DELPHI Inner Detector (ID)

Technical description

The Inner Detector (ID) is a tracking and triggering detector. It consists of two parts: the JET chamber and the Trigger Layers (TL). The JET chamber is a driftchamber, subdivided into 24 sectors of 15 degrees in phi. Each sector consists of 24 sense wires, measuring drifttime. The driftgas used is CO2 with a small admixture of Isobutane. This together with the special shaping of the driftfield, makes that for >1.0 GeV tracks coming from the interactionpoint, all 24 signals on the sense wires arrive more or less at the same time.

The TL consist of 5 cylindrical layers of 192 Straw tubes. The tubes in subsequent planes are staggered by half a cell. The cell width is about 8 mm. Pulse heights are measured, and drift times.

Full angular coverage is down to 15 (165) degrees in theta.

It looks like (here with a Z0 in it!)

Quantities measured

For each event, 3 types of information are produced :

• The JET chamber measures up to 24 R*PHI points on a track.
• The TL measure up to 5 points in R*PHI.
• trigger : see below.

Accuracy

• JET: the single wire resolution is of the order of 90 microns. Combined into a TE and after careful calibration and 'fixing', the TE resolution is of the order of 40 microns in R*PHI and about 1.2 mrad in the angle phi. The 2-track separation is of the order of 1 mm.
• TL: the R*PHI measurement is used in the trigger and to provide an R*PHI TE of approx. 150 micron resolution. It therefore also resolves the left/right ambiguity in the JET chamber.

Trigger:

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: The signals on the 5 (staggered) tubes of the TL are used in a track-finding chip to define a track pointing roughly to the interaction point. The OR of a group of 8 (times 5) tubes defines a trigger bit per 'sector'. The OR of all 24 sectors is the first level trigger ID used in DF4_I. The majority (>12) coincidence of the 24 wire signals in a JET sector defines a JET trigger bit per sector. The TL bit can be hardware ANDed with the corresponding bit from the Jet and defines an ID bit for sector i. This bit is then ANDed with the OR of the three OD sectors i-1, i, i+1. It is this ID.AND.OD for the 6 sectors covering the TPC sector gaps that is used in IDOD6 at the 2nd level trigger. The OR of the 24 TL bits forms the IDMJ1 trigger which is used at several places in DELPHI in coincidence with other detector triggers.
• T2: No special T2 trigger is being made. The T1 part ID.AND.OD for the 6 sectors covering the TPC sector gaps is used in the socalled IDOD6.
• T3: The basis of the T3 is a fast track search using histogramming of the drifttimes on the 24 wires in a JET sector, thus mimicing the hardware majority coincidence of T1. But now the average drifttime defines a rough PHI coordinate. This track candidate has then to be confirmed in space by a track found in the trigger chips of the TL. The output of the T3 is not only a bit per sector, but also the PHI of the track (resolution about 1 degree, corresponding to about 3 mm in R*PHI).