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.
