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                                    ALICE TPC Front End Electronics

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The ALICE TPC Front End Electronics in short

The ALICE TPC is a 88 m3 cylinder filled with a gas and divided in two drift regions by the central electrode located at its axial centre. A field cage creates a uniform electric field along each half of the chamber. Charged particles traversing the TPC volume ionise the gas along their path, liberating electrons that drift towards the end plates of the chamber. The necessary signal amplification is provided through avalanche effect in the vicinity of the anode wires. Moving from the anode wire towards the surrounding electrodes, the positive ions created in the avalanche induce a positive current signal on the pad plane. This current signal, which is characterised by a fast rise time (less than 1 ns) and a long tail with a rather complex shape, carries a charge that, for the minimum ionising particle, is of 4.8fC.

The readout of the signal is done by the 570132 pads that form the cathode plane of conventional multi-wire proportional chambers located at the TPC end caps. The signals form the pads are passed to 4356 front-end cards located some 10 cm away form the pad plane via flexible capton cables. In the front-end card a custom made charge sensitive amplifier transforms the charge induced in the pads in a differential semi-gaussian signal that is fed to the input of the ALTRO chip. Each ALTRO contain 16 channels operating concurrently that digitize and process the input signals. Upon arrival of a first level trigger, the data stream is stored in a memory. The maximum number of samples that can be continuously processed for each trigger (event data stream) is 1000. When the second level trigger (accept or reject) is received, the latest event data stream is either frozen in the data memory, until its complete readout takes place, or discarded. The data memory has the capacity to store up to 8 event data streams. The readout can take place any time, at a speed of 200MByte/sec through a 40-bit wide backplane bus linking The Front End Cards to the Readout Control Unit.   

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Last update 09/13/2007