1. 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
August, 2019 Islamabad, Pakistan
Please contact me for any questions

2. Gaseous detector family tree…
Which gas detector to use?

3. 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 mm-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

4. GEM: principle of operation
By applying a potential of 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.

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

11. GE1/1 historical development

12. 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

13. 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

14. 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

15. R&D: spatial resolution
σs=150 µm
Largest GEM built at that time (2011)!:
Active area 990 x ( ) 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; mm strip pitch

16. Long-Term test validation (1)

17. Long-Term test validation (2)

18. Generation IV GE1/1 Prototype Full-size
Start of production
CMS Technical Design Report for the Muon Endcap GEM Upgrade approved in June 2015!

19. GEM @ CMS: Integration schedule

20. Slice test
Five superchambers, consisting of two triple-GEM detectors, were installed on the negative endcap (YE-1) during the 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

21. 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.

22. GE1/1 QC overview

23. GE1/1 QC overview

24. 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

25. 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

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

27. 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.

28. 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

29. Super Chamber validation

30. 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 ( 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.

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

32. SPARES

35. 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].