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Slamming Through the Universe?
The Pierre Auger Observatory: Exploring Secrets of the Extreme Universe
How do you catch a glimpse of some of the fastest, rarest particles slamming through the universe? Build a really, really big window.
Scientists at the Pierre Auger (pronounced oh-ZHAY) Cosmic Ray Observatory are doing just that on the high plains of Argentina. Their goal is to unmask the mysteries of ultra-high-energy cosmic rays.
Michigan Tech's David Nitz, a professor of physics, is among the 300-plus scientists from fifteen countries participating in the project, which is managed by the Department of Energy's Fermi National Accelerator Laboratory. At Tech, he is joined in the effort by Associate Professor Brian Fick and postdoctoral research associate Johana Chirinos Diaz, along with a team of graduate students. Michigan Tech's participation in the project is funded by the US Department of Energy.
Nitz is site spokesperson for the northern hemisphere observatory, a twin of the Auger facility to be built in Colorado. He initially led design of the Auger microwave communication system, which was ultimately finalized and implemented in Argentina by scientists from the University of Leeds.
Cosmic rays are simply charged subatomic particles that fly through space, and the garden variety constantly shower the Earth. But occasionally a cosmic ray with an energy of 100 million trillion (1020 electron volts) or higher enters the atmosphere, packing an energetic punch 100 million times greater than the world's most powerful particle accelerator.
Studying these super-energetic cosmic rays would be easier if there were more of them. They connect with the Earth at the estimated rate of just one per square kilometer per century.
To catch a glimpse of these rare subatomic particles and their somewhat more common brethren, 1019 eV cosmic rays, scientists have been building the Auger Observatory, which covers three thousand square kilometers east of the Andes.
It is the brainchild of Alan Watson of the University of Leeds and Nobel laureate James Cronin of the University of Chicago, who has called ultra-high-energy cosmic rays "messengers from the extreme universe."
Both David Nitz and Brian Fick have been on the project with me since its inception back in 1992, and it's been extremely successful," Cronin said. "David is superb in electronics, and he was responsible for the front-end electronics. Brian's work has been in the characterization of the atmosphere, and he's been superb in the analysis of all the fluorescence data."
"Both fellows are well-known and appreciated among the nearly three hundred scientists in the project," Cronin added.
Devising a communications system that knits together an observatory scattered over an area the size of Rhode Island proved an arduous project. "In theory, everything seems simple, and in practice, everything is hard," Nitz notes. "But we're at the point where we are ramping down construction and moving toward analysis of data."
The Auger Observatory uses two systems to detect and study high-energy cosmic rays. The first relies on the observatory's dominant feature: sixteen hundred enclosed water tanks spread out one and one-half kilometers apart in a grid over the yellow prairie. When cosmic rays slam into the atmosphere, they trigger a cascade of subatomic particles known as an air shower. (French scientist Pierre Auger first identified these showers in 1938.) When these particles, which travel at nearly the speed of light, hit the water in the tanks, they produce a type of energy known as Cerenkov light that's picked up by sensors.
Nitz developed the so-called front-end board and its first-level trigger, which first winnow the information being sent over the communications network. "This is the crucial first step in finding the needle in the haystack of the interesting physics events," Nitz said. "We designed the electronics to do that process at Michigan Tech, along with the algorithms which are programmed into the electronics."
With colleagues at Penn State and Fermi Lab, they built the prototypes for the one thousand boards that are already harnessing data. "They're actually built at a shop in Milwaukee," he said. Then they are shipped north to Michigan Tech, where they are tested under desert-like conditions before being sent to Argentina. Of the nearly one thousand production boards tested, only three have failed in the field.
Data selected by Nitz's trigger is broadcast from the tanks to towers and sent via microwave to the observatory's central campus and then on to Michigan Tech and other institutions via the internet for analysis. By measuring the light from several tanks, scientists can estimate the energy of the cosmic ray particle and determine its trajectory.
The second detection method tracks the development of air showers by observing ultraviolet light emitted high in the Earth's atmosphere.
The charged particles in an air shower interact with atmospheric nitrogen, causing it to emit ultraviolet light. This process, called fluorescence, is invisible to the human eye but not to the Auger Observatory's optical detectors. To them, a cosmic ray appears like a light bulb rocketing through the atmosphere.
Brian Fick, Johana Chirinos Diaz, and David Nitz circumnavigate one of the Pierre Auger Observatory's sixteen hundred water tanks, which are scattered across the Pampa Amarilla (yellow prairie) in Argentina.
Michigan Tech Associate Professor Fick championed efforts to build fluorescence detectors and then to link them with the water tanks.
"The fluorescence detector and the ground array collect data independently, but they have a common trigger which integrates that data. This lets us observe the same phenomenon simultaneously and gives us a lot more information about cosmic rays," Fick said.
So where do ultra-high-energy cosmic rays originate? "There are data coming in," Nitz says carefully. "We're starting to build up a pattern of where they come from in the sky, but we don't have enough information to say for certain."
It will help when the Colorado observatory is built, Nitz says. That facility will be able to detect air showers within the northern hemisphere, so scientists can study cosmic rays hitting the Earth from all directions.
Whatever the results, collaborating on the Pierre Auger Project has been tremendously satisfying, says Nitz. "I've been able to work with so many talented, dedicated people from so many places on one of the most fascinating mysteries of the universe," he said. "It's been a wonderful experience."