Molière radius

The Molière radius is a characteristic constant of a material giving the scale of the transverse dimension of the fully contained electromagnetic showers initiated by an incident high energy electron or photon. It is related to the radiation length $X_{0}$ by the following approximate relation:

$\,\!R_{M}=0.0265X_{0}(Z+1.2),$

where $Z$ is the atomic number. The Molière radius is useful in experimental particle physics in the design of calorimeters: a smaller Molière radius means a smaller degree shower overplaps, and consequently a better shower separation.

Molière radii for typical materials used in calorimetry

• Caesium iodide: 3.8cm
• Liquid argon: 10.1cm
• Liquid krypton: 4.7cm