Subatomic Physics

62 Detectors
4.4 Bubble Chambers
Bubble chambers became popular in the 1950–1980’s as a tool to track particles
through large volumes. Since its invention by Glaser in 1952, it played a crucial
role in the elucidation of the properties of subatomic particles. (3)
Figure 4.12: Bubble chamber—
schematic diagram.
The physical phenomenon underlying the bubble
chamber is best described in Glaser’s own
words (4) : “A bubble chamber is a vessel filled
with a transparent liquid which is so highly
superheated that an ionizing particle moving
through it starts violent boiling by initiating
the growth of a string of bubbles along its
path.” A superheated liquid is at a temperature
and pressure such that the actual pressure
is lower than the equilibrium vapor pressure.
The condition is unstable, and the passage of
a single charged particle initiates bubble formation.
To achieve the superheated condition,
the liquid in the chamber (Fig. 4.12) is first
kept at the equilibrium pressure; the pressure
is then rapidly dropped by moving a piston.
A few ms after the chamber becomes sensitive, the process is reversed and the chamber
pressure is brought back to its equilibrium value. The bubbles are illuminated
with an electronic photoflash and recorded.
In the times when bubble chambers were popular for high-energy experiments,
the time during which the chamber was sensitive was synchronized with the arrival
time of pulses of particles from an accelerator. Pictures were taken and later
analyzed visually. Glaser’s first chambers contained only a few cm 3 of liquid. Development
was rapid, however, in less than twenty years, the volume increased by
more than 10 6 . Eventually bubble chambers became very large and costed millions
of dollars. They required enormous magnets to curve the paths of the charged particles.
The superheated liquid, often hydrogen, was explosive when in contact with
oxygen, and accidents did occur. Bubble chambers could produce tens of millions
of photographs/y, and data evaluation was complex.
Two examples demonstrate the beautiful and exciting events that were seen.
Figure 1.4 shows the production and the decay of the omega minus, a most remarkable
particle that we shall encounter later. Figure 4.13 represents the first neutrino
interaction observed in pure hydrogen. It was found on November 13, 1970, in the
3 L.W. Alvarez, Science 165, 1071 (1969).
4 D. A. Glaser and D. C. Rahm, Phys. Rev. 97, 474 (1955).