CATCHING SOME RAYS

By Lee Siegel

On a dark night in 1991, an incredibly powerful subatomic particle from space hit Earth’s atmosphere above Utah’s west desert, causing a faint fluorescent flash that was detected by the Fly’s Eye cosmic ray observatory built and run by the University of Utah.

University physicists had been studying particles known as cosmic rays for decades, but this one was different, carrying millions of times more energy that anything produced in the most powerful particle accelerators on Earth. If it could get through the atmosphere and hit someone, this subatomic cosmic ray particle – the most energetic ever detected – it would feel like a fast-pitched baseball to the skull.

Astrophysicists around the world have been trying ever since then to figure out what mysterious process in space generates such tremendously powerful particles, known as ultrahigh-energy cosmic rays, which are different than weaker cosmic rays from the sun, other stars and exploding stars. Many suspect they come from active galactic nuclei, where supermassive black holes at the heart of galaxies not only suck in large amounts of matter, but spew out jets of energy.

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The table-like scintillation detector on the right is among 507 now arrayed across 300 square miles of Utah desert as part of the existing, $25 million Telescope Array cosmic ray observatory operated by the University of Utah in an international effort to learn the source of ultrahigh-energy cosmic rays, the most energetic particles in the universe. In the distance on the left is the Middle Drum flourescence detector building – one of three such facilities in the existing observatory. Under a proposed $6.4 million expansion, another 460 scintillation detectors will be added so the array will cover almost 1,000 square miles, and two more fluorescence detectors will be added if funding can be obtained. The fluorescence detectors measure faint blue flashes in the sky when an incoming cosmic ray hits a gas molecule in the atmosphere. The scintillation detectors measure “air shower” particles that cascade to the ground when a cosmic ray hits a gas molecule. John Matthews, University of Utah.

Since that night in 1991, the University of Utah upgraded the Fly’s Eye at Dugway Proving Ground during 1994-1999 to better study ultrahigh-energy cosmic rays. Then, U physicists teamed up with Japanese scientists and their funding agencies and, during 2004-2008, built a much larger experiment: the Telescope Array, a cosmic ray observatory with instruments spread over 300 square miles of open desert west of Delta, Utah.

Last year, U physicists and colleagues from Japan, Belgium, Russia and South Korea reported the Telescope Array detected a “hotspot” in the northern sky near the Big Dipper that was producing a disproportionate number of ultrahigh-energy cosmic rays.

Now, to zero in on the hotspot, confirm statistically that it is real and hopefully figure out the source or sources of ultrahigh-energy cosmic rays, Japan and the U plan a $6.4 million expansion of the existing $25 million Telescope Array to cover almost 1,000 square miles.

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Lee Siegel is a senior science writer at University Marketing and Communications. If you have an interesting story idea, email him at lee.siegel@utah.edu.