Technician’s Notes Activity 10S Software Based 'Bubble chamber photographs'

Particles & Antiparticles Particle Physics Lesson 4

Lesson Outcomes To be able to describe the difference between Matter and Anti Matter in terms of rest mass. Describe the processes of pair production and annihilation. Calculate energies involved in pair production and annihilation.

Answers: Connector vacuum 3.0 600 4.0 200 glass 2.0 8.0 water 4.6 500 Medium Speed / x 108 ms-1 Frequency / x 1014 Hz Wavelength / nm Energy / x 10-19 J vacuum 3.0 600 4.0 200 glass 2.0 8.0 water 4.6 500 5.0 3.32 3.0 750 2.65 3.0 302 10 250 5.3 2.3 3.05

Video ATOM – Prof. Jim Al-Khalili explains Paul Dirac’s achievement. The Matter with Antimatter.wmv

Classification of Particles Because protons and neutrons have very similar properties they are classified as BARYONS Electrons are quite different (partly because of their mass) and are classified as LEPTONS (Leptons are Light) There are lots more baryons and leptons which we will meet later

Conservation of Leptons During any reaction between particles or decay of particles the number of leptons is conserved (this is nearly true) However, this beta decay suggests otherwise: A neutron has changed into a proton and ‘created’ an electron The orbital electrons are not involved, the baryon number has been conserved but there is an extra lepton

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 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.

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).

Electron – Positron Annihilation Find the energy equivalence of the electron Deduce the energy equivalence of the positron Show that the energy released by electron / positron annihilation is 1.64 X 10-13 J 2 photons are emitted. What is the wavelength of this e-m radiation?

Pair Production Can this process be reversed? Could colliding two photons produce an electron / positron pair? What would have to be the conditions?

Pair Production

Pair Production 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.

Activity 10S Software Based 'Bubble chamber photographs'

The image below shows a series of collisions between protons – subatomic snooker.

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.

Useful Videos Science of the impossible. Michiao Kaku. http://www.youtube.com/watch?feature=endscreen&v=178raSqg33I&NR=1