Go to work on a cosmic ray

Scientists know that we're getting space radiation with our cornflakes, but not where it comes from. A giant detector may help, says Andrew Watson
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The Independent Online
As you read your Independent over breakfast, more invisible rays will zap through it than you have cornflakes in your bowl. Day and night, this torrent of rays indiscriminately blasts through you, your home and everything around you.

Victor Hess, the Austrian physicist, discovered cosmic rays - radiation that comes down from space - in 1912, while riding in a hot-air balloon. Despite being known for more than 80 years, they are one of science's greatest enigmas, says Professor Alan Watson of Leeds University.

Scientists have little idea of where in space these rays come from, or why they exist.

One puzzle has been to identify celestial "engines" powerful enough to boost the subnuclear particles in cosmic rays up to their high energies. In the scientific journal Nature, Japanese and American scientists have just pinpointed the remnants of a star that exploded in AD1006 - a supernova - as one candidate.

Last month, an international meeting in Paris, headed by the Nobel prize-winning physicist Jim Cronin, laid out plans for an off-beat experiment to cover 6,000 sq km of the Earth's surface with an enormous cosmic ray detector. Professor Cronin, of the University of Chicago, hopes that this will offer insights into the latest twist in the cosmic ray plot, the detection of a handful of extra-high energy rays. "The number one problem is really the nature and origin of these highest energy cosmic rays," he says.

Professor Watson is Jim Cronin's lieutenant in the new detector project, and has already spent years studying cosmic rays with the Haverah Park "giant air shower detector" just outside Leeds. No one knows how to measure the high-energy ray directly. Instead, air shower detectors are used to monitor the cascade of particles streaming down to earth from the collision of a single cosmic ray particle with an atom in the upper atmosphere.

In the 20 years of its life, the Haverah Park detector has seen four super-energetic events at around the 16-joule mark. That may not seem much - it's the energy put out by a household light bulb in a quarter of a second, and would raise the temperature of your cornflakes by just a 50th of a degree (Celsius) - but for a sub-atomic particle it's huge. It is 100 million times the energy of particles in the world's most powerful atom-smashing particle accelerator, the Tevatron, near Chicago. Physics, as we understand it, says that cosmic rays shouldn't really exist at that energy.

At these high energies, says Professor Watson, cosmic rays should be soaked up by the faint "radio hiss" that fills space, the microwave background radiation left over from the creation of the universe. "This predicts a very sharp cut-off in the spectrum if the cosmic rays are coming from very far away," says Professor Watson. "The expectation, and this was predicted in 1966, was that there would actually be an end to the cosmic ray spectrum at somewhere around 7 joules. And people like me have spent their lives looking for this cut-off without success."

Out in the Utah desert near Salt Lake City, the Fly's Eye detector, a collection of mirrors housed in what look like big, tilted oil drums, discovered the highest energy particle ever detected. The Fly's Eye has been focusing light from the entire night sky on to sets of light detectors. The charged particles in the shower of debris from a cosmic ray cause the air to glow, and it is this glow that the Fly's Eye sees. "The light intensity is small, typically a few watts, so it is like watching a flashlight miles away moving at the speed of light through the air, and it's all over in 30 microseconds," says Professor Paul Sommers, one of the Fly's Eye team.

Professor Sommers explains that the Fly's Eye has seen evidence for a change in the nature of primary cosmic rays at energies around 1 joule, with those of lower energy more likely to be heavy nuclei produced in our own galaxy. Above this figure, cosmic rays are mostly protons from beyond our galaxy, he believes.

"By far the most exciting result from the Fly's Eye, however, is the super-energetic particle well above the expected energy cut-off," says Professor Sommers. It came in at a record-breaking 48 joules, the highest energy particle recorded. The second-highest energy particle was recorded in late 1993 by the Akeno array west of Tokyo, according to Motohiko Nagano of Tokyo's Institute for Cosmic Ray Research.

The world total of recorded cosmic "Big Ones" stands at eight. Professor Sommers says particles with such high energy must have originated outside our galaxy, but not too far out, as they lose energy travelling through space. "It's a tremendous challenge to explain how you can accelerate a charged particle to that sort of energy and the theorists are very, very puzzled by it," says Professor Watson. Lower-energy cosmic rays are believed to be accelerated in the shells of gas surrounding exploding stars - as in the case of the AD1006 supernova reported last week - but the theory breaks down well short of the kinds of energies seen in the super-energetic events.

That is where the new detector, the Pierre Auger Cosmic Ray Observatory, discussed in Paris last month, comes in. Named after the French discoverer of cosmic ray showers, Pierre Auger, the observatory will in fact comprise two detector arrays, one each in the northern and southern hemispheres. Each array will cover an area of 3,000 sq km, roughly the size of Lancashire.

The site of the southern detector will be Las Lunas, in Mendoza, Argentina. The northern site, which will probably be in the States, will be agreed in about six months' time. The Paris gathering was also an opportunity for prospective collaborators to commit themselves to finding cash to help meet the US$100m price tag, which Cronin believes will be spread among about 20 nations.

Pressed to justify the cost of the project, Professor Cronin says: "Two hundred years from now, we're not going to remember what level the stock market was at, but great discoveries made about the smallest things and the largest things are all going to be remembered and recorded."

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