The abbreviation ”CMS” stand for Compact Muon Solenoid. It is a general purpose detector to be operated as Large Hadron Collider at CERN in Geneva for the 2007 experiment. This cylindrical shaped detector has 14.6 meter overall diameter, overall length is 21.6 meter and 14500 tons total weight.
The three main regions of CMS detector are
The Barrel Region
The Endcap Region
Forward region
Backward Region
The Very Forward Calorimeter
Resistive Plate Chambers (RPCs)
The Resistive Plate Chambers (RPC's)
are the part of the CMS Muon System. An RPC consists of two
parallel plates made out of bakelite with a bulk resistivity of
109 - 1010 Ωcm, separated by a gas gap of
2mm. The whole structure is made gas tight. The outer
surfaces of resistive material are coated with conductive
graphite paint to form the high voltage and ground
electrodes. The readout is performed by means of aluminum strips separated from the
graphite coating by an insulating Polyethylene (PET) film.
Resistive Plate Chambers (RPCs) are very important detectors,
because of excellent time resolution which is crucial at Large
Hadrons Collider (LHC) due to the beam crossing time of 25ns.
These are the dedicated chambers used for the first level muon
trigger. The good performance of RPC is very essential in
assigning the muon to the right bunch crossing. There will
be an elastic collision after every 25ns. The event rate at LHC
is expected to be extremely high due to the large total pp
cross-section at √14TeV. The expected event rate is ~105
Hz which is unmanageable, so using level 1 trigger we
achieve a reduction by a factor of 104. This rate at
the output of level 1 is too high to be stored, therefore
another factor of 103 is applied at the high level
trigger and the final event rate is brought down to the 100 Hz.
At level 1 trigger the available time for the detector is 25 ns,
which implies fast trigger system. To achieve this goal, we need
a fast trigger system. Resistive Plate Chambers (RPCs) are good
candidates for fast triggering, where the spatial resolution is
not important but fast timing information is more important. The
shape of the RPC is trapezoidal and it covers 10 degrees in Ø.
The strips are also trapezoidal shape. The endcap RPCs are
double gap chambers with the readout strips running
perpendicular to the beam line (in Ø).
RPC's Usage in CMS
Resistive Plate Chambers (RPC's) are used in the endcaps of the Compact Muon Solenoid (CMS) detector at CERN. Due to its ease of construction and operation, low cost and the good time resolution, it has been proposed as a suitable detector for the first level muon trigger for the LHC (Large Hadron Collider) experiments, where thousands of square meters will be needed.
The low cost of production and the flexibility in segmentation make this solution attractive in experiments where there is need to equip very large surfaces.
Few conditions that have to be fulfilled by the RPC are as follows
RPC efficiency > 97%
Rate capability > 1 KHz/cm2
Operation efficiency plateau > 400V
Time resolution < 3ns
Cluster size < 3
Dead time should be few nano seconds
RPC's Production in Pakistan
Pakistan is actively involved in the production of Resistive Plate Chambers (RPCs) (RE2/2, RE2/3, RE3/2, and RE3/3) which will be used in CMS endcaps. RPC will serve as first level muon trigger. Pakistan has to prepare 288 RPCs with 10% in excess for backup purposes. Bulk production of RPCs is in progress.
The shape of the RPCs is trapezoidal because of the projectivity and it covers 10 degrees in ø. The strips are also trapezoidal in shape. The endcap RPCs are double gap chambers with the read out strips are running perpendicular to the beam line (in ø).
Some important chamber dimensions and parameters are given in table 1 below:
Where, Ro is radius from beam line (low eta), and Ri is the radius from beam line (high eta).
Station
Ri (mm)
No. of Chambers
A (mm)
B (mm)
C (mm)
D (mm)
Area (sq. m)
RE2/2
3299
36+36
1693
979
684
1687
1.403
RE2/3
5001
36+36
1961
1323
981
1954
1.917
RE3/2
3299
36+36
1693
979
684
1687
1.403
RE3/3
5001
36+36
1961
1323
981
1954
1.917
TOTAL
288
513.2
Table1: Chambers geometrical data
Pakistan Atomic Energy Commission (PAEC) and NCP are involved in the production of 288 RPCs for the endcap of CMS. The assembly of RPCs is done at site A (PAEC), after assembling, the RPCs are moved from site A to site B (NCP) for testing and quality assurance. Each RPC consists of 3 FEBs and each FEB contains 4 ASICs.
NCP has well equipped electronic laboratory for RPCs testing. RPCs are tested with the muons because these detectors have to detect the muons in the original experiment in 2007. Cosmic rays are used as a source of Muons for testing of RPCs at electronic testing laboratory, NCP. An assembled RPC at NCP lab is shown in figure 2.
Figure 2: Assembled RPC at NCP Lab
Electronic testing laboratory at NCP consists of:
Gas System and Gases (Freon, Iso-butane, and SF6)
Hodoscope (Figure 3)
High voltage and low voltage power supply(figure 4)
Gas system (figure 5)
Data Acquisition system (DAQ) (figure 4)
Miscellaneous items
The DAQ will be based on VME and NIM. At NCP we have both VME and NIM bins.
The hodoscope will consists of scintillators, which will be placed at the top and bottom of the Cosmic Ray Muon Stand as shown in Figure 3.
Figure 3: Hodoscope at NCP Lab
These scintillators are 2.0 meters long. To cover the full width of a chamber we require 8 scintillators on the top and 8 at the bottom, so in total 16 Scintillators are used. The trigger logic based on these scintillators is shown in figure 6. We are operating these scintillators between 1.0 ~ 1.6KV and to read out the signals from these scintillators preamplifiers are used.
Figure 4: HV power supply and DAQ system at NCP lab
