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Cosmic rays are charged particles or photons coming from outer space. Their energy spans over fourteen orders of magnitude, from the MeV range to at least 1020 eV. The cosmic ray flux decreases from more than 103 particles per second per m2 at GeV energies, to about one particle per m2 per year at a PeV, and further to less than one particle per km2 and per century above 100 EeV.

Therefore, at low energies, up to 1014 eV, where the flux is strong, cosmic rays can be detected in a direct way, using detectors placed on air balloons and satellites which work out of the Earth’s atmosphere, but at higher energies, they can be only studied indirectly. The Extensive Air Showers (EAS), produced by the interaction of primary cosmic rays with the atmosphere, are detected by large observation sites on the surface. Information about the primary cosmic ray must be reconstructed, starting from the features of the shower measured at the ground.

The EAS longitudinal profile is studied with Cherenkov or Fluorescence detectors; the lateral distribution with ground arrays of detectors, which are usually scintillators, RPCs or water Cherenkov stations.

The Pierre Auger Observatory was designed to study cosmic rays with energies above 1017 eV (UHECR – Ultra High Energy Cosmic Rays).

It is located in the Argentinian pampa, near the town of Malargüe in the province of Mendoza, at an altitude of about 1400 m above sea level. And is composed by a surface detector array (SD) of 1660 water-Cherenkov stations spread over an area of 3000 km2 and overlooked by 27 fluorescence telescopes (FD).)

A surface detector station consists of a polyethylene tank with a base area of 10 m2 containing 12 m3 of ultra-pure water. The Cherenkov light produced by the charged particles which cross the water is collected by three 9” diameter photomultiplier tubes (PMTs).

The collected signal pulses have different size and the sum of the signal over time is related to the energy deposited by the particle in the detector.

Thanks to the size of the signal and its arrival time in the stations, it is possible to reconstruct the energy and the arrival direction of the shower. The signal in the station is dominated by the electromagnetic and muonic components of the shower. The third component, the hadronic component, is absorbed in the Earth’s atmosphere. The muonic component, which cannot be directly measured by the SD stations but only evaluated using different techniques, is a very powerful variable to study the mass composition of UHECR.

The content of this component in the shower is related to the first hadronic interactions of the primary cosmic ray with the atmosphere. At the energies of interest for the Pierre Auger Observatory, the hadronic cross sections are not well known: they are extrapolated by the cross sections measured at the LHC, the most powerful accelerator in the world, at lower energies.

Therefore, the estimation of the muonic component is strongly dependent on the several hadronic interaction models coming from the extrapolation of the hadronic cross sections. The number of muons obtained by the Auger data with all methods is always larger that the one expected from simulations produced with any hadronic models. It is very important to measure the muonic component to constrain hadronic models and to have a reliable variable to study mass composition with high statistics.

The Xmax (depth of the maximum of the shower longitudinal profile) measured by the FD is another powerful variable for the mass composition, but the duty cycle of the FD, that can operate only during clear moon-lights nights, is ~15%, while the SD duty cycle is 100%.

The Pierre Auger Collaboration has planned an upgrade of the surface detector, AugerPrime, whose main goal is to discriminate between the muonic and the electromagnetic component.

The main part of the upgrade is the Surface Scintillator Detector (SSD), which consists of a ~4 m2 plastic scintillator detector which will be mounted on top of every SD station.

The SSD is more sensitive to the electromagnetic component, while the Water Cherenkov Detector (WCD) signal is dominated by the muonic component. The combination of these two measurements with different sensitivity will provide the number of muons for each shower. Each SSD consists of two scintillator sub-modules, each composed of extruded polystyrene scintillator bars of about 1.6 m length, 5 cm width and 1 cm thickness.

The scintillator light will be read out with wavelength-shifting fibres that are inserted into straight extruded holes in the scintillator planes, and then bundled and attached to a single photomultiplier tube.

For a shower at 1020 eV and a zenith angle of 38 degrees, the peak signal in a 4 m2 SSD at 200m from the shower core (the point of impact at ground of the shower axis, which represents the direction of the primary cosmic ray at the ground – the density of particles at the ground has its maximum near the core) is expected to be around 12,000 MIP (Minimum Ionizing Particles).

A maximum signal of 12,000 MIP for a UHECR, is therefore a reasonable upper bound for determining the dynamic range. It is necessary a PMT that is linear enough to cover the whole range.

The PMT that will be chosen for the upgrade requires high quantum efficiency at the wavelength of the scintillating light, as well as an excellent linearity range of peak anode current with good gain.

The Pierre Auger Collaboration has planned an upgrade of the surface detector, AugerPrime, whose main goal is to discriminate between the muonic and the electromagnetic component. The main part of the upgrade is the Surface Scintillator Detector (SSD), which consists of a ~4 m2 plastic scintillator detector which will be mounted on top of every SD station. The SSD is more sensitive to the electromagnetic component, while the Water Cherenkov Detector (WCD) signal is dominated by the muonic component. The combination of these two measurements with different sensitivity will provide the number of muons for each shower. Each SSD consists of two scintillator sub-modules, each composed of extruded polystyrene scintillator bars of about 1.6 m length, 5 cm width and 1 cm thickness. The scintillator light will be read out with wavelength-shifting fibres that are inserted into straight extruded holes in the scintillator planes, and then bundled and attached to a single photomultiplier tube.

Moreover, a fourth PMT with a small cathode surface will be added to the WCD to extend its dynamic range. It is important to have fewer saturated stations to reduce the uncertainty in the reconstruction of the shower.

The Small PMT (the standard PMTs are called Large PMT having a collecting area with a diameter of 230 mm) will need an excellent linearity range with good operative gain.

After the first measurements are taken with a small number of upgraded acquisition stations, the plan is that AugerPrime will be further validated and the final design details will be defined.

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