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Solar power farm begins gamma ray astronomy

Solar power farm begins gamma ray astronomy

(January 10, 2001)

Solar Two, once the largest solar power farm in the world, has been given a second life as a gamma ray observatory.

University of California researchers Tümay Tümer, Gora Mohanty, Jeff Zweerink, Harry Tom and Umar Mohideen of the Riverside campus and Mani Tripathi of the Davis campus reported preliminary work carried out at Solar Two Gamma Ray Observatory today at the American Astronomical Society meeting in San Diego.

Constructed in Barstow, Calif. by Southern California Edison and the U.S. Department of Energy, the plant generated solar power until 1998. Dr. Tümer recognized the potential to add a gamma-ray camera to the existing infrastructure. With a grant from the W.M. Keck Foundation, his team incorporated a secondary focusing mirror and photomultiplier tubes.

"The vision of the Keck Foundation, help from Edison, and hard work of our team has resulted in a unique observatory in the middle of the great Mojave Desert," Tümer said. "It would have cost tens of millions of dollars to build a detector from scratch."

Gamma rays are of great interest to astrophysicists because they offer clues to some of the mysteries in the universe, such as the birth of a galaxy, the possibility of supersymmetric dark matter, violent gamma ray bursters, and the supernova of a dying star. Gamma rays are at the extreme high end of the electromagnetic spectrum, which includes ordinary visible light, radio waves and X-rays.

Scientists detect very high energy celestial gamma rays on the ground by collecting secondary light that is emitted in a two-stage process. First, the gamma ray interacts with the upper atmosphere and quickly multiplies into a shower of electron and anti-electron particles. Second, because the gamma rays are so energetic, these particles in the air showers move faster than the speed of light in the atmosphere. As such, they emit Cherenkov light in much the same way that a plane moving faster than the speed of sound in air will produce a sonic boom. The technique for using this light for detecting gamma rays was pioneered by the Whipple telescope located in Arizona.

At ground level, the Cherenkov light is typically spread out over a circle the size of a football stadium and lasts for about a billionth of a second. The key is to collect as much Cherenkov light as possible. Solar farms, which are essentially large groups of large mirrors spread out over the ground, are particularly suited for this task.

Solar Two consists of an array of 1,800 mirrors, each with an area of more than 400 sq. ft. It is the largest such farm built in the world, and the only one capable of containing the entire Cherenkov light pool. Thus, it has the potential to be the most sensitive detector.

Gamma-ray astronomy has grown rapidly in the past decade following the observation of more than 270 gamma-ray sources made by the EGRET detector aboard the NASA Compton satellite. However, EGRET could only measure gamma rays with energies up to 10 GeV (equivalent to 10 billion volts) while the current ground-based telescopes can only measure gamma rays with energies above about 200 GeV. The Solar Two team hopes to use their telescope to explore the energy region from 10 to 200 GeV that has never been observed completely before.

Sensitive observations in this region will reveal why EGRET observed more than 270 sources while only about five sources have been detected by the ground-based experiments so far. It is especially puzzling, given that the sensitivities of the space and ground experiments are comparable.

"Either the sources are turning off at higher energies or the gamma rays are not making it to the earth," explained Zweerink. He and Mohanty, both postdoctoral researchers, have worked extensively for two years on the conversion of Solar Two to an observatory.

Prof. Tripathi and Senior Engineer Britt Holbrook of UC Davis are upgrading the Solar Two camera with a state-of-the-art electronics system. Participating with them are UC Davis graduate students Pat Murray, Mao Xing and Juan Lizarazo.

"In today's world of big science, it is quite rare for a student like me to have the opportunity to develop a detector and also conduct independent research using that device," said Murray.

The University of California provided Tripathi with a seed grant to develop new techniques for photon detection in collaboration with Dr. Steve Vernon of the Lawrence Livermore National Laboratory.

"Such a cooperative effort between universities, national labs and private foundations is an excellent model for conducting fundamental scientific research," said Tripathi.

The Solar Two team is in the midst of measuring gamma rays from the Crab Nebula, a well-known source and "standard candle" for calibrating the telescope. "We would like to invite observers from around the world to come and work at Solar Two Gamma Ray Observatory in surveying the gamma ray sky and making sensitive measurements," said Tümer.

Solar Two astrophysicists are also launching an outreach effort aimed at public education in astronomy and astrophysics especially directed to Riverside and San Bernardino counties.

For more information:

Dr. Tümay O. Tümer, research physicist at UC Riverside, (909) 787-4701, or Prof. Mani Tripathi, UC Davis, (530) 752-8785 or

Three print-quality images are available on the Web at

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