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

Abstract

We report the design and functionality of the Murchison Widefield Array Particle Detector Array (MWA PDA), an array of eight particle scintillation detectors deployed to Inyarrimanha Ilgari Bundara, the Murchison Radio-astronomy Observatory (MRO). The purpose of the instrument is to identify cosmic ray extensive air showers (EAS) occurring over the core of the MWA radio telescope, and generate a trigger to allow radio data on the event to be captured and analysed. The system also acts as a pathfinder for a much larger instrument to be deployed in the core of the low-frequency component of the Square Kilometre Array, SKA-Low, by the SKA's ultra-high-energy particles science working group. Here, we describe the instrument and associated infrastructure, which has been verified to comply with the strict radio-frequency emissions requirements of the MRO, and was deployed in November 2024. We present calibration data, which demonstrates the ability of each detector to identify individual atmospheric muons at the expected rate, and we characterise the temperature dependence of the system. We describe a sample of 35,500 EAS identified using multi-detector coincidence over a 13-day period, and show how the detector data can be used to reconstruct the arrival directions and approximate energies of these events. We conclude that the PDA can reliably trigger on and reconstruct EAS contained within the $\sim 103 \times 90$ m$^2$ core region, arriving within 20$^{\circ}$ of zenith, at primary cosmic ray energies above $\sim 4$ PeV. We have also verified that the detector array can generate triggers, allowing the capture of radio data from the MWA correlator for offline analysis.

Figures (21)

• Figure 1: An illustration of the distribution of the detectors #1--8 of the PDA, situated between the tiles of the core and of the east and south hexes of the MWA. All detectors are cabled to the power supply and fibre connection point shown. Also marked is the prototype particle detector described in previous work Prototype. Background features in the image are access roads and vegetation. Imagery from Siwei, processed by SkyFi.
• Figure 2: An illustration of the interior of a particle detector. The scintillator material is coloured blue, the bars of wavelength shifter green, and the positions of the SiPMs red. The exit point for the fibre waveguide, and gland plate for supply voltage, are located in the lower left. The RFoF emitters sit in the space to the left of the scintillator. The nine larger circles are feet located on the underside of the box; the four smaller circles on the scintillators are plastic supports keeping the scintillator in place, with the aid of plastic straps (thick grey vertical lines) kept taut with springs (thick black lines crossing the green wavelength shifter). Four handles are attached for ease of transport.
• Figure 3: A photograph of Detector 3 as deployed to the MWA.
• Figure 4: Photograph of the Bedlam digital signal processing board (top right) mounted in its rack while undergoing lab testing.
• Figure 5: Signals seen from a SiPM board in a dark box for four applied voltages.