cloud chamber experiment tracks

Charles Wilson saw tracks of single charged particles in his cloud chamber the first time in 1910. Having studied meteorology and the formation of water droplets that make clouds, he started his research on cloud formation in 1894. He made a chamber filled with water and air where the temperature could rapidly be lowered by pulling a piston that caused the air to expand. The water vapour would condense into droplets along a track of a charged particle that traverses the chamber at the right moment. The tracks could be photographed and with his invention Wilson visualised for the first time tracks of atomic particles. He received the Nobel Prize for his invention in 1927.

The Wilson cloud chamber was used to study different kinds of particles and interactions for more than 40 years and many discoveries were made.

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A cloud chamber makes the invisible visible, allowing us to see delicate, wispy proof that there are tiny particles whose story starts in outer space shooting through all of us, every minute of every day.

1 × cloud chamber Propanol (aka isopropyl alcohol or IPA) Dry ice (solid carbon dioxide)

The demonstration

The cloud chamber is prepared and placed down. Moments later, wispy streaks of cloud appear, seemingly spontaneously, inside the chamber. These tiny clouds show the path of charged particles through the chamber—and, since there are no obvious sources of charged particles around, they’re evidently natural and omnipresent…

Vital statistics

muon mass: 207× electron mass

muon flux at sea level: 10,000 muons/m 2 /minute

How it works

The base of the cloud chamber is filled with dry ice, and an absorbent material near the top thoroughly soaked in propanol. Propanol is quite volatile, and so forms a vapour at the top of the chamber. As the vapour falls, it cools rapidly due to the dry ice and the air becomes ‘supersaturated’: the propanol really wants to condense, but there is nothing for it to condense onto. Charged particles passing through the chamber cause the propanol molecules to gain an electric polarisation, and be drawn towards those particles, and one-another. This provides the impetus for them to condense into tiny liquid droplets in the chamber which show up as white streaks of cloud along the path of the particles.

This demo is often done with a radioactive source, with alpha and beta particles causing propanol to condense, but it actually works in the absence of a source too, because of cosmic ray muons passing through the apparatus. The muons are produced high in the atmosphere by protons (the ‘cosmic rays’) smashing into the nuclei of gases. These produce a variety of daughter particles, but the only ones typically long-lived enough to make it to the Earth’s surface are muons.

Muons are heavy electrons, and decay into an electon and a neutrino with a mean lifetime of 2.2 μs. This actually provides an interesting test of special relativity: muons are typically produced around 15 km up in the atmosphere, a distance which takes around 50 μs to traverse at the speed of light—over 20 muon lifetimes. Thus we’d expect barely any to make it! However, since they are travelling quite near the speed of light, time in their frame of reference is significantly dilated as seen by an observer on Earth, meaning that a significant fraction can, in fact, make it to the surface.

• YouTube: How to make a cloud chamber

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Part of a streamer chamber image from the UA5 experiment that took data at the CERN proton-antiproton collider in the 1980s.

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Cloud Chamber

The group has recently built a desktop diffusion cloud chamber that allows the paths of charged particles to be seen by the unaided eye.

It is built from inexpensive and readily available materials and is suitable for classroom demonstrations. In operation it consumes small quantities of dry ice (a couple of kg provides several hours of operation if care is taken with insulation) and small quantities of isopropanol (a few cc are enough for an hour of operation).

The chamber is made from a fish tank that can be purchased cheaply from a toy shop or pet shop. The open side of the tank is sealed with a closely-fitting thin metal plate. Before closing the tank with the plate, attach a paper tissue to the inside face opposite the plate and moisten the tissue with isopropanol. Now place the tank so that the metal plate is at the bottom and in contact with a layer of dry ice in an insulating container. Illuminate the region inside the tank, just above the metal plate, from the outside using a bright light source.

The assembled chamber can be seen in the photo. Note the use of black paper to reduce annoying reflections.

Principle of operation

The principle of operation is illustrated in the sketch (click on it for a higher resolution version). Vapour falls from the isopropanol-soaked tissue towards the very cold metal plate. After a short while you should see a mist of isopropanol droplets just above the plate where the vapour is cold enough to condense. Just above this region the vapour is super-saturated - it would like to condense but there are no nucleation centres to seed the droplets. A charged particle travelling through the super-saturated layer can cause nucleation along its path by polarising the vapour molecules and this allows droplets to form along the paths of the particles and so make them visible.

You may find that your apparatus works straight away without problem. More likely, however, you will find that it may need a little optimisation. At the bottom of this page we provide some hints that you might find helpful.

For the impatient, here are a few movies (QuickTime). Movies 1 and 2 are without any radioactive source added (just natural background radiation). The lighting is not ideal in these and the tracks are a little indistinct but you may see a few traces. For the second two movies we inserted a radioactive source (actually a thoriated welding rod) vertically into the cloud chamber. You can see the tip protruding from the bottom of a metal sheath. Thorium is an α-source (see below) and the α-particles produce quite bright traces. Two or three should be visible in each of movies 3 and 4.

Movies 5 and 6 are of particle trails in a commercially available cloud chamber by the company PHYWE . The physics department has one of these in the Cavendish museum area.

A few words on what you can see

So, after a couple of minutes to allow the conditions inside the chamber to stabilise, you should begin to see filament-like trails of droplets of various lengths. Some will be straight, some irregular, some clear and bright and others more tenuous.

Most of the droplet trails are caused by particles produced in the radioactive decay of naturally occuring elements. These are classified as α-decays , β-decays and γ-decays . α-decays produce α-particles (they are the same as the nuclei of Helium atoms), β-decays emit electrons (and neutrinos, but these are not detected by the cloud chamber) and γ-decays emit high energy photons known as γ-rays (they are also not detected).

α-particles lose their energy very quickly and stop after travelling a few centimetres in air and are easily stopped by a piece of paper. Therefore, only α-particles produced inside the cloud chamber will be visible and they will typically produce a short, but usually bright, trail of droplets.

Electrons lose their energy more slowly but the ones produced in natural radiation are unlikely to be able to penetrate through the walls of the tank. Therefore these too can only be seen if they are produced inside the chamber but they may travel much further and produce trails that span the entire width or length of the chamber.

γ-rays are neutral particles and do not polarise the vapour so they are not observed except in the rare case that they interact with an atomic nucleus. Even then, the products of the interaction may not travel far enough to be seen.

It may also be possible to see droplet trails caused by particles known as muons that are generated when high energy cosmic rays strike the top of the Earth's atmosphere. They tend to be mostly travelling vertically downwards so only leave short trails because the super-saturated vapour layer is not very deep.

Hints on the cloud chamber design

• The vertical height of the chamber affects the temperature gradient and therefore the thickness of the super-saturated layer. Our tank is about 25cm high.
• The metal plate should not be larger than necessary and no part of it should be exposed to ambient temperature as it will be very hard then to make it cool enough. Our plate is a snug fit to the tank and the whole lower part of the tank is inserted into a home-made insulating (expanded polystyrene) container that both holds the layer of dry ice and provides insulation around the lower part of the chamber. This should also be a snug fit to avoid wasting dry ice.
• A layer of foam rubber underneath the dry ice may help to level it out so that there is better thermal contact with the metal plate. When first placed in contact with the dry ice and pressed down you might hear a squealing/hissing noise as the carbon-dioxide gas tries to escape through small holes.
• The chamber should not be completely airtight to allow the internal pressure to equalise with atmospheric pressure while the chamber is cooling. However, draughts caused by air leaking inside around the metal plate should be avoided as this makes it harder to keep the plate cold and also may cause turbulence. We use a rubber gasket to seal between the metal plate and the tank but we drilled a small hole through the glass of the opposite face using a diamond drill (take care!). This also allows us to replenish the isopropanol without dismantling the chamber.
• Keep the metal plate horizontal.
• Try to avoid reflections that make it hard to see inside. We painted the inside surface of the metal plate black. Operate the chamber in a darkened room and use black paper if necessary to try to reduce reflections.
• Use a bright light source. A fluorescent desk lamp will do but we used bright white LED lights and a lens to produce an approximately parallel and horizontal beam.
• You will certainly need to play with the position of the light source to get the best effect. We positioned our lamp along one side of the tank and about a quarter of the way up from the bottom. The beam should be roughly horizontal but you may need to adjust the angle and also mask light that isn't going where you want it.

Cloud Chamber Calendar and Bookings

When not "on tour" our PHYWE cloud chamber is on permanent display in the Cavendish Museum area.

Future events highlighted in green.

This educational resource is part of the The Hunt for Dark Matter spotlight

Build A Cloud Chamber

Grade level, physical science, activity type:, visualize radiation , advanced , experiment.

All around you, and on every surface of the earth, there is radiation pummeling the atoms that make up the matter that we can see and feel. Even as you read this sentence, you are being bombarded by radiation. Pew! Pew!

But fear not, it’s completely normal. This background radiation is safe. And though it cannot be seen directly, you can build a cloud chamber to help you indirectly observe radiation and begin to understand it.

Target Grades : 9-12+ Content Areas: Physics, Engineering and Technology Activity Type : Detect radiation, design radiation shielding Time required: About 1 hour Next Generation Science Standards : HS-PS1-8

Radiation is any type of wave or particle that transmits energy and includes things like ultraviolet light, alpha radiation (particles made up of two neutrons and two protons), sound (acoustic waves), and x-rays (electromagnetic waves). Radiation that transmits large amounts of energy, called high-energy radiation or ionizing radiation, can change or damage other materials and living cells that it comes into contact with. When describing radiation exposure from man-made sources of radiation like X-rays or nuclear energy plants, what is being described is ionizing radiation.

In small amounts, however, ionizing radiation does not do significant harm, and in fact, it’s a part of our everyday life. Ionizing radiation that naturally occurs on our planet is called background radiation and is a natural part of our planet’s environment. Background radiation can come from all kinds of things, including cosmic rays from outside our galaxy, radioactive materials in the earth like uranium and radon, and even radioactive materials in our body, such as certain kinds of potassium and carbon atoms.

Even though we can’t see these different kinds of radiation directly, we can indirectly observe them when they interact with substances that we can see. Interactions are often physical collisions between a radioactive particle or wave and a non-radioactive atom. Detecting radiation by looking for its interactions with atoms is similar to how you might indirectly observe wind: you can’t see wind itself, but you can see leaves, trees, or plastic bags move in the wind and deduce that the wind is there.

What you'll need to build your cloud chamber

With the right tools, you can observe the interaction of radiation with other substances. Cloud chambers are great tools for indirectly viewing certain types of radiation like the kinds we get from our sun. Here’s how to make one.

The basic gist of a cloud chamber is this:

What you’ll need to build your chamber:

• An adult who can safely use isopropyl alcohol and dry ice
• A crystal clear plastic or glass container with a wide, tight-fitting lid to be your cloud chamber.  You’ll want something at least as big as a peanut butter jar or a deli container. If you don’t have a lid, a cookie sheet larger than the mouth of the container will work.
• A durable, absorbent material that you can squish into the bottom of the container. Try felt, wool, or a sponge
• Bubble gum or modeling clay (optional)
• Black paper cut to fit inside the lid of your container
• Dry ice and insulating gloves for safe handling (find a dry ice distributor here)
• A plastic or foam container with a rim that can hold both the dry ice and your cloud chamber. It should be about four times the size of your cloud chamber
• A bottle of 90+% isopropyl alcohol (available at most pharmacies)
• A room, closet, or large box that you can make completely dark for conducting your cloud chamber observations in
• A very bright flashlight (LED flashlight works best)
• A small bowl of warm water that can sit on top of your clear plastic or glass container
• A clock, timer, or stopwatch
• Safety glasses and lab apron
• Optional: a digital camera or cell phone camera

Materials safety notes:

• Isopropyl alcohol is toxic to ingest and also highly flammable. Make sure there are no hot surfaces or open flames in your work area, clean spills promptly, and avoid contact with skin or clothing.
• Direct skin contact with dry ice can cause burns. Avoid direct contact with dry ice by using dry, insulating gloves when you handle it. Since dry ice will produce large amounts of gas on contact with liquids, never store in a glass container and always wear safety goggles and a lab apron.
• Because isopropyl alcohol is flammable, it is important to safely allow any of the remaining alcohol in the cloud chamber to evaporate in a well-ventilated area away from open flame at the end of your experiment.

How to build your cloud chamber

• Stuff the bottom of your clear container with your absorbent material. If you need help getting it to stick, try using a small piece of modeling clay or chewed gum to stick it to the bottom. Note: isopropyl alcohol dissolves most adhesives, so you may have to troubleshoot other ways of sticking the absorbent material to the top.
• Set the piece of black paper on the inside of the tightly fitting lid and trim it so that the container can still close with the paper inside.
• Pour just enough isopropyl alcohol into your container so that the absorbent material becomes saturated but is not standing in any liquid. Carefully pour any excess isopropyl alcohol down the sink.
• Turn the container upside-down onto the lid and black paper and secure the lid, making sure that your alcohol-soaked material stays stuck to the bottom of your container. If you don’t have a tight-fitting lid, you can turn your container upside-down onto a cookie sheet, covered with black paper, and tape it around the edges to the cookie sheet. The vapor of the isopropyl alcohol will begin to fill your jar immediately.
• In a room or large box that you can make completely dark, set up your rimmed container on a solid work surface. Using insulated gloves to protect your skin, pour some dry ice into the rimmed container. Place your cloud chamber lid-side down onto the dry ice and keep it there until the lid appears frosty, about 10 minutes. (If you used a cookie sheet, rest the bottom of the sheet on the dry ice.)
• Fill a small bowl or dish with warm water, and set it on top of your cloud chamber. This warms the isopropyl alcohol so the chamber fills with vapor more quickly. You now have a totally cool cloud chamber.
• Turn off all the lights, and shine your flashlight across the bottom of your container through the side. Look inside, what do you see?

Identify the kinds of radiation in your cloud chamber, then measure the amount of background radiation.

See all those little streaks and lines forming in the mist in your cloud chamber? Those are paths from ionizing radiation! Ionizing radiation is high-energy radiation that has enough energy to knock the electrons off of other atoms it collides with. Ionizing radiation can come from cosmic rays from the sun, from the argon in our air, and from radioactive materials. Take a moment to sketch some of the different path shapes that you see. Each of these different path types (e.g., long and straight, short and straight, long and bent) is caused by a different kind of ionizing radiation. You can identify the different types of radiation with this cloud chamber field guide:

When ionizing radiation enters a cloud chamber, it interacts with atoms in the atmosphere — like hydrogen, nitrogen, and oxygen–by violently knocking off their electrons. Those atoms turn into positively charged ions, which are very attractive to the gaseous alcohol molecules in the cloud chamber! Chilling the cloud chamber on dry ice causes those gaseous alcohol molecules to crowd so close together that no matter where in the chamber ionizing radiation strikes, there will be many alcohol molecules ready to stick to the trail of positive ions it produces. The result is visible trails of condensed alcohol mist wherever ionizing radiation comes into contact with atoms in the air.

Use your cloud chamber to measure background ionizing radiation.

Using a clock or timer, try to count how many streaks of ionizing radiation you see in your chamber in a minute. Repeat this count two more times, and calculate an average “ionizing interactions per minute” for your cloud chamber. If it helps, you can print out this cloud chamber observation sheet to record your observations and do calculations.

The trails you are seeing in your chamber are a tiny window into the radiation that is buzzing in, through, and around you all of the time. Pretty cool, right?! Well, not if you’re trying to study dark matter.

If you’re looking for dark matter, background radiation is a major problem

Physicists have evidence that in addition to the known subatomic particles that make up most of the things we can see and touch, there is an entirely separate class of very small, potentially weakly interacting particles that make up the majority of our universe called dark matter. Though it comprises over 90 percent of our galaxy, dark matter is poorly understood.

Dark matter is difficult to study because it’s made of unimaginably small particles that we can’t see, and it interacts with other atoms very rarely. Detecting dark matter interactions that are so minute and rare is made especially difficult because they are grossly overshadowed by the background radiation that is constantly pouring down on our planet from cosmic rays. Our planet’s background radiation makes the search for dark matter like trying to hear a shy, whispering child in a party of shouting adults. Science Friday’s video producer, Luke Groskin, visited with scientists looking for dark matter, who describe this conundrum in the video “4850 below.”

Science Friday Documentary: “4850 Below”

In an effort to quiet the “noise” of background radiation, a long-running dark matter experiment called the LUX dark matter experiment (LUX stands for Large Underground Xenon) was built inside a giant water tank in an old mine a mile below the surface of the earth. The tank of water and mile of rock and dirt shield the experiment from background radiation by effectively putting a lot of other atoms – in the form of lots of dense materials like rock and water – between sources of radiation and the experiment.

Engineer your own cosmic ray shielding, and then test it

Now that you have a cloud chamber that works as a particle detector, and a baseline rate of ionizing radiation (“ionizing radiation interactions per minute”), you can make your own radiation shielding and test it by monitoring whether ionizing interactions are less frequent. What will you build around your cloud chamber to shield it from background radiation from sources like the sun?

Materials to try – Bags or containers of water – Metal cookie sheets – Magnets – Bricks or rocks – Safe sources of electric fields (e.g., holiday lights)

Effective cosmic ray shielding should do the following:

• Allow you to observe and count the number of ionizing interactions in your cloud chamber, even when the shielding is installed
• Avoid contact with the chamber itself, the dry ice, and the remainder of your experimental setup
• Lower the number of ionizing interactions per minute in your cloud chamber from your initial measurement

To test out your shielding design, first refresh the alcohol in your chamber, and make sure you have sufficient dry ice to keep it cold. As your cloud chamber cools, assemble your radiation shield around it.  Once everything is in place, count how many streaks of ionizing radiation you see in your chamber in one minute, and record the count. Count the number of ionizing radiation interactions in a minute two more times, and calculate an average “ionizing interactions per minute” for your cloud chamber with it’s radiation shielding. Is this average different from the rates you observed before you installed the shielding? If you had unlimited funds and space, how would you improve your design to make your shielding more effective?

Related Links

Check out these other cloud chambers for more inspiration :

• Petri-dish cloud chamber from Thomas Jefferson National Accelerator Facility
• Particle detector from Symmetry Magazine
• Compressed air cloud chamber featured at Physicsworld.com

Next Generation Science Standards: HS-PS1-8  Develop models to illustrate the changes in the composition of the nucleus of the atom and the energy released during the processes of fission, fusion, and radioactive decay.

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Ariel Zych is Science Friday’s director of audience. She is a former teacher and scientist who spends her free time making food, watching arthropods, and being outside.

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Science in School

Bringing particle physics to life: build your own cloud chamber teach article.

Author(s): Francisco Barradas-Solas, Paloma Alameda-Meléndez

Particle physics is often seen as something only for huge research institutes, out of reach of the general public. Francisco Barradas-Solas and Paloma Alameda-Meléndez demonstrate how – with the aid of a homemade particle detector – you can dispel this myth by bringing particle physics to life…

The objective of elementary particle physics is to find the basic building blocks of which everything is made and to investigate the behaviour of these building blocks. Although it can be seen as a cornerstone of science, particle physics is often neglected or poorly understood in schools, partly because it is perceived as unrelated to the things with which we interact on a daily basis. However, particle physicists detect and measure electrons, photons or muons every day with the same confidence with which all of us ‘detect’ cows, tables or aeroplanes. Furthermore, particle detectors (e.g. PET scanners) are routinely used, for example, by medical physicists to detect tumours and monitor the function of internal organs.

Here we demonstrate how to bring particle physics to life in the classroom, using possibly the simplest type of particle detector: a continuously sensitive diffusion cloud chamber. This homemade version consists simply of an airtight fish tank full of air and alcohol vapour, cooled to a very low temperature, which can be used to detect charged particles, particularly cosmic ray muons, if they have enough energy.

Elementary particles

Elementary particles are the simplest elements from which everything is made. They are not just the building blocks of matter and radiation, but also give rise to the interactions between them (for more details of elementary particles, see Landua & Rau, 2008 ). These particles carry energy and momentum, and can thus be seen by detectors. Strictly speaking, you cannot directly see any particles – instead, their passage through detectors is inferred from the effects they cause, such as ionisation (for charged particles). That is precisely what we do when we observe the condensation trail left in the sky by an aeroplane that we cannot see – and what we can do with our homemade cloud chamber.

The continuously sensitive diffusion cloud chamber

This cloud chamber is basically an airtight container filled with a mixed atmosphere of air and alcohol vapour. Liquid alcohol evaporates from a reservoir and diffuses through the air from the top to the bottom of the chamber. Cooling the base with dry ice (solid carbon dioxide, which is at a constant temperature of around –79 ºC while it sublimates) results in a strong vertical temperature gradient, so that a zone with supersaturated alcohol vapour forms close to the bottom. This sensitive layer is unstable, with more very cold alcohol vapour than it can hold. The process of condensation of vapour into liquid can be triggered by the passage of a charged particle with enough energy to ionise atoms in its path. These ions are the condensation nuclei around which liquid droplets form to make a trail.

Assembly and operation

• Straight-sided, clear plastic or glass container (e.g. a fish tank) with a base about 30 cm x 20 cm, and a height around 20 cm (other sizes can be used, but the effects may vary)
• Aluminium sheet (about 1 mm thick, same thickness as the base of the fish tank)
• Shallow tray somewhat larger than the base area of the fish tank
• Two lamps, one of them strong
• Strip of felt (about 3 cm wide and long enough to wrap around the inside of the fish tank, e.g. somewhat more than 1 m long)
• Glue (not alcohol-soluble)
• Black insulating tape or duct tape
• Isopropyl alcohol (isopropanol)

• Glue a strip of felt (the alcohol reservoir) around the insides at the bottom of the fish tank (the body of the cloud chamber). Some felt can be glued to the bottom of the tank, too.
• Cut the aluminium sheet to fit (as closely as possible) the top the fish tank, and cover one side of the sheet with insulating tape, forming a black surface.
• Soak the felt with isopropyl alcohol (but not so much that it drips down the sides of the chamber). Safety note: Do this in a well-ventilated room and remember that alcohol is flammable.
• Turn the fish tank upside-down over the aluminium sheet. Make sure the black side of the sheet faces upwards (to make the particle tracks more visible).
• Use insulating or duct tape to fasten the aluminium sheet to the rim of the fish tank, sealing the chamber so that it is airtight . This is the most critical step and must be carefully done, as the joint will become moist and very cold during operation.
• Make a flat layer of dry ice in the tray and place the chamber on top of it, making sure that its base is horizontal. To ensure good thermal contact between the metal plate and the dry ice , avoid large chunks of dry ice: flat sheets or dust are best, but small grains will do. Safety note: Dry ice is around –79 ºC and should only be handled using thick gloves.
• Keep the top of the chamber warm, for example by shining a lamp onto it. Avoid using the chamber in a cold environment, because this could prevent the correct temperature gradient from forming, meaning no tracks can be seen.
• Leave the chamber undisturbed for about 10 min, until the temperature gradient is established. Shine a bright light through the chamber at a low angle , and look at the bottom of the chamber. At first you should see only an alcohol mist falling, but gradually, charged particle tracks should appear as thread-like condensation in the mist. Note: the tracks are more visible in a darkened room.

Although any charged particle with enough energy, for example from ambient radioactivity, can leave its trail in the chamber, the majority of the tracks will be made by secondary cosmic rays : particles created when other particles (mostly protons) coming from outer space hit the upper atmosphere. Secondary cosmic rays travel at close to the speed of light and are absorbed by the atmosphere or decay in flight, giving rise to new particles including muons , which can reach the surface of Earth and are easily detected. Muons are charged elementary particles very similar to electrons except for their mass (which is two hundred times larger).

What you can do with the chamber?

In order to make the chamber really useful, we cannot limit ourselves to showing it and describing how it works. To support the explanation, we have prepared a short, simply written comic strip w1 (see below), showing how the chamber works and illustrating the origin and composition of cosmic rays through the story of a cosmic proton and its descendants.

We use this chamber at school with our 12- to 16-year-old students as part of an effort to help them see particles as real physical objects. Watching the visible trails left by invisible particles and comparing them to trails left by jet engines (in which much of the same physics is involved) is the first step in a process that we continue by introducing real data and pictures from high-energy physics into otherwise standard exercises and questions w2 , w3 ( Cid, 2005 ; Cid & Ramón, 2009 ) and that we conclude with another, more complicated, detector for school use: a cosmic-ray scintillation detector which allows students to record and study data by themselves ( Barradas-Solas, 2007 ).

Acknowledgements

The authors would like to thank Dr Eleanor Hayes, Editor-in-Chief of Science in School , for her assistance in giving the final form to this article.

• Barradas-Solas F (2007) Giving new life to old equipment. Physics Education 42 : 9-11. doi: 10.1088/0031-9120/42/1/F03

To access this article, which is freely available online, visit the website of the Institute of Technical Education, Madrid, Spain ( http://palmera.pntic.mec.es ) or use the direct link: http://tinyurl.com/y8ssyc5

• Cid R (2005) Contextualized magnetism in secondary school: learning from the LHC (CERN). Physics Education 40 : 332-338. doi: 10.1088/0031-9120/40/4/002
• Cid X, Ramón C (2009) Taking energy to the physics classroom from the Large Hadron Collider at CERN. Physics Education 44 : 78-83. doi: 10.1088/0031-9120/44/1/011
• Landua R, Rau M (2008) The LHC: a step closer to the Big Bang. Science in School 10 : 26-33. www.scienceinschool.org/2008/issue10/lhcwhy

Web References

• w1 – The comic strip (in English and Spanish) and full assembly instructions (in Spanish) are available from our website: http://palmera.pntic.mec.es/~fbarrada/cc_supp_mat.html
• w2 – See, for instance, the introductory information about the LHC and simple physical calculations which take place in all particle accelerators (Physics at LHC) on the ‘Taking a closer look at LHC’: http://www.lhc-closer.es
• w3 – The CERN website for high-school teachers ( http://teachers.web.cern.ch ) also includes a gallery of bubble chamber pictures which fits nicely into our project. See the direct link: http://tinyurl.com/yfbv8ls
• Close FE (2004) Particle Physics: A Very Short Introduction . Oxford, UK: Oxford University Press. ISBN: 9780192804341
• The Lawrence Berkeley National Laboratory’s online interactive tour, ‘The Particle Adventure: the Fundamentals of Matter and Force’: www.particleadventure.org
• The virtual visitor centre of the SLAC National Accelerator Laboratory (particularly the sections on theory, detectors and cosmic rays): www2.slac.stanford.edu/vvc
• The CERN website: http://public.web.cern.ch/public/en/Research/Detector-en.html
• Landua R (2008) The LHC: a look inside. Science in School 10 : 34-47. www.scienceinschool.org/2008/issue10/lhchow
• Barnett RM et al. (2000) The Charm of Strange Quarks: Mysteries and Revolutions of Particle Physics. New York, NY, USA: AIP Press. ISBN: 0387988971
• Treiman SB (1999) The Odd Quantum . Princeton, NJ, USA: Princeton University Press. ISBN: 0691009260

Treiman’s book is one of the best to begin tackling the subtleties of quantum mechanics in particle physics (which we have avoided in this article), including virtual and unstable particles, and the field / particle relationship.

• To learn more about cosmic rays, see NASA’s Cosmicopia: http://helios.gsfc.nasa.gov/cosmic.html
• We and many others have learned about building cloud chambers from Andy Foland’s cloud chamber page: www.lns.cornell.edu/~adf4/cloud.html
• The American Museum of Natural History’s website includes an illustrated version of the main stages of the assembly of the cloud chamber: www.amnh.org/education/resources/rfl/web/einsteinguide/activities/cloud.html
• It is not easy to explain in detail the processes of supersaturation and track formation or to justify the choice of active liquid (isopropanol, in our case), as they depend in a complicated way on – for example – ionisation energies, vapour pressures, diffusion rates and various engineering aspects of the chamber. If you want to pursue this further, see the supplementary bibliography on our cloud chamber website: http://palmera.pntic.mec.es/~fbarrada/cc_supp_mat.html
• Budinich M, Vascotto M (2010) The ‘Radon school survey’: measuring radioactivity at home. Science in School 14 : 54-57. www.scienceinschool.org/2010/issue14/radon

Francisco Barradas-Solas has a degree in physics and teaches physics and chemistry at secondary school, although he is currently on leave, working as a scientific advisor to the regional education authority of Madrid, Spain. One of his main interests is the introduction of particle physics to schools and he has taken part in several programmes for teachers organised by CERN.

Paloma Alameda-Meléndez has a degree in chemistry and teaches physics and chemistry at El Álamo Secondary School, near Madrid.

Cosmic rays consist of subatomic particles that come from space and strike Earth’s atmosphere, creating a shower of secondary particles that can be studied at the Earth’s surface. Students in secondary education can usually only read about those particles in books or study them through simulations – although the particles constantly pass through our bodies.

Here, Francisco Barradas-Solas and Paloma Alameda-Meléndez present the idea that cloud chambers can be used by students as an experimental tool, enabling them to conduct their own investigations on radiation. They also provide details about the construction of a cloud chamber, equipment that can be built at school without too much difficulty, which enables students to observe these subatomic particles in the classroom by making their tracks visible.

Vangelis Koltsakis, Greece

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PhysicsOpenLab Modern DIY Physics Laboratory for Science Enthusiasts

Cloud chamber.

May 5, 2017 English Posts , Radioactivity 20,893 Views

See the nuclear particles is possible! With a little patience, at PhysicsOpenLab  we have built a cloud chamber , a tool that allows, for example, to reveal the traces of cosmic rays that are continually “raining” on Earth, coming from the remote regions of the universe.

A cloud chamber is nothing more than a well-sealed box containing an over-saturated alcohol vapor , so concentrated to be ready to condense as soon as something happens to change its condition. That is why when the detector is crossed by cosmic particles, which are electrically charged, the alcohol vapor condenses in very small droplets. The trace of the particles becomes so immediately visible in the form of a long and thin white trace that dissolves in a matter of seconds.

Building a cloud chamber, although it requires a lot of patience, is not difficult, neither expensive. In this post we describe the construction of “our” cloud chamber.

Introduction

The cloud chamber we are building is a diffusion type cloud chamber. As mentioned above this cloud chamber is basically an airtight container filled with a mixture of air and alcohol vapor. Liquid alcohol evaporates from a reservoir and spreads inside the chamber. Cooling the base with dry ice (carbon dioxide ice at a constant temperature of -79 °C while subliming) you get an intense temperature gradient along the vertical direction. In this way, a super-saturated alcohol vapor zone is formed on the bottom. The sensitive layer is unstable: it has a quantity of cold alcohol vapor greater than it can maintain. The condensation process is triggered with the passage of the charged particle with enough energy to ionize atoms along its path. These ions are the condensing cores around which liquid droplets form a trace. The image below shows the basic equipment.

The traces left by the particles depend on the particle type and their energy. α particles (helium nuclei) leave short and wide traces as their range in the air is just a few centimeters, β particles (electrons) give rise to thin, sometimes straight lines, sometimes with rough paths, depending on whether they are scattered or not. The most “spectacular” traces: clear, long and straight are produced by cosmic muons that are very energetic particles and therefore give rise to rather intense ionization. The image below provides a representation of the typical traces visible in a cloud chamber.

History of Cloud Chambers

(From SCoolLAB DIY Cloud Chamber CERN Manual)

The cloud chamber is one of the oldest particle detectors, and it led to a number of discoveries in the history of particle physics. It also was involved in two Nobel prizes!

Charles T. R. Wilson (1869 – 1959)

Carl Anderson (1905 – 1991)

Construction

The first component of the cloud chamber is the base that is intended to contain dry ice and provide adequate insulation to ensure maximum working life with a “dry ice” charge. For the stand we opted for a solid wooden structure, further internally insulated with expanded polystyrene plates (styrodur). At the center, a foam pad was placed in order to achieve the best contact between the dry ice and the aluminum base of the cloud chamber. The picture below shows the base of our cloud chamber.

The outer dimensions of the stand are 40 cm x 40 cm, while the inside, insulated with polystyrene, has a smaller size: 30 cm x 30 cm, the space for dry ice is further reduced due to the polystyrene plates and the foam, this allows you to not “waste” too much dry ice during operation of the chamber. The image below shows the base filled with an adequate amount of dry ice.

Dry ice (or carbon snow) is obtained by expanding compressed carbon dioxide, such as that found in fire extinguishers. For a run of our cloud chamber we need at least a couple of kilograms of carbon snow. Carbon snow should be handled with care because it can cause cold burns : it is mandatory to wear protective gloves . It should also be stored in insulated containers, as shown in the image below, in order to preserve it as long as possible.

A black anodized aluminum plate, 2 mm thick, is placed above the base. This plate form the base of our cloud chamber, as shown in the following image. Aluminum is used because it is an excellent heat conductor, it is has to be black in order to highlight the traces of the particles. It is advisable to use black anodized aluminum rather than paint it as the paint easily disappears when the aluminum is cooled and comes into contact with the isopropyl alcohol.

A wooden frame is secured to the aluminum plate to contain inside the actual cloud chamber. On two sides of the frame there are two grooves that are intended to accommodate two strips of led each. These LEDs are high power, powered by 12 Vdc, and are used for lighting. On the inside sides of the wooden frame are felt strips to make the chamber more stable and to prevent the plexiglass from scratching with the surface of the wood.

In the picture below you can see a detail of the LED strip.

The cloud chamber consists of four acrylic slabs glued together. Be careful that gluing of acrylic sheets should be done with a special procedure consisting in joining the parts to be glued and applying a solvent at the joint point. The solvent penetrates between the surfaces, melting a thin layer that immediately polymerizes again, thus producing a kind of welding. The picture below shows the acrylic sheets after the bonding operation. It may be necessary to put a transparent silicon veil along the joints to ensure the air-tightness of the chamber.

On the edges of the slabs (upper edge and bottom edge) silicon gaskets are then applied (the ones used for the fixtures are fine). The chamber is then inserted into the wooden frame and rested on the aluminum base, as shown in the picture below.

The top of the chamber is closed with a lid. The cover is made of acrylic and is inserted in a wooden frame. On the acrylic slab are four heating elements, operating at 12 Vdc, which produce a temperature of 40 ° C. In contact with the heating elements is placed an aluminum plate on whose inner surface are placed the felts that are dipped with isopropyl alcohol to activate the chamber. In this way, the upper side of the chamber is maintained at a temperature of about 40 ° C while the lower side is maintained at about -79 ° C. The top cover, with the felt, acts as an alcohol reservoir. Between the top and bottom side of the chamber is also maintained a potential difference of about 1000 Vdc which is intended to “clean” the inside of the chamber from dust which could trigger unintended condensation of the alcohol.

The equipment was completed with a control box that houses the four LED strips and electrical resistors control switches, all powered by 12 Vdc.

There is also the HV generator, whose outputs are connected to the upper aluminum plate and to the base.

The following pictures show the complete cloud chamber.

Running the Cloud Chamber

For the operation of the cloud chamber, it is necessary to have an adequate amount of dry ice or carbon snow which is to be inserted into the base of the chamber, taking care to ensure a good thermal contact with the aluminum plate. The felt on the top cover is dipped with isopropyl alcohol, but be careful not to use too much alcohol otherwise it will start to “rain” alcohol. When this is done, close the chamber, start the heating resistors, the LED lights and the high voltage to deposit the dust.

After a few minutes you start to see a slight mist of very small droplets, if the drops are too much it means that too much alcohol is used. When the temperature and density of the alcohol vapor are the right ones, you can see the traces left by cosmic ray particles and background radioactivity.

If you have some sample source (alpha or beta), you can place it inside the chamber and see the traces of particles leaving the sample, as in the video below.

Maybe the right conditions are not found on the “first shot” but after a few attempts the result is ensured!

Improvements

The cloud chamber of PhysicsOpenLab (described above) has recently been improved with some significant changes.

• the aluminum cover has been replaced by a transparent acrylic cover (Plexiglas), this allows to observe the traces of the particles from above, obtaining an improved visibility.

On the side walls of the chamber two plastic trays were fixed and, directly inside this trays, the heating resistances were positioned. Thus the isopropyl alcohol can be inserted into the trays directly from the holes shown in the image below, where the resistance cables also pass.  Alcohol should be introduced by syringe. This operation can be done with the chamber closed without interrupting operation. The trays then replace the felt soaked in alcohol.

On the Plexiglas cover, on the side edges, adhesive aluminum strips have been fixed, which have the purpose, together with the metal base to create the electric field inside the chamber, useful to improve the “quality” of the particle tracks.

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Gamma Spectroscopy with KC761B

Abstract: in this article, we continue the presentation of the new KC761B device. In the previous post, we described the apparatus in general terms. Now we mainly focus on the gamma spectrometer functionality.