Particles are the fundamental constituents of matter. Some of them move freely around us: they can come from radioactivity, from the sun or from the depths of space. By studying these particles, we can better understand the rules that govern our universe. Such is the quest of my project. Problem As you can imagine, particles of this size are invisible to the naked eye. Even with the most powerful electron microscope, it’s impossible to see them. However, there’s a machine called a “fog chamber” which, by using a layer of metastable vapour, makes it possible to form a particle “screen”. In order to form this state, an alcohol must be evaporated under very specific conditions of temperature, illumination, electric and magnetic field, all without air fluctuation. If all these elements are met, it should be possible to reveal subatomic particles in the form of droplet trails in the vapor. Methodology The greatest difficulty was to reach the temperature of -25°C. My first approach was to keep things simple: use Peltier modules. These are electronic components that create a temperature difference when an electric current is applied to them. In concrete terms, the first machine consists of 12 Peltier modules that cool a 100 cm2 surface, along with all the other electrical components needed to achieve the right conditions for particle visibility. The device works and allows subatomic particles to be revealed to the naked eye. However, many problems were encountered with this prototype: temperatures are not constant, there are too many air fluctuations due to the heat of the LEDs illuminating the chamber, and the screen surface is small, making it difficult to study cosmic rays. I therefore developed a second version of the fog chamber. This uses a much more complex phase-change refrigeration principle (a closed circuit filled with propane compressed to 12 bar). It solves all the problems encountered with the first version of the machine. It also incorporates a powerful magnetic field that enables certain particles passing through the chamber to be identified, and even their energy to be estimated. The screen surface is 8 times larger than that of the previous version, making it ideal for studying cosmic rays. Results Emissions of 6 radioactive isotopes contained in samples I collected were observed in the fog chamber (Radium-226, Radon-220, Thorium-232, Uranium-238, Potassium-40, Americium-241), as well as 3 forms of phenomena involving subatomic particles (electrons bent under the influence of the magnetic field, creation of electron/positron pairs and delta rays emitted by collisions between particles). Cosmic rays could also be studied, mainly by observing muons passing through the chamber. The first fog chamber was also used to make presentations on radioactivity, to give a visual demonstration of the concepts learned in class. Discussion The results obtained with radioactive sources in the fog chamber are consistent with theory. The fog chamber makes it easy to distinguish between different types of particle (e.g. alpha particles leave a short, wide trail). The relative activity of a sample can be determined, as can the type of particles emitted, again in line with theory (Am-241 emits mainly alpha particles, while K-40 emits only betas). Conclusions The fog chamber fits perfectly into the “Physics/Technology” category, blending engineering concepts for the design and construction of the chamber with particle physics phenomena, all in a visual and intuitive experience. The project was a great success. At first, the goal was only partially achieved with the first version, then largely surpassed with the second. The problems encountered were numerous and considerable, but they all had one thing in common: they enabled me to develop my skills by overcoming them. The ambition of this adventure has pushed me to go further and further into areas I hardly knew existed.