Ar for rare-event physics using underground argon*

Underground argon with lower cosmogenic activities of ³⁹Ar and ⁴²Ar has been planned as a detector in detecting scintillation light for dark matter searches and as a veto detector in suppressing backgrounds for neutrinoless double beta decay experiments. Long-lived radioactive isotopes, ³⁹Ar and ⁴²Ar can also be produced on the surface when underground argon is pumped out from underground. Understanding the production of long-lived isotopes in argon is important for utilizing underground argon for dark matter and neutrinoless double-beat decay experiments in terms of its production, transport, and storage.  Argon exposure to cosmic rays at sea-level are simulated using Geant4 for a given cosmic neutron, muon and proton energy spectrum.

We report that the simulated cosmogenic production rate of ³⁹Ar, ⁴²Ar, and other long-lived isotopies at sea-level from fast neutrons, high energy muons, and high energy proton. Care must be taken to limit the production of long-lived isotopes when planning for using underground argon.