Performance of prototype GE11 chambers for the CMS muon spectrometer upgrade

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Abstract

The high-luminosity phase of the Large Hadron Collider (HL-LHC) will result in ten times higher particle background than measured during the first phase of LHC operation. In order to fully exploit the highly-demanding operating conditions during HL-LHC, the Compact Muon Solenoid (CMS) Collaboration will use Gas Electron Multiplier (GEM) detector technology. The technology will be integrated into the innermost region of the forward muon spectrometer of CMS as an additional muon station called GE11. The primary purpose of this auxiliary station is to help in muon reconstruction and to control level-1 muon trigger rates in the pseudo-rapidity region 1.6|η|2.2. The new station will contain trapezoidal-shaped GEM detectors called GE11 chambers. The design of these chambers is finalized, and the installation is in progress during the Long Shutdown phase two (LS-2) that started in 2019. Several full-size prototypes were built and operated successfully in various test beams at CERN. We describe performance measurements such as gain, efficiency, and time resolution of these prototype chambers, developed after years of R&D, and summarize their behavior in different gas compositions as a function of the applied voltage.

Introduction

The High Luminosity upgrade of the LHC (HL-LHC) will provide p-p collisions with center-of-mass energy of 14 TeV and instantaneous luminosity (L) up to or above 5 × 1034 cm−2 s−1. The increase in the collision rate will affect the operational conditions in HL-LHC due to the increase in pileup and radiation background. It will also pose a challenge to maintain an efficient and reliable trigger particularly in the region |η|>1.6. The high-radiation background may accelerate aging of the current muon system and may cause performance losses, dead regions and degradation of the efficiency of online event selection due to bandwidth limitations.

The CMS Collaboration is preparing for the upgrade of the current muon system scheduled in 2019 to perpetuate its high level of performance. A quadrant of the CMS muon system with existing detectors and proposed extensions is shown in Fig. 1. In the 1.6<|η|<2.4 forward end-cap region, currently only Cathode Strip Chambers (CSC) are installed. To enhance muon trigger and reconstruction capabilities, large-area GEM detectors [1], [2] will be installed in this region. These detectors play a significant role in the instrumentation of particle physics experiments and are known to have high performance with spatial resolution better than 70μm, rate capability of order MHz/cm2, and high tolerance to radiation in strong radiation background environments. The integration of these new detectors together with the existing CSC system will highly improve the muon trigger momentum resolution due to an increase in the lever arm for the measurement of the muon bending angle. In particular, the new station to be installed is GE11,1 which would be equipped with a specific type of GEM detectors named as GE1/1 chambers.

We present performance studies such as of gain, efficiency, time resolution, and discharge probability of GEM GE11 chambers and further describe their behavior for standard CMS operating conditions. The document is structured as follows: the first two sections describe the preliminary details such as the design of GE11 chambers and the CMS GEM upgrade. The third to seventh sections describe the performance studies, which are followed by the summary in which the recommended operating conditions of the GE1/1 chambers for CMS are provided.

Successive versions of the GE11 chambers have been built by improving their design in each release. Fig. 2 shows the evolution of the GE11 detectors since 2010, when the CMS Collaboration proposed their use in the muon end-cap region of the CMS detector. The latest version is generation-X whose design is discussed in Section 3. The mechanical constraints in the GE11 station require two trapezoidal types of detectors to be used to obtain maximum detection coverage. Long chambers GE11-L have a length of 128.5 cm and short chambers GE11-S have a length of 113.5 cm. The technical details of Short and Long versions, their construction and layout, can be found in [3]. Two identical GE11 detectors are combined to form a “super-chamber” to obtain two detection planes and thus maximize the detection efficiency and the redundancy of the GE11 layer.

Section snippets

Impact of GE11 upgrade on CMS

The introduction of the new station known as GE11 will cover the pseudo-rapidity region 1.6<|η|<2.2 of CMS [2] and complement the current CSC system. These new chambers are based on GEM technology and can operate at very high rates with good performance. The GE11 station will extend the path length and will provide additional hits that will help to refine the stub reconstruction and improve the momentum resolution. With the new station installed, muon direction will be measured using hit

GE11 detector design

A GEM [1] is a 50μm thick copper-clad polymer (Kapton or Apical NP) foil chemically perforated by a high density of microscopic holes. The copper cladding is on both sides of the foil with a thickness of 5μm. The holes in the foil are pierced with double cones with outer diameter 70μm, inner diameter 50μm, and pitch 140μm. Each hole acts as a signal amplifier. Three foils are cascaded to form a detector known as triple GEM to obtain a measurable signal.

The construction of the GE1/1 detectors is

Test setup

The detector under test is powered using a programmable high voltage (HV) power supply (CAEN N1470) that allows a controllable current limit (Iset), voltage ramping up and down in steps, maximum voltage, and trip time (the maximum time the current can remain over the controllable limit). The power supply delivers a current up to 1 mA with a monitoring resolution of about 50 nA, which allows identification of unusual current fluctuations. The voltages on the foils are provided through a

Beam facility

The prototype GEM detectors were tested in the CERN H4 beam, extracted from the SPS (Super Proton Synchrotron). A secondary beam consisting of pions and their decay products is produced when the primary proton beam strikes a beryllium target. The secondary beam is filtered by collimators to produce a beam of muons of energy 150 GeV, which are minimum ionizing particles (MIPs).

Test setup

The test setup, shown in Fig. 8, consists of a scintillator hodoscope for triggering, a GEM tracking system to

Rate capability

The flux in the CMS end-caps is not expected to exceed 10 kHz/cm2 and the nominal operating gain of a GE11 detector is expected to be 7 × 103 [2]. Using an intense source of X-ray photons, the rate capability is assessed by measuring the gain as a function of rate. The amplified current is measured using a pico-ammeter connected to the anode of the detector as the incident particle flux is varied using copper attenuators. The measurements (Fig. 10 (bottom)) show that the effective gain of a

Discharge probability

The GE11 detectors will operate at sufficiently high gain ( 104) to ensure maximum detection efficiency while maintaining timing performance. However, with high gains, intense particle fluxes, and densely ionizing particles, the probability increases for producing discharges that could damage the detectors. Discharges are initiated when local charge exceeds the Raether limit [21], resulting in variations in the local electric field. These variations can transform the avalanche into a

Fits to data and summary

To characterize the performance of a GE11 chamber for any drift voltage, the data for gain, discharge probability, efficiency, and time resolution are fit with parametric equations as shown in Table 1. The fits provide a good description of the data and allow interpolation to any desired value of drift voltage. Interpolated data points are obtained for the measured quantities and are displayed in the master plots of Fig. 12, Fig. 13 for ArCO2 and ArCO2CF4 gases, respectively. The CMS Region

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

We gratefully acknowledge the support from FRS-FNRS (Belgium), FWO-Flanders (Belgium), BSF-MES (Bulgaria), BMBF (Germany), CSIR & UGC (India), DAE (India), DST (India), INFN (Italy), NRF (Korea), LAS (Lithuania), QNRF (Qatar), DOE (USA) and the RD51 collaboration .

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