First experience with the HERMES silicon detector

A silicon detector system (the Lambda Wheels) is being added to the HERMES experiment at DESY. The acceptance of the front region will be enlarged in order to increase the fraction of Lambda (0), Lambda (c), and J/Psi -partic les which carl be reconstructed. The detector is located inside the ring va cuum. Two disks cover the area between 4.5 and 16cm from the beam. The dete ctor consists of 12 identical modules. This report describes some details o f the design, the cooling and the read-out of the detector. A prototype of a single module has been operated since the beginning of 2000. A signal-to- noise ratio of about 10 has been obtained for minimum-ionizing particles. ( C) 2001 Elsevier Science B.V. All rights reserved.