Particles & AntiparticlesParticles & Antiparticles

Particle Physics Lesson 4Particle 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.

VideoVideo

The Matter with Antimatter.wmvThe 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.

Magnetic field causes charged particles to bend.

The particle bent the wrong way in a magnetic field.

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

011YX A

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Also called β+ decay.

Remember is isn’t a ‘natural’ form of decay – only occurs in experiments.

Happens when there are too many protons.

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 with proton

Charge

Electron 1/2000 -1e

Positron

Proton

Antiproton

Neutron

Antineutron

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.

2mcE

Annihilation If a particle meets

with its corresponding anti-particle they annihilate.

All the mass of the particle and anti-particle mass is converted into energy in the form of photons (γ-rays).

0min producedphoton each ofenergy Minimum Ehf

Pair Production

Pair Production

0min 2 neededphoton ofenergy Minimum Ehf

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.

Where fmin is the minimum photon frequency required, E0 is the rest mass energy of the particles.

antimatter andmatter of amounts equalget you mass into converted isenergy When

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