Cloud chamber in the context of "Supersaturation"

In modern physics, antimatter is defined as matter composed of the antiparticles (or "partners") of the corresponding particles in "ordinary" matter, and can be thought of as matter with reversed charge and parity, or going backward in time (see CPT symmetry). Antimatter occurs in natural processes like cosmic ray collisions and some types of radioactive decay, but only a tiny fraction of these have successfully been bound together in experiments to form antiatoms. Minuscule numbers of antiparticles can be generated at particle accelerators, but total artificial production has been only a few nanograms. No macroscopic amount of antimatter has ever been assembled due to the extreme cost and difficulty of production and handling. Nonetheless, antimatter is an essential component of widely available applications related to beta decay, such as positron emission tomography, radiation therapy, and industrial imaging.

In theory, a particle and its antiparticle (for example, a proton and an antiproton) have the same mass, but opposite electric charge, and other differences in quantum numbers.

Vapor pressure or equilibrium vapor pressure is the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases (solid or liquid) at a given temperature in a closed system. The equilibrium vapor pressure is an indication of a liquid's thermodynamic tendency to evaporate. It relates to the balance of particles escaping from the liquid (or solid) in equilibrium with those in a coexisting vapor phase. A substance with a high vapor pressure at normal temperatures is often referred to as volatile. The pressure exhibited by vapor present above a liquid surface is known as vapor pressure. As the temperature of a liquid increases, the attractive interactions between liquid molecules become less significant in comparison to the entropy of those molecules in the gas phase, increasing the vapor pressure. Thus, liquids with strong intermolecular interactions are likely to have smaller vapor pressures, with the reverse true for weaker interactions.

Air showers are extensive cascades of subatomic particles and ionized nuclei, produced in the atmosphere when a primary cosmic ray enters the atmosphere. Particles of cosmic radiation can be protons, nuclei, electrons, photons, or (rarely) positrons. Upon entering the atmosphere, they interact with molecules and initiate a particle cascade that lasts for several generations, until the energy of the primary particle is fully converted. If the primary particle is a hadron, mostly light mesons like pions and kaons are produced in the first interactions, which then fuel a hadronic shower component that produces shower particles mostly through pion decay. Primary photons and electrons, on the other hand, produce mainly electromagnetic showers. Depending on the energy of the primary particle, the detectable size of the shower can reach several kilometers in diameter.

The air shower phenomenon was unwittingly discovered by Bruno Rossi in 1933 in a laboratory experiment. In 1937 Pierre Auger, unaware of Rossi's earlier report, detected the same phenomenon and investigated it in some detail. He concluded that cosmic-ray particles are of extremely high energies and interact with nuclei high up in the atmosphere, initiating a cascade of secondary interactions that produce extensive showers of subatomic particles.

Cloud condensation nuclei (CCNs), also known as cloud seeds, are small particles typically 0.2 μm, or one hundredth the size of a cloud droplet. CCNs are a unique subset of aerosols in the atmosphere on which water vapour condenses. This can affect the radiative properties of clouds and the overall atmosphere. Water vapour requires a non-gaseous surface to make the transition to a liquid; this process is called condensation.

In the atmosphere of Earth, this surface presents itself as tiny solid or liquid particles called CCNs. When no CCNs are present, water vapour can be supercooled at about −13 °C (9 °F) for 5–6 hours before droplets spontaneously form. This is the basis of the cloud chamber for detecting subatomic particles.

In dosimetry, linear energy transfer (LET) is the amount of energy that an ionizing particle transfers to the material traversed per unit distance. It describes the action of radiation into matter.

It is identical to the retarding force acting on a charged ionizing particle travelling through the matter. By definition, LET is a positive quantity. LET depends on the nature of the radiation as well as on the material traversed.