Detector design

• the environmental conditions at the site, which influence the spacing of optical modules, the mechanical structure of detector strings, and the expected background rates;
• the practical experience gained from the design and deployment of the prototype string;
• the need to optimise the physics performance of the detector;
• the requirement for a high level of reliability.

The detector consists of an array of approximately 1000 photomultiplier tubes in 12 vertical strings, spread over an area of about 0.1 km² and with an active height of about 350 metres. The figure below  shows a schematic view of part of the detector array indicating the principal components of the detector.

Schematic of the detector at the Antares site	Arrangement of the twelve lines on the sea bed

The basic unit of the detector is the optical module, consisting of a photomultiplier tube, various sensors, and the associated electronics, housed in a pressure-resistant glass sphere. The electronics includes a custom-built digital electronic circuit which captures and stores waveforms, pulse heights and timing information, as well as the HV power supply for the photomultiplier tubes and the network nodes for data transmission and slow control.

An optical module (c) CEA/DSM/DAPNIA  for the ANTARES collaboration
A storey is composed of three optical modules and the associated electronics. (c) Camille Moirenc for the ANTARES collaboration

The optical modules are grouped together in `storeys' of three modules and interconnected via an electro-mechanical cable (see pictures, ). In the present design the detector has 12 strings, each of which has a total height of about 350 metres and consist of 25 storeys spaced vertically by 14.5 metres. The strings are distant by about 70 metres.

The optical modules in a storey are arranged with the axis of the photomultiplier tubes 45below the horizontal. 10-inch Hamamatsu photomultiplier tubes will be used. The angular acceptance of the optical modules is broad, falling to half maximum at around 70° from the axis. This means that the proposed arrangement of OMs detects light in the lower hemisphere with high efficiency, and has some acceptance for muon directions above the horizontal. In the lower hemisphere there is an overlap in angular acceptance between modules, permitting an event trigger based on coincidences from this overlap.

The relative positions of all optical modules in the detector are given in real time by a positioning system identical to that described for the prototype string.

Each string is instrumented with several electronics containers. At every storey, there is a local control module (LCM), and at the base of each string there is a string control module (SCM). Special containers house acoustics and calibration equipment. Each of these containers constitutes a node of the data transmission network, receiving and transmitting data and slow-control commands. The functions which they support include reading sensors, adjusting slow-control parameters, the trigger, and the distribution of power, master clock and reset signals to the front-end electronics.

The individual SCMs are linked to a common junction box by electro-optical cables which are connected using a submarine. A standard deep sea telecommunication cable links the junction box with a shore station where the data are filtered and recorded.

 

The junction box on the Castor deck before its deployment (c) CNRS Dars/Papillault for the ANTARES collaboration

The trigger logic in the sea is planned to be as simple and flexible as possible. The first-level trigger requires a coincidence between any two OMs in a single storey. The second-level trigger is based on combinations of first-level triggers. Following a second-level trigger the full detector will be read out. A more refined third-level trigger, imposing tighter time coincidences over larger numbers of optical modules, will be made in a farm of processors on shore. The readout rate is expected to be several kHz, and the corresponding data recording rate less than 100 events per second.