Episode 534: Antiparticles and the lepton family

The purpose of this episode is to introduce the lepton family, and also to bring in the idea of anti-particles, which annihilate when they meet particles.

If you have had students research the questions at the end of episode 533, time must now be allowed for them to feed back what they have found. You may need to supplement their finding with some of the details mentioned below.

Leptons

Summary

  • Student presentations: Information about leptons (15 minutes)
  • Discussion: PET scans (10 minutes)
  • Student activity: Examining particle tracks (10 minutes)
  • Discussion: Summarising the main points (5 minutes)
  • Student activities: Readings (20 minutes)

Student presentations:Information about leptons
Your students should present their findings in response to the questions posed at the end of episode 533. Important points to establish:

Positron e+

Dirac’s theoretical prediction of the antiparticle to the electron is too difficult to elaborate here, but Carl Anderson’s discovery of it in cosmic rays – he discovered the muon a few years later in the same way – is worth describing.

The original cloud chamber track of Carl Anderson’s positron is shown in this famous photograph (Projecting this using a digital projector makes for more dramatic discussion.):

Discovery of the positron

Episode 534-1: Anderson’s positron photograph (Word, 63 KB)

The particle is moving up the photograph. It has been slowed down by passing through the lead plate across the centre, and the curvature of the path is caused by a magnetic field. At this stage, it’s enough to say that the particle is curved more when it is slower because the particle spends longer in the magnetic field.

Anderson could deduce, from the direction and magnitude of the curvature and the length of the particle track, that the particle was positive and had a mass not more than twice that of an electron.

The positron was the first anti-particle discovered: since then it has been found that every particle has its antiparticle.

Muon
The muon quote (‘Who ordered that?’) was from physicist Isadore Rabi – it’s whimsically supposed to be the sort of thing you say in a Chinese restaurant when you get some strange dish you don’t recognize. The muon was a problem because it had exactly the mass predicted for Yukawa’s meson, but it didn’t undergo strong nuclear interactions at all, which the meson had to do (that was its job, after all!). It turned out to be a heavy type of electron. Like the electron, it has an anti-particle (the anti-muon, m+) which is positively charged.

Neutrinos n
The problems with beta decay are worth describing in detail. Reactions such as carbon-14 ® nitrogen-14 + b- were expected to produce beta particles with identical kinetic energy: this is what happens in alpha decay. This does not happen in beta decay; sometimes a lot of the energy seems to be missing.

In 1930 Wolfgang Pauli suggested, in a famous letter to fellow physicists starting ‘Dear Radioactive Ladies and Gentlemen’, in which he wrote ‘I’ve done something terrible: I have predicted an undetectable particle’. He suggested that the ‘lost’ energy was carried away by a new particle, which must be chargeless and have virtually no mass. Enrico Fermi developed the theory of this new particle, which he called a neutrino, but it wasn’t until 1951 that Reines and Cowan discovered it at the Savannah River nuclear reactor. Current (2005) thought is that the mass of the electron neutrino is in the range 0 < mass < 3 eV/c2 (compare with the electron, me = 0.511 MeV/c2).

Like the electron and the muon, neutrinos have antiparticles. Furthermore, there are different neutrinos associated with the electron and the muon.

Because these light particles do not experience the strong force of hadrons, they form a different category of particle and given the name leptons.

There are now a total of 12 leptons: the electron, the muon, and a super-heavy version called the tau (t); a neutrino for each of these three; and six antiparticles for these six particles. The six leptons each have a lepton number of +1, while the six anti-leptons each have a lepton number of -1.

Discussion: PET scans
Take a look at PET scans and how they are made.

When a positron meets an electron, they annihilate to produce a pair of gamma ray photons, each of energy 511 keV. (Both the electron and the positron have a mass of 511 keV/c2.) This principle is used in medicine, in Positron-Electron Tomography (PET) scans. A radiochemical emitting positrons is injected into the body. When the chemical reaches the organ of interest, positrons emitted very soon meet electrons and annihilate. The scan reveals exactly where the radiochemical is by looking for a pair of gamma photons travelling in opposite directions.

You may like to ask students how they think radiochemicals which emit positrons are made: they can be led to realize that the unstable nuclei lose positive charge when a positron is emitted, and so have too many protons. This suggests that you have to fire protons into the nucleus, which is one way this is actually done.

Episode 534-2: Making PET scans (Word, 4 MB)

Student activity: Examining particle tracks
Examining particle tracks: This can be done as class discussion with a digital projector, as suggested for the Anderson photograph of the positron track shown above, or students can work individually or in pairs, using printed copies of the images or looking at them on computer screens. If you adopt the latter approach, give a little more time for the activity: it could be a homework activity. The questions are intended for students late in the post-16 level course, so you should concentrate on simple patterns:

Pair production

Gamma photons are not very ionising, so tend not to leave tracks in bubble chambers or cloud chambers.

Gamma photons of enough energy (2 × 511 keV) can produce an electron-positron pair (provided they are near nuclei at the time: don’t emphasize this point.)

Particles and anti-particles – protons and antiprotons in this case – can annihilate with production of energy, and also ‘new’ mass in the case of big particles.

In a magnetic field, charged particles follow curved paths, with opposite charges curving in opposite directions.

Episode 519-3: Particle tracks (Word, 612 KB)

Episode 534-3: Annihilation and pair production: Bubble chamber pictures (Word, 5 MB)

Discussion: Summarising the main points
Establish the main points of this episode:

The electron is one of a small family of fundamental particles called leptons, which are quite different from the nuclear particles (hadrons) of episode 533.

Particle have anti-particles, with opposite value of charge, lepton number and (by implication) baryon number and strangeness as well).

Student activities: Readings
Here are a number of supplementary readings that you may care to use to broaden students’ background knowledge:

Episode 534-5: The discovery of beta decay (Word, 35 KB)

Episode 534-6: Three poems about particles (Word, 29 KB)

Episode 519-3: Particle tracks (Word, 613 KB)

Download this episode
Episode 534: Antiparticles and the lepton family (Word, 5 MB)