The Molière radius is a characteristic constant of a material giving the scale of the transverse dimension of the fully contained electromagnetic showers

Particle shower

In particle physics, a shower is a cascade of secondary particles produced as the result of a high-energy particle interacting with dense matter. The incoming particle interacts, producing multiple new particles with lesser energy; each of these then interacts in the same way, a process that...

 initiated by an incident high energy electron

Electron

The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...

 or photon

Photon

In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...

. By definition, it is the radius of a cylinder containing on average 90% of the shower's energy deposition. It is related to the radiation length

Radiation length

In physics, the radiation length is a characteristic of a material, related to the energy loss of high energy, electromagnetic-interacting particles with it.-Definition:High-energy electrons predominantly lose energy in matter...

  by the following approximate relation: , where is the atomic number

Atomic number

In chemistry and physics, the atomic number is the number of protons found in the nucleus of an atom and therefore identical to the charge number of the nucleus. It is conventionally represented by the symbol Z. The atomic number uniquely identifies a chemical element...

. The Molière radius is useful in experimental particle physics

Particle physics

Particle physics is a branch of physics that studies the existence and interactions of particles that are the constituents of what is usually referred to as matter or radiation. In current understanding, particles are excitations of quantum fields and interact following their dynamics...

 in the design of calorimeters

Calorimeter (particle physics)

In particle physics, a calorimeter is an experimental apparatus that measures the energy of particles. Most particles enter the calorimeter and initiate a particle shower and the particles' energy is deposited in the calorimeter, collected, and measured. The energy may be measured in its...

: a smaller Molière radius means better shower position resolution, and better shower separation due to a smaller degree shower overlaps.

Molière radii for typical materials used in calorimetry

• Caesium iodide
  Caesium iodide
  Caesium iodide is an ionic compound often used as the input phosphor of an x-ray image intensifier tube found in fluoroscopy equipment....
  
  : 3.8 cm
• Liquid argon
  Argon
  Argon is a chemical element represented by the symbol Ar. Argon has atomic number 18 and is the third element in group 18 of the periodic table . Argon is the third most common gas in the Earth's atmosphere, at 0.93%, making it more common than carbon dioxide...
  
  : 10.1 cm
• Liquid krypton
  Krypton
  Krypton is a chemical element with the symbol Kr and atomic number 36. It is a member of Group 18 and Period 4 elements. A colorless, odorless, tasteless noble gas, krypton occurs in trace amounts in the atmosphere, is isolated by fractionally distilling liquified air, and is often used with other...
  
  : 4.7 cm