Properties and performance of the prototype instrument for the Pierre Auger Observatory

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Abstract

Construction of the first stage of the Pierre Auger Observatory has begun. The aim of the Observatory is to collect unprecedented information about cosmic rays above 1018eV. The first phase of the project, the construction and operation of a prototype system, known as the engineering array, has now been completed. It has allowed all of the sub-systems that will be used in the full instrument to be tested under field conditions. In this paper, the properties and performance of these sub-systems are described and their success illustrated with descriptions of some of the events recorded thus far.

Introduction

The Pierre Auger Observatory has been conceived to measure the flux, arrival direction distribution and mass composition of cosmic rays from 1018eV to the very highest energies with high statistical significance over the whole sky. To achieve this coverage, the Observatory will have instruments located at two sites, one in each of the Northern and Southern Hemispheres. The astrophysical interest in this energy range is well known, stemming largely from the expectation of spectral features in the decade above 1019eV. In particular, it has been predicted [1], [2] that the energy spectrum should steepen sharply above about 6×1019eV because of the interaction of primary cosmic rays with the microwave background radiation. There is considerable controversy [3], [4] about the existence, or not, of the predicted steepening, commonly known as the Greisen–Zatsepin–Kuzmin (GZK) cut-off. It is clear, however, that there are cosmic rays with energies well beyond 1020eV and major issues are the flux of these events and the accurate measurement of the spectral shape. It is known that the spectrum of cosmic rays extends to at least 3×1020eV. Recent reviews are available [5], [6].

Above 1020eV, the rate of events is about 1km−2sr−1century−1, so that vast areas must be monitored to collect a large statistical sample. The Pierre Auger Observatory has been planned as a pair of arrays, each of 3000km2. The design for the Southern Observatory calls for 1600 water-Cherenkov detectors, arranged on a triangular grid, with the sides of the triangles being 1.5km, overlooked from four sites by optical stations, each containing six telescopes, designed to detect air-fluorescence light. The water tanks respond to the particle component (mainly muons, electrons and positrons and photons at the distances of importance) and the fluorescence cameras measure the emission from atmospheric nitrogen, which is excited by the charged particles of the shower as they traverse the atmosphere. Both techniques, already used for many years to study extensive air showers [7], [8], are brought together in a ‘hybrid’ detector to observe showers simultaneously with different techniques. The array of water tanks is known as the surface detector (SD), while the optical stations form the fluorescence detector (FD).

The surface array will have the following properties:

  • 100% duty cycle.

  • A well-defined aperture that is independent of energy above 1019eV.

  • Uniform coverage in right ascension on a daily basis.

  • A response that is largely independent of weather conditions.

  • The quality of the data for each event improves with energy.

  • Sensitivity to showers arriving at large zenith angles.

  • In situ calibration of the detectors by cosmic ray muons.

  • Measurement of the time structure of the arriving signals, which is sensitive to the mass of the primary particles.

The fluorescence detectors can be operated during clear nights with little moonlight and have the following characteristics:
  • Every event above 1019eV is registered by at least one fluorescence detector: 60% of these events will be recorded by two or more fluorescence detectors. Essentially, every trans-GZK event will be a stereo event. Multiple station coverage improves the energy resolution.

  • A coincidence of a single detector of the surface array with a single fluorescence telescope constrains the shower geometry as precisely as a stereo fluorescence detector.

  • The longitudinal development profile is measured directly.

  • The fluorescence detectors provide a more direct measure of the shower energy. The small, unseen, fraction of the total energy carried by neutrinos and muons depends somewhat on the mass of the primary particle as well as on the hadronic interaction model.

The design for the Observatory was developed through a series of workshops, starting in Paris in 1992, and culminating in a 6-month study at Fermi National Accelerator Laboratory in 1995. The design is well suited to resolve the discrepancies at the high energy end of the cosmic ray spectrum that have been reported by the AGASA surface array [3] and the HiRes fluorescence detector [4].

Presently, the Southern Hemisphere Observatory of the planned pair is being built in Argentina. The first phase of the project, the construction and operation of a prototype system, known as the engineering array, has now been completed. This has allowed all of the sub-systems that will be used in the full instrument to be tested under field conditions. The Engineering Array comprises 32 fully instrumented water tanks and two FD telescopes at one site. In the following we describe the properties and performance of the sub-systems and illustrate their success with a description of some of the events recorded thus far.

Preliminary details concerning some of the material discussed here were presented at the 2001 International Cosmic Ray Conference in Hamburg and in the Design Report [9].

Section snippets

Site of the Southern Observatory

The goal of constructing a hybrid instrument led to the specification of the site characteristics. These include the need for location at an altitude between 500 and 1500m above sea level (asl). In addition, the communications and deployment requirements make a relatively flat site desirable while the need to detect the faint fluorescence signals requires a location with optical characteristics close to those sought by astronomical telescopes. After evaluating several sites in Argentina,

Surface detectors of the engineering array

A photograph and a schematic diagram of an EA surface detector unit are shown in Fig. 2a and b, respectively. Each Cherenkov detector consists of a rotationally moulded polyethylene tank, 3.6m diameter and 1.55m high, enclosing a liner filled with 12000l of high purity water. During the period of EA operation, combinations of 8 and 9in. diameter PMTs were used with two types of viewing windows. The water in each tank is observed through three hard (polycarbonate) or soft (clear polyethylene)

Fluorescence detector

A single fluorescence detector unit comprises six telescopes, each located in independent bays, overlooking separate volumes of air. Two prototype fluorescence telescopes of the EA were installed in bays 4 and 5 of the Los Leones building. A schematic cross-sectional view of one fluorescence telescope is shown in Fig. 6. A circular diaphragm, positioned at the centre of curvature of the spherical mirror, defines the aperture of the Schmidt optical system. UV transmitting filters are installed

Data communication system

The data communications system of the Observatory consists of two integrated radio networks organised as a two-layer hierarchy. One network, the microwave network, is of high capacity and provides communication from the FD sites to the central campus. It also supplies a series of distributed concentration nodes for data that are sent to and from individual surface detectors via the second network, a wireless Local Area Network (LAN).

Overview

The Central Data Acquisition System (CDAS) has been running with the EA since March 2001. The system was designed to assemble the triggers from the surface array detectors, to allow control of these detectors and to organise the storage of data. It is constructed using a combination of commercial hardware and custom-made, high-level, software components. The system is designed to run continuously, with minimum intervention, and can be expanded for use with the full 1600 detector array, without

Performance of the Engineering Array

In this section, we demonstrate the performance of the Engineering Array by presenting data relating to some shower events. Where energies are mentioned in association with events, these should be regarded as indicative only.

Conclusions

It has been demonstrated, through the successful operation of the engineering array of the Pierre Auger Observatory, that the design formulated in 1995 works extremely well. This has been shown through a description of the operation of various sub-components of the Observatory and by the detection of a wide range of events that satisfy a variety of trigger conditions. Of particular importance are the hybrid events in which measurements were made in the same showers with the fluorescence

Acknowledgements

We are very grateful to the following agencies and organisations for financial support: Gobierno de Mendoza, Comisión Nacional de Energı́a Atómica y Municipalidad de Malargüe, Argentina; the Australian Research Council; Fundacao de Amparo a Pesquisa do Estado de Sao Paulo, Conselho Nacional de Desenvolvimento Cientifico e Tecnologico and Fundacao de Amparo a Pesquisa do Estado de Rio de Janeiro, Brasil; National Science Foundation of China, Ministry of Education of the Czech Republic (projects

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