If you look back at the cloud chamber exhibit, we saw that particles could interact with the gas in the chamber and the trails left behind showed us when particles traversed through the chamber. The cloud trails, shapes, and sizes could then be used to visually determine what type of particle had passed through the chamber. A similar experiment can be done to see particle interactions, but rather than using cloud vapour, a different object is utilized: bubbles!
A bubble chamber is an apparatus that is full of a given liquid and kept at high pressure. When a particle passes through the chamber, it can create bubbles to be detected. Depending on the shape, size, and position of the bubbles, what type of particle travelled through the detector can be determined. Bubbles are also going to pop after a short time, and inside the detector we can capture the sound of the bubble’s creation and popping. The specific sound of the creation of the bubble forming is able to give scientists clues about the type of the particle detected, with each particle type giving slightly different sounds.
What is the liquid inside the detector and why must it be kept at a high pressure? How are the bubbles created?
Different liquids can be used to different effects determined by their properties in order to detect particles with the lowest background noise and cost. The liquid must be kept steady at a very precise pressure that puts it in a precarious state of wanting to boil, but just being shy of the energy required to do so. By carefully keeping the liquid in this balanced motionless state, the liquid is perfectly balanced between remaining calm liquid and beginning to boil into a vaporous state. When a particle like an alpha particle, or a neutrino coming from the sun, passes through the detector, it provides enough energy to the trail of liquid it passes through to cause it to boil. This then quickly releases a bubble (or a series of bubbles) that can be detected. The amount of bubbles produced, the relative size, and even the sound created when the bubble(s) form can then be used to figure out which type of particle it was that passed through the detector! Once the bubbles are formed, the device then quickly repressurizes itself to stop all the liquid from being disturbed and sets the device back to its state before the bubble was created, ready to detect again! The image to the right showcases a number of observed particle interactions passing through a chamber causing different bubble formations.
These detectors are currently being used in attempts to discover dark matter particles, as well as different types of neutrinos. Scientists are in labs right now developing new technologies with these chambers to try and discover a currently theoretical process called neutrinoless double-beta decay. You can check out a number of experiments currently using bubble chambers here as well as watch a YouTube video on the bubble chamber device used in the PICASSO experiment.