Particle physics

• How did rutherford estimate the maximum radius of a nucelus

  Equated the kinetic energy of the alpha particle to the electric potential energy at the closest approach to the nucleus
• Isotope


  A nucleus of an element with the same number of protons but different number of neutrons
• Two generation 1 quarks and leptons and their charges

  • Quarks - up (charge +2/3 e), down (charge -1/3 e)
  • Leptons - electron (charge -1 e), electron neutrino (0 charge)
• Measuring momentum from a particle track

  If its in a magnetic field, measure the radius of the curvature of the track
• Alpha scattering experiment observations - Most

  Most alpha particles passed through the gold foil undeflected as the nucleus occupies a very small volume compared to the volume of an atom
• Alpha scattering particle observations - Some

  Some alpha particles were deflected through small angles when they passed close to the nucleus showing that charge is concentrated at the centre of the atom in the nucleus
• Alpha scattering particle observations - Few

  A few alpha particles were deflected through large angles when they passed very close to the nucleus or made a direct collision showing that mass is concentrated at the centre of the atom, in the nucleus
• Conservation laws of particle interactions

  Mass-energy, Momentum, Charge, Bayron number, Lepton number
• Which particle mediates electromagnetic force

  A photon
• What particles wont leave tracks in a cloud or bubble chamber

  Neutral particles
• Rutherfords measurements of scattering experiment

  The angle of deflection of the alpha particles
• Alpha scattering experiment conclusion

  Most of the atom is empty space with most of the mass concentrated in a small nucleus
• Atomic mass unit definiton

  It is 1/12th the mass of a Carbon-12 atom
• Thermionic emission


  The emission of electrons from the surface of a heated metal
• Conclusion from some alpha particles being scattered was

  The nucleus is positively charged
• What happens to the lifetime of particles at relativistic speeds, from an observers point of view

  Particle lifetime appears to increase from the point of view of an observer
• Momentum from a bubble chamber photo is determined by

  A magnetic field perpendicular to the particle velocity produces circular motion, the radius of the tracks can be used to determine particle momentum using r = p/BQ
• Pair annihilation


  If a particle and antiparticle such as an electron and positron meet, they will annihilate and create a photon
• How many quarks and leptons are there in the standard model

  12 + 12 = 24
• Baryons


  Baryons are composed of three quarks or three anti quarks.

  • They are not fundamental
• Mesons


  Mesons are composed of a quark and an antiquark

  • They are not fundamental
• What was measured in Rutherfords scattering experiment

  The angle of deflection of the alpha particles
• Role of the magnetic field in a bubble or cloud chamber

  A magnetic field perpendicular to the particle velocity produces circular motion, the radius of the tracks can be used to determine particle momentum or charge using r = p/BQ
• A photon is the standard model particle which mediates the electromagnetic force
• How did rutherford estimate the maximum radius of the nucleus

  He equated the KE of the alpha particle to the electric potential energy at the closest approach to the nucleus
• Relation of particles and antiparticles, and what happens when they meet

  Particles and antiparticles have the same mass and opposite charge. When they meet they annihilate
• Why are high energies needed to probe the inside of the nucleons

  From E=mc^2 large energies are needed to create particles with large masses, high energy particles have short wavelengths and so can image small particles
• Relativistic


  Speeds are a significant fraction of the speed of light (>10%)
• Nucleon 


  A proton or neutron
• Role of an electric field in a bubble or cloud chamber

  The electric field can be used to accelerate particles. The deflection can be used to find their mass F = ma = EQ
• What is the starting point of charged particles in a cyclotron?

  Centre of the cyclotron
• How are charged particles accelerated in a cyclotron?

  By the potential difference across the gap
• What is the orientation of the magnetic field in relation to the particle's path?

  At right angles to the path
• What type of path do charged particles follow in the dees of a cyclotron?

  Circular path
• What happens to the potential difference across the gap during each half cycle?

  It changes direction
• What effect does the increase in velocity have on the radius of the charged particles' path?

  The radius increases
• What remains constant regarding the time spent by protons in each dee?

  The time is constant
• What does the constant time spent in each dee imply about the frequency of the potential difference?

  The frequency of the potential difference is constant
• See image

  • Photon is neutral because it doesn't leave a trail
  • Particles are oppositely charged because they curve in opposite directions to conserve charge
  • Using FLHR the top particle is positive and the bottom particle is negative
  • Both particles have the same momentum because they have the same radius
  • The photon enters from the left because the momentum of the two particles is to the right

  
• What does radius curvature of the track give

  • Charge
  • Momentum - including a changing momentum, usually a spiral as kinetic energy decreases with ionisation