Drift Chambers
Behind the open dipole magnet 8 driftchambers are used for track reconstruction. Each chamber contains a double layer of hexagonal drift cells, so that each particle track will hit at least two drift cells in each chamber it passes. The chambers come in four different orientations, two chambers have vertical wires and measure the x-coordinate, two chambers have horizontal wires and measure the y-coordinate, and four chambers have wires tilted by ±9° against vertical, measuring an u- respectively v-coordinate, used to disambiguate between true and false combinations of multiple hits in the x and y chambers.
The individual chambers have slightly different sizes and wire numbers due to their different orientations (see table). The two layer drift cell geometry is identical for all chambers. To create a nearly symmetric electrical field additional field wires are introduced on both sides of the drift cell layers. The spacing between to anode wires of the same layer is 17mm.
| chamber | angle | sens. area / mm² | height / mm | width / mm | signal wires | cathode wires |
| X | vertical | 2456x1396 | 1965 | 2867 | 288 | 868 |
| Y | horizontal | 2483x1232 | 1779 | 3053 | 144 | 436 |
| U /V | ±9° | 2592x1765 | 2335 | 3139 | 304 | 916 |
The wires are made from gold plated tungsten with a diameter of 25 μm. The anode (signal) wires are kept on a ground potential, drift cells are defined by the hexagonally arranged cathode
wires, typically set at high voltages around U = −2800 V. Additional 200 μm diameter gold plated beryllium bronze field-forming wires surround the double-layer of drift cells to assure an equal field distribution and to shield against external field distortions. All wires are soldered to PCB boards and additionally glued with epoxy glue to increase the mechanical stability.
In contrast to the front tracking detectors the drift chambers do not have a central hole for the photon beam to pass trough. To avoid signal overflow or damage of the chambers a central spot of ≈5 × 5 cm² has been made insensitive by galvanising additional gold withinthis area to the signal wires, increasing the wire diameter to approximately 100 μm. This sufficiently reduces the gas amplification within the spot to prevent signals from the passing photon beam.
The chambers are operated with a mixture of 70% Argon and 30% CO2. After mixing the gas is distributed to the chambers individually and vented after passing the chamber. As the flow of 2 l/min in total is small enough this method was chosen above a more complicated recycling of the gas.
The readout of the chambers is done with the CROS-3 (Coordinate Read Out System, third generation) developed by PNPI Gatchina. It consists of four different types of cards:
- CSB - CROS-3 system buffer
- CCB16 - CROS-3 16-channel concentrator board
- CCB10 - CROS-3 10-channel concentrator board
- AD16 - 16-channel amplifier/discriminator card
The AD16 amplifier/discriminator boards are directly attached to the drift chambers. The digitized signals of the frontend boards are send via an LVDS link to the CCB10 concentrators. The CCB10 concentrators transmit their signal to the single CCB16 card. Finally an optical fiber connects the readout system to CSB system buffer implemented as a PCI-card. This cost effective setup minimizes the need for interconnects between the chambers.