Molly Ayala
Cloud chambers are devices that allow us to visualise the tracks left by cosmic radiation, environmental radioactivity, or radioactive samples. They were invented in the early 1900s by the Scottish physicist Charles Thomson Rees Wilson and played a prominent role in experimental particle physics from the 1920s to the 1950s. Building your own cloud chamber can be a mesmerising and satisfying project, and while CERN's cloud chambers took many years to plan and build, you can make a simple cosmic-ray detector at home with the right materials and safety precautions.
| Characteristics | Values |
|---|---|
| Purpose | To detect cosmic rays and visualize the tracks left by cosmic radiation, environmental radioactivity, or radioactive samples. |
| Principle | When a charged particle passes through the chamber, it knocks electrons off gas molecules, leaving a trail of ions. Mist droplets form around these ions, creating a visible cloud track that can be studied to determine the energy and charge of the particle. |
| Required Materials | Metal sheets, foam insulation, dry ice, alcohol, lights, high voltage source, glass container, cooling system, etc. |
| Safety | Handling dry ice and isopropanol can be dangerous; adult supervision is recommended. |
| Cost | Approximately 250-300 € for materials, excluding tools. |
| Time | Building a cloud chamber can take a significant amount of time, with some projects spanning two years or more. |
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What You'll Learn
Use the right materials, like metal sheets, wires, and isopropanol
To build an electric cloud chamber, you'll need to gather the right materials. Metal sheets, wires, and isopropanol are among the most important components.
For the metal sheets, aluminium is the preferred choice. You'll need an aluminium base to support the chamber and create the electric field, and adhesive aluminium strips are fixed to the Plexiglas cover to generate the electric field inside the chamber. This electric field is crucial for improving the quality of particle tracks and preventing background "rain" from obscuring the sensitive region.
The wires, or resistance cables, are used to heat the isopropyl alcohol trays. These cables pass through the holes in the plastic trays, where the alcohol is inserted. The alcohol should be introduced carefully with a syringe, even when the chamber is closed, to avoid interrupting its operation.
Isopropanol, or isopropyl alcohol, is essential for creating the supersaturated vapour necessary for the cloud chamber's functionality. The alcohol evaporates and spreads inside the chamber, and when cosmic particles pass through, they interact with the vapour, leaving visible traces. The lower boiling point of isopropyl alcohol helps with evaporation, and its high purity contributes to larger cloud formations.
In addition to these key materials, you'll also need acrylic or Plexiglas sheets to construct the chamber walls and a lid. A wooden frame will support the entire structure. Remember to exercise caution when handling dry ice and isopropanol, as they can be dangerous if not used properly.
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Ensure a temperature below -25°C
To ensure the temperature is below -25°C, you can use a Peltier cooler. Peltier coolers are solid-state heat pumps that can be used to cool cloud chambers. They are a good alternative to dry ice, which is non-renewable.
To achieve the required temperature, you can cascade two Peltier elements and run the bottom one at a higher voltage. You can experiment with different configurations, such as using two or three TECs (thermoelectric coolers) at different voltages. However, a two-stage TEC element might be a more effective option, requiring only a single voltage.
The TEC should be attached to a CPU cooler with heat pipes. CPU coolers are designed to work with the heat pipes facing upwards, but they can also be used upside down, providing sufficient cooling power. The TEC is attached to the CPU cooler with thermal grease or paste, which helps with thermal coupling and achieving lower temperatures.
After assembling the TEC, copper plate, and enclosure, connect everything to the power supply and measure the temperature. If you do not reach a temperature below -25°C, check your thermal coupling and ensure the TEC is facing the correct direction.
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Create a high-voltage field with a power supply
To create a high-voltage field with a power supply for your electric cloud chamber, you will need a high-voltage DC (direct current) power supply. The voltage required is in the thousands, typically above 1kV.
There are several ways to achieve this high voltage. One method is to use a small Whimhurst Machine or a small Van de Graaff generator, which can be either built or purchased. These are safer options compared to a high-voltage DC power supply, which can pose safety risks if not handled properly.
Another option is to harvest the high-voltage step-up board from an electric fly swatter, as demonstrated by Aidan Lawrence. The circuit involves using a pair of AA batteries to supply 3V DC, which is then converted into a high-frequency alternating current using a transistor circuit. This AC current is then stepped up in voltage using a transformer before being converted back to DC using a bridge rectifier. Finally, a capacitor stores the high-voltage DC energy for use in the cloud chamber.
Once you have your high-voltage source, you can create an electrostatic field within the cloud chamber by connecting the leads from the power source to two pieces of aluminium foil or similar conductive material placed on the top and bottom of the chamber. This setup will cause the paths of charged particles within the chamber to curve upward or downward, depending on their charge and the polarity of the foil plates.
It is important to exercise caution when working with high voltages. Ensure that you understand the nuances and dangers associated with high-voltage equipment and always prioritize safety in your setup and handling procedures.
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Soak the sponge with isopropanol and place a sample on the plate
To build an electric cloud chamber, you will need to soak the sponge with isopropanol and place a sample on the plate. Here is a step-by-step guide for this process:
Firstly, ensure you have the right materials. You will need isopropanol, also known as isopropyl alcohol, which can be found in drug stores as a common disinfectant or cleaning agent. It is important to use pure alcohol for this experiment, as any water content can lead to condensation and freezing issues. To handle the isopropanol, take the necessary precautions as its vapours are toxic and highly flammable.
Next, prepare the sponge by thoroughly soaking it in isopropanol. The sponge will be placed above the high-voltage grid, and its position can be adjusted with PVC pipes. The sponge's distance from the cold plate is important, as it determines the amount of alcohol that will evaporate. A tip to consider is using felt instead of a sponge, as it can absorb and release alcohol more uniformly.
Once the sponge is in place and soaked, you can proceed to put the sample on the plate. The sample should be placed in the middle of the copper plate, which is located directly under the high-voltage grid. This sample can be any radioactive source, such as alpha or beta particles, or even a few pieces of thoriated welding rods. Ensure that the sample is secure and stable to avoid any disruptions during the experiment.
After placing the sample, you can switch on the chamber. Observe the formation of a mist of alcohol at the bottom after a few minutes, indicating that the chamber is functioning. The visibility of the tracks can be enhanced by switching on the high voltage, making the tracks more focused and distinct.
By following these steps and adjusting the components according to your specific setup, you should be able to effectively soak the sponge with isopropanol and place the sample on the plate in your electric cloud chamber.
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Observe cloud tracks with a light source and black background
To observe cloud tracks, a black background is ideal. A tangential light source is then required to illuminate the white droplets against the black background. The light source should be positioned to the side, rather than directly above, to create the desired effect.
The cloud tracks are the result of an ion trail left by radioactive particles. This trail provides an optimal trigger for condensation and cloud formation. The droplets are visible as a "cloud" track that persists for several seconds while the droplets fall through the vapour.
The tracks have distinct shapes, depending on the particle that created them. For example, an alpha particle track is thick and straight, while a beta particle track is wispy and shows more evidence of deflections by collisions. These tracks can be studied to determine the energy and electric charge of the particle that produced them.
In a cloud chamber, a tray of alcohol is heated to form alcohol vapour. The vapour then condenses around the trail of ions created by a cosmic ray, which can be seen as a visible streak of tiny droplets.
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Frequently asked questions
A cloud chamber is a device that allows you to visualise the tracks left by cosmic radiation, environmental radioactivity, or radioactive samples.
You will need access to the right materials, such as metal sheets, foam-core poster boards, black spray paint, a glass fish tank, a cooler, LED lights, a high-voltage electric fly swatter, wires, isopropyl alcohol, and a 3D printer.
First, assemble the housing and place the high-voltage grid and sponge on top of the cold plate. Connect the wire, seal the chamber with glass, and soak the sponge with isopropanol. Place a sample in the middle of the copper plate and switch on the chamber. You should see a mist of alcohol forming, and tracks should appear.
Be careful when handling dry ice and isopropanol, as they can be dangerous. It is recommended to have a teacher or guardian present to help with the setup and operation of the chamber.
