This help file tries to describe the various lead-scintillator counters that have been installed in DELPHI to cover holes in the electromagnetic calorimetry. (HPC in the barrel, FEMC forward.)
The barrel electromagnetic calorimeter (HPC) consists of 144 modules, arranged in 6 rings with 24 sectors. Gaps between modules are about 1 cm in phi and z, except at (theta) 90 degrees, between the third and fourth rings, where there is a 7.5 cm gap to accomodate a stiffening ring for the cryostat. This central z-gap and all the phi cracks point straight back to the interaction region and make it possible for particles to escape undetected. (The other z-gaps are much less serious but require some special care in shower reconstructions.)
The cable ducts between the barrel and the end caps also point towards the interaction region, thus creating rather large holes around (theta) 40 and 140 degrees.
Already from the beginning, this gap has been covered by lead-scintillator counters, which are read out via wave-length shifters and fibers to PM tubes mounted outside the magnetic field. Until 1994, the 24 signals were digitized by two Fastbus cards identical to those used for the HPC scintillators. From 1995, the signals are digitized by the same LeCroy ADC that is used for the phi taggers.
There is some space between the HPC and the cryostat, where lead-scintillator counters can be installed to cover the phi cracks. Installation requires extraction of HPC modules and was foreseen to take place during the large-scale HPC interventions (92/93 and 93/94) that were planned for curing the ageing problem (by chamber rewiring and graphite inserts in the brass pads). This enormous intervention was cancelled, however, when graphite test modules showed no significant improvement and a rewired module had an unexpectedly large ageing speed. Only the worst modules were repaired or replaced, but enough modules were removed to allow phi tagger installation in the critical regions around the DELPHI feet that are not covered by TOF.
Like the 90° taggers, the phi taggers are connected to PM tubes mounted outside the magnetic field. The signals are digitized in a LeCroy Fastbus ADC. Currently, 36 channels exist out of the planned 192.
These holes of complicated geometry between barrel and end caps can be covered by three concentric rings of lead-scintillator counters mounted at increasing radii in the space between the barrel and forward RICHes. Wavelength-shifting fibers connect the scintillator to so-called fine-mesh PM tubes (Hamamatsu), which can operate in the magnetic field. The signals are digitized in a LeCroy Fastbus ADC.
Installation is made difficult by the presence of RICH support structure and all the cables. As of spring '95, both the A- and C-side rings are essentially complete, except for a few impossible sectors. Currently, 46 channels are read out.