The Lambda Wheels are two wheel-shaped arrays of silicon strip counters, which are mounted inside the HERA electron beam vacuum approximately 50~cm downstream of the HERMES internal target. The Lambda Wheels enlarge the acceptance for the detection of slowly-decaying hadrons produced in various semi-inclusive reaction channels by a factor of two to four. Moreover, the precision by which the polarisation of the Lambda-zero hyperon can be determined is greatly improved, because the false asymmetry (which is large if the standard HERMES acceptance is used) vanishes when the Lambda Wheels are employed.

The detector consists of 12 modules, and has an outer diameter of about 33~cm. Each module has two double-sided wedge-shaped silicon counters, cut out of 6'' wafers, with acccompanying readout electronics based on the HELIX-2.2 chip. Cooling is provided by a closed loop filled with ethanol, which in turn is cooled by a set of Peltier elements.

The HERMES Lambda Wheels. All support materials and readout electronics are located at the outer diameter of the system staying clear of the standard HERMES spectrometer acceptance.

In the year 2000 a prototype module was installed inside the HERA vacuum chamber, using the already available support structure of the Lambda Wheels. The efficiency of this prototype module was found to range from 95 to 97%, depending on conditions. The average track multiplicity was determined to be only 1.2 tracks per module (after cuts). Hence, there are no concerns about possible tracking ambiguities.

Following the upgrade and recommissioning of HERA (in 2001-2002), the Lambda Wheels were reinstalled. The figure shows a picture of the detector after its installation inside the vacuum chamber. Unfortunately, the commissioning of the Lambda Wheels in 2002 -- 2003 was hampered by the much too high fringe field of the newly installed transverse-target magnet. During the short periods when this magnet was switched off, the Lambda-Wheel electronics were tuned and by the end of this period the Lambda-Wheel data could be included in the normal data stream.