The Pierre Auger Observatory installation is finishing at Malargüe, Argentina. It aims at measuring particle showers produced by ultra high energy particles. It is an hybrid detector that takes advantage of complementary observational techniques: a Surface Detector (SD) coupled with air Fluorescence Detectors(FD).

The Surface Detector consists in 1,600 water tanks. Each tank is equipped with 3 photomultiplier tubes (PMTs - Photonis XP1805) that detect the Cherenkov light produced when particles cross the tank.

The production of the PMT bases was carried out under the responsibility of IPN Orsay and INFN Torino, Italy. Each group produced 2 500 bases. The base design was under the responsibility of the R&D Detection group (RDD).

The shower identification is performed through the observation of several tanks. As a consequence, the smallest signal to be observed is the response to a single muon. The average charge of events is around 50 photoelectrons and the counting rate is 2 kHz. The dynamic range required for the measurements is 2×10⁴. It is achieved by the simultaneous measurement of the anode and the amplified last dynode signals.

Calculations showed that a purely resistive base fulfill those requirements. The components were chosen for a good reliability (20 years of use). Because of the price of some components, their values were adjusted with simulations with Spice under Cadence Analog Workbench.

The high voltage is locally produced on the base by a module powered with 12 Volts. Since this voltage is produced by solar cells, the total power absorption allowed for the base is 500 milliWatts. The high voltage power supply module was not commercially available at that time. Prototypes were developped by several companies. Among them, SDS and ETL, met the requirements with a satisfactory price. The company that now produces it, ETL, was chosen through a particular tender.

The principle of the amplification on the last dynode for the low energy signals was validated in November 2000 by measurements on the a tank in Malargüe, Argentina. Thousands of signals were recorded to be used in the electronics simulation chain. The validation of the base line stability after a large pulse was achieved with a Nitrogen laser exciting a CsI(Tl) crystal.

Measurements were performed with photomultiplier tubes to verify the single electron response (with a pulsed LED) and the temperature stability (measurements with the RDD climate test cabinet).

Due to the distance to the experimental site, it was decided to install a tank in Orsay to test rapidly the bases and to study the detection for the calibration of the simulations performed in Orsay.

The RDD group studied and ordered a tank that is installed at IPN Orsay since 2001. The major change from the Auger design is the opening system, in order to allow more frequent access to the photomultiplier tubes. Its installation was designed to study the tank response to determined particle trajectories: it is possible to install scintillators above and under the tank. The photomultipliers are linked by 25 meters of cable from the measurement room. Anyway, the tank can be adapted to receive and test all the Auger electronics.

The data acquisition is performed by a CAMAC system and a digital oscilloscope driven by a GPIB. The GPIB sequencing and data analysis are performed by programs written in C++.

The base production is performed in an external company. After stuffing, the bases undergo a thermal cycling to detect early failures. A functional test is then performed. It includes the detection of short circuits, the correctness of the voltage divider and of the power supply module, and verifies the value of the coupling capacitors by checking the output shapes after a pulse injection on the anode and dynodes pins. The test bench was developped by INFN Torino, and the software modified by RDD to adapt to other instruments.

Preseries of 170 and 300 pieces were achieved in 2002 to validate the fabrication processes. A series of 1 200 pieces started in March 2003 and finished in December 2003. A last series of 850 pieces was produced in 2004. The Italian part of the production was achieved in the beginning of 2005.