Scintillation and Ionization in Liquefied Noble Gases

This presentation will summarize the fundamental properties of liquefied noble gases as radiation detection media. Liquefied noble gases are used as detector materials in particle physics, nuclear physics, astrophysics and medical imaging experiments. They are easily purified, can be scaled to large masses in a straightforward manner, and scintillate brightly from ionizing radiation. The scintillating species are excited diatomic molecules (excimers), which are efficiently produced in both singlet and triplet spin states. The decay of a noble gas molecule to two free atoms results in photon emission; the energies of these photons are insufficient to excite the ground-state atoms in the bulk liquid, allowing them to travel to the edges of the liquid and be detected. The heavier noble liquids, such as liquid xenon, liquid krypton, and liquid argon also exhibit large ionization yields and electron drift speeds. The noble liquids also allow efficient discrimination against electron recoil backgrounds, both through measurement of the ionization/scintillation ratio and through measurement of the fraction of scintillation light that is emitted by short-lived (singlet) molecules relative to the total (singlet+triplet).