A trial detector for ‘cosmic rays’ with a CSIRO-built ‘backend’ (processor) has been set up alongside the Murchison Widefield Array (MWA) at CSIRO’s Murchison Radio-astronomy Observatory (MRO) in Western Australia.
The new detector will let the MWA study particles with energies up to 10 million times greater than the Large Hadron Collider can produce.
The ‘proof of concept’ detector – a luggable 50-kg box – is the first step in developing a set of detectors for the low-frequency Square Kilometre Array (SKA1-low) telescope, which is to be built at the MRO.
Despite their name, most cosmic rays are particles, commonly protons or iron nuclei. Some have extremely high energies, and the origins of these high-energy particles are mysterious.
When cosmic rays enter the atmosphere they create a cascade of secondary particles. These secondary particles generate a burst of largely coherent radio emission that lasts less than a microsecond but which could be detected by a radio telescope.
The radio bursts generated by cosmic rays can give the most precise measurement of the primary particles’ masses, and hence the mix of particle types. Knowing the particles’ masses will let us better gauge the directions from which they are arriving, and so help determine their origins.
The trial detector was installed alongside the Murchison Widefield Array (MWA) telescope by its builder, Dr Justin Bray (University of Manchester), Curtin University PhD student Alex Williamson and the MWA operations team.
For the first few months the detector will just be monitored for performance and weather-fastness.
In the longer term, it will be used to trigger the MWA to capture the radio pulse associated with an incoming cosmic ray. This experiment will run in tandem with the MWA’s normal observing program, needing no extra observing time.
The final goal is to create a set of particle detectors that trigger SKA1-low.
Equipping SKA1-low to study cosmic rays is an initiative of the SKA’s High Energy Cosmic Particles Focus Group, which has members from ten countries. The group was founded and initially led by Dr Bray and Dr Clancy James (ICRAR/Curtin University). Dr James coordinated the deployment of the equipment at the MRO. Dr Tim Huege and Dr Andreas Haugs (Karlsruhe Institute of Technology/KASCADE Grande Collaboration) have also made major contributions to the project.
The MWA and SKA experiments are aimed at high-energy cosmic rays, ones with energies around 10^16 eV. These are thought to originate both in our Galaxy and beyond it. They reach the MRO at the rate of about 15 per square kilometre per hour.
Dr Bray built the detector at the University of Manchester with Dr Ralph Spencer, a former professor of physics at the University and one of the pioneers of radio studies of cosmic rays.
The trial detector’s backend is the Bedlam board designed by CASS engineer Dr Paul Roberts, which captures nanosecond pulses. Dr Roberts originally designed the board for the LUNASKA experiments, which used the Parkes telescope and ATCA to look for radio waves generated by ultra-high-energy particles hitting the Moon.
The trial detector had to meet the MRO’s stringent RFI (radio frequency interference) requirements, which ruled out many technologies normally used in these devices. The system backend was tested for RFI at CASS’s Marsfield laboratory before being shipped to Western Australia; the detector and power supply were tested at Curtin University.
The radio emission from the cascade is strongest at low frequencies and so low-frequency telescopes, such as LOFAR, the MWA and SKA1-low, are needed to detect it. The observations also need a large field of view, which these telescopes provide.
At MWA frequencies, the emission is expected to be partially decoherent and contain information on the cascade’s structure and the particle interactions within it.