The surface detector system of the Pierre Auger Observatory

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Abstract

The Pierre Auger Observatory is designed to study cosmic rays with energies greater than 1019eV. Two sites are envisaged for the observatory, one in each hemisphere, for complete sky coverage. The southern site of the Auger Observatory, now approaching completion in Mendoza, Argentina, features an array of 1600 water-Cherenkov surface detector stations covering 3000km2, together with 24 fluorescence telescopes to record the air shower cascades produced by these particles. The two complementary detector techniques together with the large collecting area form a powerful instrument for these studies. Although construction is not yet complete, the Auger Observatory has been taking data stably since January 2004 and the first physics results are being published. In this paper we describe the design features and technical characteristics of the surface detector stations of the Pierre Auger Observatory.

Introduction

Cosmic rays with energies near 1020eV have been a continuing mystery since Linsley reported the first such event in 1963 [1]. As yet there are no identified sources and no convincing mechanisms for accelerating particles to these energies. Interaction with the cosmic microwave background (CMB) constrains protons of 1020eV to come from distances not greater than about 50 Mpc [2], [3]. Similarly constrained are other primaries: heavier nuclei lose energy by photo-disintegration and pair production, and photons due to pair creation [4]. Furthermore, the flux of cosmic rays at these highest energies is very low (less than one event per km2 per century per sr), so that their detailed study requires detectors covering large areas.

The Pierre Auger Observatory was designed for a high statistics, full sky study of cosmic rays at the highest energies [5]. It utilizes an array of surface water-Cherenkov detectors combined with air fluorescence telescopes, which together provide a powerful instrument for air shower reconstruction. Energy, direction and composition measurements are intended to illuminate the mysteries of the most energetic particles in nature.

On dark moonless nights, air fluorescence telescopes record the development of what is essentially the electromagnetic shower that results from the interaction of the primary particle with the upper atmosphere. The surface array measures the particle densities as the shower strikes the ground just beyond its maximum development. By recording the light produced by the developing air shower, fluorescence telescopes can make a near calorimetric measurement of the energy. This energy calibration can then be transferred to the surface array with its nearly 100% duty factor and large event gathering power [6], [7]. Moreover, independent measurements with the surface array and the fluorescence detectors alone have limitations that can be overcome by combining the results of their measurements.

The water-Cherenkov detector was chosen for use in the surface array because of its robustness and low cost. Furthermore, water-Cherenkov detectors exhibit a rather uniform exposure up to large zenithal angles and are sensitive to charged particles as well as to energetic photons which convert to pairs in the water volume. Their use in surface arrays was proven to be successful in previous experiments [8].

Each of the 1600 surface detector stations includes a 3.6 m diameter water tank containing a sealed liner with a reflective inner surface. The liner contains 12 000 l of pure water. Cherenkov light produced by the passage of particles through the water is collected by three nine-inch-diameter photomultiplier tubes (PMTs) that are symmetrically distributed at a distance of 1.20 m from the center of the tank and look downwards through windows of clear polyethylene into the water. The surface detector station is self-contained. A solar power system provides an average of 10 W for the PMTs and electronics package consisting of a processor, GPS receiver, radio transceiver and power controller. The components of the surface detector station are shown in Fig. 1.

In this paper we describe the design features and performance of the surface detector hardware. This description includes the detector tanks, liners and accessories and the pure water production, as well as the most relevant steps for assembly and deployment of the surface detectors. We conclude with an overview of the technical performance of the system. The electronics system of the surface detectors will be described in a companion paper [9].

The Southern site of the Auger Observatory, now under construction in the Province of Mendoza, Argentina, is over 85% completed. Active detectors have been recording events in a stable operation mode since January 2004 [10].

Section snippets

Design considerations

The low event rate of the highest energy cosmic rays requires an area large enough to accumulate good statistics in a reasonable time. By covering an area of 3000km2 at the Southern Site, the aperture achieved with the surface array for zenith angles less than 60 will be 7350km2sr. By including events with larger zenith angles, up to 80, the aperture can be increased by 30%[11]. The detection efficiency at the trigger level reaches 100% for energies above 3×1018eV[12]. This energy is

Tanks

The water-Cherenkov detectors have a cylindrical shape for the water volume, which is the simplest and least expensive to manufacture. The top of the tank is rather complex in order to provide rigidity both for mounting external components such as the solar panels and for people standing on top of it, and to provide space inside the tank for the photomultiplier assemblies and cabling. The tanks do not exceed 1.6 m in height so that they can be shipped over the roads within transportation

Solar panels and batteries

Power for the electronics is provided by a solar photovoltaic system. The power system provides the required 10 W average power. A 24-V system has been selected for efficient power conversion for the electronics.

Using the available insolation data for the Auger site, it was found that a suitable power system can be obtained with two 55 Wp panels3 and two 105 Ampere-hour (A h), 12 V batteries. Power is

Development and design

Tank liners are right circular cylinders made of a flexible plastic material conforming approximately to the inside surface of the tanks. The liners fulfill three functions: they enclose the water volume, preventing contamination or the loss of water and providing a barrier against any light that enters the closed tank; they diffusively reflect light traversing the water; and they provide optical access to the water volume for the PMTs, such that PMTs can be replaced without exposing the water

Water quality specification

Each surface detector contains 12 000 l of ultra-pure water. The high water purity is required for two purposes: to achieve the lowest possible attenuation for UV Cherenkov light, and to guarantee stability of the water during the 20 years of operation of the detectors.

For these reasons, the detector water needs to be deionized and completely free of microorganisms and nutrients. After consulting experts in pure water production, it was established that the best achievable water quality requires a

Detector assembly

The assembly of the surface detector stations is done in the Assembly Building located at the Central Campus of the Observatory in Malargüe. The different components are received and assembled into a complete detector in this building, which provides workspace for eight detectors at a time.

When received, tanks are unloaded and inspected. Using a template, holes are drilled to guide the hatchcover screws and the hatches are closed to keep water and dirt out of the tank during outdoor storage.

Maintenance and operation

As of September 2007, more than 1400 surface detector stations are operational. Typically more than 98.5% of the stations are operational at any time. The technical staff distributes its time between deployment of new detectors and maintenance and repair of down stations.

Only seven liners were observed to leak shortly after installation. In these cases, which constitute the worst failure mode, the tank is emptied and brought back to the Assembly Building for replacement of the interior

Conclusions

In conclusion, with over 1400 commissioned detectors in the field, some of which have already been operational for over five years, much insight on their performance has been gained. All components of the above-described detector hardware have fully met our design expectations. The design has proven sufficiently robust to withstand the adverse field conditions and failure rates are less than expected. Data taking is ongoing and the first scientific results have already been published. The

Acknowledgements

The successful installation and commissioning of the Auger Surface Array would not have been possible without the strong commitment and effort from the technical and administrative staff in Malargüe.

We are very grateful to the following agencies and organizations for financial support: Gobierno de la Provincia de Mendoza, Comisión Nacional de Energía Atómica, Municipalidad de Malargüe, Fundación Antorchas, Argentina; the Australian Research Council; Fundação de Amparo a Pesquisa do Estado de

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    Pierre Auger Collaboration, Av. San Martín Norte 306, 5613 Malargüe, Mendoza, Argentina.

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