The Pierre Auger Observatory – the world's largest cosmic-ray observatory – is set for a $14m upgrade that will also see its operations extended until 2025. The AugerPrime upgrade will involve installing scintillation detectors alongside the 1660 existing water Cerenkov detectors. This will enable more precise measurements of the mass of particles that make up cosmic rays, as well as help to identify the origin and nature of such particles.

The facility, which covers an area of 3000 km2 in Argentina, measures the cascade of secondary particles produced when a cosmic ray hits the Earth's atmosphere. By observing these particles, astronomers can obtain information on the mass, direction and energy of the original cosmic ray.

Recent results have shown that at the highest energies – 1018–1021 eV – the number of cosmic-ray particles decreases much faster than at lower energies. A better understanding of the mechanisms responsible for this flux suppression should help infer the maximum energy of cosmic rays and identify what is responsible for accelerating cosmic rays to such high energies. However, this requires more detailed measurements of the highest energy rays.

Unique capabilities

The new 4 m2 scintillators will complement the existing Cerenkov water detectors and enable electrons and muons to be separated in the secondary shower more efficiently. Researchers will also be aided by a 23 km2 area of new buried muon detectors. "The ratio of electrons to muons turns out to be very sensitive to the mass of the primary particle," says Pierre Auger Observatory spokesperson Karl-Heinz Kampert. "AugerPrime will address a number of fundamental scientific problems that cannot be addressed anywhere else within the next decade or more."

Measuring the fluxes of both muons and electrons should make it easier to identify cosmic rays that are high-energy protons. This is important because they are deflected less by cosmic magnetic fields and do a better job of pointing back to their distant sources than cosmic rays that are heavy, highly charged, nuclei.

"It has been said that identifying the sources of cosmic rays is the 'holy grail' of our field," says Gordon Thomson, co-principal investigator for the Telescope Array cosmic-ray observatory in Utah. "This is exactly the aim of the AugerPrime project. The technique of counting both muons and electrons in air showers has been used successfully in previous experiments, and I believe it will work very well for the Auger experiment also."

Better electronics

Work will start on the upgrade next year, which will also include faster and more powerful electronics to facilitate the new detector components and enhance the overall performance of the observatory.