A Particle Detector Array deployed to the Murchison Widefield Array in the Murchison Radio-astronomy Observatory

This paper presents the design, deployment, and performance verification of the Murchison Widefield Array Particle Detector Array (MWA PDA), an eight-detector system that successfully identifies cosmic ray extensive air showers to trigger radio data capture and serves as a pathfinder for future Square Kilometre Array instruments.

The Gist

The Big Picture: Catching Invisible Rain

Imagine the Earth is constantly being pelted by invisible rain. This isn't water, though; it's cosmic rays—high-speed particles from deep space that smash into our atmosphere. When one of these giant particles hits the air, it creates a massive explosion of smaller particles, like a pebble dropped in a pond creating a ripple. Scientists call this an Extensive Air Shower (EAS).

The problem? These showers happen incredibly fast and are hard to catch. To study them, we usually need huge, expensive detectors. But this paper is about a clever new tool built right next to a giant radio telescope called the Murchison Widefield Array (MWA) in the Australian outback.

The Solution: A "Tripwire" for the Radio Telescope

Think of the MWA radio telescope as a giant, sensitive ear trying to listen to the faint "crack" of a cosmic ray hitting the atmosphere. But the universe is noisy (like static on a radio), and the telescope can't record everything all the time because the data stream is too huge.

The scientists built a Particle Detector Array (PDA) to act as a tripwire or a motion sensor.

• The Setup: They placed eight "buckets" (detectors) on the ground in a specific pattern, spaced about 50 meters apart, right in the middle of the radio telescope's core.
• The Job: When a cosmic ray shower hits the ground, it splashes particles into these buckets. If three or more buckets get hit at almost the exact same time (within a microsecond), the system knows, "Hey, a big event just happened!"
• The Trigger: This "tripwire" sends a signal to the radio telescope to start recording. It tells the telescope, "Freeze! Capture the radio signal from this exact moment."

How the Detectors Work: The "Light Catchers"

Inside each of those eight buckets is a special sandwich made of plastic and light-shifting bars.

1. The Plastic: When a particle from space zips through the plastic, it makes the plastic glow with a tiny flash of light (scintillation).
2. The Light Shifters: These flashes are hard to catch directly. So, the detectors use special bars that absorb that light and re-emit it as a different color, guiding it like a hallway of mirrors toward a sensor.
3. The Sensors: At the end of the hallway are SiPMs (Silicon Photomultipliers). Think of these as incredibly sensitive eyes that can see a single photon (a particle of light). When they see the flash, they send an electrical signal.

The Desert Challenge:
The MWA is in a hot, dry desert. Electronics hate heat. The scientists had to design these buckets to survive scorching temperatures (up to 45°C) and keep them from picking up radio noise that would ruin the telescope's data. They even built a special "shielded cabinet" (like a Faraday cage) to keep the power supply quiet.

The Results: A Successful Test Run

The team deployed this system in late 2024 and let it run for about two weeks. Here is what they found:

• It Works: The system successfully caught 35,500 cosmic ray events.
• The "Rain" Pattern: By looking at which buckets got hit and when, they could figure out where the cosmic ray came from (like triangulating a sound) and how much energy it had.
• The Temperature Quirk: They noticed the detectors got "jumpy" when it got hot. Just like a person might get more energetic in the heat, the sensors created more "noise" (false signals) when the temperature rose. They learned how to adjust for this.
• The Energy: Most of the events they caught were from cosmic rays with energies around 4 to 15 PeV (that's quadrillions of electron volts—enough energy to power a lightbulb for a few days, all packed into a single subatomic particle!).

Why This Matters

This project is a pathfinder. It's a test run for a much bigger instrument that will be built for the Square Kilometre Array (SKA), the world's largest radio telescope, which is currently being built in the same location.

By proving that they can build a particle detector that doesn't interfere with the radio telescope, they have paved the way for future studies. This will allow scientists to:

1. Pinpoint the source: Figure out exactly where these high-energy particles are coming from.
2. Understand the physics: Learn how these particles interact with the atmosphere.
3. Listen to the universe: Capture the radio "echo" of these explosions, which might reveal secrets about the composition of the universe that we can't see with optical telescopes.

In a Nutshell

The scientists built a smart, solar-powered "tripwire" made of eight light-catching buckets in the Australian desert. When a cosmic ray explosion hits the ground, the buckets ring the alarm bell, telling the giant radio telescope to start recording. They proved it works, survived the heat, and are ready to help us listen to the loudest explosions in the universe.

Technical Summary

1. Problem Statement

• Scientific Context: Cosmic rays with energies above $\sim 10^{15}$ eV create extensive air showers (EAS) when interacting with the atmosphere. Studying these showers via their radio signatures offers precise measurements of shower development and cosmic ray composition.
• The Challenge: The Square Kilometre Array low-frequency telescope (SKA-Low) and its precursor, the Murchison Widefield Array (MWA), operate in the Murchison Radio-astronomy Observatory (MRO), a radio-quiet zone with strict electromagnetic emission limits.
• The Gap: Previous attempts to detect cosmic rays using the MWA alone (via voltage capture systems) were limited by:
  • Extremely high raw data rates, allowing only sporadic data recording.
  • Difficulty distinguishing cosmic ray events from Radio Frequency Interference (RFI) without an independent trigger.
  • The need for a dedicated particle detector array that generates triggers without violating the MRO's strict radio-frequency (RF) emission standards.

2. Methodology

The authors designed, built, and deployed the MWA Particle Detector Array (MWA PDA) to the core of the MWA.

A. Hardware Design

• Detector Array: Consists of eight particle detectors distributed approximately 50 meters apart within the MWA core (a $103 \times 90$ m$^2$ region).
• Scintillator Configuration:
  • Each detector contains four $47.5 \times 47.5 \times 3$ cm slabs of Bicron BC416 plastic scintillator.
  • Sandwiched between the scintillator slabs are Bicron BC482A wavelength shifter bars ($47.5 \times 3 \times 1$ cm).
  • Photodetection: Four Silicon Photomultiplier (SiPM) arrays (Onsemi J-series) are installed per detector. Two view the ends of the wavelength shifter bars, and two view the scintillator directly through windows. This dual-view design allows for both low-gain (wavelength shifter) and high-gain (direct scintillator) responses to extend dynamic range.
• Signal Transmission & Power:
  • To maintain radio silence, electrical signals are converted to optical signals immediately at the detector using Radio Frequency over Fibre (RFoF) modules.
  • Power is supplied via long coaxial cables with low-pass filters to prevent RF noise leakage.
  • All components are housed in shielded enclosures tested to meet MIL-STD461F standards (emissions 20 dB below the limit).
• Data Acquisition (DAQ):
  • Located in the MWA Control Building, the system uses a Bedlam digital signal processing board.
  • It digitizes signals at 1.024 GHz and applies coincidence logic.
  • Trigger Logic: An event is recorded if 3 out of 8 detectors exceed an ADC threshold within a 4 $\mu$s time window.

B. Calibration and Analysis

• Single Particle Response: Calibrated using a muon paddle to verify uniform response across the scintillator. The system was tuned to a nominal operating voltage of 28 V to minimize thermal dark counts while maintaining sensitivity to atmospheric muons.
• Reconstruction Algorithms:
  • Arrival Direction: Calculated using planar wavefront approximation and $\chi^2$ minimization of arrival times (corrected for fiber delays).
  • Core Position & Energy: Modeled using the Greisen approximation to the NKG lateral distribution function (LDF). Energy was estimated by integrating the muon count ($N_\mu$) and converting to primary energy ($E_\mu$).

3. Key Contributions

1. Radio-Quiet Deployment: Successfully demonstrated a particle detector array that operates in a radio-quiet observatory without generating detectable RF interference, meeting MRO/CSIRO compliance.
2. Integrated Trigger System: Developed a system capable of triggering the MWA correlator to capture voltage data for specific cosmic ray events, solving the "blind search" limitation of previous MWA-only methods.
3. Dual-Gain Detector Design: Implemented a novel scintillator/wavelength-shifter/SiPM arrangement that provides both low-gain (for high-density shower cores) and high-gain (for sparse peripheral particles) responses, extending the dynamic range of the array.
4. Pathfinder for SKA-Low: Validated the infrastructure and methodology required for the future ultra-high-energy particle science working group of the SKA.

4. Results

• Deployment: The array was deployed in November 2024 and operated continuously for 13 days (Feb 18 – Mar 2, 2025).
• Event Statistics:
  • Recorded 36,868 raw events.
  • After excluding data from a failed detector (Detector 8) and applying quality cuts (requiring 5 detectors for reconstruction), 1272 high-fidelity events were analyzed.
• Performance Metrics:
  • Detection Rate: Approximately 2 events per minute (raw), reduced to ~0.72 events/hour for high-fidelity events after selection cuts.
  • Energy Threshold: The array is 100% efficient for primary cosmic ray energies $> 4$ PeV arriving within $20^\circ$ of zenith ($\theta_z < 0.35$) within the core region.
  • Reconstruction: Successfully reconstructed arrival directions and core positions. The energy spectrum showed a power-law slope of $-2.85 \pm 0.14$ (consistent with primary cosmic ray spectra), though the raw energy proxy ($E_\mu$) was found to overestimate true energy by a factor of $\sim 6.5$ due to electromagnetic component contributions.
• Environmental Factors: Observed a strong diurnal correlation between event rates and ambient temperature (25°C–45°C), attributed to SiPM gain variations and power cable resistance changes.

5. Significance

• Scientific Impact: The MWA PDA enables the first systematic study of cosmic ray radio signatures in the 80–300 MHz range with a dense antenna array, bridging the gap between LOFAR (lower frequency) and higher-energy observatories.
• Technological Validation: It proves that complex particle physics instrumentation can be co-located with sensitive radio telescopes without compromising radio-quietness, a critical requirement for the SKA.
• Future Outlook: The system serves as a direct pathfinder for the SKA-Low particle detector array. The successful implementation of the trigger mechanism allows for the capture of "voltage data" (raw antenna voltages) for specific events, enabling detailed offline analysis of the radio emission physics of air showers, including polarization and shower development tomography.