placed periodically around the ring to
produce a magnetic field perpendicular to the
This gives the centripetal acceleration
needed to make the electrons follow the
More complicated magnets,
◦ such as quadrupoles,
with four pole tips.
Are placed at regular intervals along the
These act on the beam like lenses in a beam
◦ and are used to keep the bunches well focussed,
◦ and travelling in the centre of the storage ring
structure as desired.
A cyclotron is a device used;
◦ to accelerate charged particles to high energies,
◦ generally so they may collide with atomic nuclei,
◦ and produce a nuclear reaction.
There are three main parts of a cyclotron:
1. Ion Source
A beam of protons;
◦ or sometimes deuteron,
which is heavy hydrogen.
Can be charged particle
◦ Positive Ion
◦ Negative Ion
How can you create these ions?
Produced in a cyclotron.
A small amount of hydrogen gas;
◦ is introduced into an evacuated chamber,
◦ which is then heated,
via an electric filament.
Electrons which have gained thermal energy;
◦ from the heating process.
Liberated from the filament;
◦ called thermionic electrons,
◦ collide with gas molecules.
Electrons are then ejected from the gas;
◦ molecules which are then ionised.
Originally they are H2
◦ later become H+,
Modern ion sources are generated from an
◦ external to the cyclotron,
◦ vacuum is not compromised.
2. Semicircular Metal Containers (‘dees’)
Originally, two hollow copper electrodes;
◦ shaped like the letter ‘D’,
◦ their straight edges facing each other were used.
A large ac P.D. is applied between the dees.
The P.D. creates an electric field;
◦ in the gap between them,
◦ that is continuously changing.
As the dees are closed hollow metal
◦ they have no electric field inside of them.
The dees are in a magnetic field;
◦ produced by an electromagnet.
This means that there is a magnetic field;
◦ within the dees.
Within the gap there exists;
◦ an electric and magnetic field.
3. Evacuated Outer Chamber
The dees are placed;
◦ within an outer evacuated container.
As it is a vacuum;
◦ no energy loss will occur,
◦ due to collisions with gas molecules,
◦ they are not scattered away from their target.
If they were to collide with the walls of the
◦ the dees may become radioactive.
This makes maintenance much more
Consider the motion of a positive ion;
◦ inside the dees of a cyclotron.
The dees are placed between;
◦ poles of an electromagnet.
The ions are not shielded from;
◦ magnetic field,
◦ unlike the electric field.
This means the ions are affected;
◦ inside the dees,
◦ in the gap between them.
To make the ions move in circular path;
◦ uniform magnetic field is needed,
◦ perpendicular to the plane of the dees.
Polarity of the field is important;
◦ to make the ions move in the right direction.
The force is such that;
◦ always acting towards the centre of the circle,
◦ causing centripetal acceleration.
In the diagram below;
◦ at the particular instant shown,
◦ its velocity is down.
As the force must always act towards the
centre of the circle;
◦ determine the direction of the magnetic field,
◦ using the right hand rule.
The field must be in a direction which is;
OUT of the page.
The function of the electric field;
◦ accelerate the ions to high energies.
The longer the ions is in the electric field;
◦ the higher the energy.
The function of the magnetic field;
Make the ions move in a circular path;
◦ it repeatedly comes under the influence of the
◦ increases the energy level.
1. Radius of the circular arc described by an
◦ is proportional to its speed, therefore:
2. The time for an ion to complete one
◦ same irrespective of the speed of the ion.
The path followed by an ion is shown below.
Consider the ions in positions a, b and c;
◦ shown above and in more detail below.
At position a;
◦ +ive ion is affected by the electric field,
◦ experiences a force to the right,
◦ accelerates to the right.
Once it leaves the electric field;
◦ and enters the right hand dee,
◦ it experiences no force,
◦ therefore its speed is constant.
The magnetic field causes the ion to;
◦ move in a semicircular path.
At a time later, it reaches position b.
The frequency of the ac current is match so;
◦ that when the ion arrives at b,
◦ the E field has reversed.
During the time in the electric field;
◦ the speed increases further.
Now that the ion is in the left hand dee;
◦ the path followed is semicircular.
It now has a greater radius than previously;
◦ but it also has a higher speed.
This means that the time taken to return to
◦ will be exactly the same.
The a.c. current has;
◦ again reversed the electric field,
◦ by the time the ion gets to position c.
The acceleration process is repeated for many
As the ions pass through the gap;
◦ their speed increases,
◦ so must their kinetic energy.
This means work is done.
◦ W = q V
Change in kinetic energy is also equal to the
W = K
If we assume that the ions have very little
kinetic energy to begin with;
◦ their kinetic energy will increase,
◦ by q V,
◦ for each pass of the gap.
As there are two passes of the gap per
◦ their kinetic energy per revolution,
◦ is 2q V.
Originally, cyclotrons were used as;
◦ research devices in nuclear reactions.
They are now used to produce;
◦ radioactive materials for a number of purposes.
Hospitals use them for producing
◦ that undergo rapid radioactive decay.
The decay is so rapid;
◦ they must be produced on site,
◦ at the time of use.
The protons are used to bombard stable
To produce radioactive forms of these
They are then combine with glucose and are
given to the patient.
The radioactivity can then be detected;
◦ bodily functions that use the above chemicals,
◦ can be monitored.
A medical diagnosis can then be made.
At higher doses, the radioactive elements can
◦ as a treatment to kill cancers,
◦ in organs where the element,
◦ tends to concentrate.
Previously we stated:
The radius of the circular arc described by an
ion is proportional to its speed.
We can apply Newton’s second law;
◦ F = ma,
as it applies to circular motion
And F = qvB
therefore, upon rearrangement:
As the mass m;
◦ charge q,
◦ magnetic field B,
◦ are all constant,
We also stated:
The time for the ion to complete one
semicircle is the same irrespective of the
speed of the ion.
From before, if the speed doubles;
◦ radius doubles.
This also doubles the;
◦ circumference (2 r).
Mathematically, this can also be shown to be
The velocity of an object undergoing circular
motion is given by:
Rearranging for T
From we can substitute for r.
This shows that the period is;
◦ independent of speed,
◦ or radius.
Constant for ions of;
◦ a constant charge,
◦ in a given magnetic field.
The period refers to the time;
◦ for one complete revolution,
◦ not the time in a dee.
That is half a period.
Any nuclear reaction caused by a collision;
◦ requires a certain amount of energy.
It is critical to know the kinetic energy;
◦ with which the ions leave the cyclotron.
We previously stated that if we know the P.D;
◦ and the number of revolutions,
◦ we can calculate the kinetic energy.
While this is correct;
◦ the number of revolutions is not normally known.
An alternative method is required.
K = ½mv2
If we rearrange equation
Substituting into formula for K:
This indicates that the kinetic energy of an
◦ of given charge,
◦ and mass.
Only depends on the radius;
◦ of the final circle,
◦ magnitude of the magnetic field.
This can be understood due to two points.
If the magnetic field increases;
◦ the radii decreases,
◦ ions make more revolutions,
◦ more crossings of the gap between the dees.
At each crossing;
◦ they are accelerated,
◦ to higher kinetic energy.
Increasing the magnetic field results in;
◦ increase of the kinetic energy,
◦ of the emerging ions,
◦ at a given radius.
If the P.D. is increased;
◦ the ions gain more speed with each crossing of the
◦ and so make circles with larger radii,
◦ and make fewer revolutions.
This means that a larger P.D. does not result
◦ a larger kinetic energy,
◦ of the emerging ions at a given radius.
What is a Synchrotron?
A very large, circular, megavoltage machine
about the size of a football field.
Inside there is a vast, circular network of
interconnecting tunnels and high tech
Uses electricity to produce intense beams a
million times brighter than the sun.
Produced when high-energy electrons are
forced to travel in a circular orbit inside the
synchrotron tunnels by ‘synchronised’
application of strong magnetic fields.
Beams travel at just under c.
Light they produce is filtered and adjusted to
travel into experimental workstations.
Provides information about the molecular
structure of materials.
Used to research diseases,
◦ make plants more productive
◦ metals more resilient.
What is synchrotron light?
e/m radiation emitted when
electrons, moving near c, are forced to
change direction under the action of a
e/m radiation is emitted in a narrow cone in
the forward direction, at a tangent to the
Synchrotron light is unique in its intensity
and brilliance and it can be generated across
the range of the e/m spectrum.
Properties of synchrotron light
High brightness: synchrotron light is
extremely intense (hundreds of thousands of
times more intense than that from
conventional x-ray tubes) and highly
Wide energy spectrum: synchrotron light is
emitted with energies ranging from infrared
light to hard x-rays.
Highly polarised: the synchrotron emits
highly polarised radiation, which can be
linear, circular or elliptical.
Emitted in very short pulses: pulses emitted
are typically less than a nano-second (a
billionth of a second), enabling time-resolved
Electrons are generated in the centre (electron gun) and
accelerated to 99.9997% of c by the linear accelerator (linac).
The electrons are then transferred to the booster ring, where they
are increased in energy.
They are then transferred to the outer storage ring.
The electrons are circulated around the
storage ring by a series of magnets separated
by straight sections. As the electrons are
deflected through the magnetic field created
by the magnets, they give off electromagnetic
radiation, so that at each bending magnet a
beam of synchrotron light is produced.
At each deflection of the electron path a beam of
light is produced. The effect is similar to the
sweeping of a search light. These beams can be
captured and focussed to a specific wavelength
appropriate for a particular technique.
Intensity of light can be significantly
increased by the use of 'insertion devices‘
◦ in the straight sections of the ring.
There are two classes of insertion devices.
Multipole wiggler (MPW)
Cone of light is emitted at each bend in the
◦ cones of light superimpose on each other,
◦ intensity increasing with the number of bends.
At the peak of each wave a beam of light is
Beams reinforce each other and appear as a
broad beam of incoherent synchrotron
light when viewed in the horizontal plane
ahead of the wiggler.
Second type of device is an undulator.
Uses less powerful magnets to produce
◦ gentler undulations of the beam.
Light cones just overlap and interfere with
◦ so that certain wavelengths of light are enhanced
◦ perhaps by 10,000 times.
Wavelengths can be changed by
◦ altering the gap between the component magnets
◦ so that the light is tunable to specific wavelengths.
The poles produce less deflection of the electron beam. This results
in a narrow beam of coherent synchrotron light, with certain
frequencies amplified by up to 10,000 times.
Current ('third generation') designs aim to
optimise the intensity that can be obtained
from insertion devices.
Attention is given to the size and positioning
of the straight sections that accommodate
the insertion devices.
The Australian Synchrotron is an advanced
'third generation' design.
Uses the three different types of light sources
◦ bending magnets,
◦ multipole wigglers,
The charged particles travelling around the
synchrotron accelerate each time their direction
When a charged particle accelerates, it emits e/m
When it emitted in the X-Ray part of spectrum,
there is a loss of energy
The radiation is very intense, polarised and
Useful for X-Ray diffraction or other expts
investigating properties of materials
Advantage of synchrotron
Can accelerate particles in opposite
Energy is much higher than hitting a fixed
LHC does the same thing
On much larger scale and performs other
High acceleration means radiation loss for
As acceleration is a one off event, if collision
does not take place, particles are lost
Follows circular path therefore more chance
Difficulties construction big Ds and magnets
there high energies not reached.
Good for medical apps and is cheap to build
2 particles of equal mass
◦ Proton & antiproton
Energy available for new particle =
◦ Rest energy + Ek of both particles
Not all energy can be used to produce new
◦ Momentum must be conserved
2 = 2EMc2 + (Mc2)2 + (mc2)2
EA = energy available
M = rest mass of target
m = rest mass of accelerated particle
E = total energy of accelerated particle
Energy in eV
Created by American Scientist Donald Glaser
Won 1960 Nobel Prize for invention of Bubble Chamber
Uses hydrogen in pressurised container
Temperature below boiling point
Can only form liquid around impurities
◦ None available
As ions move through, bubbles form
◦ Vapour trails seen and photographed
Placed in magnetic field
◦ Particles deflected and identified
Particles collide and produce other particles
◦ Need large cloud chamber to capture interactions
◦ Can’t be done
Once a particle triggers a vapour trail
◦ Cloud chamber must be reset
High energy collisions can produce γ – rays
Will not leave a track as a particle would
Single photon enters through window
Window coated & absorbs photon
Accelerated towards +ive dynodes
Each time collision with dynode
◦ More electrons liberated
Signal then strong enough to measure change
High energy γ – rays will not liberate electron
Some materials will emit visible light when
they absorb a γ – ray.
If paced before window detection can be
Ions conduct electricity
Concept used in G-M
Will detect particle but
Use lots of G-M tube
around room allows
tracking of path
This is a wire chamber
GM tube consists of metal cathode
surrounding a centre electrode
The front of the tube is a thin Mica window
sealed to the metal cylinder. The thin mica
window allows the passage and detection of
the weak penetrating alpha particles. The GM
tube is first evacuated then filled with
Neon, Argon plus Halogen gas.
The GM tube is put into an initial state (ready
to detect a radioactive particle), by applying +
500-volt potential to the anode (centre
electrode) through a ten mega ohm current
A 470K-ohm resistor is connected to the
metal wall cathode of the tube and to ground.
The top of the 470K resistor is where we see
our pulse signal whenever a radioactive
particle is detected.
In this initial state the GM tube has a very
However, when a radioactive particle passes
through the GM tube, it ionizes the gas
molecules in its path and creates a
momentary conductive path in the gas.
This is analogous to the vapour trail left in a
cloud chamber by a particle
Electron liberated from the atom by the
particle, and the positive ionized atom both
move rapidly towards the high potential
electrodes of the GM tube.
They collide with and ionize other gas atoms,
creating a momentary avalanche of ionized
These ionized molecules create a small
conduction path allowing a momentary pulse
of electric current to pass through the tube
allowing us to detect the particle.
This momentary pulse of current appears as a
small voltage pulse across the 470 K ohm
The halogen gas quickly quenches the
ionization and the GM tube returns to its high
resistance state ready to detect more
Like a GM Tube except it has multiple wires
The data gives arrival time and the track of
◦ Breaks down matter to smallest number of
fundamental units to construct all known particles
High energy particle collisions
◦ Support many predictions
Quark confinement means
◦ Not possible to detect free quarks
Using high energy electrons can show
◦ Hadrons & mesons are made of smaller particles
Scattering of high-energy particles
◦ By particles that make up nucleons.
◦ Scattered particles lose energy to target
By analysing scattering angles can show:
◦ Nuclei are made of smaller charges knocked apart
At higher energies
◦ Baryons scatter electrons in a way
◦ Consistent with being made of 3 point charges
◦ Mesons scatter in a way
◦ Consistent with being made of 2 point charges
◦ The size of charges is 1/3 e or 2/3e.
Total momentum of quarks
◦ much more than expected
◦ Implies other particles in nucleus - Gluons
Findings support quark model
No free quarks were found
◦ Due to confinement principle
Particles found are
◦ Consistent with standard model
Force holding quarks together is very strong
Caused by exchange of gluons
If force increased as distance between quarks
◦ Matter would collapse to nothing
Believe at short range
◦ Force tends to zero
Quarks have asymptotic freedom
Quarks which make up a hadron
◦ Won’t be held tightly by strong nuclear force
◦ Free to move about inside particles
◦ Like apples in a bag
Getting quarks out of bag takes lots of
◦ Not moving them around inside
Results of deep inelastic scattering
◦ Suggest small charges responsible for scattering
◦ Inside hadrons
◦ Are free to move around
Related to weak interaction
◦ Mediated by W+, W & Z+ bosons
Certain interactions can be predicted
An electron and a neutrino can collide
This is a weak interaction
◦ involves W exchange particle.
The resultant particles do not change charge
◦ called neutral currents
Difficult to observe
◦ First seen in 1973 at CERN bubble chamber.
Without neutral currents
◦ Holes would appear in standard model
Finding it validated the electroweak theory.
In the beginning (before 10-43s)……
◦ No time
◦ No space
Just a singularity of very dense matter
What was there before this?
Nobel prize for first correct answer
At the beginning of time 4 forces were
What was the temperature?
Can tell from energies required to produce
Originally (after 10-43s) 1032 K
10-35 s – Grand Unification Era (GUT)
Cooled to 1027 K (1014 GeV).
Particles & antiparticles form radiation
◦ Photons produce particle – antiparticle pairs
◦ Particles – antiparticles can then decay
Result more particles than antiparticles
Temp now 1015 K (100 GeV)
◦ Particle accelerators can reach these energies
Quarks now form
◦ protons, neutrons
◦ Other baryons & mesons.
Nuclear Reaction Era
Temp now 109 K (0.1 MeV)
Protons & Neutrons combine to make nuclei
◦ 90% Hydrogen
◦ 10% Helium
Now below 4000 K (0.4 eV)
Electrons combine with nuclei to form atoms
Gravity pulls atoms together
Forms larger elements
3 K (0.0003 eV)
Still hot places
Most of universe is cold
Originally matter slightly dominated over
Once energies (temp) of photons was below
that which could produce pair production
◦ Matter dominated
Have ignored gravity
Because when mass small
◦ Force is neglibgible
However, when distance is tiny
◦ force is very large
This doesn’t fit with matter
◦ being made of small particles
String Theory addresses problem
Instead of point masses
Particles are made of short strings
◦ 10-35 m
This is an alternative to quantum theory
Mode of vibration represents properties
◦ Such as mass
Only certain harmonics allowed
◦ Therefore only certain particles allowed
Need extra dimensions (difficult to visualise)
Elementary Particle Physics is opening up new
One new theory is called the;
◦ Grand Unified Theory
A Nobel Prize beckons you if you wish to
continue in this area