In order to demonstrate the feasibility for operating silicon detectors close to a high-intensity (peak currents up to 50 mA) electron beam under ultra-high vacuum (UHV) conditions a test detector was built. A drawing of the system is shown in the figure. The detector has three octagonal silicon counters with 1~mm wide strips on each side. The strips are read-out using an APC64 chip. Special care was needed to ensure that all materials used are UHV compatible. For that reason a special type of hybrid was developed consisting of an oxygen-free copper plate with a layer of kapton glued to each side. The copper ensures a good heat conduction which is important for cooling purposes, while the kapton layer is used to build the electronic circuitry.

Three-dimensional view of the Silicon Test Counter (STC). The top part of the picture shows the cover plate with the opening towards the target cell. This window is covered by an open grid (not shown) with a spacing of 5 x 5 mm2. In the bottom figure the following parts can be identified: three octagonal silicon detectors supported by rectangular carrier boards, cabling (black) and the cooling bar, which extends below the counters. The overall size of the STC is 421 x 142 x 63 mm3

The STC was a prototype only, which was installed to demonstrate the feasibility of a new detection technique. The proof of principle has been given. Events coincident with a deep-inelastic scattered lepton in the HERMES detector were successfully identified. The momentum distribution of spectator protons in the deuteron (resulting from a deep-inelastic scattering event on a neutron) could even be determined. Sufficient data were collected to extract the first momentum distribution of spectator protons from deep-inelastic scattering events on deuterium (see the distribution ).