Pierre Auger Observatory

On the vast plain known as the Pampa Amarilla (yellow prairie) in western Argentina, a new window on the Universe is taking shape. There, the Pierre Auger Cosmic Ray Observatory, designed to study the highest energy particles in the Universe is being completed and, at the same time, taking data. Already it is the largest cosmic ray detector ever built. It is designed particularly to study cosmic rays above 1019 eV. By detecting and studying these rare particles, the Auger Observatory is tackling the enigma of their origin and existence.

The cosmic rays properties are measured by two independent detector systems, the Surface Detector (SD) and the Fluorescence Detector (FD).  The combination of these two  complementary techniques forms a powerful hybrid system.

The Observatory is designed to study the extensive air showers (see link to cosmic rays) and, when completed, will consist of 1600 water-tanks spread over 3000 km2, roughly 30 times the area of Paris, viewed by fluorescence detectors. This vast area is needed to catch the rarest events as even at 1019 eV the rate is only about 1 per km2 per year while a decade higher the rate is at least a factor of 100 smaller!

The plan of the final layout shown in figure 1. Each point indicates the site of one of the water-tanks. The fluorescence detectors are positioned on small hills at the each of the area covered with the tanks. At these locations (Leones, Coihueco, Morados and Loma Amarilla) are four buildings that house sophisticated photomultiplier cameras that collect fluorescence light, produced after the excitation of atmospheric nitrogen by the shower as it propagates through the atmosphere.

 
 
Figure 1: The layout of the Pierre Auger Observatory with 1600 water-tanks over-looked by 4 fluorescence detectors.
  
 
The Observatory is unique in its use of two methods to study the extensive air-showers simultaneously. An outline of the principle of detection of showers, using this ‘hybrid’ approach, is shown in figure 2. Slight differences in the detection times at different tank positions help scientists determine the trajectory of the incoming cosmic ray. The detection times are measured using GPS receivers located at each water-tank. To the fluorescence detectors, a cosmic ray looks like a UV light bulb rocketing through the atmosphere at the speed of light, with an ever-increasing brightness that can reach up to four watts as the cascade grows to its maximum size. The Auger Observatory's fluorescence detectors are much more sensitive than the human eye and can "see" distant air showers develop. Occasionally, a cascade will occur in a place where two, or even three, fluorescence detectors can record it. This allows very precise measurements of the direction the cosmic ray came from.
 
 
Figure 2: The conceptual design of the Pierre Auger Observatory. A fluorescence detector over looks an array of water-Cherenkov detectors.
  
 
Each water-tank holds 12 tonnes of clear water and detects the Cherenkov light produced by particles passing through the tanks using three large photomultipliers. One of the tanks is shown in figure 3 where the solar panels used to provide power, the GPS receiver and the antenna used to send information to a central point can be clearly seen.
 
 
Figure 3: One of the water-tanks of the Pierre Auger Observatory. The solar panel, used to power the electronics, and the communications mast can be seen. The mast holds a radio antenna and a GPS receiver. In the background are the cordillera of the Andes.


 
 
It is important to check the accuracy of measurements of signals size and arrival times in the water tanks. This is done using pairs of tanks located close to each other. Such a pair is shown in figure 4 which shows two of the tanks deployed during early studies with a prototype array.
 
 
Figure 4: Two adjacent tanks that are used to make accurate estimates of the measurement accuracy of signal and times. These tanks were used in the prototype array and are of slightly different design to the final style seen in figure 3.

In the background the fluorescence site at Los Leones can be seen some 10 km distant.



  
 
The equipment housed in the fluorescence buildings (FD Buildings) is quite complex and power is supplied by conventional methods. Each building houses 6 telescopes designed to detect the fluorescence light produced by the cosmic ray showers. Components of one telescope - a camera and the associated mirrors and filter window are shown in figure 5. The faint fluorescence light comes through a special filter which is designed to shield out much of the background light from stars. It also acts as a shield to prevent dust entering the clean room in which the sensitive photomultiplier camera is located. The light is reflected from mirror of 11 m2 area onto the camera.
 
The very faint fluorescence emission can be related to the number of particles in the shower, thus allowing the growth and decay of the event to be monitored. However, this requires a sophisticated program of atmospheric monitoring and a battery of instruments has been assembled to do this. Some of the information obtained may be of use to atmospheric scientists. The cameras are able to detect light from the most energetic events even when they fall as far as 25 kilometres away.
 
Employing these two complementary observation methods provides the Auger Observatory with high quality information about the types of particles in the primary cosmic rays. Comparing results from the different types of detectors also helps scientists reconcile the two sets of data and produce the most accurate results about the energy of primary cosmic rays. The fluorescence detectors are able to detect the total energy of an air shower, which is approximately equal to the energy of the primary cosmic ray. Total cosmic ray energy is more difficult to determine with the surface detectors, which sample a small fraction of the energy of an air shower. Comparing data from the two methods is similar to comparing the results of a political poll and the results of an actual election, allowing scientists to better understand data from both detection methods and work on increasing the accuracy of both techniques. While the fluorescence detectors only work on clear, moonless nights, the surface detectors are always operating regardless of atmospheric conditions.
 
 
Figure 5a: A view of the 3.5 m x 3.5 m spherical mirror (left) and the aperture/filter through which light is received The picture was taken during the prototype phase when two types of mirror were under test.


 
 
 
Figure 5b: One of the 24 cameras used to photograph the fluorescence light The camera mount can be seen in figure 5a. There are 440 photomultipliers in each camera.


 
 
The Auger Observatory is in the final stages of construction and has begun to collect data near Malargüe, Argentina, a town in Mendoza Province that lies just east of the Andes Mountains. A matching site will also be built in south-eastern Colorado, providing nearly uniform coverage of the skies in the northern and southern hemispheres. If cosmic rays are found to arrive from specific directions, the Auger Observatories will be able to identify and study possible cosmic ray sources all over the sky with equal sensitivity. If discrete sources are not found, the full-sky coverage provided by the two sites will be essential for determining whether cosmic ray arrival directions are characterized by subtle large-scale patterns in the universe, or whether they are completely arbitrary. Some discussion of the latest measurements relating to these issues can be found at Astrophysics Results from the Pierre Auger Observatory.
A sample of events from the Auger Observatory can be found at Public Event Explorer.
The Auger project was first proposed by Jim Cronin and Alan Watson in 1991. Today more than 300 physicists from 90 Institutions around the world are involved in the work of the Observatory. The 17 participating countries shared the $50 million cost of construction, each providing a part of the construction costs. The current spokespersons are Giorgio Matthiae (Italy) and Paul Sommers (USA). Jim Cronin and Alan Watson are now spokesperson emeriti.