||Dr C N Booth
In a superheated liquid, bubbles start to form on nucleation centres -
surface irregularities, dirt or positive ions - in the liquid.
Bubbles can therefore form along the track of a charged particle, and make
it visible. This was the principle exploited by Glaser in 1952 in
the first bubble
chamber, which proved of great importance in particle physics experiments
for many years.
A typical bubble chamber is filled with liquid hydrogen (forming a very
simple nuclear target) at a temperature above the normal boiling point
but held under a pressure of about 10 atmospheres by a large piston to
When particles have passed, and possibly interacted in the chamber, the
piston is moved to reduce the pressure, allowing bubbles to develop along
After about 3 milliseconds have elapsed for bubbles to grow, the tracks
are photographed using flash photography. Several cameras provide
stereo views of the tracks.
The piston is then moved back to recompress the liquid and collapse the
bubbles before boiling can occur.
Features of bubble chambers include
this is a true 3-D detector, rather than one providing a set of projections;
an extremely good spatial resolution (~0.1 mm in large chambers, even better
in specialised small chambers);
the bubble density is proportional to the primary ionisation
this leads to a Poisson distribution, rather than a Landau distribution
with its long tail,
particle identification can thus be achieved, separating pions and kaons
up to ~0.9 GeV/c, and pions and protons up to
the chamber may be mounted in a magnetic field, allowing accurate momentum
At high momentum, the error is dominated by uncertainties in the optical
constants, distortions, etc.;
At low momentum, it is dominated by multiple
Coulomb scattering as the particle passes through the dense liquid.
Holographic recording of the bubble chamber can lead to a resolution as
good as 6 microns in small chambers (used for studying very short-lived
particles such as B-mesons).
Other liquids can also be used, to provide an alternative target.
E.g. if deuterium is used, interactions with both protons and neutrons
Heavier liquids are useful for detecting photons, but may not be appropriate
as the primary target. With a "track sensitive target", a central
region of liquid hydrogen in a transparent box or bag is surrounded by,
for example, neon to convert photons into e+e− pairs.
(In practice, a mixture of liquids may be required to maintain appropriate
boiling temperatures and pressures in the two compartments.)
Additional external detectors can be used to trigger the flash and film
advance, to select particular types of events.
Film measurement and event reconstruction is slow, even with semi-automatic
measuring machines (such as Sweepniks, spiral readers, etc.).
(This typically limits experiments to 105 to 106
The sensitive period is quite long, so the beam intensity must be limited.
Though the camera can be triggered, the chamber itself cannot. A
typical cycling rate is about 10 Hz or less (though it can be faster
for very small chambers), normally matched to the pulsed beam extraction
rate from the accelerator.
For the above reasons, it is difficult to search for and study very rare
The liquid in the chamber acts as both target and detector, so bubble chambers
cannot be used with modern colliding-beam machines.
Picture from the Stanford 1 m hydrogen bubble chamber, exposed to
8.8 GeV/c antiprotons.
A careful study of this photograph reveals the reaction to be ,
the slow proton is identifed by its heavier ionisation,
the K0 subsequently decays into a pair of charged pions,
the antineutron annihilates with a proton a short distance downstream from
the primary interaction, to produce three charged pions,
the neutral pion decays into two photons, which (unusually for a hydrogen
chamber) both convert into e+e− pairs,
(external particle detectors were used to identify the charged kaon).
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