Measurement of the Cosmic Muon Flux at the Stawell Underground Physics Laboratory

We report the first measurement of the underground cosmic muon flux at the Stawell Underground Physics Laboratory. The measurement uses eight EJ200 plastic scintillator panels, equipped with Hamamatsu R13089 PMT pairs at the ends, which are the primary components of the muon veto system for the upcoming SABRE South experiment. This study uses approximately 236 days of data collected between 2024 and 2025, with the muon veto system arranged in a telescopic configuration. SUPL, located 1025 metres below ground at the Stawell Gold Mine in Victoria, Australia, has a flat overburden of approximately 2.8 km of water equivalent, which significantly reduces the muon flux. The overburden is well characterised through data collected by the mining company, allowing tight constraints on systematic uncertainties. The measured muon flux is f = (6.33 +/- 0.04(stat) +/- 0.35(sys)) x 10-8 [s(-1) x cm(-2)]. This measurement is in excellent agreement with simulations, with a relative uncertainty an order of magnitude smaller than the modelling uncertainty.