Cloud Chambers: Revealing Invisible Particles
When you hear the word radiation, you likely associate it with danger. What’s not commonly known is that the discovery of radiation prompted the quantum physics revolution and transformed our understanding of the universe. Today, radiation is its own field of scientific research, which in its infancy allowed us to detect “background radiation.” In fact, it’s known today that these so-called background radiation particles are harmless and are sailing through your body right now as you’re reading this.
The existence of background radiation remained a mystery until 1896. Among others, we can thank Henri Becquerel for the discovery while investigating possible connections between X-rays and phosphorescent crystals. A phosphorescent substance will glow when exposed to certain wavelengths of light. It can take in the light and release it at a longer wavelength.
When phosphorescent uranium salts and a photographic plate covered with opaque paper were positioned near each other, Becquerel discovered that the plate fogged, a result of spontaneous radioactivity. In 1903, Becquerel was given half of the Nobel Prize for Physics, while the other half was awarded to Pierre and Marie Curie for their own study of radiation.
Their combined research led to the discovery that all atoms can be categorized as stable or unstable. An atom may be unstable due to unbalanced forces between particles or one too many protons or neutrons within the nucleus.
When the nuclei of an atom spontaneously release subatomic particles and/or energy, it is in an effort to stabilize itself and regain balance. When this happens, we say the atom is undergoing radioactive decay. Through this process, the energy it releases is referred to as radiation. Thus, it is fair to say that radioactivity is a property that pertains to individual atomic nuclei.
Radioactivity can be observed through a simple yet groundbreaking device called a cloud chamber, an invention developed by Scottish physicist Charles Thomson Rees Wilson throughout the first decade of the 20th century. Cloud chambers are most often used to detect background radiation — radiation that is present in a given environment. Specifically, it detects ionizing radiation. Ionizing radiation is composed of ionizing particles, which are particles that contain energy capable of ionizing other atoms; they can remove electrons from atoms or molecules in their path. This type of radiation can be harmful, however, the doses we are exposed to in our daily lives are too small for any real damage. Part of this is due to Earth’s protective magnetosphere. Composed of magnetic fields, it encases our planet and shields us, limiting the amount of cosmic rays that penetrate its border. However, when cosmic radiation does make it through, it’s in such low doses that it doesn’t create any harm.
Once these particles make it through the atmosphere, the majority of them have decayed into muons (a type of lepton identical to the electron, except it has a mass around 200 times larger) which barely interact with matter. According to CERN, “The rate of muons arriving at the surface of the Earth is such that about one per second passes through a volume the size of a person’s head.”
The cosmic rays that compose background radiation, which cloud chambers are designed to detect, are mostly composed of muons. However, they can detect a range of particles, including alpha and beta particles, gamma radiation, and pretty much anything electrically charged. However, a cloud chamber does not reveal the particle itself. Rather, it reveals a trail from a particle’s past trajectory as the particle traveled through the cloud chamber. The trajectory is revealed through condensation, visible as a cloud streak, unveiling the invisible particle’s path through the chamber.
How does it work?
Cloud chambers take advantage of the fact that these particles ionize others around them. A supersaturated vapor is used as the detecting medium, as it condenses tiny liquid droplets around the newly produced ions that form as charged particles pass by.
Original cloud chambers used air saturated with water and a glass chamber that allowed you to adjust the volume. When the bottom of the chamber is pulled down, the additional space allows the vapor to expand, which results in the cooling of the chamber as a whole. The energy for the expansion comes from the internal energy of the gas. Internal energy is a sum of all the kinetic energy of the molecules that make up the gas. When the chamber is expanded, the gas molecules bump into each other and the chamber less frequently, resulting in a lower internal energy and a lower temperature. So, as the chamber expands, the temperature drops. This thermodynamic difference results in the water vapor nearly condensing as it becomes supersaturated.
A supersaturated substance is one that has already been saturated. A change in temperature creates the conditions necessary for the supersaturation to occur. As the temperature drops, the substance can no longer hold the amount of vapor it was previously holding. The only way for the substance to get out of its unstable state is to release some of the vapor through precipitation or condensation.
If an ionizing particle passes through the vapor in a cloud chamber, the newly formed ions function as points of condensation for the surrounding vapor. This results in the formation of visible clouds, hence the name, cloud chamber.
Modern cloud chambers are also known as diffusion chambers and they work slightly differently. Instead of water they use an alcohol, such as propanol. This is because propanol is very volatile, meaning it evaporates easily at normal temperatures. And instead of relying on a change in volume and internal energy for the temperature drop, they use dry ice to cool the chamber’s base. An absorbent material, such as a tissue is soaked in the propanol and placed at the top of the chamber.
The top of the chamber is significantly warmer than the chamber base, so when the vapor from the propanol falls to the bottom, it cools rapidly and reaches a point of supersaturation. Once again, as ionizing particles pass through the chamber, they form new ions within the chamber, allowing the vapor to condense into liquid droplets in the chamber and reveal the particle’s path.
Certain properties can be determined by the nature of a cloud trail left behind by a particle. For example, repeated changes of direction indicate interactions with gas molecules, and curves in the trajectory of a particle can be observed if an electric or magnetic field is present in the chamber. The direction of the curve reveals the charge of the particle, as positive and negative particles curve in opposite directions.
This is how Carl Anderson proved the existence of antimatter in 1932. Using a cloud chamber surrounded by a magnetic field, he observed a particle with the exact same properties as an electron but with a positive charge. This particle is known today as a positron.
Cloud chambers laid out the foundation for entire fields of research. They proved the existence of a new category of particles which became crucial to our understanding of the universe and the fundamental nature of our reality.
