Episode 539: Deep inelastic scattering

At higher electron energies, two things happen:

  • The electrons penetrate deeper into the nucleus and scatter off sub-units within protons and neutrons
  • The electrons lose energy (they undergo inelastic collisions); this energy is ‘converted’ into mass as a jet of pions is produced

Hence higher energy electrons give us information about the structure of nucleons.


  • Student experiment: Analogue of electron scattering by quarks (20 minutes)
  • Discussion: Deductions from electron scattering (10 minutes)

Student experiment: Analogue of electron scattering by quarks
Magnets are concealed in a box. These represent ‘charges’ which are probed using a freely-suspended magnet.

The important point here is that each of the ‘charges’ represented by the magnet poles will affect the trajectory of the target ‘charge’ of the moving magnet pole in a more complex way than a single one would.

Episode 539-1: Probing arrangements (Word, 36 KB)

Discussion: Deductions from electron scattering
Students should appreciate that the necessarily complex analysis of particle paths from deep electron scattering indicates that the neutron and proton are not simple point charges but contain simpler structure within them.

Electrons must be given very large kinetic energies to penetrate nuclei. Those energies can be sufficiently great that some of the energy can be converted (via E = mc2) into the mass of new particles. This results in the electrons losing energy – this is why the scattering is inelastic – to produce pions. Jets of pions are typical of the ‘events’ seen in particle accelerators colliding protons and anti-protons.

At this stage you could ask the open question: if, as indicated by deep inelastic scattering, neutrons and protons are each made of three particles called quarks, what’s the smallest number of quarks you need?

Obviously you could have any number of quarks, but there must be more than one, or else neutrons and protons would not be different. The simplest model would have two different types.

Assuming there are just two types of quark, then possibilities could be AAA and BBB (chargeless A and +e/3 for B) or AAB and BBA, which is actually correct, with A being d (-e/3) and B being u (+2e/3) The latter (correct) version also explains other particles.

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Episode 539: Deep inelastic scattering (Word, 48 KB)