Bubble Chamber Lesson 7
Introduction for the teacher: During the first part of the lesson we review some features of bubble
chamber pictures such as: recognizing beam tracks, identifying electrons, identifying interactions,
identifying neutral decays. We now show the bubble chamber picture. The pupils describe what
they see, trying to identify beam tracks and electrons. This part is used to introduce positrons and
photons, pair creation and Compton effect. The next part of the lesson is dedicated to
bremsstrahlung. In the last part the pupils may try their knowledge on a more complicated
picture.
Objectives:
Students learn about the positron. They discover its properties like mass and charge and learn
about antiparticles.
Students learn about different QED phenomena, i.e. pair creation, Compton effect and
bremsstrahlung.
Suggested lesson introduction:
The lesson starts by reviewing the different features of bubble chamber pictures that the students
should be familiar with from the former lessons. The most important features to be useful in this
lesson are identification of electrons and other particles (beam; protons) and of neutral decays.
After the repetition students see a projection of the linked bubble chamber picture. They will see
that this picture looks different from the pictures seen so far. Students shall describe the picture.
They might have difficulties in find beam tracks (what is not important for this lesson) and they
will identify electrons and misinterpret even some positrons as electrons.
Remark: Some students will notice here that there are different curved “electrons” in this picture.
This would be a nice way to start the following discussion. If not push the students to find out the
direction of the magnetic field (a uniform field is given), different students by looking at different
“electrons” (some of course looking at positron tracks) will find different answers. They might
assume that the magnetic field is not uniform but we will interrupt here and confirm that it is
uniform.
To avoid further problems in this point just tell the students the direction of the magnetic field
(which of course is known in an experiment) and let them work out the charge of some of the
particles (especially for these two). They will identify the electrons and some positive particles.
But what are these positive particles? The answer proton can be discussed by comparing known
proton tracks with the tracks here. If momentum of particles was introduced in earlier lessons
they can find out that these particles have a much lower momentum than a “usual” proton has. By
looking on many pictures one will find (except the sign of the charge) no difference between
electrons and these particles. It turns out that this particle really has the same mass as the electron
but positive charge. So we have really discovered a new particle. It is kind of a “positive charged
electron”, which is called positron. Pupils now can identify all positrons and electrons in the
picture.
It depends on the teacher whether he wants to give some more informations about antiparticles to
the students. It might be possible to report about the historic development that leads to the
discovery of the positron.
Now we want to learn where the positron is coming from. We focus on the arrangement of
positrons and electrons. Students discover that there are pairs of positrons and electrons and there
are “lone” electrons but no lone positrons. They remember the pictures they have seen about
neutral decays, so there must be a neutral particle coming from somewhere that “decays” into a
positron and an electron.
Depending on the knowledge of the pupils they can imagine or they are told that it is a highenergy photon that creates an electron positron pair. This event is called pair creation. It is only
possible if there is a nucleus nearby that carries away some momentum.
If students are familiar with the relativistic formula E = m c2 they can calculate the minimum
energy (about 1MeV) that is necessary for this event. It is furthermore possible to calculate that a
photon can’t create an electron positron pair without one more particle (nucleus nearby) that
carries some momentum away.
As we now know the photon as reason for the pair creation we can ask for the origin of the
photon. The students follow the direction where it came from. With the hint that its origin is quite
close to the pair creation point they can be guided to the closest electron spiral (picture). The
students understand that the photon was emitted by the electron with two consequences: the
photon itself produced later an electron positron pair; the electron lost energy and momentum.
The curvature of the electron track therefore differs before and after the emission of the photon as
to be seen in the picture.
The same discussion can now follow for one of the Compton electrons on the picture. (Reason for
the observable electron is a scattered photon, track of the photon before scattering, assumed track
after scattering). Here a positron had emitted the photon shortly before. Students learn the
emission of photons is a frequent event for fast electrons and positrons. Electrons will decelerate
by this process due to the loss of energy. The emitted radiation (photons) is called bremsstrahlung
(from the German words bremsen = decelerate and Strahlung = radiation).
The pupils can now look for all the other pair creation and Compton electrons on this picture.
Depending on time and curriculum teachers can give more information about the probabilities for
photons to occur pair creation or Compton effect (may be even photo effect is included to this
discussion) depending on photon energy or/and surrounding material.
One more interesting event is recorded on this picture: A cosmic ray entered the bubble chamber
short before the picture was taken and left a braod track (including one electron track) from up to
down in the picture.