Episode 539: Deep inelastic scattering
Lesson for 16-19
- Activity time 30 minutes
- Level Advanced
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.
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 accelerated to very high speeds to penetrate nuclei. The energy involved is so great that some of it can be converted (via E = m × c 2) into the mass of new particles. This results in the electrons dissipating 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 +e3 for B) or AAB and BBA, which is actually correct, with A being d (-e3) and B being u (+2e3) The latter (correct) version also explains other particles.