the chamber when the piston is fully withdrawn, the pressure
at its minimum, and the liquid superheated. Then, about one
millisecond later, an arc light flashes, illuminating the trails of
bubbles formed by charged particles. The delay between minimum
pressure and the flash allows the bubbles to grow large enough to
show up on the photographs. Meanwhile, the piston moves back in
towards the chamber, increasing the pressure again, and the film in
the cameras is automatically wound on to the next frame. It then
takes about a second for the chamber to ‘recover’ and be ready for
the next expansion. Thus the bubble chamber shows where
the particles have been, enabling their behaviour to be studied
at leisure.
In a magnetic field, a charged particle’s trajectory will curve, the
direction revealing whether the particle was positively or negatively
charged, and the radius of the curve revealing its momentum. So we
can deduce the charge and momentum; if you know a particle’s
momentum and velocity, you can calculate its mass and hence
its identity.
One method of pinpointing the velocity used two scintillation
‘counters’, which produced a flash of light each time a charged
particle passed through. Each tiny burst of light was converted to a
pulse of electricity, which was then amplified to produce a signal. In
this way, two or more scintillation counters could reveal the
flightpath of a particle as it produced flashes in each counter, and
from the time taken to travel between the two counters, the
particle’s speed could be determined.
However, such techniques did not help solve the identification
puzzle in the case of a bubble chamber picture. Often the only way
was to assign identities to the different tracks, and then to add up
the energy and momentum of all the particles emerging from an
interaction. If they did not balance the known values before the
interaction, the assumed identities must be wrong, and others must
be tested, until finally a consistent picture was found. This was
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Detectors: cameras and time machines