S. Gautam, Panjab University, Chandigarh, July 20-21, 2007

Centre of Advanced Study in Physics, Panjab University, Chandigarh

S. Gautam, Panjab University, Chandigarh, July 20-21, 2007

RPCs in CMS

Supreet Pal Singh

India-CMS Meeting, July 20-21, 2007

BARC, Mumbai

Centre of Advanced Study in Physics, Panjab University, Chandigarh

CMS will use three types of gaseous particle detectors for muon identifica-tion: Drift Tubes (DT) in the central barrel region, Cathode Strip Chambers (CSC) in the endcap region and Resistive Parallel Plate Chambers (RPC) in both the barrel and endcaps. The DT and CSC detectors are used to obtain a precise measurement of the position and thus the momentum of the muons, whereas the RPC chambers are dedicated to providing fast informa-tion for the Level-1 trigger

RPC system at CMS detectorRPC system at CMS detector

RE1/2, RE1/3 of end-cap RPC (totally 144 detectors);

RPC(Resistive Plate Chambers) is in fact a special purpose detector providing

fast information for the Level-1 trigger. Its geometry follows the muon detector

geometry. As stated earlier in addition to RPC there are two other types of

detectors used for muon identification: Drift Tubes (DT) chambers in the barrel

and Cathode Strip Chambers(CSC) in the endcap region. These muon detectors

consists of four stations interleaved with the iron return plates. They are

arranged in concentric cylinders around the beam line in the barrel region, and

in disks perpendicular to the beam line in the endcaps.

So we have a barrel divided in 5 wheels ,12 wedges and 4 stations and two end-

caps divided in 4 stations(disks) with rings of 36 or 18 wedges. Stations are

numbered 1 to 4 starting from the one nearest to the beam line. In the barrel, we

have RPC chambers below each Muon station. MS1 and MS2 will have also

RPC chambers on the outer surface.

The total number of RPC barrel chambers are 480: all the chambers can be seen

in this schematic view that clarifies also why there are 96 chambers for each

helm . (The CMS reference system has the x axis pointing to LHC center, the y

axis pointing up and the z axis along the beam )

I. All the end-cap RPC chambers have a trapezoidal shape with

radial strips.

II. Their number is 540. They are organized in rings and two or

more rings form a disk. Each ring contains 36 or 18 chambers

(i.e. each chamber sees a sector of 10 or 20 degrees).

III. The first station has three rings of 36 chambers, the other three

stations have an outer ring of 36 and an inner ring of 18.

Basics of Basics of ResistiveResistive

Plate ChamberPlate Chamber

Resistive Parallel Plate Chambers

Resistive Parallel plate Chambers are fast gaseous detectors whose

information is at the base of the triggering process. RPCs combine

a good spatial re-solution with a time resolution of 1 ns,

comparable to that of scintillators. The RPC is a parallel plate

counter with the two electrodes made of very high resistivity

plastic material.

The high gain and thin gap result in a small but very precise delay

for the time of passage of an ionizing particle. The high resistivity

electrodes are transparent to the electric signals generated by the

current of the avalanche: the signals are picked up by external

metallic strips. The pattern of hit strips gives a measure of the

muon momentum which is used by Trigger

• Effective area: ~ 0.6X0.6 m2;• Double gas gaps: 2 mm;• Read-out strip plane between two gas-gaps;• bakelite resistivity: ~ 10 12- 10 13 cm .

Block diagram of an RPC

The electric field inside a RPC is uniform. Electrons made free by the ionizing particle near the cathode generate a larger number of secondary electrons (exponential multiplication). The detected signal is the cumulative effect of all the avalanches. A proper threshold setting allows the detection of a signal dominated by the electrons generated near the cathode. The threshold setting determines to a large extent the time delay of the pulse, the time resolution and also the efficiency. With a proper choice of the resistivity and plate thickness, the rate capability can reach several thousand Hertz per cm2.

The drawing shows the simplicity of an RPC detector:

one of the two resistive plates holds a glued array of

small 2mm thick spacers having a typical pitch of 10 cm.

Also glued on the plate is the border that will guarantee

the chamber tightness. The second plate is then placed

on top and the detector is completed.

Basics of Resistive Plate ChamberBasics of Resistive Plate Chamber

The signal is induced on the read-out electrodes.

Basics of Resistive Plate Chamber: working Basics of Resistive Plate Chamber: working modemode

Avalanche: The electric field is such that the electron energy is larger than the ionising potential

Streamer:The electric field is intense enough to initiate a spark breakdown

CMS-RPC will work at avalanche mode, to ensure the proper operation at very high rate.

– Good timing performance comparable to that of scintillator (~ 1-2 ns)– Space resolution sufficient for CMS muon trigger purpose (~ cm ) – Simple design & low cost– Arbitrary readout geometry – Good rate capability (~ several kHz/cm2)

Why RPC?

RPC has been used in L3, BaBar, Belle experiments. All 4 LHC experiments will use RPC for muon system. STAR used MRPC as TOF

Assembly procedure of RE1/2 RPCAssembly procedure of RE1/2 RPC

Brief outline of work done at BARC

● The wires were cut for RE*/2 and RE*/3 RPCs.

● Total no. of wires cut - 1300 wires.

● 1000 wires for 10 RE*/2 and 300 wires for 3 RE*/3.

Some interesting facts about

CMS detector

General: The total mass of CMS is approximately 12500 tonnes - double that of ATLAS (even though ATLAS is ~8x the volume of CMS).

Silicon Tracker: The CMS tracker comprises ~250 square metres of silicon detectors - about the area of 25m-

long swimming pool.

The silicon Pixel detector comprises (in its basic form) more than 23 million detector elements in an area of just over 0.5 square metres.

Electromagnetic Calorimeter (ECAL):

The lead tungstate crystals forming the ECAL are 98% metal (by mass) but are completely transparent.

The 80000 crystals in the ECAL have a total mass equivalent to that of ~24 adult African elephants - and are supported by 0.4mm thick structures made from carbon-fibre (in the endcaps) and glass fibre (in the barrel) to a precision of a fraction of a millimetre.

Hadronic Calorimeter (HCAL) : The brass used for the endcap HCAL comes from recuperated artillery shells from Russian warships

Solenoid Magnet :

The CMS magnet will be the largest solenoid ever built. The maximum magnetic field supplied by the solenoid is 4 Tesla - 100000 times the strength of the magnetic field of the earth. The amount of iron used as the magnet return yoke is roughly equivalent to that used to build the Eiffel Tower in Paris. The energy stored in the CMS magnet when running at 4 Tesla could be used to melt 18 tonnes of solid gold.

Data Acquisition:

The "slow control" data rate of an LHC experiment (readout of temperatures, voltages, status etc.) is comparable to the total LEP data rate. During one second of CMS running, a data volume equivalent to 10,000 Encyclopaedia Britannica is recorded . The data rate handled by the CMS event builder (~500 Gbit/s) is equivalent to the amount of data currently exchanged by the world's Telecom networks

Computing: The total number of processors in the CMS event filter equals the number of workstations at CERN today (~4000 - how many failures per day?!)

Thanks