Pierre Auger Observatory

An International Facility to Study the Highest Energy Cosmic Rays

Southeast Colorado has been chosen as the site in the Northern Hemisphere to house a world-class observatory.

On the vast plain in western Argentina known as the Pampa Amarilla, a new window on the universe is taking shape. There the Pierre Auger Observatory has begun its study of the universe’s highest energy particles. The observatory consists of an array of 1,600 particle detectors spaced approximately one mile apart within an approximately 35 mile by 35 mile area. The observatory also includes a series of fluorescence telescopes housed in a series of four observatory buildings around the perimeter of the detector array.  Scientists have also planned a second observatory to be located in the Northern Hemisphere in Southeastern Colorado.  The array will be located within the region outlined in dark blue in the map below. This represents a rare opportunity for Southeast Colorado to house one of only two such observatories in the world, a truly world-class research facility.

What are cosmic rays?

Cosmic rays are very energetic charged particles that rain down on us constantly from space and constantly bombard the earth. They are usually protons, but can also be larger nuclei, and they are accelerated by dynamic magnetic fields such as those on the sun, in solar wind, and fields blasted out by supernova explosions in our own Milky Way Galaxy. The particles or cosmic rays can have a wide range of energies, and many, especially those with lower energy levels, are well understood.

Scientists have various ways of measuring energy levels, and the unit of the electron volt (eV) is used to measure energy possessed by these cosmic particles. Their energy could also be expressed in more familiar units of energy like calories, BTU, or kilowatt hours. These particles could possess energy in the range from 10¹² to 10²⁰ eV. For those not accustomed to using exponents or scientific notation, l0¹² eV would be equivalent to a 1 followed by 12 zeroes, or 1,000,000,000,000 (one trillion) eV. The weakest ones have energy of about a billion electron volts, which is about the minimum energy needed for a particle to get from beyond the solar system through the magnetized solar wind. Just a few thousand of these atom-sized particles with energy of 10²⁰ eV would have enough energy to power an ordinary 60-watt light bulb for an hour.

For the higher energy regime cosmic rays there are few explanations and there is no scientific consensus about where they originate. Scientists have built special particle accelerators to duplicate these particles, but the highest energies they have achieved are around 10¹² eV. The Auger Observatory is designed to study particles in the range of 10¹⁹ or 10²⁰ eV, which have around ten million times more energy than the particles produced on Earth. The Observatory will gather the data needed to solve the puzzle of where they originate. Until it is solved, it will remain one of the top mysteries of our universe.

How do cosmic rays work?

The atmosphere of the Earth acts as a natural transducer and amplifier for the energy in the cosmic rays. When they enter the atmosphere, they interact with molecules, mostly nitrogen, and produce a shower of smaller, lower-energy particles. As they continue on their pathway toward earth, that series of lower energy particles interacts again, each one breaking up into even lower energy particles, producing a cascade much like that illustrated on the diagram above.

Once they reach an elevation of around 3,500 to 4,000 feet, the cascade has broken the original particle into a shower of tens of thousands, perhaps even millions or billions, of particles that can be detected. That shower will be spread over an area of just a few square miles. Southeast Colorado has been chosen as the site for the project because it is at both the correct latitude and altitude.

How often do they occur?

These remarkable cosmic rays are extremely rare. The lower energy particles occur much more often, but for the higher energy particles above 10¹⁹ eV, the arrival rate is only one particle per square kilometer per year. One square kilometer is equivalent to about one third of a square mile. For the highest particles detected so far, 10²⁰eV, the rate is only one per square kilometer per century.

How many cosmic rays strike the ground each second?

At the lower energy levels, around a billion eV, tens of thousands of the particles strike the ground each second. But as the energy level increases, the number of particles becomes more and more rare. At l0¹⁹eV, only about one particle per square kilometer arrives at the upper atmosphere per year. Above 10²⁰eV, only about one particle per square km arrives each century. Therefore, to find and measure these rare events, a high energy cosmic ray study needs a truly giant detector, like the 35 mile by 35 mile array proposed for Southeastern Colorado.

How are they detected?

There are two means by which the particles can be detected -- fluorescence telescopes and specially designed tanks spaced about one mile apart in an array that covers a 35-mile by 35-mile area. When the particles interact with nitrogen atoms in our atmosphere, they fluoresce, or emit ultraviolet light that can be detected by the fluorescence telescopes that are part of the observatory. The Auger telescopes view the air showers from a distance of about 15 km (9.3 miles), seeing what looks like a UV light bulb descending through the atmosphere at the speed of light. The detectors track the cascade development by measuring the brightness of the emitted light.

The particles also penetrate the detector tanks and interact with highly purified water inside. They are initially traveling at almost the speed of light, but when they travel through water, they travel at a slightly slower speed, which produces an electromagnetic shock wave known as the Cherenkov Effect. That shock wave interacts with water molecules, again producing light that can be detected by three large photomultiplier tubes located within the detector tanks. The data is recorded by special equipment in the tanks and is sent to a central observatory site by radio transmitters built into each tank. Since the tanks contain their own solar cells, batteries, and radio transmitters, no wires are needed to connect them to the central observatory site. Scientists do not need to physically visit the tanks to collect data, and usually only visit the tanks once every five years to replace batteries. The information is shared with scientists around the world using the Internet and other means of communication.

The Pierre Auger Observatory in Malargue, Argentina includes a visitor center, pictured at the left. Its North American counterpart located in S.E. Colorado, will also include a visitor center located on the Lamar Community College campus. As the tanks are located on private land, they are not available for the general public to view. 
 

Why spend so much money and effort studying these particles?

Something out there – no one knows what – is hurling these incredibly energetic particles around the universe. Do these particles come from some unknown super powerful cosmic explosion? From a huge black hole sucking stars to their violent deaths?  From colliding galaxies?  From the collapse of massive invisible relics from the origin of the universe?  Scientists love such a mystery, because solving such a mystery in nature means the opportunity to learn something about the universe. We don’t yet know the answers, but we do know that solving this mystery will take scientists another step forward in understanding the universe. Information that can be gathered by the Auger Observatory is akin to information gathered in other projects like the Hubble space telescope in that it can unravel mysteries about nature and our universe.

This mystery has prompted the formation of the consortium of more than 300 physicists from 55 institutions in 19 participating countries. Each shares a small part of the total cost of the project, which is estimated at approximately 50 million dollars. The United States’ share of project cost is funded mostly through the National Science Foundation and the U.S. Department of Energy.

Frequently asked questions:

What is this project?  It is a pure science project very much like a standard observatory. A standard observatory might use a telescope to look at starlight. The Auger Project does not observe starlight. Instead it measures a different kind of natural particle that arrives at the earth from outer space. The particles measured are mostly likely the hard central core, or nucleus, of atoms. When one of these particles arrives at the atmosphere, it makes a shower of other particles, and it is this shower of millions of other particles that are measured in the tanks.

The Pierre Auger Project, an international collaboration involving about 300 physicists and engineers from about 19 countries in North and South America and Europe, is named after the physicist who, in the 1930s, discovered this type of particle. In general, the particles are called “cosmic rays.”

Why do tanks need to be placed in Southeast Colorado?  The altitude is just right to make the measurements, the skies are clear and dark, without much stray light from towns, and that is also needed for the project. The land is fairly flat, which is good for the communication between the tanks and the central data center. The latitude is also in the range the scientists need to view the northern sky -- and having a detector in the northern hemisphere to go with the one in Argentina will allow scientists to view the whole sky.

Why would the project last 20 years?   The project is an observatory, so scientists have to wait and observe what nature sends them. The most interesting events are very rare, happening about twice per square miles every hundred years. In one year, with the whole detector, scientists will record just 30 or so of the most interesting (and most mysterious) events, so they need a lot of years to gather a significant amount of data.

Will the detectors attract more cosmic rays?   No. The cosmic rays will come no matter whether the project is built or not, the scientists will only observe what happens naturally.

Are the rays harmful?  Cosmic rays are a natural occurrence, and are not harmful.

Is there anything in the tanks that is harmful to cattle or crops? No. The tanks are built of polyethylene, a type of plastic, or fiberglass, and are filled with pure water. They are about 12 feet in diameter, about five feet tall, and hold about 3,000 gallons of water. Should one of them develop a leak, the worst thing that could happen is that it will water the plants growing around the tanks.

Excerpts from: Pierre Auger Observatory brochure, produced by SECED.      This fact sheet is printed by Prowers County Development, Inc.  719.336.2384.

Resources: Southeast Colorado Enterprise Development, Inc. 719.336.3850; Baca County Commissioners 719.523.6532; Bent County Commissioners 719.456.2223; Prowers County Commissioners 719.336.8025