Transcript Today`s Objectives - RanelaghALevelPhysics

Particles & Antiparticles
Particle Physics Lesson 4
Today’s Objectives

Compare rest mass energies of particles and
anti-particles

Describe the processes of pair production and
annihilation.

Calculate energies involved in pair production
and annihilation.
Video

The Matter with Antimatter.wmv
Particle Physics



Particle physics is concerned with
fundamental particles, which means that the
particles can't be broken down any further..
It used to be thought that protons, neutrons and
electrons were the fundamental particles of
matter.
However it has been found that nucleons
(proton and neutron) are made up of smaller
particles, so nucleons are now not fundamental.
Paul Dirac (1902-1984)

Wrote an equation
obeyed by an electrons
which had a kind of
mirror image solution

Predicted the existence
of anti-particles in 1928.
Discovery of the Positron




Cosmic ray experiment.
Charged particles leave
an ion trail as they pass
through a super-cooled
gas.
Causes vapour to
condense to leave vapour
trails – like jet plane.
The particle bent the wrong
Magnetic field causes
way in a magnetic field.
charged particles to
bend.
Antiparticles




Each particle has an
antiparticle. However,
antiparticles are not
found in normal matter,
but arise in:
high-energy collision
experiments,
interactions with cosmic
rays,
radioactive decay.
Positron Emission

Also called β+ decay.

Remember is isn’t a ‘natural’ form of decay –
only occurs in experiments.

Happens when there are too many protons.
A
Z
X  Y   
A
Z 1
0
1
Notes





an antiparticle has the same mass as its particle,
a particle and its antiparticle have equal but
opposite charge
an unstable particle and its antiparticle have the
same lifetime.
some neutral particles and their antiparticles are
identical (e.g. photon)
other neutral particles and antiparticles are not
identical.
Complete this table
Particle
Mass compared Charge
with proton
Electron
Positron
Proton
Antiproton
Neutron
Antineutron
1/2000
-1e
Albert Einstein

From Einstein’s special theory of
relativity:-

Equivalence of energy and mass:
Energy can turn into mass
Mass can turn into energy



Have a go! Calculate the rest
mass energy of a proton in eV.
E  mc
2
Annihilation

If a particle meets with
its corresponding antiparticle they annihilate.

All the mass of the
particle and anti-particle
mass is converted into
energy in the form of
photons (γ-rays).
Minimum energy of each photon produced  hf min  E0
Pair Production
Pair Production
When energy is converted into mass you get equal amounts of matter and antimatter

In pair production a photon creates a particle and
a corresponding anti-particle (usually e+ and e-).

Only occurs if one photon has enough energy to
produce the total rest mass of the two particles.
Minimum energy of photon needed  hf min  2E0

Where fmin is the minimum photon frequency
required, E0 is the rest mass energy of the particles.
Cloud Chamber Pictures

Can you see where pair
production happens?

Photons enter from the
top of the image.
Cloud Chamber Pictures
The top interaction includes a “knock on” electron.
Example Questions




What is the rest mass energy of an electron?
What is the minimum energy required for pair
production of an electron and a positron to take
place?
What is the minimum frequency required for
pair production of an electron and a positron to
take place? … and the wavelength?
To which part of the electromagnetic spectrum
does this correspond?
Application - PET Scanners

Positron Emission
Tomography

Positron emitting isotope
is given to patient –
some reaches the brain.

Positrons only travel a
few mm before
annihilating with electron
and photons produced
are detected.
Albert Einstein

Showed that the mass of a
particle increases as its speed
increases with the energy
supplied to the particle to its
increase mass by E=mc2.

The mass of a particle when
it is stationary its rest mass
(m0) corresponds to rest
energy m0c2