Particle Physics

Question 1

Atomic Structure

Answer 1

• Proton number = number of protons in nucleus
• Neutron number = number of neutrons in the nucleus
• Proton mass = 1.67262*10^27 kg
• Neutron mass = 1.67493*10^-27 kg
• Electron Mass = 0.00091*-27 kg
• nucleus density = about 10^16 kgm^-3

Question 2

Alpha Scattering

Answer 2

• lead to disproving plum pudding model as Rutherford fired alpha particles at gold sheets
• repelled very few particles, but 0.01% were scattered over 90 degrees so the nucleus must be small but dense
• as alpha particles are positive the nucleus must also be positive

Question 3

Thermionic emission

Answer 3

• the releasing of electrons due to heat (used in electron guns)
• resulting beam can be manipulated by a magnetic/electric field
• force on electron in magnetic field = BeV
• radius of electrons circular path = momentum/Be, as circular path is only in one equipotential line no work is done to keep particle in motion

Question 4

Cyclotrons

Answer 4

• proton accelerator, emits them from a central emitter in a spiralling pattern due to a strong magnetic field facing vertically downwards
• proton has a spiral path and is accelerated towards the first semi-circular ‘dee’ across the gap by an electric field
• as it follows the path round it accelerates towards the second dee as the field across is negative. as it accelerates the radius of its path increases
• when it is in the next dee the field swaps direction
  (frequency of alternating current must be perfectly timed for this using 2(pi)f = Be/m) so it continues to accelerate circularly until the radius is wide enough for it to escape at a high velocity

Question 5

Linac

Answer 5

• Linear electron accelerator that uses tubes with electric fields instead of dees, tubes must increase in length as electron accelerates so that by the time it leaves the tube the alternating current has flipped and it is attracted to the next tube
• this doesn’t need a magnetic field, just electric ones in the tubes and a source of electrons

Question 6

Particle detectors

Answer 6

• used to show tracks left by ionising particles
• alpha particles leave fatter and shorter trails, while beta particles leave long thin trails
• radius of path in magnetic field = momentum/Be
• neutral particles will have straight paths we cannot see
• acceleration = chargevoltage/distance between plates in electric fieldmass

Question 7

Bubble/cloud chambers

Answer 7

• bubble chambers contain low pressure liquid hydrogen that ionises very easily, forming vaporisation bubbles
• Cloud chambers have a pad soaked in volatile liquid (ethanol) with an electric field between the particle source and the sides of the chamber that has the liquid on the brink of ionising
• when ionised, the pressure is reduced and the ethanol condenses in a trail

Question 8

Shapes of trail

Answer 8

• decreasing spirals = losing energy, fast particles leave thin trails slow particles leave thick trails
• if two trails are at right angles, the mass of the colliding particles must be identical

Question 9

Relativistic equation

Answer 9

• Energy = mass*speed of light^2
• rest mass of particle = mass when it isn’t moving
• change in energy of the particle corresponds with an increase in mass of change in energy/c^2
• in collisions momentum and mass-energy are conserved