05.4b - Bubble Chambers

Where do the bubble chamber pictures come from?

From the 1920s to the 1950s, the primary technique used by particle physicists to observe and identify elementary particles was the cloud chamber. 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Today, both cloud chambers and bubble chambers have largely been replaced by other types of detectors that produce digital signals and work at a much faster rate.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.

This activity is based on images recorded by the 2 m bubble chamber at CERN on 10 August 1972. The bubble chamber was exposed to a beam of protons from CERN’s proton synchrotron PS with a momentum of 24 GeV/c. The magnetic field of 1.7 Tesla is pointing out of the page for all images. This bubble chamber was 2 m long, 60 cm high, and filled with 1150 litres of liquid hydrogen at a temperature of 26 K (-247°C). After its closure, this bubble chamber has been donated to the Deutsche Museum in Munich. The original pictures as well as the pictured with coloured tracks can be found online: https://cds.cern.ch/record/2307419

A large cylinder is filled with a liquid at a temperature just below its boiling point. Then, the pressure inside the cylinder is lowered by moving a piston out to increase the chamber volume. In this way, the liquid enters a metastable phase, the so called superheated state. Any disturbance will now cause the creation of bubbles when parts of the liquid enters the gaseous state. High-energy electrically charged particles leave a track of ionized molecules when penetrating the chamber. These ions will trigger the vaporization process, and a line of bubbles will form along the particle track. Once the newly formed bubbles have grown large enough, cameras mounted around the chamber capture the event. Afterwards, the piston is moved inwards again to increase the pressure and get rid of the produced bubbles to make the chamber ready for the next particles. A magnetic field penetrates the chamber to allow momentum measurements through the radius of curvature of the deflected particle tracks.

Describe the difference between the two pictures and explain why both phases are needed to operate a bubble chamber.