Gaseous detectors at the LHC experiments: GEM technology (lesson #3) G. M. I. Pugliese (INFN and Politecnico of Bari) The 8th School on LHC Physics 19 -20 August, 2019 Islamabad, Pakistan gabriella.pugliese@cern.ch Please contact me for any questions

Gaseous detector family tree… Which gas detector to use?

GEM: principle of operation Electrode The GEM (Gas Electron Multiplier) has been invented by F. Sauli in 1997 (NIM A386 (1997) 531). It consists of a thin (50 μm) polymer foil, metal-coated on both sides and pierced with a density of holes (typically 50-100mm-1), inserted between a drift and charge collection electrodes which act as the cathode and the anode. Conversion and drift Induction Amplification and detection are decoupled Electrode

GEM: principle of operation By applying a potential of 400-500 V between the two copper sides, an electric field as high as ~100 kV/cm is produced into the holes. Electrons released in the upper region drift towards the holes and acquire sufficient energy to cause ionizing collisions. A fraction of the electrons produced in the avalanche’s front leave the multiplication region and transfer into the lower section of the structure where can be collected by the electrode. The signal on the anode is generated only by the collection of electrons. Gains up to 1000 can be easily reached with a single GEM foil.

Optimization of GEM geometry The holes diameter and shape have a direct influence on the performance and long-term stability of operation of the detector. To ensure high gains: the optimum hole diameter should be comparable to the foil thickness. While narrower holes results in larger fields for a given voltage, losses on the walls compensate for the increased gain. 70 μm is the selected value Effective gain: ratio of the detected to the primary ionization charge

Multi-GEM structure The fraction of amplified electrons transferring to the gas gap following a first electrode can be injected and multiplied in a second foil, and yet again in a cascade of GEM electrodes; structures of up to five multipliers have been successfully studied. The noticeable advantage of multiple structures is that the overall gain needed for detection can be attained with each of the electrodes operated at much lower voltage, therefore much less prone to discharges

GEM @ LHCb A triple-GEM detector has been installed in the Central Region of the first Muon Station of the LHCb experiment at CERN, for which the requirements are: MUON SYSTEM: MWPC & GEM in M1R1 12 Triple GEM chambers active area of 20 × 24 cm2

GEM LHCb: gas mixture optimization The intrinsic time spread: s(t) = 1/nvdrift, where n is the number of primary clusters per unit length and vdrift is the electron drift velocity in the ionization gap. 9.7ns 5.3ns 4.5 ns 4.5ns Ar/CO2/CF4 (45/15/40): 10.5 cm/s @ 3.5 kV/cm 5.5 clusters/mm fast & non flammable Garfield simul. To achieve a fast detector response, high yield and fast gas mixtures are necessary

The CMS GEM project The CMS GEM project represents a major step in the evolution of the GEM technology, both in terms of detector size and quantity, from a small number of medium-size detectors to a large number of large-size detectors. Sensitive area will be about 350 m2, totaling 720 detectors modules and about 1000 m2 of GEM foils

The GE1.1 station GE1.1 SuperChamber 36 Super chambers will be mounted in front of ME1.1 chambers. Each SC consists of two tripleGEM detectors covers φ = 10

GE1/1 historical development

New GEM foils manufacture validation GEM foils are produced using the photolithographic technology as for printed circuit board construction. Early GEM electrodes were produced using a double mask procedure. Single-mask technology was proposed and validated to large foil production. Single-mask technology validated Kapton etching using the copper mask Copper etching by chemical solution Gain = 104

GEM Foil Stretching and Assembly Current state-of-the-art: Self-stretching assembly without spacers (CERN) Readout PCB Tightening the horizontal screws tensions the GEMs GEMs Drift electrode Detector base pcb Allows re-opening of assembled detector for repairs if needed 2012

R&D: timing performance σt=4 ns First small size prototypes (2010): Custom made HV divider for standard triple-GEM Clear effect of gas mixture, and induction and drift fields Timing resolution of 4 ns reached

R&D: spatial resolution σs=150 µm Largest GEM built at that time (2011)!: Active area 990 x (220-445) mm2 Gap sizes: 3/2/2/2 mm Single mask foils with spacer frames 35 HV sectors per GEM foil, ~ 100 cm2 each 1024 channs, 4 η partitions, 2 columns; 0.8-1.6 mm strip pitch

Long-Term test validation (1)

Long-Term test validation (2)

Generation IV GE1/1 Prototype Full-size Start of production CMS Technical Design Report for the Muon Endcap GEM Upgrade approved in June 2015! https://cds.cern.ch/record/2021453?ln=it

GEM @ CMS: Integration schedule

Slice test Five superchambers, consisting of two triple-GEM detectors, were installed on the negative endcap (YE-1) during the 2016-2017 extended winter shutdown of the CERN LHC run. Slice test achievement: Start acquiring installation and commissioning expertise Integrate GEM into the CMS online system Provide the system's operation conditions 4 uperchambers (Δ𝜑 = 40°) in Slot 1 are to measure muon rate, while 1 Superchamber 1 (Δ𝜑 = 10°) in Slot 2 to test v3 electronic and a new GEM HV system. Event Display: two muons (red lines), associated with hits on one of the five GE1/1 slice test SC (blue trapezoidal boxes) have been reconstructed. GE1/1 performance: Detection efficiencies for muons as a function of η partition (η= 1~8) and distribution strip multiplicity

GE1/1 Production Sites Given the large number of detectors and the complexity of GEM technology, a very comprehensive and stringent quality control process has been established to ensure performance that meet the CMS requirements.

GE1/1 QC overview

GE1/1 QC overview

The GEM lab Pakistan site has been validated by the CERN inspection GEM lab at NCP The GEM lab Pakistan site has been validated by the CERN inspection team on June 08, 2018

GEM Assembly and QC Defined protocols and QC&QC applied on the GEM construction and all data stored in DB. QC5-effective gain measured with the x-ray tube

Pakistan GEM Site (2) Eleven GE1/1 detectors have been assembled, tested and shipped to CERN

GE1/1 QC results Gas leak and HV test results for all G1/1 chambers done in all GEM assembly cites. The pressure drop is modeled by the function P(t)=P0exp(-t/τ). The parameter τ quantifies how fast the overpressure inside the detector decreases as a function of the time. Τ>3 hours guarantee a maximum leak in the gas flow rate around 0.02 l/h. Deviation between the measured resistance and the resistance extracted using the Ohm’s law. Deviation greater that 3% is not excepted.

Super-chamber assembly The optimization of the HV point is driven by the super-chamber operation • Each super-chamber (made up of a pair of GE1/1 detectors) will be powered with a single power supply when operating in CMS • Choice of the proper detector pair and operating HV point is crucial • Detailed pairing procedure for selecting the detectors in the super-chambers " Sort thet detectors by gain " Select the detectors pairs with similar gain " Calculate the HV point to set the super-chamber gain to 104

Super Chamber validation

Conclusions The GEM technology have been successfully operating at LHCb experiment CMS GEM project has been approved in 2015 as Muon Upgrade project and currently all the GEM detectors 160 (144+16 spares) for the first station CMS Endcap have been assembled. All GE1/1 detectors satisfy the criteria. Installation will starts in September 2019 System ready for a successful operation in the CMS experiment, even in the HL-LHC era.

Conclusion of my lessons Which gas detector to use?... Do we have the answer?

SPARES

Optimization of GEM geometry The detailed shape of the holes plays an important role in determining the detector performances. 3 geometry considered: double-conical shape permits one to reach high gains but shows a slow gain increase at startup due to charging-up of the insulating surfaces during operation. Conical shape shows a large increase at start up. a cylindrical shape offers a more stable operation, but is more prone to discharges at high gains [45].