Evaluation of cosmogenic production of $^{39}Ar$ and $^{42}Ar$ for rare-event physics using underground argon

TL;DR

This study simulates cosmogenic isotope production in underground argon to determine safe handling durations for dark matter and neutrinoless double beta decay experiments, ensuring minimal background interference.

Contribution

It provides detailed simulated production rates of $^{39}Ar$ and $^{42}Ar$ in underground argon, establishing time limits for surface exposure to maintain experimental sensitivity.

Findings

Production rates of $^{39}Ar$ and $^{42}Ar$ are quantified through simulation.

Maximum surface exposure durations are set at 954 days for $^{39}Ar$ and 1702 days for $^{42}Ar$.

Results inform handling and storage protocols for underground argon in rare-event physics.

Abstract

Underground argon (UAr) with lower cosmogenic activities of $^{39}Ar$ and $^{42}Ar$ has been planned as a detector in detecting scintillation light and charge collection using time projection chambers for dark matter searches and as a veto detector in suppressing backgrounds for neutrinoless double beta decay (0$\nu\beta\beta$) experiments. Long-lived radioactive isotopes, $^{39}Ar$ and $^{42}Ar$, can also be produced on the surface when UAr is pumped out from a deep well. Understanding the production of long-lived isotopes in Ar is important for utilizing UAr for dark matter and 0$\nu\beta\beta$ experiments in terms of its production, transportation, and storage. Ar exposure to cosmic rays at sea-level is simulated using Geant4 for a given cosmic ray muon, neutron, and proton energy spectrum. We report the simulated cosmogenic production rates of $^{39}Ar$, $^{42}Ar$, and other long-lived isotopes at sea-level from fast neutrons, high energy muons, and high energy protons. Total production rates of 938.53/kg$_{Ar}\cdot$day and 5.81$\times$10$^{-3}$/kg$_{Ar}\cdot$day for $^{39}$Ar and $^{42}$Ar are found from our simulation. Utilizing these production rates, we set a time limit of 954 days constrained by the production of $^{39}$Ar for UAr to be on the surface before it compromises the sensitivity for a dark matter experiment. Similarly, a time limit of 1702 days constrained by the production of $^{42}$Ar is found for a 0$\nu\beta\beta$ experiment.