World's largest cosmic-ray observatory selects upgrade proposal

The Pierre Auger Observatory in Argentina is to be upgraded to help identify the origin of the most energetic subatomic particles known to science.

By Elton Alisson, in Buenos Aires

Agência FAPESP – The 17 countries that participate in the Pierre Auger Collaboration, including Brazil and Argentina, have selected a proposal to upgrade the world’s largest facility for observing cosmic rays, installed near the town of Malargüe in Mendoza Province, Argentina, about 1,000 km from Buenos Aires.

Details of the proposal were presented during a round-table session on large-scale scientific collaborations held on April 8 as part of FAPESP Week Buenos Aires.

Hosted in Buenos Aires by FAPESP in partnership with Argentina’s National Scientific & Technological Research Council (CONICET), the event brought together researchers from São Paulo State and several higher education and research institutions in Argentina to discuss an increase in scientific collaboration between the two countries.

“The Pierre Auger Observatory is a good example of a major international scientific collaboration, with Argentina and Brazil participating effectively in the design, construction operation and enhancement of the detector system’s performance throughout the ten years since its inception,” said Carola Dobrigkeit Chinellato, chair of the Auger Publications Committee and a professor at the University of Campinas’s Gleb Wataghin Physics Institute (IFGW-UNICAMP).

“The upgrade will enhance the observatory’s technological capabilities so that we can supplement the data collected on the ultra high energy cosmic particles we observe with the detectors we have there at present,” Chinellato told Agência FAPESP.

Some 500 scientists from the 17 participating countries work with the observatory, which began collecting data in 2004 after over ten years of planning. Brazil’s share in the collaboration is about 5% and Argentina’s roughly 8%.

Participation by researchers from São Paulo State is supported by FAPESP. Other Brazilian research funding agencies support participation by researchers from other states.

“Brazil and Argentina are the main partners in the Pierre Auger Collaboration,” said Alberto Etchegoyen, director of the Astroparticle Detection Technology Institute subordinated to Argentina’s National Atomic Energy Commission (CNEA), in a presentation during the event.

According to the researchers who are participating in the collaboration, in ten years of operation the Pierre Auger Observatory has enabled them to observe dozens of cosmic rays in the energy region above 100 billion billion electron-volts (10²⁰ eV), and has confirmed that there is a strong suppression of the flux of cosmic rays reaching Earth at energies above 55 billion billion electron-volts (5.5 x 10¹⁹ eV).

Among the questions researchers hope to answer thanks to the upgrade is what causes this suppression of the most energetic subatomic particles hitherto known to science, especially whether it is due to energy loss during extragalactic propagation or the existence of an upper energy limit for particles in their galactic or extragalactic sources.

“If we can identify the origin of the flux suppression, we’ll be able to find ultra high energy cosmic ray sources or source regions ,” Chinellato said.

“The key to achievement of this goal is enhanced identification of the composition of primary cosmic rays, especially at the highest energy levels.”

According to participating researchers, identifying the source and composition of these astroparticles is a major challenge because they can only be measured indirectly.

When an ultra high energy cosmic particle reaches Earth’s atmosphere it collides with the nucleus of an air atom, producing new particles that in turn collide and interact. The resulting particle cascade, called an extensive air shower, contains a billion particles or more.

The Auger Observatory studies the ultraenergetic cosmic rays that reach Earth by measuring the extensive air showers they produce in the atmosphere.

The upgrade program to be implemented at the observatory is expected to help researchers answer fundamental questions about the nature of ultraenergetic cosmic rays.

“In order for the observatory to achieve these scientific goals, it’s important to improve the sensitivity of its particle detectors and extend observations into the energy region in which the flux of ultra high energy cosmic rays is suppressed,” Chinellato said.

Selected proposal

According to Chinellato, members of the collaboration presented five proposals to upgrade the Auger Observatory. The proposals were evaluated by a committee of outside experts.

Each proposal involved a different technique for identifying and quantifying muons in air showers. Muons are elusive subatomic particles that are created when cosmic rays hit air molecules high in the upper atmosphere.

Muon quantification will help researchers obtain information on the composition of the highest-energy cosmic rays, Chinellato explained.

The selected proposal, known as the Scintillator Surface Detector Array, calls for the installation of additional 2 m² detectors on top of each of the observatory’s 1,660 surface detectors and will cost US$12 million, equivalent to one-fifth of the observatory’s construction cost.

Spread over an area of 3,000 km² on a vast plain overlooked by the Andes, the detectors (which operate uninterruptedly) are polyethylene tanks filled with 12,000 liters of purified water and instrumentalized with photomultiplier sensor tubes.

When particles in an air shower pass through the water in the tank, they emit light that can be measured by the photomultipliers.

Antennas mounted on top of each tank transmit the data by radio to the main campus of the observatory in Malargüe, in western Argentina. From there the data is sent for analysis to some 500 researchers in other parts of the world.

The observatory’s existing surface detectors are already fairly sensitive to the muons present in air showers. The addition of new detectors with scintillators will enhance the resolution and enable researchers to measure particles more precisely.

“Prototypes of the scintillation detectors are already being tested at the observatory,” Chinellato said. “We expect most of the new detectors to have been installed within two years, so that we’ll be able to measure ultra high energy cosmic rays more precisely without interrupting the observatory’s operations.”