Tracking Subatomic Particles

The tracks of elementary particles were first made visible in particle detectors called bubble chambers. They were invented in the early 1950s and thereafter used for research mainly in the 1960s and 1970s. Scientific analyses of the particles were based on photographs of the tracks. Bubble chambers were replaced in the early 1980s by electronic detectors, which were able to feed their information directly to computers.

The Functional Principle

A huge vessel is filled by a heated transparent liquid, very close to boiling point. During the experiment a piston is slowly pulled out of the bubble chamber to decrease the pressure within. This brings the heated liquid to a meta-stable phase. If a charged particle then crosses the chamber, some of the surrounding liquid’s molecules become ionized along the track and the liquid around these ions is vaporized, forming microscopic small bubbles. The bubbles continue to grow in size while the piston is being retracted. At the end of the experiment the tracks remain visible as dotted lines and can be photographed from different angles.

 Consequent scientific analysis is based upon the photographs. The size of the bubbles can be measured and is proportional to the specific energy loss of the corresponding particle and from which the particle type may also be inferred. Additionally the entire bubble chamber is located within a strong magnetic field that bends the tracks of charged particles. The radius of the track can be measured, and this is proportional to its energy. Since subatomic particles often have a limited lifetime before they decay into other particles, the length of their path is proportional to their lifetime.

The Beauty of Bubble Chamber Pictures

Bubble chambers make the footprints of invisible particles visible and have inspired the imaginations of many researchers. For analytical purposes physicists normally shoot particles into the bubble chamber, that is by placing a radioactive source close to or within the chamber or by directing a particle beam at the chamber. The subsequent pictures visualize their route and the route of their products of decay through the chamber.

Cosmic particles falling from space to earth frequently stray into the chamber, their tracks overlaying the picture. Additionally elementary particles can spontaneously occur from and consequently vanish into the vacuum, their tracks also overlaying the picture.

Since subatomic particles “feel” the existence of other particles (e.g. charged particles attract or repulse each other, or the decay of a particle is influenced by others surrounding it), they all interact with each other, with their daughter particles, with cosmic rays and with the variety of particles which frequently exist within the vacuum, like a very complex cosmic dance.

Dr. Uwe Flagmeyer, High-Energy Physicist at CERN 1995—2002

© Sabine Bürger 2015