1. The µ-RWELL technology
LABORATORI NAZIONALI DI FRASCATI
The µ-RWELL technology
G. Bencivenni,
LNF – INFN

2. OUTLINE Muon detectors for future colliders The micro-RWELL
The performance
Future perspectives & Summary

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 environment (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 (for CepC, as well as for SppC, FCC-ee and FCC-hh)
R&D for HL-LHC (CMS & LHCb phase-2 muon upgrade) could be clearly a good starting point

4. 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

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 is implemented (MPDG_NEXT- LNF) suitable for LHCb-Muon upgrade
a standard readout PCB
(*) DLC = Diamond Like Carbon
High mechanical & chemical resistant material 5

6. Principle of operation
Applying a suitable voltage between top copper layer and DLC the “WELL” acts as multiplication channel for the ionization.
top copper layer
kapton
resistive stage
Insulating medium
Pad/strip r/out HV  r t
Not in scale
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, but an appropriate grounding of the resistive layer with a suitable pitch solves this problem (see High Rate scheme)

7. Technology 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 )

8. The Low Rate scheme 1 2 3 Copper layer 5 µm Kapton layer 50 µm
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)

9. The High Rate scheme 1 2 3 4 Copper layer 5 µm Kapton layer 50 µm
DLC layer: 0.1 – 0.2 µm
( M/☐) 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

10. Towards the High Rate layout
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

11. DLC - sputtering
DLC sputtering on Kapton foils (w/copper on one side) is Be-Sputter Co., Ltd (Japan).
non preliminary dried
kapton samples
non preliminary dried
kapton samples
The different trends of the two curves seems to be related with the humidity trapped by the kapton before the sputtering process.

12. 2017 R&D Program
Technology Transfer to ELTOS of the single-layer (large area - CMS) PCB-RWELL manufacturing: in progress w/good results (final kapton etching still at CERN)
Technology Transfer to TECHTRA for the etching of the amplification-stage (kapton etching): just started (tuning the procedure … )
Ageing GIF++ of large area single-layer RWELL + small area double-layer (v1.0) RWELL: in progress (till end of the year)
Construction of prototypes based on double-layer layout + an alternative simplified scheme: 10x10 cm2 with pad size of 0.6x0.8 cm2
Test H4-SPS (5 – 19 Jul) : efficiency/cluster-size (pad r/o) studies
Test PSI (4 –18 Dec - tbc) with a high intensity hadron beam in current mode:
- rate capability with a particle flux up to 20 MHz/cm2
- discharge probability
- integrating a charge equivalent to some years of HL Colliders ? 12

13. Single-layer performance
Space resolution (mm)
RWELL = (52+-6) µm
@ B= 0T after TRKs contribution subtraction

14. Rate capability with X-rays (double layer)
Double resistive layer w/ 1x1 cm2 through-vias grounding pitch
local irradiation
for m.i.p.×7
Φ = 3.4 MHz/cm2; Φ = 2.8 MHz/cm2; Φ = 1.6 MHz/cm2
Local irradiation is practically equivalent to global irradiation

15. Beam Test (CMS/LHCb collaboration)
H8 Beam Area (18th Oct. – 9th Nov 2016)
Muon/Pion beam: 150 GeV/c
3 -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 (never done before)

16. Time Performance (single/double-layer)
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 is dominated by the fee (measurement done with VFAT2).

17. Performance vs Rate (single/double-layer)
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 >3ˣ3 cm2 (FWHM) (medium size irradiation, ~10 cm2 spot)
Double resistive layer
(High Rate scheme)
Single resistive layer
(Low Rate scheme)

18. Ageing test: GIF++ (INFN-BO, LNF)
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) 18

19. Ageing test: GIF++ (single/double-layer)
I(G=4000)  10 nA/cm2 corresponding to an equivalent mip rate of  250 kHz/cm2
Low gain
High gain
Filter scans
Detectors currents are constant/stable during the operating time gates
The double layer has integrated ~35 mC/cm2

20. Technology Transfer (single-layer)
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 has been 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 DONE
Construction of the first 1.8x1.2m2 (GE2/1) µ-RWELL (only M4) /2017 ONGOING
DONE
1.8x1.2m2 (GE2/1) µ-RWELL
ONGOING
~40 times larger than small protos !!!
~200 times larger than small protos !!!
mock-up
TT to ELTOS

21. 2018 R&D program (very preliminary)
Technology Transfer to ELTOS for PCB-RWELL manufacturing + TECHTRA for amplification-stage (kapton) etching, with construction of prototypes
ENGINEERING of the HIGH RATE layout (double layer or other simplified scheme) with construction of prototypes
Test H4(H8)-SPS for prototypes characterization
Test PSI with high intensity hadron beam in current mode (in case of no confirmation of the Dec-2017 TB slot)
RD_FA RWELL-Group composition (preliminary):
G.Bencivenni (20%), G. Felici, M.Gatta, G.Morello, M.Poli Lener
… a MAECI project has been recently submitted: “Design, Industrialization and Production of an innovative Micro Pattern Gas Detector (MPGD) based on the micro-RWELL technology“

22. COLLABORATION OF SEVERAL INFN (/RD_FA) GROUPS:
- BARI
- BOLOGNA
- FERRARA
- FRASCATI
- TORINO
Global budget of the project: k€
INFN co-funding (w/salary): k€
INFN (RD_FA, CMS, BESIII): k€
MAECI: k€

23. SUMMARY
The µ-RWELL is a compact, single-amplification stage, simple to assemble & suitable for large area muon apparata, MPGD:
gas gain > 104
intrinsically spark protected
rate capability >> 1 MHz/cm2 (HR version)
space resolution ∼ 60µm
time resolution ∼ 5.7 ns
R&D/engineering (RD_FA task):
Low rate (<100kHz/cm2 – CepC, FCC-ee) :
small and large area prototypes built and extensively tested
Technological Transfer to industry is ongoing
High rate (>1 MHz/cm2 - SppS, FCC-hh):
first prototypes show promising performance
the engineering still to be started