Identify tracks of subatomic particles from their ‘signatures’ in bubble chamber photos – a key 20th century technology for studying particle physics.
What is the Universe made of? What holds it together? How will it evolve? Particle physicists are fascinated by these big eternal questions. By studying elementary particles and their fundamental interactions, they try to identify the puzzle pieces of the Universe and find out how to put them together. Our current understanding is summarised in the Standard Model of particle physics, one of the most successful theories in physicsw1.
But how do we know anything about these particles, all of which are much smaller than the atom? From the 1920s to the 1950s, the primary technique used by particle physicists to observe and identify elementary particles was the cloud chamber (Woithe, 2016). By revealing the tracks of electrically charged subatomic particles through a supercooled gas, with cameras used to capture the events, researchers could work out the particles’ mass, electric charge and other characteristics, along with how they interacted. However, in 1952 the bubble chamber was invented, and this soon replaced the cloud chamber as the dominant particle detection technology. Bubble chambers could be made physically larger, and they were filled with a much denser material (liquid rather than gas), which made them better for studying high-energy particles.
Today, both cloud chambers and bubble chambers have largely been replaced by other types of detector that produce digital signals and work at a much faster rate. So while photos from bubble chambers are no longer the technology of choice for professional physicists, they can still enrich the discussion of particle physics in the classroom.
The key component of a bubble chamber is a superheated liquid. When electrically charged particles pass through a bubble chamber, they ionise the molecules in the chamber medium. The ions trigger a phase transition and the superheated liquid vaporises, creating visible tracks as bubbles form along the particle’s path. Once the newly formed bubbles have grown large enough, cameras mounted around the chamber capture the event.
Importantly, a uniform magnetic field runs through the chamber, which produces a force on moving electrically charged particles, making them move in curved paths – and creating ‘signature’ shapes for different particles. Measuring the radius of curvature allows a particle’s momentum to be calculated, providing further clues to its characteristics.
We have developed several activities for advanced high-school students, in which they study bubble chamber photographs and try to work out for themselves what they show. You can find our original worksheet describing these activities (including solutions and additional information for teachers) on the CERN websitew2.
The photographs were produced by the 2 m-long bubble chamber at CERN in 1972. This chamber was filled with 1150 litres of liquid hydrogen cooled to 26 K (–247°C). In its 12 years of operation, 20 000 km of photographic film were produced to capture the particle collisions.
In this article, we present three simple but intriguing activities for students aged 16–19 as an introduction to particle track analysis, using the bubble chamber images. Before starting the activities, students should be familiar with the basics of particle physics (especially the properties of protons, electrons, positrons, photons and, if possible, neutrinos).
The first two activities focus on the identification of some typical particle tracks based on the behaviour of electrically charged particles in magnetic fields. Activity 3 builds on these activities to look at particle transformations. Depending on students’ prior knowledge, it will take approximately 1 hour to carry out all the activities.
Note: when working with bubble chamber pictures, high-resolution images are crucial to allow the identification of individual tracks. You can download full-sized versions of the images in the additional materials section of this article.
In this activity, students identify the electric charge of particles based on the direction of curvature of their tracks in a magnetic field. They also compare the speed of electrons, based on the radius of curvature of their tracks.
As a preliminary to this activity, students will need to understand the following facts relating to particle physics:
For this activity, the only materials needed are the images and information in figures 1, 2 and 3. Each student or group of students will need colour printouts of the two bubble chamber photos (figure 2 and figure 3), which can also be downloaded from the additional materials section. In all the bubble chamber images, the particles enter the chamber from the left, and the magnetic field points out of the page.
Ask the students to work through the tasks below, using the materials provided.
These rules are shown in figure 1.
Remember, the particles enter the chamber from the left, and the magnetic field points out of the page (on a printout).
The answers are as follows:
Because the blue track curves downwards in the photo in photo 1, there must have been a force pointing downwards. Now we try both the left-hand and right-hand rules with the following information taken from the photo:
This configuration of fingers works only with the right hand, thus the blue track was caused by a positively charged particle.
The red track curves upwards in the photo in photo 1, so there must have been a force pointing upwards, which leads to the following configuration of fingers:
This configuration works only with the left hand, thus the red track was caused by a negatively charged particle.
Why do particles leave spiral tracks in a bubble chamber? On their way through the liquid, electrically charged particles constantly lose kinetic energy – for example, because they ionise the hydrogen molecules on their way. A lower kinetic energy then leads to a progressively smaller track radius in a magnetic field.
In photo 2, the same procedure identifies tracks 1, 2 and 3 as belonging to negatively charged particles, whereas track 4 was caused by a positively charged particle.
For the speeds, track 2 belongs to the electron (with the highest speed), followed by track 1, then track 3 (with the lowest speed). This is because the lower the speed of the particle, the smaller the radius of curvature of its track. This relationship can be derived for particles as follows:
The force on the electrically charged particle (charge q) moving with speed v perpendicularly to a magnetic field B is described as:
FL = q x v x B
This force acts as centripetal force, Fc , and leads to a circular particle track with radius r. The centripetal force needed to keep an object on a circular path with radius r depends on the mass m of the object, and the square of its speed v, thus:
Fc = m x v2/r
So FL = Fc
Therefore: q x v x B = m x v2/r
So r = (m x v)/(q x B)
Thus the radius of curvature is directly proportional to the speed of the particle.
Note that we are assuming here that the particles are non-relativistic, i.e. they are moving much more slowly than the speed of light. However, tracks in bubble chamber photos are typically made by relativistic particles moving at speeds close to that of light. In this case, there is a relativistic factor that changes this relationship.
In the next activity, students use their understanding of track characteristics to identify specific particle ‘signatures’ (track types) in bubble chamber photos.
Three different types of track are shown in table 1, together with the particle identities, signature descriptions, and explanations in terms of the processes that produced the tracks. This information enables students to identify particles in the bubble chamber images that follow.
For this activity, the only materials needed are the images and information in table 1 and figure 4. In all these images, the particles enter the chamber from the left, and the magnetic field points out of the page.
|Description||Upward-curving track, starting at another visible particle track||Downward-curving track (positron) starting ‘out of nowhere’, together with an upward-curving track (electron)||Downward-curving track, starting at the visible track of another particle|
|Production process||An electrically charged particle enters the chamber and interacts with an electron in the liquid.||A photon transforms into an electron-positron pair. (The photon does not leave a track.)||An electrically charged particle enters the chamber and interacts with a proton in the liquid.|
Ask the students to tackle the following tasks using the materials provided.
|Upper blue track|
|Lower blue track|
The correctly completed table is shown in table 3.
|Green track||✓||Track curves upwards.|
|Upper blue track||✓||Track curves upwards.|
|Lower blue track||✓||Track curves downwards and appears together with an electron track.|
|Purple track||✓||Track curves downwards and starts at another track.|
Interpreting particle transformations is what made bubble chambers famous: many of the particles produced in a bubble chamber are not stable, but transform in time to other particles. However, working out transformation events is usually more difficult for students than simply identifying specific track types, because it requires additional knowledge about the fundamental interactions described by the Standard Model of particle physics. We suggest using a simple example (figure 5) and providing step-by-step instructions to work through.
Again, the only materials needed are the image in figure 5 and the information provided previously. As usual, the particles enter the chamber from the left, and the magnetic field points out of the page.
Ask the students to answer the following questions, using the materials provided.
Which type of pion caused the green track? Explain your answer.
The answers are as follows:
More information about this transformation and the Feynman diagrams that can help to understand this process can be found in the CERN student worksheetw2.
As these activities show, bubbles chamber images are a great way to make particle physics accessible to high-school students. Using these images, students can discover the identity of particles by working out their characteristics for themselves.
Meanwhile, the ongoing endeavour to understand the missing puzzle pieces of our Universe continues. And as well as helping to train the particle physicists of tomorrow, bubble chambers have recently found a new role in current research: the detection of dark matter particles – for example, in the PICO project in Canadaw3. Here, the bubble chamber’s relatively slow rate of response (compared to new digital technologies) is not a problem, because – unlike the cascades of particles produced each second at CERN – no signals from dark matter have yet been detected.
Finally, for a completely different approach to particle physics, the unique spirally patterns found in bubble chamber pictures can inspire a range of artistic ideas – from Christmas decorations made using the technique of paper quilling to fabric design. What other fun ideas can your more creative students come up with?