They are burgundy, hexagonal, 2.5 metres high and just over 4 metres wide, and everyone passing them where they stand by the bicycle path from Växjö campus to the lake Trummen has surely wondered what they are. The answer is that these two cisterns are prototypes for 1,242 similar ones that are to detect gamma-rays from space.
The cisterns are made of a sandwich of carbon fibre and polyvinyl chloride (PVC) and they belong to the Astroparticle Physics research group at Linnaeus University. Specifically to the ALTO project, that is developing a so-called wide-field Very High Energy (VHE) gamma-ray detector. This detector is designed to consist of 1,242 cisterns filled with water to be installed at very high altitude in the Southern hemisphere.
Besides 25 tonnes of water, each tank contains a photomultiplier. Using this, you can study the universe by detecting footprints of air showers produced by high-energy gamma-rays.
– When a charged particle having a speed larger than the speed of light in water enters the tank, Cherenkov photons (blue light), are created along the direction of the particle. These photons created in water can be detected by an ultra-sensitive photon detector positioned at the bottom of the tank, says Yvonne Becherini, associate professor at Linnaeus University and project manager for ALTO.
Prototypes for testing
The research group has worked on the project for a few years and using the prototype setup, Yvonne and her colleagues are able to test if the design they propose is a good and cost-effective solution.
– Our idea is that we can improve substantially the sensitivity by adding a second detector below the concrete table that the cisterns are placed on. This second detector will tag the passage of muon particles and help us to distinguish between the signals generated by gamma-rays and those generated by the background of cosmic rays.
The cisterns were delivered in April. The tests begun late in May, once the cisterns were filled with tap water, and will last for a couple of years. The researcher won't, however, be able to detect gamma-rays on site in Växjö.
– No, unfortunately not, says Yvonne Becherini. To do that we need a large number of cisterns; in our design we have 1,242. What we are able to detect with 2-4 cisterns is the background of cosmic rays.
Active galactic nuclei
The scientific focus of the Astroparticle Physics research group is active galactic nuclei. An active galaxy has an extremely massive black hole at its centre, which can attract matter from the surrounding disk through the gravitational force. This process also produces a bipolar jet coming out from the black hole.
– We still do not know how this works, but the process carries a large amount of energy. Through the study of gamma-rays, that is photons with very high energy, here on Earth, we can detect what happens far away in the Universe and thus gain more knowledge of the Universe and its structure.