Observing cosmic rays and radioactive particles using parts from Aliexpress
At some point in physics class you learn about ionizing radiation and find yourself staring at this image:
Somehow, people were able to take photos of radioactive particles over 100 years ago using this apparatus:
Surely with advances in modern technology (and AliExpress) we can achieve the same at home? Let’s get started.
Before building anything, we should understand how a cloud chamber works: supersaturated vapor is produced in a chamber, so that when a charged radioactive particle passes through the vapor it ionizes gas molecules along its path. These ions act as nucleation points where the vapor condenses, creating a misty trail (superficially similar to airplane contrails).
To create the supersaturated vapor, we will need to boil our vaporization liquid at the top of our chamber and sufficiently cool it at the bottom of the chamber. This temperature gradient is what causes supersaturation: the warmer air can hold more vapor than the cooler air, so as the air from the top of the chamber falls toward the cold end of the volume, it reaches a state where it holds more vapor than it can ordinarily hold at that temperature - it becomes supersaturated.
Due to the process of vapor being continuously evaporated at the top of the chamber, diffusing downwards (due to gravity) and progressively cooling until supersaturated, this apparatus is known as a “diffusion cloud chamber”.
The vaporization liquid which is most commonly used is isopropyl alcohol, due to its reasonable boiling point (82C), ease of purchase, and low cost. We will use the same.
To achieve the cooling, a common method is to place dry ice underneath the chamber, creating a very cold floor. This is cheap and simple, but is a short-term, one-time solution. The dry ice is a consumable and lasts a few hours at most. There is a much more interesting approach to this problem which can run continuously and on-demand: thermoelectric cooling with Peltier coolers.
By applying a voltage across a stack of n-type and p-type semiconductors, a temperature gradient is created. With a sufficiently large stack and adequate cooling of the hot side (to extract as much heat as possible), we can create a cold floor beneath our volume.
With these fundamentals figured out, our process will be the following:
Our chamber needs to achieve a temperature below -26°C (the temperature at which alcohol vapor supersaturation occurs), which means the floor of our chamber needs to be cooled even more than that (likely under -35°C). To achieve this, multi-stage cooling is required.
As always we begin by surveying other successful projects, and this one provides a useful diagram of their thermoelectric cooling system:
We will take a similar approach with our cooling system (but will design and print a 3D-printed structure, instead): a stack of 3 thermoelectric cooling stages (Peltier modules), one water cooling stage, and one air cooling stage. In this arrangement, heat is “pumped” from the metal floor of our chamber through two layers of weaker thermoelectric coolers before being pumped through a more powerful thermoelectric cooling layer - this should prevent any individual Peltier module from being overwhelmed. The watercooling layer removes moves most of the heat away from the cooling stack (to an air-cooled radiator), and an air-cooled heatsink handles any residual heat below the watercooling block.
The cooling system begins with the metal floor of our chamber. Wherever metal is required in our cooling system, we will use copper due to its high thermal conductivity (one of the highest among commonly-used materials). The only drawback is it is relatively expensive.
We want to maximize our particle viewing area, so we will use two 1/4" thick copper blanks side-by-side to achieve a 4" x 4" floor:
To maximize contrast when viewing particles, we will want our floor to be as dark as possible. We will place black aluminum foil tape on top of the copper to achieve this:
Every metal surface which interfaces with another layer of the cooling stack will need to be as flat as possible for maximum thermal transfer. The brilliant Main Presenter of the Tech Ingredients YouTube channel describes a classic lensmaker grinding process to achieve this in his video on thermal interface materials:
As a final step to maximize thermal transfer we will use Noctua NT-H2 thermal compound between all thermal interfaces:
The active elements of the thermoelectric cooling system will be cheap and readily-available Peltier cooling modules. These come in a standard size of 40mm x 40mm, (roughly 1.5" x 1.5") so a 2 x 2 arrangement will work nicely under our 4" x 4" copper floor. We will arrange stacks of 3 modules each, with the most powerful module at the bottom:
Running each layer of the Peltier stack at a different voltage will require multiple DC power supplies:
The watercooling system is a simple design:
Heat is taken out of the thermoelectric cooling system via two copper cooling blocks underneath the Peltier stack:
The hot water (and cool water on the other side of the loop) pass through two PVC impellers to measure flow rate (with temperature sensors installed to monitor water temps):
The hot water passes through a large radiator, which will have 3 industrial fans attached to it:
The water is pumped from a large cylindrical reservoir using a generic pump:
Everything is connected using soft silicone tubing, backed up with small hose clamps:
The assembled watercooling setup:
We will use a large copper heatsink to radiate any residual heat from the Peltier stack which has not been extracted by the watercooling system:
We build a 3D-printed base to house the entire chamber cooling stack, with small 12V fans blowing across the copper heatsink final stage:
With the cooling system complete, we can now focus on measuring the temperature of our chamber floor.
Our target temperature at the chamber floor is below -35°C, which is too low for most common temperature sensors (commonly-available temperature sensors have reduced accuracy below -20°C to 0°C). Affixing a Type-T thermocouple to a hole drilled into our chamber floor is an ideal solution to this problem. With a range of −200°C to +350°C, we will have no problems with accuracy at any temperature we will conceivably encounter.
A Type-T thermocouple is easily obtained:
But actually measuring temperatures with the thermocouple is a much harder (read: expensive) problem - digital thermometers compatible with Type-T thermocouples aren’t cheap. Building our own digital thermometer is quick and inexpensive, however.
For the microcontroller we will use my favourite, the Arduino Pro Micro:
To interface with the thermocouple we will use a Universal Thermocouple Amplifier from Adafruit:
And for the display we will use the ubiquitous backlit 1602 (16 characters, 2 lines) LCD display:
We wire everything up on a protoboard:
And design a 3D-printed case:
The final product:
We are now ready to design the diffusion chamber itself.
The diffusion chamber needs to enclose our copper floor within a sealed transparent cover. At the top of the chamber we will need a heating element to evaporate alcohol. The copper floor will need side-lighting to enhance contrast when viewing particle trails. And lastly, we can add a high-voltage ion scrubber to improve the performance of the cloud chamber.
A mock-up of what the assembled chamber will look like:
The cover is an easy problem to solve: 5 sheets of 6"x6" acrylic (2mm thick), bonded together using Weld-On #4:
Where the cover touches the floor, we will use neoprene foam to ensure an air-tight seal:
Side-lighting with white LEDs will maximize contrast when viewing particle trails. Two rows of 5mm super-bright white LEDs will do the trick:
A 3D-printed mount for the LEDs is designed to snap into place on the chamber floor:
Evaporating alcohol at the top of the chamber is simple. Felt squares are soaked in alcohol and a simple loop of resistors is used to heat the felt. The felt is held on a wire mesh, which is raised off the chamber floor using delrin rods:
The rubber stoppers holding the wire mesh in place are an unusual item: cattle castration rings.
By applying a high-voltage electric field across the chamber volume, we can achieve better viewing of particle trails due to a variety of effects:
To achieve this, we harvest an electric fly-swatter for its high-voltage power supply and metal grid:
We cut out the metal grid and place it above the viewing area. The copper floor of our chamber is our ground reference.
With all of the components ready we can assemble the full cloud chamber:
The watercooling system is working as expected, we can add ice packs to the air-cooling system for even better performance:
We test the lowest temperature we can achieve:
-37.72°C is more than sufficient for supersaturation. We are ready to observe particle trails.
Background radiation:
Thoriated tungsten welding rods:
Being able to see the path of radioactive particles is a luxury, usually we are only able to detect their presence through the horrifying click of a Geiger counter (or similar detector). What if we combined the two? By placing a Geiger counter close to the viewing area, we could observe the particle trail and hear a confirmation of the particle hitting the Geiger–Müller tube. This is a ludicrous idea and I do not believe the correlation is so simple (e.g. there are radioactive particles coming from every direction at all times), but it is a fun experiment to try.
Like everything else, we start by making our own Geiger counter.
Thanks to the Chernobyl incident, eBay is littered with surplus Soviet SBM-20 Geiger–Müller tubes:
Using the SBM-20 as a detector, there are countless examples of simple geiger counter circuits. We will use this one as inspiration. The circuit is based on a 555, and pulses from the SBM-20 tube will be used to drive an LED and a speaker.
We build the circuit on stripboard, and lasercut a clear acrylic case:
The final product:
Placing the Geiger counter next to the viewing area of the cloud chamber, we achieve our goal: capturing the trail of a particle before it hits the Geiger–Müller tube: