Liquid argon is used as the target for neutrino experiments and direct dark matter searches.
The interaction of a hypothetical WIMP particle with the argon nucleus produces scintillation light that is detected by photomultiplier tubes.
Two-phase detectors also use argon gas to detect the ionized electrons produced during the WIMP-nucleus scattering.
As with most other liquefied noble gases, argon has a high scintillation light yield (~ 51 photons/keV[33]), is transparent to its own scintillation light, and is relatively easy to purify.
Compared to xenon, argon is cheaper and has a distinct scintillation time profile which allows the separation of electron recoils from nuclear recoils.
On the other hand, its intrinsic beta-ray background is larger due to 39
Ar contamination, unless one uses underground argon sources which has much less 39
Ar contamination. Most of the argon in the Earth’s atmosphere was produced by electron capture of long-lived 40
K (40
K + e− → 40
Ar + ν) present in natural potassium within the earth.
The 39
Ar activity in the atmosphere is maintained by cosmogenic production through 40
Ar(n,2n)39
Ar and similar reactions.
The half-life of 39
Ar is only 269 years. As a result, the underground Ar, shielded by rock and water, has much less 39
Ar contamination.[34]
Dark matter detectors currently operating with liquid argon include DarkSide, WArP, ArDM, microCLEAN and DEAP-I.
Neutrino experiments include Icarus and MicroBooNE, both of which use high purity liquid argon in a time projection chamber for fine grained three-dimensional imaging of neutrino interactions.