# What Do Gargamelle and Picasso Have in Common?

Nope.  Nothing to do with the arch-nemesis of the Smurfs or with an avant-garde artistic masterpiece, unlike the top picture appears to suggest…  Actually, the Gargamelle on the left is at CERN and takes its name after the giantess in the works of satirist François Rabelais: she was Gargantua’s mother!  The Gargamelle is a historical ‘bubble chamber’ detector however…

# The Principle of the Bubble Chamber

1952, US physicist Donald A. Glaser invents the bubble chamber.  1954, the first tracks are observed in John Wood’s 1.5-inch (~ 3.8 cm).  Don Glaser goes on to win the 1960 Nobel Prize in Physics, aged just 34.  The bubble chamber can be used to detect the motion of electrically charged particles flowing through it.

A bubble chamber is created by filling up a large cylinder with a transparent liquid heated to just below its boiling point.  Typically, liquid hydrogen is used.  As electrically charged particles flow into the vessel, a piston is activated to decrease the inner pressure of the bubble chamber.  At this point, the liquid enters a superheated state.

Charged particles create ionisation tracks, around which the superheated liquid vaporises and generates a trail of microscopic bubbles.  Around a track, the bubble density is proportional to a particle’s energy loss.  As bubbles expand when the chamber space increases, they become large enough to be seen by an observer or photographed by a camera.

The bubble chamber is permeated by a constant magnetic field, causing the charged particles to travel in helical paths, with a radius determined by their charge-to-mass ratios and velocities.  Assuming the charge magnitude of all known long-lived subatomic particles is comparable to that of an electron, the radius of their curvature is proportional to their momentum.  As a result of measuring the radius of that curvature, the particle’s momentum can be determined.

Bubble chambers have been broadly used from the mid-1950s until well into the 1970s.  Several cameras can be mounted around a bubble chamber, so as to allow for a three-dimensional image to be recorded.  Devices with resolutions down to a few micrometres (μm).

Although successful, they have proved of limited use to very-high-energy experiments.

The drawbacks of using a bubble chamber include:

• Photographic readouts are inconvenient compared to 3D sets of electronic data, when it comes to repeating an experiment many times,
• The super-heated phase must be ready at the precise moment of collision,
• Bubble chambers are not large or massive enough to permit the analysis of high-energy collisions,
• High-energy particles have path radii that are typically too large for enabling a precise estimation of momentum in a small chamber.

# Crazy Particle Trails

Working with bubble chambers has provided experimental data on the lifetimes and decay modes of a range of particles such as the K° meson.  More recently, it has led to the discovery of weak neutral currents – relevant to the way in which subatomic particles interact through the weak force – that paved the way for the discovery of the W boson and the Z bosons in 1983.

CERN provides this fantastic tool: “A step-by-step tutorial on how to read bubble chamber pictures“.  Want to learn even more?  Check out The Particle Adventure website.

Still, what do Gargamelle and Picasso share in common?  😛

# Looking for WIMPs

Bubble chambers have also been used in recent research on WIMPs (Weakly Interacting Massive Particles) at COUPP (Chicagol and Observatory for Underground Particle Physics)  and PICASSO (Projet d’Identification de CAndidats Supersymétriques SOmbres) experiments.

COUPP is an experiment (E961) in the underground MINOS (Main Injector Neutrino Oscillation Search) near detector hall at Fermilab to demonstrate the  performance of a 30-litre, 60-kilogramme, heavy liquid, room temperature, bubble chamber as a Dark Matter detector.

The PICASSO project is a dark matter search experiment presently installed and taking data in the SNOLAB underground laboratory at Sudbury, Ontario, Canada.

If a dark matter particle hits a nucleus in a tiny super-heated droplet, the atom recoils and deposits its energy in a heat spike, which in turn triggers a phase transition.

The subsequent mini-explosion gives an acoustic signal lasting about 4 milliseconds  and can be recorded easily with piezoelectric transducers.

Professor Don Glaser died earlier this year in February 2013, at the age of 86.

Nowadays, bubble chambers have mainly been replaced by wire chambers and spark chambers.