The Outer Detector (OD) is composed of 24 planks of 5 layers x 32 columns
of 4.7 meters long drift tubes operating in the limited streamer mode
(section 1.65x1.65 cm2). The electrons produced by a charged particle
crossing a tube drift toward the wire (high voltage of 4.4 kV) and give
a signal with short raise time and high amplitude (typically 3ns and 80mV).
Situated at 2 meters from the beam axis, it makes up the precision of the
momenta of charged particles measured by the TPC.
5800 electronic channels in total give 3 space points plus 2 r-
points per track.
20% (contiguous channels in r-phi) of the plank 6 is known to be dead.
It looks like
For each event, 3 types of information are produced :
- the drift distances : for each tube in the detector, the drift distance
of the produced electrons is derived from their drift time
recorded in TDCs.
- the z values : all the tubes in the 3 internal layers give a
z value. It is obtained from the difference in
time between the arrivals of the signals at both ends of the tubes.
- trigger : see below.
- The precision over the measure of the r-(phi)
coordinate is 100(mu) per track.
- The accuracy of the z measurement is of 4.4 cm.
Dedicated electronics is used to produce the local trigger
results for the first and second level. ID trigger data are correlated
with the OD ones in order to reduce the rate due to cosmic and beam gas
events. The third level is done by software. The IDOD trigger is
very useful to cover the cracks between the TPC sectors.
- T1: 15degres segmentation (one plank). One 24-bit word giving
the planks hit on the basis of simple track reconstrucution.
It's then correlated with ID 24 r-phi bits.
- T2: The compatibility with the first level is ensured to take into account
the correlation with ID and a cut on a quality factor is applied (namely the
number of matching z values in each trigger track must be greater that 2).
DELPHI uses IDOD6 in order to cover the TPC cracks (IDOD6 is the multiplicity
seen in the 6 OD planks facing each of the TPC crack).
- T3: Will be able to give the t0 online soon?