Requirements

The following requirements had to be fulfilled during the design of the Inner Tracker system:


Radiation and rate environment. The Inner Tracker covers the region from 6 cm till 27 cm, this is the region with the highest occupancy. Particle flux was assumed to be in the range from $2\times10^{3} mm^{-2} sec^{-1}$ in the outer edge till $2\times10^{4} mm^{-2} sec^{-1}$ near to the beam pipe. It was foreseen in the HERA-B proposal, that the detector has to deal with interaction rates up to 40 MHz. This could be translated to a radiation dose of $\approx$ 1 Mrad/year in the hottest area, the detector should work under such conditions for several years and should show stable performance and high efficiency during this time.


Granularity. The granularity was chosen based on the requirement that the number of hitted strips per event should not exceed 5-10%. The amount of fake tracks produced by the pattern recognition program can be sufficiently higher than the acceptable level, in case of higher multiplicity events. The high occupancy will also affect the efficiency of the first level trigger. In order to fulfill this requirement the pitch size was chosen to be $\approx300 \mu$m with the strip length up to 25 cm and about 350 $\mu$m pitch size for chambers with the strip length up to 27 cm.


Spatial resolution. The spatial resolution was required to be $\sigma_x\approx100\mu$m in the bending plane of the spectrometer magnet, the resolution perpendicular to this plane should be $\sigma_y\approx1$mm. The resolution along the proton direction should be $\sigma_z\approx3$mm. All these requirements led to the decision to choose for the pitch size of the chambers inside the magnet and in front of the RICH vessel 300 $\mu$m and for the chambers situated between RICH and ECAL a pitch of 350 $\mu$m. The coordinate $x$ is measured directly by the $0^0$ wires the other coordinates are provided by chambers orientated in three stereo views (available $0^0,+5^0,-5^0$ views). The stereo angle was selected in such a way that the designed resolution in the $y$ plane is reached and the amount of random coincidents is limited to an acceptable level.


Signal speed. The normal event ( interaction of protons in a single bunch with the target) can occur every 96 ns. The electronics should be fast enough to be able to deal with this rate.


Trigger information. Some of the Inner Tracker superlayers have to provide informations for the FLT system. The FLT has to make a decision whether an event has to be kept or not in about 10 $\mu$sec. The front-end electronics of the Inner Tracker has to store the full hit information for the time period needed for the FLT to make its decision.


Magnetic field. A part of the Inner Tracker should operate in the magnetic field. The strength of the magnetic field is about 0.85 T. The resolution and efficiency of the chambers placed in the magnetic field have to fulfill the requirements mentioned above. No magnetic materials could be used during construction.


Radiation length. The critical point is the material used for the detector production. Therefore detectors and supports and also cables and gas pipes should be as thin as possible (measured in radiation length $X_0$), in order to suppress multiple scattering, bremsstrahlung and photon conversion. All these effects have a large influence on the momentum resolution and quality of the tracking.