Every day, the Earth is bombarded by gamma rays travelling all the way from the depths of space. These rays, in the form of photons, tell cosmic secrets about violent celestial events, such as stellar explosions or black-hole feasts.
Now, researchers are setting up a new pilot project to detect the highest-energy gamma rays arriving in the southern hemisphere – using the humble water tank.
“Capturing photons with energies well above peta electronvolts (1015 eV) – the highest-energy photons – from the universe is key to unlocking its darkest secrets,” says the University of Adelaide’s Dr Jose Bellido Caceres, an astronomer in the School of Physical Sciences.
Gamma rays produce energetic secondary particles that emit a special light – called Cherenkov light – as they pass through water and air in the Earth’s atmosphere, which can be recorded with the right equipment. One way of capturing the Cherenkov light is through dense arrays of water tanks that cover a large area, each with a sensor inside.
Gamma rays down under
“Although there are observatories capable of detecting peta electronvolts gamma rays (PeVatrons) located in the northern hemisphere, none are yet installed in the southern hemisphere and so we have no measurements of peta electronvolts gamma rays produced by pulsars and supernova remnants from the southern parts of our galaxy, from where we have the best view,” says Bellido Careces.
Bellido Careces will lead a team to set up the Southern Widefield Gamma-ray Observatory (SWGO) in Arequipa, Peru, the first of its kind capable of surveying these high-energy gamma rays in the southern hemisphere.
“Peru is an ideal location for the proposed SWGO due to its high-elevation sites that are near water resources,” he says.
“A high-elevation site enables the secondary particles to be detected before they are completely absorbed by the atmosphere, and large water resources are required to fill the water tanks.”
The SWGO will consist of 6500 water tanks, produced by South Australian company Aquamate, spread over a one-square-kilometre area. The tanks are little different to the ones you might see in a backyard.
“The custom-built tanks contain a geomembrane that creates a light-tight environment, the perfect condition for the sensors to record the Cherenkov light produced by the secondary particles as they arrive,” says Bellido Caceres.
The project could place South Australian technology at the forefront of southern hemisphere gamma-ray detection and astronomy.
“We are excited by the opportunity to use our world-class expertise to design and build large prototype water tanks for the SWGO pilot project,” says Danny Di lorio, CEO and owner of Aquamate.
Deborah Devis is a science journalist at Cosmos. She has a Bachelor of Liberal Arts and Science (Honours) in biology and philosophy from the University of Sydney, and a PhD in plant molecular genetics from the University of Adelaide.
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