PHYS UNIT 4: PARTICLE PHYSICS

The Thomson model – 19^th century

• Atom was a positively charged globule w/ negatively charged electrons sprinkled in it – Plum pudding

Rutherford’s experiment disproved the Thomson model

• Stream of alpha particles from a radioactive source were fired at very thin gold foil
• Recorded the no. of alpha particles scattered at diff angles
• If the Thomson model was right, the flashes would have been seen within a small angle of the beam
• Observed that alpha particles occasionally scatter at angles greater than 90’ – only possible if they’re striking something more massive than themselves

Rutherford’s model of the atom – the nuclear model

• Most of the charged, fast alpha particles went straight through – atom is mostly empty space
• Some of the particles were deflected back through significant angles – centre of atom must be tiny but contains a lot of mass – named this the nucleus

• The alpha particles were repelled – nucleus must have a positive charge
• Atoms are neutral overall so the electrons must be on the outside of the atom – separating one atom from the next 

Atoms are made up of protons, neutrons and electrons

• Nucleus – protons and neutrons
• Orbiting this core are electrons

Particle

Charge

Relative mass (u)

Proton

+1

1

Neutron

0

1

Electron

-1

0.0005

1u = one twelfth the mass of an atom of carbon-12 = 1.66 x 10^-27 kg

Proton number = no. of protons in nucleus

• Sometimes called the atomic no. and has symbol Z
• This defines the element
• In a neutral atom, the no. of electrons = no. of protons
• Element’s reactions and chemical behaviour depends on no. of electrons 

Nucleon number = total no. of protons and neutrons

• Also called mass no. and has symbol A 

Electron guns produce electrons by thermionic emission

• When you heat a metal, its free electrons gain a load of thermal energy
• Give them enough energy and they break free from the surface of the metal – thermionic emission
• Once emitted, they are accelerated by an electric field in an electron gun
• A heating coil heats the metal cathode – electrons that are emitted are accelerated towards the cylindrical anode by electric field set up at high voltage. Some electrons pass through a little hole in the anode, making a narrow electron beam – move at constant velocity because there is no electric field beyond the anode

Electronvolt is defined using accelerated changes

• Kinetic energy that a particle with charge Q gains when it’s accelerated through a pd of V volts is QV joules = definition of the volt (JC-1)
• Replace Q in equation w/ charge of an electron, e = 1/2mv^2 = eV
• 1 electronvolt is the kinetic energy carried by an electron after it has been accelerated through a pd of 1 volt.
• So, energy in eV of an electron accelerated by a pd is: energy gained by electron (eV) = accelerating voltage
• 1eV = 1.6 x 10^-19J

Linear particle accelerators (linacs) cause high-energy collisions

• Long straight tube containing a series of electrodes 
• High-frequency alternating current is applied to electrodes so that their charge continuously changes between + and –
• Alternating current is timed so that the particles are always attracted to the next electrode in the accelerator are repelled from the previous one
• A particle’s speed increases each time it passes an electrode
• The high-energy particles leaving a linac collide with a fixed target at end of tube

A cyclotron is a circular particle accelerator

• Uses 2 semicircular electrodes to accelerate protons or other charged particles across a gap
• Takes up much less room because it is circular
• An alternating pd is applied between the electrodes – as particles are attracted from 1 side to the other their energy increases (accelerated)
• A magnetic field is used to keep the particles moving in a circular motion
• Combo of magnetic and electric fields makes the particles spiral outwards as their energy increases

The mass of particles increases w/ speed

• Special relativity – as you accelerate an object you increase its mass
• Only when you get close to the speed of light that it starts to have a big effect
• Means that as an object, like a particle in an accelerator, travels faster and faster its mass gets greater and greater. As the mass of particle increases, it gets harder to accelerate it.
• This effect limits how much you can accelerate a particle in the cyclotron. Protons can be accelerated to energies of energies of around 20MeV – if you want higher energies than that you need to use a synchrotron 

Hadrons are particles that feel the strong interaction

• Has to be a strong force holding nucleons together
• Made up of quarks
• 2 types – baryons and mesons

Protons and neutrons are baryons

• They are nucleons with different electric charges
• The proton is the only stable baryon, all baryons except protons decay into protons. Most physicists think that protons don’t decay
• The no. of baryons in a reaction is called the baryon number. Protons and neutrons each have a baryon no. of +1. Total baryon no. in any particle reaction never changes

Mesons we need to know about are pions and kaons

• All mesons are unstable and have baryon no. B = 0
• Pions are the lightest mesons: have 3 versions - +, 0 and -. Discovered in cosmic rays. You get lots of them in high-energy particle collisions like those at CERN
• Kaons are heavier and more unstable. You get +, 0 and -.
• Mesons interact w/ baryons via the strong interaction 

Leptons don’t feel the strong interaction

• Are fundamental particles – only way they interact w/ other particles is via the weak interaction and gravity (and the electromagnetic force if they are charged)
• Electrons, muons and taus
• Muons and taus are unstable – decay into ordinary electrons 
• They each come w/ their own neutrino
• Neutrinos have 0 or almost 0 mass and 0 electric charge – they don’t do much. A neutrino can pass right through the earth without anything happening to it

Have to count the 3 types of lepton separately 

Neutrons decay into protons – really just an example of beta decay

Antiparticles were predicted before discovered

• Paul Dirac wrote an equation obeyed by electrons and found a kind of mirror image solution
• It predicted the existence of a particle but w/ opposite electric charge – the positron
• Positron turned up later in a cosmic ray experiment, they are antileptons, have identical mass to electrons but w/ a positive charge

Every particle has an antiparticle

• Same mass but opposite charge 

You can create matter and antimatter from energy

• E = mc^2
• In nuclear reactions, the mass of the particles you start with might be more or less than the mass of the particles you end up with – happens when energy is converted to mass or mass to energy – total mass and energy is conserved
• When energy is converted into mass you have to make equal amounts of matter and antimatter
• Fire 2 protons at each other at high speed and you’ll end up w/ a lot of energy at the point of impact. This energy can form more particles. If an extra proton is created, there has to be an antiproton made to go w/ it. It’s called pair production
• Kg and joules are too big to be used for this so MeV is used for energy and atomic mass units, u or eV/c^2 are used for mass

Each particle-antiparticle is produced from a single photon

•  Only happens if 1 gamma ray photon has enough energy to produce that much mass
•  Also tends to happen near a nucleus – helps conserve momentum
• Usually get electron-positron pairs because they have a relatively low mass

Opposite of pair production is annihilation

• Particle meets its antiparticle = annihilation
• All the mass is converted into energy

Mesons are their own antiparticles

Pion- meson is just the antiparticle of the Pion+ meson and the Pion0 meson is its own antiparticle. 

Quarks are fundamental particles

• Building blocks for hadrons

Quarks and antiquarks have opposite properties

Baryons are made up from 3 quarks

• Evidence for quarks – hitting protons w/ high energy electrons – the electrons scattered showed that there were 3 concentrations of charge (quarks) inside the proton
• Proton = uud
• Neutron = udd

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Mesons are a quark and antiquark

• Pions  = combinations of up, down, anti-up and anti-down quarks
• Kaons have strangeness so you need to put in s quarks as well (s quark has a strangeness of S =-1)

No such thing as a free quark – not possible to get a quark by itself – quark confinement. Only pair production occurs so you get mesons

Five properties conserved in all particle reactions

• The total charge after reaction must = total charge before
• Same goes for baryon number
• Mass-energy is conserved – the total energy of the reaction if you convert the masses of all the particles into energy
• Momentum has to be conserved too
• Conservation of lepton number is a bit more complex: the 3 types of lepton no. have to be conserved separately

Charged particle leave tracks

• When charged particle passes through a substance it causes ionisation – electrons are knocked out of atoms. The particle leaves a trail of ions as it goes
• Easiest way to detect particle is if you somehow make the trail of ions show up and then take a photo

 Cloud chambers and bubble chambers detect charged particles

• Cloud chambers work using a supercooled vapour – something that is still a gas below its usual condensation temp. The ions left by particles make the vapour condense and you get ‘vapour trails’. Heavy, short tracks mean lots of ionisation, so those will be the alpha particles. Fainter, long tracks are beta particles
• Bubble chambers are a bit like cloud chambers but in reverse. Hydrogen is kept as a liquid above its normal boiling point by putting it under pressure. If the pressure is suddenly reduced, bubbles of gas will start to form in the places where there is a trail of ions. You have to take the photo quickly before the bubbles grow too big.
• Both chambers only show up charged particles 

Charged particles are affected by magnetic fields

• Will experience a force – making particle follow a curved track
• Radius of a charged particle’s curved track, r = p/BQ – the larger the curve radius. The greater the particle’s momentum
• Positive and negative particles curve opposite ways – can find out which is which using fleming’s left hand rule
• You see spirals – interactions w/ detector decrease the energy (and so the momentum) of particle
• You can also use this equation to find the magnetic field you need to keep a charge in a particular radius of circular path – handy when dealing w/particle accelerators

Charge, energy and momentum always conserved 

Neutral particles only show up when they decay or interact

•  If you see a V shape starting in the middle of nowhere, it will be 2 oppositely charged particles from the decay of a neutral particle.
• The distance from the interaction point to the V depends on the half life of the neutral particle. Longer lived particles travel further on average before they decay – have to be careful
• The particles are travelling close to the speed of light so they experience relativistic time dilation – means that time seems to run more slowly for the moving particle than it does for you – seem to survive for much longer than normal

Real bubble chamber photos can be intimidating

• Start by finding the incoming beam – the straight lines. Several particles will go straight through without doing anything – ignore them
• Look for a little spiral coming from one of the straight tracks. It shows a knock-on electron – an electron that’s been kicked out of one of the hydrogen atoms. Knock- on electrons tells you 2 things: which way the particles are going and which way negative particles curve
• Find a point w/ several curved tracks coming from it – that’s a reaction. You can identify positively and negatively charged particles from the way they curve

Calculate the particle’s momentum using r = p/BQ

Cloud chambers and bubble chambers aren’t used anymore

Nowadays physicists use detectors that give out electric signals that are sent straight to a comp. It’s a bit easier than examining photos. Modern detectors include drift chambers, scintillation counters and solid state detectors.