High School Teachers



What is particle physics? 

The aim of particle physics is to study the basic building blocks of matter and the forces they exert on each other. The basic idea behind many experiments has been the following: 

By shooting something you know into an object, and measuring what comes out, you can infer something about what went on inside. There is nothing special about this approach - you do it when you have an X-ray! 

In particle physics, one is interested in finding out what a neutron or proton is made of. We shoot in a beam of particles from a machine called an accelerator and measure what comes out in a detector. 

What actually happens is controlled by the forces that act between the particles, the rules of quantum mechanics and relativity, and conservation laws. 

Here we discuss the bubble chamber, a detector that was invented by Donald Glaser in 1952, and which served the particle physics community with distinction for about 40 years. 

Bubble chambers have recently become extinct. They could not cope with the huge event rates of current fixed target experiments; nor could they be used with colliding beams. Nevertheless, like dinosaurs, they are remembered fondly! They served the particle physics community for 40 years. 

As experiments grew, needing millions of photographs to address current issues, larger groups of (about 10) collaborating laboratories emerged, paving the way for the huge collaborations of today's experiment - which typically involve about a thousand scientists from over a hundred laboratories around the world. 

Bubble chambers are particularly remembered for their enduring images, which not only have a beauty in their own right, but which also demonstrate in a believable way the `reality' of esoteric phenomena taking place in a few billionths of a second. LINK. 

The bubble chamber 

If two aeroplanes with vapour trails behind them were to approach each other, circle around, and then go their separate ways, the fact that they had done so would be apparent for quite a while. A permanent record of the encounter could be obtained by taking a photograph of the vapour trails. 

The bubble chamber consists of a tank of unstable transparent liquid - often superheated hydrogen (which provides a source of proton targets). - in which passing charged particles initiate boiling as a result of the energy they deposit (by ionizing atoms) as they force their way through the liquid. (We see here that the bubble chamber is both target and detector: the protons are the target studied; the electrons `detect' the passage of charged particles - via the Coulomb interaction which ionizes the atoms.) 

The few eV needed to ionize the atoms is small compared with the energies of the particles involved in the interactions, and so the particles are not deviated much from their curved paths in the magnetic field in which the bubble chamber is placed. ( Electrons are an exception LINK.) 

Click here to find a detailed discussion about processes in the liquid.

Briefly, the bubble chamber works as follows: 

  1. The liquid is prepared and held under a pressure of about 5 atmospheres. 
  2. Just before the beam arrives from the accelerator the pressure is reduced to about 2 atmospheres, making the liquid sensitive to charged particles. 
  3. The beam particles pass through the bubble chamber, some interacting, in a few nanoseconds. By the end of the bubble chamber era, bubble chambers provided a path of up to about 4 metres. LINK to pictures.
  4. The bubbles formed are allowed to grow for a few ms, and when they have reached a diameter of about 1 mm, a flash photograph is taken (on several views so as to enable the interactions to be reconstructed in 3-dimensions). 
  5. The pressure is increased again to clear the bubbles and await the arrival of the next beam of particle. 

It is worth mentioning that many interactions produce photons (usually via the decay of pi0 particles within the first bubble), which, being neutra,l leave the bubble chamber undetected. Occasionally, however,a photon, in the Coulomb field of a proton nucleus, `materialises' into a positron-electron pair. bDue to their low mass, both positrons and electrons lose energy rapidly by a process known as `bremsstrahlung'; this leads to an inwardly spiralling track by which these particles are recognised. 

Since the probability of a photon producing an e+e- pair is inversely proportional to the square of the charge on the nucleus causing it, some experiments have used liquids of higher atomic number than hydrogen; for example, a mixture of neon and hydrogen.


CERN and High School Teachers Programme at CERN
Last modified: 25 July 2001