Measurement of the cosmic muon flux at the Stawell Underground Physics Laboratory

This paper reports the first measurement of the cosmic muon flux at the Stawell Underground Physics Laboratory using eight plastic scintillator panels, yielding a value of (6.33 ± 0.04_stat ± 0.35_sys) × 10⁻⁸ s⁻¹ cm⁻² that aligns well with simulations and features significantly reduced uncertainty compared to modeling.

The Gist

Imagine the Earth is a giant, noisy room filled with invisible "ghosts" constantly raining down from the sky. These ghosts are cosmic rays, high-energy particles from deep space that smash into our atmosphere and create a shower of secondary particles, including muons. Muons are like energetic, ghostly bullets that can punch right through mountains, buildings, and even your body without stopping.

For scientists trying to find the most elusive things in the universe—like Dark Matter—these muon ghosts are a huge problem. They create "noise" that drowns out the tiny, precious signals scientists are looking for. To solve this, scientists built a secret bunker deep underground called the Stawell Underground Physics Laboratory (SUPL) in Australia. It's 1,025 meters (about 3,300 feet) below the surface, buried under a massive mountain of rock. Think of it as a soundproof basement in a hurricane; the rock acts as a thick blanket, blocking most of the cosmic ghosts.

But here's the catch: even in a deep bunker, some muons still get through. Before scientists can start their real experiments, they need to know exactly how many ghosts are sneaking in. This is like a security guard needing to know exactly how many people are trying to sneak past the fence before they can trust the alarm system.

The Experiment: Building a "Muon Net"

To count these sneaky muons, the scientists used the very first tool they installed: a Muon Veto System.

• The Net: They set up eight large, flat panels made of special plastic that glows when hit by a particle (like a firefly that flashes when touched).
• The Eyes: At each end of these panels, they attached super-sensitive cameras (called Photomultiplier Tubes) that can see even the faintest flash of light.
• The Setup: They arranged these panels in two layers, one above the other, like a sandwich. This created two "telescopes."

How it works:
Imagine a muon is a bullet flying through the air.

1. If it hits the top sandwich layer, the plastic glows.
2. If it keeps going and hits the bottom layer, that plastic glows too.
3. If the "cameras" see a flash in the top layer and the bottom layer at almost the exact same time, the computer says, "Aha! That's a muon passing straight through!"
4. If it only hits one layer, or if the flashes are too weak, the computer ignores it as background noise (like a random spark).

The Journey: From Surface to Deep Underground

The scientists didn't just trust the underground setup immediately. They first took the detectors to the surface (where the "ghost rain" is heavy) to test them. They verified that the panels could catch almost every single muon that hit them (99.4% efficiency). It was like testing a fishing net in a stormy ocean to make sure it doesn't have any holes before taking it to a quiet lake.

Then, they took the net down to the Stawell mine. They collected data for about 236 days (roughly 8 months). During this time, they also monitored the temperature, pressure, and even vibrations in the mine to make sure nothing was messing with their counts.

The Big Reveal: Counting the Ghosts

After crunching the numbers and correcting for things like the angle of the muons and the thickness of the rock above them, the scientists found the answer:

The muon flux at Stawell is about 6.33 muons per square centimeter every second.

To put that in perspective:

• If you held a postage stamp-sized piece of paper flat in the air at the bottom of the mine, you would expect about 6 muons to punch through it every second.
• This number is incredibly precise. The scientists were so confident in their measurement that their "margin of error" was tiny—much smaller than the uncertainty in their computer simulations.

Why Does This Matter?

Think of the Stawell laboratory as a high-end camera trying to take a picture of a faint star in a very dark sky.

• The Problem: The "sky" (the underground lab) still has some light pollution (muons) coming through.
• The Solution: By measuring exactly how much light pollution there is, the scientists can now build a computer filter to subtract it out.
• The Result: Now that they know the "noise" level, they can turn on their main experiment (the SABRE South experiment) to hunt for Dark Matter with a clear view.

The Takeaway

This paper is essentially the "calibration report" for a new, world-class science facility. It proves that the Stawell Underground Physics Laboratory is deep enough to be a quiet place for sensitive experiments and that the scientists have the tools to accurately count and filter out the cosmic noise. It's the first step in a long journey to potentially solve one of the biggest mysteries in physics: What is Dark Matter?

Technical Summary

Here is a detailed technical summary of the paper "Measurement of the cosmic muon flux at the Stawell Underground Physics Laboratory."

1. Problem Statement and Context

The Stawell Underground Physics Laboratory (SUPL), located 1,025 meters underground in a gold mine in Victoria, Australia, is the first deep underground physics laboratory in the Southern Hemisphere. It is designed to host sensitive particle physics experiments, most notably the SABRE South experiment, which aims to directly detect dark matter.

A critical challenge for such experiments is the environmental background caused by cosmic rays. Even at significant depths, cosmic muons penetrate the rock overburden, creating secondary particles (neutrons and gammas) that can mimic dark matter signals. To mitigate this, experiments require a muon veto system to tag and reject muon-induced events.

The Core Problem: Before the SABRE South experiment can begin its primary science run, the baseline cosmic muon flux at SUPL must be precisely measured. This measurement serves two purposes:

1. Background Characterization: To quantify the environmental background affecting all future experiments at the site.
2. System Validation: To perform an in-situ test of the SABRE South data acquisition (DAQ) system and the muon veto hardware.

2. Methodology

Detector Setup

The measurement utilized the muon veto system components intended for the SABRE South experiment:

• Hardware: Eight EJ-200 plastic scintillator panels (5 cm thick, 3000 mm × 400 mm).
• Readout: Each panel is coupled to two Hamamatsu R13089 Photomultiplier Tubes (PMTs) at opposite ends to enable position reconstruction via timing.
• Configuration: For this study, the panels were arranged in a telescopic configuration consisting of two layers separated by 68.9 cm. Two telescopes were set up with orthogonal orientations to measure incident angles (though angular analysis is reserved for future work).
• Data Acquisition: Signals were processed by a CAEN V1743 digitizer (3.2 GS/s sampling rate). Events were triggered by coincident signals within a 50 ns window for single panels and a 120 ns window for multi-panel coincidences.

Data Collection

• Duration: Approximately one year of data collected between April 2024 and March 2025.
• Phases: Data was collected in three phases with varying PMT high-voltage (HV) settings (1500 V, 1600 V, and gain-matched settings) to ensure stability and optimize noise rejection.
• Environmental Monitoring: A slow control system monitored temperature, pressure, and seismic vibrations to correlate with muon rate variations.

Analysis and Reconstruction

• Event Selection:
  • Charge Variable: To cancel position-dependent light attenuation, a "combined charge" ($Q_{com}$) was calculated as the geometric mean of the charges from the two PMTs on a panel.
  • Background Rejection: A Landau distribution fit was applied to the combined charge spectrum. A lower threshold ($x_0 - 3\xi$) was set to reject low-energy background (gamma rays from natural radioactivity) while retaining muon signals (expected energy loss > 10 MeV).
  • Telescope Trigger: A valid muon event required a coincidence between at least one top panel and one bottom panel of the same telescope, both passing the charge threshold.
• Efficiency ($\epsilon$) Determination:
  • Single-panel efficiency was measured using above-ground data where the flux is higher.
  • Three panels were stacked vertically. The efficiency was calculated as the ratio of triple-coincidence events (all three panels) to double-coincidence events (top and bottom only), ensuring muons traversed the full scintillator thickness.
• Geometric Acceptance ($\alpha$) Determination:
  • Simulations were performed using the pyrate framework for detector geometry and the pumas muon transport code (running in backward mode) for muon propagation through the mine overburden.
  • The simulation incorporated detailed lithology data (basalt and sandstone densities) from the Stawell Gold Mine.
  • The acceptance was calculated by integrating the simulated flux over the detector's angular response.

Systematic Uncertainties

The analysis accounted for several systematic effects:

1. Edge Effects: Uncertainty regarding muons that only partially traverse the scintillator thickness but still trigger the detector.
2. Material Density: Variations in the density of the overburden (basalt and sandstone) used in the simulation.
3. Telescope Orientation: Uncertainty in the precise azimuthal alignment of the detector relative to the flux anisotropy.

3. Key Contributions

• First Measurement: This is the first reported measurement of the cosmic muon flux at the Stawell Underground Physics Laboratory.
• Veto System Validation: It successfully demonstrated the functionality of the SABRE South muon veto system, proving its capability to detect muons with high efficiency in an underground environment.
• Methodological Rigor: The paper provides a comprehensive breakdown of efficiency and acceptance corrections, including a detailed treatment of systematic uncertainties related to geological modeling and detector geometry.
• Baseline Data: It establishes a crucial baseline for background subtraction for all future experiments at SUPL.

4. Results

The final measured muon flux ($f$) at SUPL is:
$$f = (6.33 \pm 0.04_{\text{stat}} \pm 0.35_{\text{sys}}) \times 10^{-8} \, \text{s}^{-1}\text{cm}^{-2}$$

Key Performance Metrics:

• Single-Panel Detection Efficiency: $\epsilon_2 = 99.42 \pm 0.03_{\text{stat}} \pm 0.23_{\text{sys}} \%$.
• Geometric Acceptance: $\alpha = 0.483 \pm 0.026$.
• Stability: The muon rate was found to be stable throughout the data-taking period and independent of the PMT high-voltage settings.

Comparison with Simulation:
The measured value is in excellent agreement with the simulated expectation:

• Simulated Flux: $f_{\text{sim}} = (4.8 \pm 2.3) \times 10^{-8} \, \text{s}^{-1}\text{cm}^{-2}$.
• The large uncertainty in the simulation ($\pm 2.3$) is dominated by uncertainties in the overburden material densities. The measurement has a significantly smaller relative uncertainty (an order of magnitude smaller) than the modeling uncertainty.

5. Significance

• Dark Matter Search Readiness: The result confirms that the SABRE South experiment's veto system is operational and effective. The measured flux allows for accurate prediction of the background rate for dark matter searches, which is essential for setting sensitivity limits.
• Southern Hemisphere Context: As the first deep underground lab in the Southern Hemisphere, this measurement fills a critical gap in global muon flux data, which is necessary for comparing results with Northern Hemisphere experiments (e.g., in Italy or the US) and testing atmospheric modulation models.
• Geological Modeling Validation: The agreement between the measured flux and the simulation (which relies on specific mine density data) validates the geological models used for the Stawell Gold Mine, improving confidence in future background estimates for neutron and gamma backgrounds.
• Precision: The statistical precision of the measurement ($\sim 0.6\%$) is high, making it a robust reference point for the community. The systematic uncertainty ($\sim 5.5\%$) is well-characterized, primarily driven by the overburden density, which is a known quantity for this specific site.

In conclusion, this work successfully establishes the environmental baseline for SUPL and validates the critical veto infrastructure required for the upcoming SABRE South dark matter search.