Feature Article
Build Your Own
Cloud
Chamber
SHOILI PAL
What are elementary particles? How can we build
our own Cloud Chamber to observe elementary
particles at home?
Basic Terminology
W
E have been hearing so much
about particle accelerators and
particle detectors the last few
years. The best-known particle accelerator
possibly is the Large Hadron Collider (LHC)
at the European Council for Nuclear
Research (CERN) in Switzerland. The
particle detectors associated with it are
CMS, ALICE, ATLAS etc. Experiments done
on the Large Hadron Collider have been
making headlines for the last few years.
Will they re-create the Big Bang? Will they
find the Higg’s boson (the only elementary
particle that hasn’t been observed so far)?
Will they, in fact, find newer particles that
we haven’t thought of yet?
These experiments are the cutting
edge of science. But is it possible to make
a little particle detector at home? Wouldn’t
it be exciting if, instead of being limited to
the theoretical knowledge of textbooks,
we could see elementar y particles for
ourselves?
It is indeed possible to make a cloud
chamber at home and view elementary
particles like electrons and muons from
the cosmic rays incident on the earth.
But before we build a cloud chamber, we
need to understand a few concepts
associated with these experiments. What
are elementar y particles? What is an
electron? What is a muon? What are
cosmic rays? And what is a cloud
chamber? Let us tr y to answer these
questions before we discuss how to make
a cloud chamber by ourselves.
Elementar y particles are the basic
building blocks of all matter; they are not
composed of any further sub-particles. All
these particles can be classified as
fermions or bosons on the basis of a
characteristic called their spin. For the
purposes of this article we do not need to
know what spin is. Fermions have halfinteger value of spins and bosons have
integer spins.
In the Standard Model of particle
physics, there are twelve elementar y
fermions, six quarks and six leptons. The six
leptons are three neutrinos and three
electron-like particles. The three electronlike leptons are the electron, the muon
and the tau particle. All three have
negative charge and their spin is half.
Fermions are thus grouped into three
generations, each generation consisting
of a charged lepton and its associated
neutrino. Particles of all generations have
similar properties; only the mass increases
many times with each generation.
Thompson discovered the electron in
1896. This was the first evidence of the
43
existence of subatomic particles. The
electron is now considered the first
generation lepton. Its mass is approximately
9.1x10-31 kg. A few years later, Rutherford
demonstrated the existence of the
positively charged atomic nucleus. The
existence of the neutron was shown by
Chadwick in 1932. In this way, the model
of an atom with a nucleus of protons and
neutrons and electrons orbiting around the
nucleus was built.
Muons were discovered by Carl
Anderson and Seth Neddermeyer in 1936.
They observed a particle that deflected in
the same fashion as an electron in a
magnetic field but curved less sharply. The
muon is the second-generation lepton
represented by the Greek letter ì. Its mass
is approximately 200 times that of an
electron’s.
But it was observed that many
detectors detect a small signal even when
not near any known source. The signal
increased when the experiments were
performed in balloon-borne experiments
high up in the atmosphere. Scientists
concluded the existence of cosmic rays
from this. Cosmic rays gave them the
opportunity to study high-energy particle
interactions.
Cosmic rays are rays of high-energy
subatomic particles that originate in outer
space. Primary cosmic rays are those that
originate from extra-solar astrophysical
sources. They contain protons (hydrogen
nuclei), alpha particles (helium ions) and
SCIENCE REPORTER, JULY 2012
Feature Article
C.T.R. Wilson – Inventor of
the Cloud Chamber (far left)
Wilson’s original cloud
chamber apparatus (left)
A cloud
chamber
consists of a
sealed
environment
containing a
super-saturated
vapour of water
or alcohol.
beta particles (electrons). Primary cosmic
rays interact with interstellar matter to
create secondar y cosmic rays. As the
cosmic rays travel through the earth’s
atmosphere they collide with atoms in the
upper atmosphere. When a proton from a
cosmic ray collides with atomic nuclei in
the upper atmosphere, particles called
pions and kaons are formed. Charged
pions decay into muons and neutrinos. The
muons continue travelling towards the
earth.
Now that we have answered
questions about particles and cosmic rays
let us come to the cloud chamber. What is
it and how does it work?
Cloud Chamber
The cloud chamber is a particle detector
invented by British scientist C.T.R. Wilson in
the early twentieth century, for which he
received the Nobel Prize in Physics in 1927.
A cloud chamber consists of a sealed
environment containing a super-saturated
vapour of water or alcohol. The vapour
condenses on the ionizing particle (alpha
or beta), thus leaving a trail. The tracks can
be seen with the help of a tangential light
source.
The bottom of the chamber is cooled.
This can be done in several ways. One
method is by pumping cold water through
it. A device called a Peltier device can
help cool it further and achieve a
temperature of around -350C. This creates
a temperature gradient between the top
and bottom of the chamber. Alcohol
placed in the chamber rises up the lining
of the chamber and reaches the top part
where it is warmer, so it evaporates. It then
SCIENCE REPORTER, JULY 2012
starts to diffuse downwards where it is
cooled. This leads to the chamber being
super-saturated.
Cosmic radiation and electrically
charged particles are incident on the
chamber. They ionise the vapour by
stripping the atoms of the alcohol in their
path of their electrons. This leaves behind
positively charged atoms which act as
nuclei for the vapour to condense upon.
Therefore, the ion trail left by the particles
is a good trigger for condensation leading
to formation of a trail along the path of
the particle. The trails can be seen from
around fifteen minutes after we complete
the set-up. The particles can be identified
from the tracks. We will see how.
Alpha particles have a big mass and
a charge of +2. The positive charge
means they cause lots of ionization within
a short distance of travel. Each time the
alpha particle rips an electron off a
molecule, it loses some energy and slows
down a bit. An alpha particle might cause
about 100000 ionizations before it loses all
its energy. When this happens, it grabs two
electrons and becomes a helium atom.
An alpha particle will ionize most air
molecules it passes close to. Cloud trails
of alpha particles are typically short and
fat. The alpha particle has a heavy mass
which makes it difficult to change its
direction. Thus, a straight path is observed.
With high-energy beta, the ionisations
are much further apart. The beta’s single
negative charge means it ionizes every
hundred or so air molecules it comes
across. Again, each ionisation costs the
beta particle some energy and it slows
down. The tracks made by low-energy
44
beta particles are thicker than those made
by high-energy beta particles.
A skinny track going straight indicates
a muon and if the track bending sharply
indicates a muon decay.
So, now we know how a cloud
chamber works and how to identify tracks
of different particles. We can even fix a
camera above the cloud chamber and
take videos and later identify the different
tracks from stills.
Make your own Particle
Detector
Let’s now see how we can make our own
cloud chamber at home.
We need a transparent plastic
container for the body and a metal plate
for the bottom surface of the cloud
chamber. The metal plate should be black
to provide a suitable backdrop for the
tracks. However, if you can’t get hold of a
black metal plate then just paint it black
or wrap it with black tape. Keep some putty
or tape at hand to attach the container
and plate to each other.
Next, we need a piece of felt. The felt
will be used to introduce the alcohol into
the chamber. And then you need some
dry ice to cool the chamber. The local
ice cream vendor parlour can help you
out with getting dry ice if you don’t know
where to get it. And to keep the dry ice
you need an insulating box made from
wood or thermocol. Get some ethyl or
isopropyl alcohol from your school’s
chemistry laboratory. Find a torch with a
strong beam to provide the lighting. And
you now have all your raw materials ready.
Feature Article
Materials to build your own cloud chamber
Plastic cup
Painted piece of soda can
Felt
Dropper
Dry ice
Track of an alpha particle in a cloud chamber
(above left)
Track of a beta particle in a cloud chamber (left)
How do we put together a working
cloud chamber from all these things? Stick
the felt to the bottom of the plastic box.
Soak it with the alcohol using a dropper.
Remember that the alcohol should not
touch your bare skin. Also, it is poisonous
so do not try to drink it.
Put the box upside down on the metal
plate. The felt-lined bottom is now on top.
Seal the box to the plate using the tape.
Make sure the arrangement is airtight;
otherwise you won’t get proper tracks. Now
take the dry ice and put it below the metal
plate. This sets up the temperature
gradient and you should be able to see a
sort of ‘rain’ at the bottom of the chamber.
Remember to handle the dr y ice
wearing gloves otherwise you will get a
burn from the extremely cold ice. Also,
the room should be well ventilated
because when the dry ice melts carbon
dioxide will be formed.
Once you have completed all these
steps turn off the lights in the room. Point
the flashlight at the chamber from the side
and wait for the tracks to appear.
Good luck!
A contemporary cloud
chamber
Ms Shoili Pal is a graduate in Physics from the
Miranda College, University of Delhi. Address:
CF-333 Saltlake, Kolkata-700064; Email:
[email protected]
45
SCIENCE REPORTER, JULY 2012