Barrel RPC Chamber consists of 2 double-gaps, each equipped with a common plane of 96 strips read-out by 6 front-end boards. The two double-gaps have different lengths and are staggered to reduce the dead zone. Double-gap are made by 2 superimposed gaps with the spacers overlapped in order to do not produce gap deformation. The gap is made by two 2 mm thick bakelite plates (resistivity 1-3·1010 ·cm) kept at a fixed distance of 2 mm by insulating spacer (diameter of about 10 mm). The chambers will at 9.2-9.5 KV with a Freon based gas mixture (96.2 C2H2F4, 3.5% i-C4H10, 0.3% i-SF6).

strip planes

HV

Gas Gap

GND

backeliteGas Gap

Gas GapGas Gap

The chamber is a self-supporting mechanical structure producing a gentle pressure (about 15 Kg/m2) in order to keep the strip planes in place and in contact with the two gaps.

Production and Test of 150 Barrel RPC Chambers of the CMS experiment at LHC collider

The Compact Muon Solenoid is a general purpose p-p detector at the Large Hadron Collider

Pierluigi Paolucci – I.N.F.N. of Naples (Italy)Dip. di Fisica & INFN di Bari (Italy), Dip. di Fisica & INFN di Pavia (Italy), IRNE & Univ. of Sofia (Bulgaria), Peking Univ. (China)

The LHC bunch crossing frequency is 40 MHz and with a Luminosity of 1034 cm-2s-1 the average number of inelastic interaction in a crossing is 17.3. A rate reduction of a factor 4·105 is needed to reach the recording capability of 100 Hz. The barrel has 3 independent L1 muon trigger systems (DT-RPC-CSC), each generating 4 muon candidates with an associated Pt and track quality. The Global Muon Trigger receives this data and properly combines them to generate 4 final muon tracks. The combined trigger efficiency (see fig.2) is always greater than 90%. The estimated Muon Trigger Rate in the barrel region (||<1.04) goes from 102 Hz up to 105 Hz as function of the Pt (at L = 1034 cm-2s-1) and the RPC trigger system is able to trigger muon with a Pt > 6-8 GeV/c with an efficiency greater than 90% (see fig.3).

The Barrel Muon System is made by 5 wheels, divided in 12 sectors with 4 stations or magnet iron gaps (called for the RPC system RB1, RB2, RB3 and RB4) each. The iron gaps will be filled with a sandwich made by RPC-DT-RPC in the station RB1-RB2 and RPC-DT in the station RB3-RB4.

RB4

RB3

RB2

RB1 DTRPC

• area covered 2.400 m2

• number RPC chambers 480• number of strips 75.000• number of front-end cards 4.700

RPC system numbers

About 150 RPC chambers have been produced (General Tecnica, Bari and High Tech) and tested (Bari and Pavia) in Italy, corresponding to 1450 gaps and 400 double-gaps. A very accurate Quality Control check is made on the single gaps, double gaps and chambers.

Chamber production trend

Hz/cm2

reconstructioncounting

eff = 98.7%

<Dark Current> = 5mANoise Rate < 6 Hz/cm2

Mean Global Efficiency is measured using two different methods (fig.5):

The Counting method calculates the efficiency as the ratio between the number of chamber signals in a fixed time window and the number of triggers.The Reconstruction method uses reconstructed cosmic tracks in order to reduce the number of “fake” events generated by cosmic showers, fake triggers and multi-particle events.

Muon detector is designed to trigger and identify muons and measure their momentum. It consists of four sub-detectors: Drift Tubes (DT) in the barrel region, Cathod Strip Chambers (CSC) in the endcap region and Resistive Plate Chambers (RPC) as dedicated trigger detectors in the barrel/endcap.

105 Hz

103 Hz Trigger efficiency .vs.

Trigger Rate

Trigger efficiency .vs. Pt

fig.1 fig.2 fig.3

Two Cosmic rays telescopes have been built in Bari (10 chambers) and Pavia (5 chambers) in order to study in details the performances of the RPC chambers. One more “short” test will be done at the ISR (CERN) before installing them.

Chamber efficiency, cluster size, noise rate and dark current are measured for every chamber and the results are stored in the RPC database (http://www.ba.infn.it/rpc)

The trigger is made by two planes of scintillators, placed on the opposite ends of the structure. The DAQ is based on a custom VME TDC running at 40 MHz in common stop mode and with a bin size of 25 nsecThe gas mixture is: 96.2% C2H2F4, 3.5% i-C4H10, 0.3% i-SF6

The Cluster size (fig.6) is defined as the number of contiguous strips of the same chambers.The Noise Cluster Rate (fig.6) and the Noise rate (fig.4) are measured using the number of hits and clusters found outside the trigger window (3 TDC bins = 75 ns).

<Cluster Noise Rate> = 2.4 Hz <Cluster Size> = 3 strips <efficiency> = 98.7 %

The dark current and efficiency curves as function of the HV working point are very similar showing a very good reproducibility of the RPC chambers.

fig. 1 fig. 2fig. 3

fig. 4

fig. 5

fig. 6