Amanda, the detective

A device to trace neutrinos

Published: Tuesday 15 May 2001

amanda is the acronym for the 'Antarctic Muon and Neutrino Detector Array'. Amanda can detect elusive objects like neutrinos. It's made of ice, buried deep inside the polar ice cap in the Antarctic and has already produced results that might allow scientists to understand the furthest reaches of the universe better. First results from amanda report the presence of high-energy neutrinos, coming possibly from the edges of our universe. Neutrinos are amongst the strangest of elementary particles. They are either massless or at best have a very tiny mass and carry no electric charge. They are produced during radioactive decay, beta decay to be specific, and cannot be detected easily. The reason for their elusiveness is that they interact very weakly with ordinary matter.

In fact, a neutrino could pass right through our planet without interacting with even a single atom among the billions and billions of atoms constituting the Earth. Neutrinos reach the Earth from three sources. A majority of them are produced in the atmosphere when very high-energy cosmic rays interact with the atmosphere. The other source of neutrinos is the Sun where they are produced as a by-product of the fusion reactions that generate energy to give us light and heat. Finally, some exploding stars, called supernovae, produce these particles upon explosion. Apart from these sources, scientists conjecture that copious amounts of neutrinos are produced in the regions of our universe where violent phenomenon occur. These include regions close to black holes (where matter is being sucked in from the neighbourhood) and the very edges of our visible universe where it is still very young and violent churning is taking place. This region of the universe is so far away that light or radio waves from it are very difficult to detect. Thus detecting such high-energy neutrinos can provide a unique opportunity to astronomers to test their theories regarding the origin and structure of our universe. A neutrino 'telescope' is vital to such research. Several such detectors are working and have provided scientists valuable information on, besdies other things, the energy generation mechanism of or Sun,

But the existing neutrino detectors have not been large enough to detect the rare, high-energy neutrinos. To overcome this, amanda was built in 1997 in the Antarctic. The huge detector comprises 302 photomultiplier tubes suspended in 20,000 cubic metres of ice. It is suspended two kilometres under the polar ice cap. The photomultiplier tubes are devices that detect and amplify light.

When a neutrino passes through the transparent ice, it interacts with some of the atoms in the ice and produces another particle called a muon and some energy in the form of a bluish light. This glow is 'called Cerenkov radiation' and can be detected by photomultiplier tubes. However, to make sure that the detector really only detects the high-energy neutrinos coming from far away and not those produced in the atmosphere over the Antarctic, the tubes are pointed towards the North Pole. In this way, only those neutrinos, which have passed through the earth, are detected.

amanda was operational for 138 days and during this time the photomultiplier tubes recorded 263 high-energy neutrinos. The scientists have produced a sky map showing the origins of each of these particles.

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