Charge Calibration of the ANTARES high energy neutrino telescope

The detector consists of an array of 900 large area photomultipliers (PMTs), Hamamatsu R7081-20, enclosed in pressure-resistant glass spheres to constitute the optical modules (OMs) [2], and arranged on 12 detection lines. An additional line is equipped with environmental devices. Each line is anchored to the sea bed and kept close to vertical position by a top buoy. The minimum distance between two lines ranges from 60 to 80 m. Each detection line is composed by 25 storeys, each equipped with 3 photomultipliers oriented downward at 45 • with respect to the vertical. The storeys are spaced by 14.5 m, the lowest one being located about 100 m above the seabed. From the functional point of view, each line is divided into 5 sectors, each of which consists typically of 5 storeys. Each storey is controlled by a Local Control Module (LCM), and each sector is provided with a modified LCM, the Master Local Control Module (MLCM), which controls the data communications between its sector and the shore. A String Control Module (SCM), located at the basis of each line, interfaces the line to the rest of the apparatus. Each of these modules consists of an aluminum frame, which holds all electronics boards connected through a backplane and is enclosed in a water-tight titanium cylinder.

The full-custom Analogue Ring Sampler (ARS) has been developed to perform the complex front-end operations [3].This chip samples the PMT signal continuously at a tunable frequency up to 1 GHz and holds the analogue information on 128 switched capacitors when a threshold level is crossed. The information is then digitized, in response to a trigger signal, by means of two integrated dual 8-bit ADC. Optionally the dynamic range may be increased by sampling the signal from the last dynode. A 20 MHz reference clock is used for time stamping the signals. A Time to Voltage Converter (TVC) device is used for high-resolution time measurements between clock pulses. The ARS is also capable of discriminating between simple pulses due to conversion of single photoelectrons (SPE) from more complex waveforms. The criteria used to discriminate between the two classes are based on the amplitude of the signal, the time above threshold and the occurrence of multiple peaks within a time gate. Only the charge and time information is recorded for SPE events, while a full waveform analysis is performed for all other events. The ARS chips are arranged on a motherboard to serve the optical modules. Two ARS chips, in a “token ring” configuration, perform the charge and time information of a single PMT. A third chip provided on each board

The bare ARSs were calibrated at IRFU-CEA/Saclay. There, the transfer functions of the Amplitude to Voltage Converter (AVC) have been measured. This AVC transfer function is an important parameter for the correction of the walk of the PMT signal and also for measurement of the amplitude of each PMT pulse. The principal component of this bench is a pulse generator which directly sends signals to a pair of ARSs operating in a flip-flop mode. The generated pulse is a triangle with 4 ns rise time and 14 ns fall, somewhat similar to the electrical pulse of a PMT with variable amplitude. The tranfer functions of the dynamic range of the ADCs are linear and parametrised by their slope and intercept. The distributions of these two parameters for a large sample of ARS chips are presented on figure 2 and3 and one can see that they are homogeous which enables to use the same parameters for all ARSs.

Specials runs reading the PMT current at random times allow to measure the corresponding so-called pedestal value of the AVC channel. Besides, the photoelectron peak can easily be studied with minimum bias events since the optical activity due the 40 K decays and bioluminescent bacteria produces, on average, single photons at the photocathode level. The knowledge of the photoelectron peak and the pedestal is used to estimate the charge over the full dynamical range of the ADC. The integral linearity of the ADC used in the ARS chip has independently been studied using the TVC channel and shows satisfactory results [4]. An example of in situ charge

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