After successful test of a small prototype chamber the complete set of chambers for the experiment was manufactured by the PNPI in Gatchina and transferred to Bonn. Now 8 drift chambers are mounted behind the spectrometer magnet. 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. High voltage is supplied to the chambers from a CAEN SY2527 crate via specially designed chamber mounted fuse boards, that split the HV supply to several channels per chamber. This fuse boards allow to monitor the current for each sector of 32 sense wires of a chamber individually, and protect the chamber by switching off the HV supply if any one sector exceeds the current limit. Interfacing to this cards is done via an optically isolated USB interface, with full integration into the experiments slow control. The photon beam has to penetrate the DCs. To avoid counting overload by secondary e+ e- pairs insensitivity spots of 5 × 5cm2 are realized at the centre of the chambers by anodizing 6 of the sense wires with gold, thus increasing the total diameter to 100μm in the desired area. The effect of this procedure has been successfully tested at PNPI.
The distance of the chambers from the target ranges from 3.7m for the first chamber up to 4.7m for the last. For accurate positioning and simplified handling the chambers are hanging from two support beams attached to the magnet. Four of the chambers will be rotated by ±9degree around the beam axis. With two of the remaining chambers having horizontal wires and the other ones vertical wires four different wire orientations are obtained.
|chamber||angle||sen. area [mm2]||height [mm]||width [mm]||sen. wires||field wires|
|Y||0°||2456 × 1368||3189||1642||169||484|
|X||90°||2456 × 1232||2866||1965||288||868|
|U||99°||2728 × 1825||3157||2395||320||964|
|V||81°||2728 × 1825||3157||2395||320||964|
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.
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:
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.
To investigate the performance of the chambers and the readout system a prototype chamber was build by PNPI Gatchina and tested within a diploma thesis . During the tests the electronics of the prototype chamber performed without problems up to event rates of several kHz. The position resolution and efficiency could be investigated with high energetic electrons at the beamline E tagging system. A preliminary drift time relation x(t) was obtained through the drift time spectrum and verified in a separate measurement with a pixel detector. Using this relation a position resolution was well below the designed 300μm. This prototype chamber is now used in an advanced lab course experiment.