Bubble Chamber Lesson 1
Prior knowledge required


Current (moving charged particles) in magnetic field experiences a force
Bubble Chamber (optional, can be covered in this lesson)
Duration of lesson

50 minutes
Level of students

GCE ‘A’ Level or equivalent (17 – 18 years old)
Lesson Objectives



Students can understand that tracks in a bubble chamber are produced by
collisions between incoming particles and electrons in atoms.
Students can identify collisions between incoming beam particles and protons in a
bubble chamber.
Students can describe the tracks made by electrons and protons in a hydrogenfilled bubble chamber.
Introduction
Bubble chamber photographs provide an insightful introduction to the exotic short-lived
particles that emerge from all high energy accelerator experiments. When we turn on the
LHC at CERN in 2008 to study proton-proton collisions at the highest energies ever, most
of the particles that the physicists will detect in their modern electronic detectors will be
those first identified in bubble chambers and their precursors, cloud chambers and
photographic emulsions.
How bubble chamber works
When charged particles force their way through superheated liquid hydrogen (other
superheated liquids may be used) in a bubble chamber, they leave behind trails of tiny
bubbles.
The energy to start bubble formation comes from electrons that have been knocked out of
hydrogen atoms by the passing charged particles. When the bubbles have grown to
approximately 1 mm in diameter, flash photographs from different directions are taken.
This is needed to enable the tracks to be reconstructed in 3-dimensions.
Quantitative analysis 1: Does a particle slow down in a bubble
chamber?
A charged particle travelling at nearly the speed of light deposits about
25 keV mm1 – negligible amount of energy loss when compared with the
kinetic energy of 4.2 GeV of the passing K  beam particles in photograph 1.
For more details of how the energy is deposited, click here.
Unless otherwise stated, all photographs show tracks of particles entering a liquid
hydrogen bubble chamber placed inside a large magnet with the magnetic field
perpendicular to the page.
Photograph 1
Photograph 2
Description of Photographs
Photograph 1 shows four tracks of K  (mass of negative kaon is 493.7 MeV/c 2 ; mass of
proton is 938.3 MeV/c 2 ) entering the bubble chamber from below. The left-most K  beam
particle gives an electron such a big kick that the electron itself was able to make its own
little spiralling track.
Fact…
Except for the amount of energy involved, this electron is produced by the
Coulomb interaction, as is the case for the electrons that just produce the
tracks.
Of greater importance to physicist is to study the collision between K  and a proton which
is produced by the strong (nuclear) interaction. The short dark stopping track (sometimes
very short depending on the energy of the proton – it stops when it has lost all its energy
ionizing atoms and making bubbles) on the left-most track is made by a proton that was
struck (gently) by a K  beam particle. Notice that it curves in the opposite direction to the
spiralling electron track.
The remaining three K  beam particles – although one may legitimately claim that their
tracks are not strictly parallel – pass through with no apparent interactions.
Photograph 2 shows a particularly well-defined spiral track (and many other smaller ones)
produced by an electron that was struck by one of twelve passing K  beam particles
entering from below. Due to its small mass, the electron rapidly loses its energy due to
bremsstrahlung effect, and hence produces spiral track with decreasing radius.
Suggested Teaching Strategy
Key point…
In particle physics experiments, beams of particles with known
characteristics are fired at a known target, physicists then can analyze what
happens when these particles interact.
Go through the workings of a bubble chamber if your students do not have any prior
knowledge in this area.
Show the class photograph 1 and 2 on an overhead projector. Give a short introduction to
the lesson based on the above.
You may say…
“Photograph 1 and 2 show experiments performed to analyze the properties
of fundamental particles. With photographs like these, physicists are
uncovering the most fundamental properties of matter. Our objective today is
to use basic physics principles to uncover the story that this photograph
tells.”
You may ask…
“What are the fundamental particles?”
… Answer
Quarks (up, down, charm, strange, top and bottom), leptons (electrons,
muons and taus), and gauge bosons (photons, W and Z bosons and gluons).
You may say…
“It may be seen in later lessons that collisions like these can produce some
of the new unexpected particles that eventually led to the discovery of
quarks – the building blocks of neutrons and protons.”
Students begin by describing what they see in each photograph as a class. Repeat for
photograph 2.
Useful Tips…
Some tracks appear straight on first inspection. However, observing these
tracks at grazing small angles will reveal that they actually have slight
curvatures, indicating that these are tracks produced by charged particles.
Distribute the worksheet to your students. Divide your class into two groups so that each
group can answer the questions in the worksheet using one of the bubble chamber
photographs. Emphasize that every mark on the photographs must be considered to be
meaningful. Your students will nominate a representative to present their answers to the
other group at the end of their discussion.
By using classical physics analogy – the trajectory of cannonballs – you can ask your
students to imagine that the lines represent the paths of cannonballs. You may ask the
following guiding questions when they work on the worksheet.
Guiding questions
How many cannonballs were
fired?
What physical quantities might be
used to describe a moving
cannonball?
Possible response for photograph 1
Four
Do the tracks curve?
Why do they curve?
Yes.
There must be forces acting on them.
[Lorentz force on charged particle
moving in magnetic field will be covered
in lesson 2.]
The 3 right-most ones continue
undeviated (except for a gentle curving
to the right – hard to see).
The left-most one seems to strike two
objects:
 shown by the spiral; this seems a
very gentle collision because the
cannonball continues along its
original path.
 shown by the dark stopping track;
this collision is more significant
because the cannonball is deflected
to the left slightly.
What happens to the
cannonballs?
Mass*, velocity*, kinetic energy,
momentum.
*these quantities are not measured
directly (see later lesson)
Useful Tips…
Video clip – collisions using billiard balls.
Quantitative Analysis 2: Why are there so few particle
interactions?
A collision of a charged particle with the nucleus of a hydrogen atom is
entirely random – a bit like hitting a very small bulls-eye with a random throw
at the dart board. The probability of success is equal to the ratio of the area
of the bulls-eye to the area of the board.
You may ask…
“Estimate the number of throws of the dart needed before it hit the bullseye?”
… Answer
The following shows a simple calculation of the number of interactions we
expect to see for each charged particle travelling in a typical bubble chamber.
Relative size of proton to hydrogen atom,
rp
rH atom

1015
 105
10
10
[An analogy would be the proton is the size of a pinhead in an atom the
size of a football field!]
Probability of hitting any proton 
 rp 2
r
2
H atom
 10 10
0.5
 5  109
10
10
(The length of each bubble chamber photograph is about 0.5 m)
No. of hydrogen atoms per track along the 0.5 m path 
Number of collisions per track  5  109  1010  0.5
Roughly speaking, one would expect one collision with a proton for every two
tracks.
Worksheet Discussion
Photograph 1
Explore the two special interactions on the left-most track – your students should note that
there are differences: the path of the incoming particle does not appear to change after
the first interaction but does so at the second. Verify that the particle veers left.
You may ask…
“Would you agree that both collisions were gentle? Which is the less
gentle?”
… Answer
The first collision is a very gentle collision because the cannonball continues
along its original path while, after the second collision, the cannonball is
visibly deflected to the left.
Fact…
Particle physicists call gentle collisions ‘peripheral’.
Photograph 2
In addition to the many small spiral tracks produced by electrons knocked out of atoms,
one spiral track stands out as the electron was given a larger “kick” by the incoming beam
particle.
For both
Examine the direction of curvature of all the lines. Positive particles curve to the left while
negative particles curve to the right.
[This may not be true for all the bubble chamber photographs as the direction of magnetic
field may be different.]
At the End of the Lesson
The students can
 appreciate that the production of bubbles is the detection principle that enables
physicists to study the particle emerging from a collision between a beam particle
and a nucleus.
 recognize all spirals electron tracks in a bubble chamber.
 recognize all short dark stopping proton tracks in a bubble chamber.
 recognize that a proton track curves in the opposite way to that of the electron.
The motion of charged particles in magnetic field will be discussed in Bubble Chamber
Lesson 2.
You may say…
“Both photographs show hundreds of occurrence of the K  beam particles
gently knocking electrons out of the atoms to start up boiling and bubble
formation.”
“There are many examples of K  beam particles giving electrons in atoms
such a big kick that these electrons were able to make their own tracks.
These interactions are electromagnetic in nature.”
“In photograph 1, there is one example of what a physicist is interested in
studying – a K  hitting a proton – the nucleus of the hydrogen atom. This is a
strong interaction”
Further Discussion
You may ask…
“Which physics principles/concepts might be applied to understand what is
happening?”
Link to Worksheet for Lesson