Particle physics would be nothing
without detectors, but this is what they
used to look like - a cloud chamber. Filled
with supersaturated alcohol vapor, when
energetic charged particles travel
through it they rip off the electrons of
the air molecules leaving behind an
ionized trail of gas. These ions then
attract the alcohol acting as
condensation centers which allow a cloud
to form. Particle physicists used to take
photographs of these cloud tracks and
use those to work out what was happening.
Bubble chambers worked in much the same
way but instead of supersaturated vapour
it was superheated transparent liquid
and instead of clouds forming it was
visible bubbles. But things now are quite
a bit different!
The experiments as part of the LHC are
incredibly complicated so how do
particle detectors work in the 21st
century? ATLAS is one of the two
general-purpose experiments at the LHC.
Its inner detector tracks particles just
like a cloud chamber, but by using pixel
detectors and semiconductor trackers
it's silicon chips that are doing this
electronically. It also has a combined
transition radiation detector and straw
tracker which detect particles by using
materials with lots of different
refractive indices and a combination
of gas and wires respectively. The outer
detectors measure the energy deposited
by the particles in these calorimeters. And the experiment produces a tremendous
amount of heat, which has to be cooled by
all this water. The ALICE experiment,
which looks at heavy ion collisions at
the LHC, also uses pixel detectors
similar to ATLAS and this is what they
look like. But this is its main tracker,
which works a bit like a wire chamber
filled with gas that gets ionized and
wires to send the signals. LHCb, an
experiment focusing on bottom quarks,
also has one of these - here it is. It also
has two optical detectors picking up the
light produced by the particles through
scintillation and Cherenkov radiation.
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