Preparation for cloud chamber workshops

What happens during this workshop?

Students learn about the principles of particle detection. They build a cloud chamber using the equipment provided, observe and categorize particle tracks, and discuss their observations with the rest of the group and a CERN scientist.

Before taking part in this workshop, students should already know about:

  • different types of particles, such as proton, neutron, electron, photon, (muon, positron, neutrino) and their properties
  • radioactivity and natural radiation (alpha, beta, gamma)
  • the different states of matter, evaporation, condensation
  • ionisation

Key concepts

  1. Elementary particles: An elementary particle is a particle without substructure according to our current understanding. Example of elementary particles are electrons, muons, and photons. We describe protons and neutrons as particle systems, because they are made of quarks. The alpha particle is a special system, because is consists of 2 protons and 2 neutrons – also known as a helium nucleus.
  2. Particle properties: We distinguish particles by their properties such as mass and electric charge.
  3. Ionisation: Ionisation is the process by which the equal number of protons and electrons in a neutral atom changes: Either an electron leaves an atom after absorbing enough energy, or an electron joins an atom.

Helpful material

  1. Find out which of the following are elementary particles, sort them according to their mass, and find out which of them have an electric charge: electron, muon, neutron, alpha particle, carbon, photon, proton
  2. What happens during an ionisation process? Select one or more:

a) A neutron in the atomic nucleus absorbs enough energy to leave the atom.
b) An electron in an atom absorbs enough energy to leave the atom.
c) An atomic nucleus splits.
d) An electron joins an electrically neutral atom.

Summary and link to CERN physics

Ionising particles are part of our everyday life although we can’t see them. They can be made visible using relatively simple equipment. Cosmic particles colliding with the upper layers of the Earth’s atmosphere produce showers of new particles, some of which reach ground level. Naturally-occurring radioactive elements in the air (Radon) emit alpha particles.

The cloud chamber was used to discover the positron (Nobel Prize 1932) and the muon (Nobel Prize 1936). Today, cloud chambers are only used in education. Modern particle detectors at CERN (e.g. ATLAS) use similar principles of particle detection as cloud chambers (though different techniques) to explore the properties of particles.