Basic Guide to Bubble Chambers

How do bubble chambers detect particles?

A bubble chamber often contained liquid hydrogen. Charged particles entering the chamber would interact with the electrons transferring some of their energy via the Coulomb force. This initiated boiling and thus led to bubbles being formed. The electrons of the hydrogen atoms thus acted as the “detectors” within the chamber. Only charged particles cause tracks in bubble chambers, as neutral particles will not interact via the Coulomb force.

However, we do not just want to see a beam of particles passing through a chamber. We want to see what happens when nuclear particles interact with each other. The beams of particles interact with the protons of the liquid hydrogen in the bubble chamber and so the bubble chamber contains both the “detector” particles and the “target” particles.

How are the particles moving?

The beams of particles are parallel to begin with. Some may then undergo collisions and the path will change. So you need to look for beams that do not go directly from one side of the picture to another.

In this case the red beam track is the only one of the parallel tracks that does not continue through the chamber, showing that it has collided with a proton. Two tracks emerge from the collision.

What charge is the original beam?

If we are not told what the original beam is we may sometimes be able to tell what its charge is from just looking at the picture.

A visible beam has to be made up of charged particles. These will be bent within a magnetic field and so the direction of curvature will tell you the charge of the particle. Often a beam will collide with an electron, knocking it on with more energy than usual. This then leaves a track of its own within the detector that is quite distinctive.

Example of a knock-on electron (or delta ray).

There are often a few examples of this appearing on a picture and this shows the way that the negative charges are bent in the magnetic field. You can then compare this with the direction of curvature of the particle beam to determine the charge of those particles. Using Fleming's left hand rule you can also deduce the direction of the magnetic field in or out of the picture.

Knowing the direction of the magnetic field you can then find the charges of all particles.

Nature has been kind! The particles we observe are either plus or minus the charge of the electron.

Why are some tracks darker than others?

The darkness of the track depends purely on the velocity of the particle. The slower it is going the more time it has for the Coulomb force generated to interact with the nearby detector electrons and so the more bubbles will be produced. A fast particle will not have time to produce significant effects on the electrons and so it will produce less bubbles and the path will be fainter.

Sometimes you will observe a path that becomes very dark and then disappears. This would indicate a particle gradually slowing down and then stopping.

Track t is considerably darker than the beam tracks and stops in the bubble chamber. This is a typical track for a proton.

Why are some tracks straighter than others?

The straightness of the tracks is related to the momentum of the particle. This in turn is related to both the mass and velocity of the particle. The greater the momentum the straighter the path will be. A very curved path will indicate that the particle has low momentum and a straight path that it has high momentum.

It is important to remember that the radius of curvature is not dependent purely on velocity or mass, but momentum.

The two tracks going left from point e have similar curvatures indicating similar momentum. However, the lower track is much darker showing lower velocity. It is therefore a particle with larger mass than the upper particle and is most probably a proton. The upper particle is probably a pion.

Do the particles interact or decay?

Whenever two particles interact or a particle decays the law of charge conservation needs to be obeyed. The total charge of the starting states has to equal the total charge of the ending states. This can be used when determining whether an event is an interaction or decay.

In this case one charged particle enters the event and two charged particles leave it. This would appear to violate charge conservation. This can be accounted for, however, by the presence of a proton in the chamber. This is an example of a collision of a negative particle with a stationary particle, the positive proton. So charge within the event is conserved.

  Now we can look at an example of the decay of a charged particle to see how this differs.

As incoming and outgoing tracks have the same charge, this is a decay; there is no need to invoke the proton. You must remember that, in order for momentum to be conserved, at least one extra neutral particle must have been produced in the decay.

When you see a beam track that has a 'kink' (sudden change in curvature) like the one above at some point along its trajectory this indicates a decay of a charged particle.

The 'kink' produced by the decay may be very pronounced or can be very slight.

This picture shows a track with 2 kinks indicating 2 points of decay. The particle coming in from the right is a p+ that stops and then decays as follows:   muon track is very short because it does not have much kinetic energy when emitted from a stationary pion; in hydrogen a muon from the decay of a stopping pion only travels about 1.1cm before stopping and decaying into an electron.     typical spiralling signature

You can often see tracks starting in the middle of the picture with no apparent particle track leading up to it. This would show an event that was initiated by a neutral particle. When deciding whether the tracks were the result of the decay of a neutral particle or the collision of a neutral particle with a charged particle within the bubble chamber use the same principles of visible charge conservation that were used before.

This kind of event is commonly called a ‘vee’ and a useful exercise when you observe a ‘vee’ is to see if it lines up with a previous event in the bubble chamber. This could then provide evidence for the production of a neutral particle that left the interaction point without producing a track and then decayed into two other charged particles.

This picture shows how the observed ‘vee’ could be lined back up to show that it had come from a previous ‘kink’. This helps to identify the neutral particle produced and adds to our knowledge of the particles produced at the initial interaction.

Why was more than one picture taken?

When the original bubble chamber pictures were taken, two or more photos of the same event were taken, from different angles. This is because the event is happening in 3 dimensions and so just one 2 dimensional picture will not give us all the information we need to understand what is happening.

There are at least two situations in which more than one view of the event is needed to interpret it visually.

When a particle track just stops in the middle of the picture this could be because the particle has stopped or because it has moved into the glass of the bubble chamber window. You can check for this using two pictures by comparing the position of the track with the crosses marked on the glass as reference points. If the path stops in the same place on all the pictures with respect to these points then the particle has moved out of the chamber. If the path stops at a different place with respect to the reference points then the particle has stopped within the chamber.

The second situation in which more than one picture is necessary is when two events seem to happen at the same point in one picture. 

How do we actually identify particles?

When studying the interactions and existence of different strongly interacting particles we obviously need to be able to identify the exactly which particle is producing each track. This is done by using the information gained on charge and momentum (and, to a lesser extent, velocity and kinetic energy) directly from the particle tracks as well as the type of decay and collision patterns that the particle produces. The use of energy and momentum conservation is also essential.

These can be put together to show the characteristic signatures of particles in the bubble chamber.

As we go on you will see that the signatures of some of the particles are quite complicated and involve different steps. These steps do not always happen within the bubble chamber and so it is not always possible to identify all the tracks, just narrow it down to a few possibilities.