Silicon Drift Detectors have been adopted to equip the two middle layers of the Inner Tracking System (ITS) of the ALICE experiment at LHC. The SDDs will be mounted on linear structures called ladders each holding 6 detectors for layer 3, and 8 detectors for layer 4. The layers will be composed of 14 and 22 ladders, respectively. Each detector will first be assembled together with its front-end electronics as a unit which can be fully tested before being mounted on the ladder.
The prototypes of the ALICE drift detector are the result of an extensive R&D work started in 1992 and carried on by the INFN DSI project in collaboration with CANBERRA Semiconductors. The aim of the project was the production of a large area SDD with integrated high voltage divider. The final result of this effort is a detector whose dimensions are 6.75 X 8.0 cm2 with a sensitive-to-total-area ratio of 86%. You can find here a detailed description of the detector.
The program initially developed by DSI has found a natural continuity within the ALICE ITS collaboration. The first prototype, realized during 1998, was called ALICE-D1. It has an hexagonal shape to allow for the minimization of the overlapping of adjacent SDDs in the ladder assembly. The sensitive area is 75.3mm wide and 35.0mm long with a sensitive-to-total-area ratio of 88%.
At the end of 1999, a slight modification on the metalization mask was done in order to achieve a higher immunity against environmental perturbations. This detector was called ALICE-D1B.
The last step towards the final design has been called ALICE-D2. The slight differences from the previous detector are focused to minimize the external voltage supplies and to facilitate the calibration of the detector.
Once, the final production is launched, testing of the detectors will be the next issue. Calculating on a yield of 50%, about 500 detectors will have to be characterized at best. First a visual inspection has to be done to check for interruptions or shorts in the metal. The second step is to check both the current at the anodes and the linearity of the potential on the divider (detector selection criteria). We are studying a mechanical holder probe whose task will be to allow alignment of the detector to a set of two probe cards able to have contact with the detector safely from both faces (probe pad coordinates). In this way we will be able to check the detectors and choose only the well-performing ones with minimal risk of damage to the detector during this operation.
The next step towards the final assembling is the connection of the High Voltage microcables to the detector. These cables are designed and produced by the Scientific and Technological Research Institute of Instrument Engigneering, Kharkov, Ukraine.
The read-out electronics and the ladder assembling procedure are both developed in Torino. (http://www.to.infn.it/esperimenti/alice/)
