1. LABORATORI NAZIONALI DI FRASCATI
The µ-RWELL:
from R&D to industrialization
G. Bencivenni ,
P. De Simone, G. Felici, M. Gatta , G. Morello ,
M. Poli Lener
LNF-INFN
G. Bencivenni, LNF-INFN

2. OUTLINE WP7: Muon detectors for future colliders Why a new MPGD
The µ-RWELL: description and principle of operation
Detector industrialization: Low-rate vs High-rate layout
Financial request
G. Bencivenni, LNF-INFN

3. Muon Detectors for future colliders
The future colliders (CEPC, SppC and FCC – hh) requires for extremely large muon detectors :
~10000 m2 in the barrel
m2 in the end-cap
300 m2 in the very forward region
The detectors have to be operated in high background (very large uncertainties depending on shielding, actual structure,etc.):
O(1 – 10 kHz/cm2) in the barrel
O(10 – 100 kHz/cm2) in the end-cap
O(1 MHz/cm2) in the forward region
Taking into account the surface and the expected rates gaseous detectors and in particular MPGDs is the natural solution (straight-forward for CEPC, requiring a significant R&D for the harsher conditions of SppC & FCC-hh)
R&D for HL-LHC (CMS & LHCb phase-2 muon upgrade) is clearly a good starting point
G. Bencivenni, LNF-INFN

4. stability under heavy irradiation construction/assembly procedures
Why a new MPGD
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

5. The µ-RWELL architecture
The µ-RWELL detector is composed by two elements:
the cathode and the µ-RWELL_PCB
Drift/cathode PCB
Copper top layer (5µm)
DLC layer ( µm)
R ̴ MΩ/□
Rigid PCB readout electrode
Well pitch: 140 µm
Well diameter: µm
Kapton thickness: 50 µm 1 2 3
µ-RWELL PCB
G. Bencivenni et al., 2015_JINST_10_P02008
The µ-RWELL_PCB is realized by coupling:
a “suitable WELL patterned kapton foil as “amplification stage”
a “resistive stage” for the discharge suppression & current evacuation:
“Low particle rate” (LR) < 100 kHz/cm2: single resistive layer  surface resistivity ~100 M/ (CMS-phase2 upgrade – SHIP  muon CEPC)
“High particle rate” (HR) ≥ 1 MHz/cm2: more sophisticated resistive scheme must be implemented (MPDG_NEXT- LNF & LHCb-muon upgrade  muon SppC & FCC-hh)
a standard readout PCB
G. Bencivenni, LNF-INFN

6. Principle of operation
top copper layer HV
kapton r t 
A voltage V between the
top copper layer and the grounded resistive foil, generates an electric field of ~100 kV/cm
into the WELL which acts as multiplication channel
The charge induced on the resistive foil is
dispersed with a time constant, RC, 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 kapton, 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 by a local voltage drop around the avalanche location.
As a drawback, the capability to stand high particle fluxes is reduced, but an appropriate grounding of the resistive layer with a suitable pitch solves this problem (High Rate scheme)
G. Bencivenni, LNF-INFN

7. @ B= 0T after TRKs contribution subtraction
Performance
Ar/ISO=90/10 70 50 55 30
Ar/ISO =90/10, G = 2000 – point-like irradiation
X-ray
RWELL = (52+-6) µm
@ B= 0T after TRKs contribution subtraction
G. Bencivenni, LNF-INFN

8. The two detector layouts
Low Rate
High Rate
single resistive layer with “edge detector” grounding
“2D” current evacuation
“large current path to ground”
 higher resistance to ground
 large Voltage drop spread
 large gain non-uniformity
 low rate < 100 kHz/cm2
implementation:
kapton foil + PCB coupling
R&D completed(*), engineering on-going (started w/ELTOS – 2016)
double resistive layer with “through vias” grounding with a O(1cm2) pitch
“3D” current evacuation
“short current path to ground”
 lower resistance to ground
 small Voltage drop spread
 small gain non-uniformity
 high rate ≥ 1 MHz/cm2
implementation:
multi-layer flex w/through-vias + PCB coupling
R&D almost completed(*), engineering ready to be started
G. Bencivenni, LNF-INFN

9. The µ-RWELL_PCB for Low Rate (CMS/SHiP)
Copper layer 5 µm 1
Kapton layer 50 µm
DLC layer: µm ( M/)
DLC-coated kapton base material
Pre-preg #106 (50 µm) 2
Krempel (25 µm) epoxy – film for surface flattening
PCB (1-1.6 mm)
DLC-coated base material after copper and kapton chemical etching (WELL amplification stage) 3
G. Bencivenni, LNF-INFN

10. Towards detector industrialization (I) LR scheme (LARGE AREA)
In the framework of the CMS-phase2 muon upgrade we are developing large size µ-RWELL. The R&D is performed 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
Mechanical study and mock-up of 1.8x1.2 m2 (GE2/1) µ-RWELL
Construction & test of the first x1.2m2 (GE2/1) µ-RWELL / /2018
~40times larger than small protos !!!
~300 times larger than small protos !!!
1200 mm
Four PCB µ-RWELL spliced with the same technique used for large ATLAS MM + only one cathode closing the detector
450
Splicing zone
< 0.5 mm wide
1.2x0.5m2 (GE1/1) µ-RWELL
1.8x1.2m2 (GE2/1) µ-RWELL
G. Bencivenni, LNF-INFN

11. Test Beam results Very preliminary G. Bencivenni, LNF-INFN
Small u-RWELL
Large u-RWELL
G. Bencivenni, LNF-INFN

12. The High Rate µ-RWELL
G. Bencivenni, LNF-INFN

13. LHCb-muon Requirements @ 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)
Long stability up to 6 C/cm2 accumulated charge in 10 y of operation
Pad cluster size < 1.2
Expected max rate MHz/cm2 (*)
Active area cm2
Pad Size cm2 (*)
Rate/Pad
MHz
# pad/gaps
# gaps
#chambers
(with 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; a factor 2 more in Y, to halve the rate/Pad
X, Y/2 w.r.t. present logical pads in M3R1 and M3R2
Same particle flux as expected at future hadron colliders

14. The µ-RWELL_PCB for High Rate (LHCb)
Copper layer 5 µm 1
Kapton layer 50 µm
DLC layer: 0.1 – 0.2 µm (50 – 200 M/) 2
2nd resistive kapton layer with ∼ 1/cm2 “through vias” density
DLC-coated kapton base material 3
2nd resistive kapton layer (25µm)
insulating layer (25µm)
pad/strips (9-18 µm thick) readout on standard PCB (1 – 1,6 mm)
“through vias” for grounding
DLC-coated base material after copper and kapton chemical etching ( WELL amplification stage) 4
G. Bencivenni, LNF-INFN

15. Industrialization of the HR layout
The industrialization of the HR version of µ-RWELL requires for a Company able to work on flexible substrate (Kapton etching , multi-layer kapton structure with through-vias … )
A good candidate (in Europe) seems to be the TECHNOLOGY TRANSFER AGENCY TECHTRA (Poland)
They are specialized in GEM manufacturing: producing small and medium size GEM foils (double-mask technique) for CERN
They have all the toolings for single-mask GEM foil (machine for continuous etching of kapton, laminator, UV exposure machine … testing facilities and clean room)
They are interested in developing and engineering the µ-RWELL
G. Bencivenni, LNF-INFN

16. Program
Start the engineering phase of the double-resistive layer layout, with TECHTRA (flex photolithography) + ELTOS (rigid photolithography) ( ) 2017: preliminary tests to fine tune the overall procedure then start the manufacturing of small/medium size (M2R1) HR protos 2018: consolidation of the manufacturing procedure and construction of large size HR proto M2R2/M3R2 (30x65 cm2)
G. Bencivenni, LNF-INFN

17. Financial Request (2017) GOAL: n. 2 M2R1 - like size - 25x30 cm2
0.6x0.8 cm2 pad size
1500 chs/gap, partially instrumented w/VFAT2
Costs Estimate:
DLCed foil 50 µm thick (ampl.stage + 1st res-layer)
+ 25 µm thick 2nd res-layer(Japan): k€
Preliminary tests of double resistive-layer (on 10x10 cm2): k€
n.2 M2R1-like protos (including DRIFT electrodes & frames): 6 k€
fee (n. 500 chs – 4 VFAT2 boards) + TURBO board: k€
___________
10 k€
Missioni (contacts with TECHTRA/ELTOS) : k€
Test Beam (4 people – 2 weeks) k€
12 k€

18. SPARE SLIDES
G. Bencivenni, LNF-INFN

19. Test Beam results Very preliminary G. Bencivenni, LNF-INFN
Small u-RWELL
Large u-RWELL
G. Bencivenni, LNF-INFN

20. Readout-PCB production @ ELTOS
GE1/1 – PCB-readouts manufatured at ELTOS
1,2 m
0,5 m
G. Bencivenni, LNF-INFN

21. DLC sputtering on Kapton foils
DLC sputtering on large Kapton foils (w/copper on one side) Be-Sputter Co., Ltd (Japan)
Foil 1 (800A)
Foil 2 (1300A)
Foil 3 (1800A)
Foil 4 (1500A)
Foil 5 (1500A)
Average Surface Resistivity M/
433±90
68±9
41±12
122±22
180±17
Ar/ISO=90/10
G. Bencivenni, LNF-INFN

22. gluing the DLCed foils on the readout–PCBs @ MDT (Milano)
Coupling the DLCed Kapton with r/o-PCBs
gluing the DLCed foils on the readout–PCBs @ MDT (Milano)
G. Bencivenni, LNF-INFN

23. GEM detector 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 1
HV active divider
5 (All on GEM #1) 1%
CARIOCA-GEM
TOTEM 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

24. The µ-RWELL vs GEM (Garfield simulation)
GEM – Ar:CO2 70:30 gas mixture
Signal from a single ionization electron in a GEM.
The duration of the signal, about 20 ns, depends on the induction gap thickness, drift velocity and electric field in the gap.
Signal from a single ionization electron in a µ-RWELL.
The absence of the induction gap is responsible for the fast initial spike, about 200 ps, induced by the motion and fast collection of the electrons and followed by a ~50 ns ion tail.
Schema di una gem …
µ-RWELL – Ar:CO2 70:30 gas mixture
G. Bencivenni, LNF-INFN

25. Muon System upgrade phase – 1b & beyond
LS3: phase-1b new, high rate, muon chambers for busy regions
LS4: phase-2 luminosity upgrade at ~2x1034 cm-2 s-1
muon detector response at 2×1034cm-2s-1 is seriously affected by the increased rate
new shielding to reduce the rates on M2, we do expect a rate reduction of ≈ 50%
new pads detectors in M2R1, M3R1 (and M2R2, M3R2) pad size X, Y/2 w.r.t. present logical pads
IB removal  this would allow to cancel the ghost pads rate
detailed MC studies must be done to define the final configuration
profit of the very long shutdown LS3 to consolidate our Muon System with new chambers in the innermost regions 
design and technology must face the luminosity upgrade of phase-2
G. Bencivenni, LNF-INFN

26. Strongly reduced but not completely suppressed
MPGDs: stability
The biggest “enemy” of MPGDs are the discharges.
Due to the fine structure and the typical micrometric distance of their electrodes, MPGDs generally suffer from spark occurrence that can be harmful for the detector and the related FEE.
S. Bachmann et al.,
NIMA A479(2002) 294
241 Am  souce
Strongly reduced but not completely suppressed
GEM
discharge probability
Efficiency & discharge probability MM
10 GeV/c proton
A. Bay et al.,
NIMA 488 (2002) 162
Efficiency
G. Bencivenni, LNF-INFN

27. voltage drop due to sparking
Technology improvement: resistive MPGD
For MM, the spark occurrence between the metallic mesh and the readout PCB has been overcome with the implementation of a “resistive layer” on top of the readout itself . The principle is the same as the resistive electrode used in the RPCs: the transition from streamer to spark is strongly suppressed by a local voltage drop.
The resistive layer is realized as resistive strips capacitive coupled with the copper readout strips.
voltage drop due to sparking
G. Bencivenni, LNF-INFN

28. NS2(CERN): no gluing but still stretching …
MPGDs: the challenge of large area
NS2(CERN): no gluing but still stretching …
A further challenge for MPGDs is the large area:
the construction of a GEM requires some time-consuming (/complex) assembly steps such as:
the stretching of the 3 GEM foils (with quite large mechanical tension to cope with, 1 kg/cm)
the splicing of GEM foils to realize large surfaces is a demanding operation introducing not negligible dead zones (~3 mm). The width of the raw material is limited to cm.
similar considerations hold for MM:
the splicing of smaller PCBs is possible, opening the way towards the large area covering (dead zone of the order 0.3 – 0.5 mm).
The fine metallic mesh, defining the amplification gap, is a “floating component” stretched on the cathode (~1 kg/cm) and electrostatically attracted toward the PCB
Possible source of gain non-uniformity
Handling of a stretched
mesh
G. Bencivenni, LNF-INFN

29. The two detector layouts (II)
single layer
double layer d d’
upper layer
(1cm)d’ r r
d (50cm)
conductive vias
(*) 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
inferior layer
Ω ~ ρs x d/2πr
Ω’ ~ ρs’ x d’/πr
Ω/ Ω’ ~ (ρs / ρs’) x d/2d’
If ρs = ρs’  Ω/ Ω’ ~ d/2d’ = 25
(*) Morello’s model: appendix A-B (G. Bencivenni et al., 2015_JINST_10_P02008)
G. Bencivenni, LNF-INFN

30. µ-RWELL: B≠0 with Ar/ISO=90/10
CC analysis
June 2015 – θ=0°, B= 0 T
Dec 2014 – θ=0°, B= 0.5 T
June 2015 – θ=0°, B= 1 T
June θ=0°
For θ=0° and 0 < B < 1 T σ < 180 µm and ε > 98%
G. Bencivenni, LNF-INFN