The recent advances in Si and diamond detector technologies give hope of a simple solution to the radiation hardness problem for vertex trackers at the LHC. In particular, the CERN RD39 collaboration has recently demonstrated that operating a heavily irradiated Si detector at liquid nitrogen (LNFormula: 0) temperature results in significant recovery of Charge Collection Efficiency (CCE).

Among other potential benefits of operation at cryogenic temperatures are the use of large low-resistivity wafers, simple processing, higher and faster electrical signal because of higher mobility and drift velocity of carriers, and lower noise of the readout circuit.

A substantial reduction in sensor cost could result.

The first goal of the approved extension of the RD39 program is to demonstrate that irradiation at low temperature in situ during operation does not affect the results obtained so far by cooling detectors which were irradiated at room temperature. In particular, we shall concentrate on processes and materials that could significantly reduce the final detector cost.

The second goal is to demonstrate the operation of existing radiation-hard CMOS readout electronics at LNFormula: 1 temperature, and to measure discrete device characteristics at these temperatures, so that their parameters can be extracted and optimised circuits can be designed.

The design and fabrication of optimised circuits, however, is not planned at this stage.

The third goal is to demonstrate that low-mass cooling at LNFormula: 2 temperature is feasible at a reasonable cost, and that the electrical and optical feedthroughs of a large system can be mastered. The extended programme also consists of common projects with the NA60, COMPASS and TOTEM experiments at CERN.
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