5. THE MUON DETECTOR


Endcap muon system consists of four stations and based on multilayer Cathode Strip Chambers (CSCs). Specific experimental conditions impose different requirements for performance of the endcap stations. The ME1/1 forward muon station is located at the end of superconducting solenoidal magnet and should provide the unique spatial resolution for muon momentum measurements and a good matching of the track information between the muon system and central tracker. Other ME 2-4-4 endcap muon stations are located between the iron disks of the magnet return yoke in the region of lower magnetic field and lower background rates.


Back


5.1 THE FORWARD MUON STATION ME1/1

Forward muon station ME1/1 is located behind the endcap hadron calorimeter (HE) in the nonuniform magnetic field up to 3.5 Tesla and cover the pseudorapidity range of
= 1.6 - 2.4, where the expected background rates are up to 1 kHz/cm2.

Station should provide the muon coordinate spatial resolution of 75 , time resolution of order of a few ns and a few mm azimuthal coordinate measurement accuracy in order to have 1st level trigger decision in the range of from a few GeV up to 100 GeV. The detector with such dimensions and such performance have not been achieved ever. This task required a new methods of high energy muon detection which was developed by Russian scientists. Based on this investigation the CSC chambers were proposed for implementation in the muon system of the CMS detector. Scientists from JINR in collaboration with Belarus scientists participate in ME1/1 design and development.

R&D

R&D on CSC implementation in CMS muon system carried out in Dubna since 1993. A number of multilayer CSCs with different structure, shape and dimensions were developed and tested including full scale 6 layers prototypes with radial strip structure P1 and P2.

In cooperation with Belarus engineers and physicists a few generation of readout electronics were designed and developed. What allow to attract the intellectual and industrial potential of Belarus - JINR member state country.

Tests results of large size prototypes up to 3m length have shown that CSC proved the spatial resolution 50-80 and could be used in large area precise muon detectors. Special methodical program was devoted to study the strong magnetic field influence on CSC spatial resolution.

Adequate simulation was done in order to describe the spatial and time resolution dependence from physical gas processes, magnetic and electrostatic field influence, gas mixtures, gas gain and so on.

Experimental investigation of the possibility to compensate effects of magnetic field was done in high energy beams and cosmic rays tests in superconductive 3 Tesla magnet. Fig. 5.1. shows the spatial resolution over CSC sensitive area.

CSC study with correlated background closed to real experimental conditions is performed in magnetic field in Integrated Test of the End cap CMS detector Segment prototype, which includes electromagnetic calorimeter with preshower detector, hadron calorimeter and muon station ME1/1. CSC single layer muon detection time spectrum has the nonsymmetrical shape and about 40 ns full width on the level of 1%. The logic combination of signals (OR out of six) from layers provide significant improvement of the station timing parameters. Timing resolution of order of 2 ns for the 6 layers CSC is shown in Fig. 5.2.

Time information analysis shows the possibility of unambiguous bunch crossing identification with correlated and uncorrelated background using 1st, 2nd and even 3d ME1/1 signals. Integrated test data analysis shows that for 300 GeV muons passing through hadron calorimeter up to 25% of events have electromagnetic secondaries, which decrease the spatial resolution. For such events special track reconstruction algorithm was designed. It's provide 92% muon track reconstruction efficiency (see Fig. 5.3.) with acceptable time resolution.


Different types of center cluster finding algorithms for Level 1 trigger decision were studied in integrated test. It was shown that resolution is less than half strip and can be used in muon trigger.

First tests of the CSC and muon trigger rate capability basing on cathode readout information were performed in 1996. It was shown that for the present estimation of ME1/1 background level and in the case of new type of read-out electronics implementation one can achieve the acceptable value of the ME1/1 local track reconstruction efficiency. Optimization of radiation shielding in order to decrease background rates is in progress. Fig. 5.4. shows coordinate reconstruction efficiency with trigger channel with precision of 1/2 strip vs. uncorrelated background rates. There is insignificant muon trigger efficiency degradation up to background rates 200 kHz/strip.




Full scale prototype P3.

Fig. 5.5. shows ME1/1 sector full scale prototype P3 which was build in 1996 and consist of six trapezoidal CSCs with radial strips.

At the development stage of the P3 the tooling for serial production was used. High flatness self supporting panel is one of the most important CSC construction element. Technology of the production of such panels was designed in Dubna. First panels prototypes was used in P3 construction. Fig. 5.6. shows special machine to produce radial strips. The diamond disk is used for strips milling.

First tests of the P3 prototype with radioactive sources was done at Dubna. The final test in magnetic field is planed to be in summer '97 at CERN.

ME1/1 electronics

CSC should provide 0.55% precision of strip charge measurement in order to have a good muon track reconstruction. In this case the signal to noise ratio of the electronic channel should be not less then 200. This required the low noise amplifier. CSC front end consists of eight 96-channels cathode read out cards and sixteen 24-channels anode read out cards, which connected to one motherboard which provide interfacing with trigger and data acquisition system. Total number of ME1/1 read out channels is 27648 for anode and 55296 for cathode.

The experimental conditions of ME1/1 are different from other endcap stations and demand specific requirements for front end electronics. Set of ASIC's for cathode and anode readout which meet the ME1/1 requirements was designed in cooperation with NCPHEP Minsk and consists of:

  • 16-channels charge sensitive preamplifier - shaper (shaping time 100 ns) with head n-p-n transistor, tail cancellation and gain control;

  • 16-channels cathode fast channel shaper - discriminator (shaping time 30 ns) with threshold and output pulse width control;

  • 8 channels anode preamplifier - shaper - discriminator (shaping time 15 ns) with threshold and output pulse width control;

  • 16 channels CMOS Analog memory (16 x 152) with multiplexed output.

ASIC'ís was manufacturing by Integral company - the leader of the Belarus electronics industry. The topology of the 16-channels charge sensitive preamplifier - shaper is shown in fig.5.7.



Main parameters of ASIC's are presente in table 1.



Further plans

The Muons TDR has to be completed by October 1997. The work on the chapters related to the ME1/1 station is in the progress.

The following activities have to be completed before mass production of chambers in 1998:

  • investigations of the P3 prototype on the high magnetic field at high energy muon beams. The study should confirm the correctness of the technical decisions. Results also would be useful to make corrections at production level for the P4 prototype.

  • assembling of the P4 prototype in December 1997.

In the same time , the work to develop mass production technology is in progress. The study of CSC high rate capability will be continued. It is planned to produce 96-channel cathode readout card equipped with a slow channel for precise position measurements and with a fast one for the trigger purposes.


Back

5.2 ENDCAP MUON SYSTEM

The PNPI group of physicists and engineers is involved in the Endcap Muon (EMU) collaboration with US institutions. Group continues its activity in Romp;D program on Cathode Strip Chambers (CSC). PNPI should produce 114 six-layer muon chambers (from the total number of 482 chambers in the EMU US project), perform complete tests of these chambers at PNPI, deliver the chambers to CERN and install them into the CMS detector. Most of the materials and components for mass production will be supplied by the US CMS collaboration. Total cost accounted as PNPI contribution in the CMS Cost Matrix is 1 MCHF.

PNPI group plays a leading role in design and construction of the EMU CSC prototypes. Most of the design work is done by PNPI designer on their visits to the FNAL. From 1997 the design work will be continued also at PNPI. The construction of the prototypes was performed at FNAL under leadership of PNPI physicists. During 1996 full scale trapezoidal CSC prototype (P1) of 0.8m x 1.2m x 3.3m was constructed and tested.

The PNPI group also participates in the tests of the P0 prototype (test beam at CERN in 1996) and data analysis.

The PNPI engineers designed the anode electronics (main responsibility) and now testing the produced chips of amplifier discriminator.

In PNPI was performed the optimization of the gas mixture for CSC using the following gases: Ar, CO2, CF4, and C4H10. The gas gain was studied as a function of HV in the range of (3.6 - 4.2) kV for different gas mixtures. The maximum value of 105 at HV = 4 kV was obtained for the gas mixtures (30% Ar + 50% CO2 + 20% CF4) and (30% Ar + 65% CO2 + 5% CF4).

In 1996, the CSC ageing test station was constructed, and the tests of the CSC prototype ageing were performed using the 90Sr- source of high intensity. The first results have shown that the change of the gas gain at the radiation dose up to 10 Coulomb per cm did not exceed 5% for the gas mixture (30% Ar + 50% CO2 + 20% CF4) at HV = 4 kV. The dark current at the dose of 10 Coulomb per cm increases by a factor of approximately 10.

Back