1. The µ-RWELL for the Muon System Upgrade
LABORATORI NAZIONALI DI FRASCATI
The µ-RWELL for the Muon System Upgrade
G. Bencivenni
P. de Simone, G. Felici, M. Gatta, G. Morello, M. Poli Lener
LNF – INFN
(Italy)

2. OUTLINE Requirements The detector The performance
Future perspectives & Summary

3. Expected max rate MHz/cm2 (*)
2×1034cm-2 s-1
Rate up to 3 MHz/cm2 with an additional filter in front of M2
Efficiency for single gap > 95% within a BX (25 ns)
Operation stability up to 6 C/cm2 accumulated charge in 10 y (G=4000, in CF4,16cl/gap)
Pad cluster size < 1.2
Expected max rate MHz/cm2 (*)
Active area cm2
Pad Size cm2 (*)
Rate/Pad
MHz
# pad/gaps
# gaps
#chamber
(2 gaps)
M2R1 3
30x25
0.63x0.77
1.5
1536 24 12
M2R2
0.5
60x25
1.25x1.58 1
768 48
M3R1
32.4x27
0.67x1.7
M3R2
0.15
64.8x27
1.35x3.4
0.7
384
(*) average rate is about 50% of maximum rate
(*) X, Y/4 w.r.t. present logical pads in M2R1-R2
X, Y/2 w.r.t. present logical pads in M3R1 and M3R2
~12 m2
G.Bencivenni, LNF - INFN
LHCb - Upgrade Meeting, 30 May 2017, Elba 3

4. Motivation
The R&D on µ-RWELL is mainly motivated by the wish of improving the
stability under heavy irradiation
& simplify as much as possible
construction/assembly procedures
G.Bencivenni, LNF - INFN
LHCb - Upgrade Meeting, 30 May 2017, Elba

5. The detector architecture
Copper top layer (5µm)
DLC layer (<0.1 µm)
R ̴100 MΩ/□
Rigid PCB readout
electrode
Well pitch: 140 µm
Well diameter: µm
Kapton thickness: 50 µm 1 2 3
µ-RWELL PCB
Drift cathode PCB
G. Bencivenni et al., 2015_JINST_10_P02008
The µ-RWELL is composed of only two elements:
the µ-RWELL_PCB and the cathode
The µ-RWELL_PCB, the core of the detector, is realized by coupling:
a “suitable patterned kapton foil” as “amplification stage”
a “resistive layer” for discharge suppression & current evacuation:
“Single resistive layer” (LR) <<100 kHz/cm2: single resistive layer  surface resistivity ~100 M/☐ (CMS-phase2 upgrade; SHIP)
“Double resistive layer” (HR) > 1 MHz/cm2: more sophisticated resistive scheme must be implemented (MPDG_NEXT- LNF) suitable for LHCb-Muon upgrade
a standard readout PCB
(*) DLC = Diamond Like Carbon
High mechanical & chemical resistant material 5
G.Bencivenni, LNF - INFN
LHCb - Upgrade Meeting, 30 May 2017, Elba

6. Principle of operation
top copper layer
kapton
resistive stage
Insulating medium
Pad/strip r/out HV  r t
Not in scale
Applying a suitable voltage between top copper layer and DLC the “WELL” acts as multiplication channel for the ionization.
The charge induced on the resistive foil is
dispersed with a time constant, τ = ρC ,
determined by
the surface resistivity, 
the capacitance per unit area, which depends on the distance between the resistive foil and the pad readout plane, t
the dielectric constant of the insulating medium, r [M.S. Dixit et al., NIMA 566 (2006) 281]
The main effect of the introduction of the resistive stage is the suppression of the transition from streamer to spark
As a drawback, the capability to stand high particle fluxes is reduced,
G.Bencivenni, LNF - INFN
LHCb - Upgrade Meeting, 30 May 2017, Elba 6

7. The µ-RWELL_PCB for High Rate (LHCb)
Copper layer 5 µm 1
Kapton layer 50 µm
DLC layer: 0.1 – 0.2 µm 2
2nd resistive kapton layer with ∼ 1/cm2 “through vias” density
DLC-coated kapton base material 3
2nd resistive kapton layer
insulating layer
pad/strips readout on standard PCB
“through vias” for grounding
DLC-coated base material after copper and kapton chemical etching ( WELL amplification stage) 4
G.Bencivenni, LNF - INFN

8. Main detector features
The µ-RWELL is a single-amplification stage, intrinsically spark protected MPGD characterized by:
simple assembly procedure:
only two components  µ-RWELL_PCB + cathode
no critical & time consuming assembly steps:
no gluing
no stretching (no stiff & large frames needed)
easy handling
suitable for large area with PCB splicing technique w/small dead zone
cost effective:
1 PCB r/o, 1 µ-RWELL foil, 1 DLC, 1 cathode and very low man-power
easy to operate:
very simple HV supply  only 2 independent HV channels or a trivial passive divider (while 3GEM detector  7 HV floating/channels )
G.Bencivenni, LNF - INFN
LHCb - Upgrade Meeting, 30 May 2017, Elba 8

9. Detector performance Results from the R&D on µ-RWELL optimized
as tracking device (strip readout)
G.Bencivenni, LNF - INFN
LHCb - Upgrade Meeting, 30 May 2017, Elba

10. WELL kapton thickness = 50µm
Gas gain
Prototypes with different resistivity (40-35 MOHM/☐) have been tested with an X-Ray gun (5.9 keV) with Ar/CO2/CF4=45/15/40 gas mixture (same used for the GEM in M1R1) and characterized by measuring the gas gain in current mode.
WELL kapton thickness = 50µm
G.Bencivenni, LNF - INFN
LHCb - Upgrade Meeting, 30 May 2017, Elba

11. Rate capability with X-rays
Double resistive layer w/ 1x1 cm2 through-vias grounding pitch
local irradiation
for m.i.p.×7
max rate
Φ = 3.4 MHz/cm2; Φ = 2.8 MHz/cm2; Φ = 1.6 MHz/cm2
Local irradiation is practically equivalent to global irradiation
G.Bencivenni, LNF - INFN
LHCb - Upgrade Meeting, 30 May 2017, Elba

12. Beam Test (CMS/LHCb collaboration)
H8 Beam Area (18th Oct. – 9th Nov 2016)
Muon/Pion beam: 150 GeV/c
3 µ-RWELL prototypes
MΩ /□
VFAT (digital FEE)
Ar/CO2/CF4 = 45/15/40
Beam
GEM Tracker 1
N° 2 µ-RWELL protos
10x10 cm2
40-35 M/☐
Double resistive layer scheme
400 µm pitch strips S3 S1 S2
GEM Tracker 2
N° 1 µ-RWELL proto
100x50 cm2
70 M/☐
Single resistive layer scheme
800 µm pitch strips
Trigger=S1+S2+S3
GOAL: time resolution measurement
G.Bencivenni, LNF - INFN
LHCb - Upgrade Meeting, 30 May 2017, Elba

13. Time Performance
97%
5.7ns
Different chambers with different dimensions and resistive schemes exhibit a very similar behavior although realized in different sites (large detector ELTOS)
The saturation at 5.7 ns seems to be dominated by the fee (measurement done with VFAT2).
To be compared with a measurement done with GEM in (LHCb), giving a
σt = 4.5 ns with VTX chip - (NIM A 494 (2002) 156).
G.Bencivenni, LNF - INFN
LHCb - Upgrade Meeting, 30 May 2017, Elba 13

14. Efficiency in 25 ns time window
Single gap !
With an average efficiency in 25 ns of about % w/out any optimization of the detector working point and the electronics
G.Bencivenni, LNF - INFN
LHCb - Upgrade Meeting, 30 May 2017, Elba

15. Ageing test: GIF++ (LNF, INFN-BO)
In principle the only detector component to be validated under irradiation is the DLC-based resistive stage.
The set-up, including different scheme of µ-RWELLs, has been installed at the GIF++ irradiation area.
Detectors are operated with both Ar/CO2 and Ar/CO2/CF4 gas mixture at a gain 4000
Test will last about 6-9 months
Large area (CMS) Single resistive layout
Double resistive layout (high rate)
Single resistive layer scheme (reference chamber) 15

16. Ageing test: GIF++
I(G=4000)  10 nA/cm2 corresponding to an equivalent mip rate of  250 kHz/cm2
Detector Gain MUST be increased as much as possible at least to equalize the expected peak current in M2R1 that should be ~ 60 LHCb – LS2
Source off
Low gain
High gain
Currents constant/stable during the operating time gates
The double layer has integrated ~10 mC/cm2 up to now
G.Bencivenni, LNF - INFN
LHCb - Upgrade Meeting, 30 May 2017, Elba 16

17. Summary
The first prototypes of µ-RWELL based on the double resistive layout show very promising performance:
gas gain > 104
safe operation
rate capability > 1 MHz/cm2
time resolution ∼ 5.7 ns
efficiency/25 ns ∼ % (single gap)
G.Bencivenni, LNF - INFN
LHCb - Upgrade Meeting, 30 May 2017, Elba

18. 2017 R&D Program
Construction of new prototypes based on double layer layout, with pad readout (M2R1 type):
– 10x10 cm2 with pad size of 0.6x0.8 cm2
Test H8-SPS (4 – 18 Oct) to measure:
- time resolution
- efficiency in 25 ns
Test PSI (4 –18 Dec - tbc) with high intensity hadron beam in current mode to measure:
- rate capability with a particle flux up to 20 MHz/cm2
- discharge probability
- integrating a charge equivalent to some years of LHCb
(soft) Ageing GIF++ : in backgorund … (till end of the year)
G.Bencivenni, LNF - INFN
LHCb - Upgrade Meeting, 30 May 2017, Elba 18

19. Towards HL
2018 – 2019: full scale prototyping & engineering of M2R1/M2R2
> 2018: study if and how the VFAT3 chip (GEMs for CMS phase-2 upgrade) can be adapted to LHCb needs
2020: design finalization & module-0s construction
2021 – 2023: detector construction
2024: ready for installation
G.Bencivenni, LNF - INFN
LHCb - Upgrade Meeting, 30 May 2017, Elba

20. Backup slides

21. Front end Electronics: VFAT-3 chip
VFAT3 chip (128 ch. &130 nm CMOS tech.) is currently under design for the readout of triple-GEM detectors of the CMS experiment
(phase 1 upgrade )
TRIGGER (fixed latency) path
9 bits output trigger bus
VFAT FEATURES
Selectable peaking time
Rate capability = 1 TPEAK = 25 ns
Time resolution  6 TPEAK = 25 ns
Noise eRMS ≤ 1500e (∼ ¼ fC) @ TPEAK = 25 ns & a pad capacitance < 20 pF
Chip readout clock rate 40 MHz using an 8 bit wide bus & a transfer 640 MHz
The triggering unit provides a "Fast OR" combinations of groups of 2 or 4 channels for internal self-triggering purpose.
Tpeak [ns]
Delay time Td [ns] 25 15 50 29 75
43.4
100
57.8
Even though the peaking time of the VFAT3 is higher than Carioca, thanks to the Constant Fraction Discriminator it’s possible to achieve high trigger efficinecy in the bunch crossing window of 25 ns.
G.Bencivenni, LNF - INFN
LHCb - Upgrade Meeting, 30 May 2017, Elba
D. BADONI 21

22. Stiff structure with honeycomb sandwiched r/o & cathode PCBs
Possible layout for M2R1
M2R1 chamber with two µ-RWELL gaps r/o by 24 VFAT3 chips (equivalent to 3k channels) should fit in the current M2R1 available space.
R/O & µ-RWELL
Cathode
Honey-comb
gas gap
3mm 5 1 7 17
Gas gap cross section
Stiff structure with honeycomb sandwiched r/o & cathode PCBs
VFAT
VFAT
VFAT
VFAT
VFAT
VFAT
bi-gap Design 68
G.Bencivenni, LNF - INFN
LHCb - Upgrade Meeting, 30 May 2017, Elba

23. Muon System Upgrade phases
LS3: phase-1b new, high rate, muon chambers for busy regions
LS4: phase-2 luminosity upgrade at ~2x1034 cm-2 s-1
Exploiting the very long shutdown LS3 to consolidate the Muon System with new chambers in the innermost regions.
G.Bencivenni, LNF - INFN
LHCb - Upgrade Meeting, 30 May 2017, Elba

24. Discharges: µ-RWELL vs GEM
single-GEM
µ-RWELL
Discharge Amplitude (nA)
Ar/CO2 = 70/30
test with X-ray
the µ-RWELL detector reaches discharge amplitudes of few tens of nA, <100 max gain
the single-GEM detector reaches discharge amplitudes of ≈ 1µA (of course the discharge rate is lower for a triple-GEM detector) 24

25. Ageing test: GIF++
I(G=4000)  10 nA/cm2 corresponding to an equivalent mip rate of  250 kHz/cm2
Detector Gain MUST be increased as much as possible at least to equalize the expected peak current in M2R1 that should be ~ 60 LHCb – LS2
G.Bencivenni, LNF - INFN
LHCb - Upgrade Meeting, 30 May 2017, Elba 25

26. The Low Rate scheme (CMS/SHiP)3 1 2
DLC layer: µm ( M/☐)
Kapton layer 50 µm
Copper layer 5 µm
DLC-coated base material after copper and kapton chemical etching (WELL amplification stage)
DLC-coated kapton base material
PCB (1-1.6 mm)
Insulating medium (50 µm)
G. Bencivenni, LNF-INFN - INSTR2017, Novosibirsk
02/03/2017

27. Performance vs Rate
The detectors rate capability (with Ed=3.5 kV/cm) has been measured in current mode with a pion beam and irradiating an area of >3 x 3 cm2 (FWHM) (medium size irradiation, ~10 cm2 spot)
Double resistive layer
(High Rate scheme)
Single resistive layer
(Low Rate scheme)
G.Bencivenni, LNF - INFN
LHCb - Upgrade Meeting, 30 May 2017, Elba

28. Technology Transfer to Industry
In the framework of the CMS-phase2 muon upgrade we have developed large size µ-RWELLs , in strict collaboration with Italian industrial partners (ELTOS & MDT).
The work is performed in two years with following schedule:
Construction & test of the first x0.5m2 (GE1/1) µ-RWELL DONE
Mechanical study and mock-up of 1.8x1.2 m2 (GE2/1) µ-RWELL ONGOING
Construction of the first 1.8x1.2m2 (GE2/1) µ-RWELL (only M4 active) /2017 near future
DONE
1.8x1.2m2 (GE2/1) µ-RWELL
~40 times larger than small protos !!!
~200 times larger than small protos !!!
mock-up
ELTOS tests

29. Towards the High Rate scheme
single resistive layer, edge grounding,
2D evac.
current
top layer
conductive vias
bottom layer d’
(1cm)d’
double resistive
layer, 3D grounding r d
d (50cm)
(*) point-like irradiation, r << d
Ω is the resistance seen by the current generated by a radiation incident in the center of the detector cell
Ω ~ ρs x d/2πr
Ω’ ~ ρs’ x 3d’/2πr
Ω/ Ω’ ~ (ρs / ρs’) x d/3d’
If ρs = ρs’  Ω/ Ω’ ~ ρs/ρ’s * d/3d’ = 50/3 = 16.7
(*) Morello’s model: appendix A-B (G. Bencivenni et al., 2015_JINST_10_P02008)
02/03/2017
G. Bencivenni, LNF-INFN - INSTR2017, Novosibirsk

30. µ-RWELL exhibits a gas gain larger than a single-GEM
µ-RWELL vs GEM: gain
Single-GEM:
only 50% of the electron produced into the holes contributes to the signal, the rest of the electron is collected by the GEM bottom side
the signal is mainly due to the electron motion, the ion component is largely shielded by the GEM foil itself
 effective gain < intrinsic gain
(Of course for a triple-GEM the signal losses are more important due to the multiple transfers of charge among GEM foils)
µ-RWELL:
100% electron produced into the amplification stage is promptly collected on the resistive layer
the ionic component, apart ballistic effects, contributes to the formation of the signal
further increase of the gain is achieved thanks to the resistive stage (which, quenching the discharges, allows to reach higher amplification field inside the blind holes)
 effective gain = intrinsic gain
µ-RWELL exhibits a gas gain larger than a single-GEM
(Ar/CO2=70/30)
Only electronic component
electronic component
ion component
G.Bencivenni, LNF - INFN
LHCb - Upgrade Meeting, 30 May 2017, Elba

31. Cluster Size vs layer resistivity
Ar/ISO=90/10
400 µm pitch strips
6 mm LHCb pad
The use of low resistivity increases the charge spread (cluster size) on the readout strips.
Of course for large size pad r/out (as in LHCb muon), most of the charge is collected on a single pad. Then the expected Cluster size for LHCb should be 1
G.Bencivenni, LNF - INFN
LHCb - Upgrade Meeting, 30 May 2017, Elba

32. Stretching up to 8 clock cycles
Electronics and DAQ
VFAT2 front-end electronics (from CMS group)
Readout board “TURBO” developed by Siena and PISA INFN for TOTEM (*)
VFAT2 is a triggering and tracking front-end ASIC characterized by:
Preamplifier-shaper-comparator readout chains (128) to detect signals above a programmable threshold.
“Fast-OR” lines that merge channels to provide the fast hit detector signal (asynchronous wrt the scintillator trigger)
Digital information for tracking purpose (synchronization in step of 25 ns with a programmable latency)
128
channels
Gain 60mV/fC
Shaping Tine 22ns
LV 1A Latency 6.4ms
Stretching up to 8 clock cycles
ANALOG AND
ASYNCRONOUS
DIGITAL AND
SYNCRONOUS
Amplification
and
Shaping
Comparison
with a programmable
Threshold
Synchronization
And
Time stretching
Data Storage
and Transmission
FAST OR Trigger
output
(*) P. Aspell "VFAT2 Operating Manual, Internal note :
"
G.Bencivenni, LNF - INFN
LHCb - Upgrade Meeting, 30 May 2017, Elba

33. VFAT TRIGGER DIGITAL CHAIN & CABLING
VFAT OUTPUT STAGE (TRIGGER PURPOSE)
The triggering unit can access all the 128 bits (channels) before the synchronization unit by means of trigger output bus (fixed latency path)
An 8 bit wide bus together with a 640 MHz clock is used to transfer the128 chip channels 40 MHz clock rate (8 bits x 16 transfers = 128 bits)
The triggering unit provides a "Fast OR" combinations of groups of 2 or 4 channels for internal self-triggering purpose.
CABLING REQUIREMENTS
R-WELL
6 BOARDS
(9 bits + ctrl)/BOARDS
LV/HV dedicated lines
M1R1-GEM
24 BOARDS
(8 bits + ctrl)/BOARDS
LV/HV dedicated lines
R-WELL
6 BOARDS
(9 bits + ctrl)/BOARDS
LV/HV dedicated lines
R-WELL
6 BOARDS
(9 bits + ctrl)/BOARDS
LV/HV dedicated lines
µ-RWELL
6 BOARDS
(9 bits + ctrl)/BOARDS
LV/HV dedicated lines
24 BORADS
µ-RWELL detectors requires the same number of cables used for the current GEM in M1R1
G.Bencivenni, LNF - INFN
LHCb - Upgrade Meeting, 30 May 2017, Elba

34. GEM detectors currently running @ HEP
Experiment
Instrumented area (m2)
Gas Mixture
Gain
Flux (MHz/cm2)
HV-type
# lost sector for shorts
% damaged area
Front-End Electronics
COMPASS 2
Ar/CO2
4000
<1
HV passive divider
???
APV25
LHCb
0.6
Ar/CO2/CF4
8000/ 4000 1
HV active divider
5 (All on GEM #1) 1%
CARIOCA-GEM
TOTEM
8000 6
percent level
VFAT2
KLOE2 4
Ar/i-C4H10
12000
0,01
7 independent ch; then active divider 61
(8 GEM#1,
28 GEM#2,
25 GEM#3) 5%
GASTONE
A damaged GEM sector could required for the replacing of a whole a detector gap!!
G.Bencivenni, LNF - INFN
LHCb - Upgrade Meeting, 30 May 2017, Elba

35. GEM: the discharge problem
The biggest “enemy” of MPGDs are the discharges.
Due to the fine structure and the typical micrometric distance of their electrodes, GEMs generally suffer from spark occurrence that can eventually damage the detector and the related FEE.
241 Am  souce
Strongly reduced but not completely suppressed
GEM
S. Bachmann et al.,
NIMA A479(2002) 294
G.Bencivenni, LNF - INFN