1. LABORATORI NAZIONALI DI FRASCATI
WP13.2.4: the µ-RWELL detector
G. Bencivenni
LNF-INFN, Italy

2. Improving their performance in terms of time and space resolution
MPGDs: state of the art
MicroPattern Gas Detectors (MPGD) due to the high rate capability and fine space resolution are ideal tools for :
fundamental research (Compass, LHCb, Totem, KLOE, Jlab, LHC experiments upgrades)
applications beyond science (medical, industrial, neutron ...)
In spite of the recent relevant progress in the field, still dedicated R&D studies are needed towards:
stability under heavy irradiation (discharge containment)
simplified construction technologies, a MUST for very large scale applications in the various fields of application
Improving their performance in terms of time and space resolution

3. MPGDs: stability MM GEM
Due to the fine structure and the typical micrometric distance of their electrodes, MPGDs generally suffer from spark occurrence that can eventually damage the detector and the related FEE.
GEM
Strongly reduced but not suppressed
discharge probability
Efficiency & discharge probability MM
10 GeV/c proton
A. Bay et al.,
NIMA 488 (2002) 162
Efficiency
S. Bachmann et al.,
NIMA A479(2002) 294

4. Technology improvements: resistive Micromegas
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.
by R.de Oliveira TE MPE CERN Workshop
The resistive layer is realized as resistive strips capacitive coupled with the copper readout strips.
voltage drop due to sparking
the steramer, discharging a limited area around its location, is automatically quenched and the transition to spark is strongly suppressed.

5. MPGDs: construction issues (I)
A further limitation of such MPGDs is correlated with the complexity of their assembly procedure, particularly evident in case of large area devices.
NS2(CERN): no gluing ...
but still stretching …
The construction of a GEM requires some time-consuming (complex) assembly steps such as the stretching (with quite large mechanical tension to cope with, 1 kg/cm) of GEM foils. The splicing/joining of smaller detectors to realize large surfaces is difficult unless introducing not negligible dead zones (2÷3 mm). The width of the raw material is limited to 60 cm.
Similar considerations hold for MM: the splicing /joining of smaller PCBs is possible, opening the way towards the large area covering (dead zone of the order 0.2÷0.3 mm).
The fine metallic mesh, defining the amplification gap, is a
“floating component”, because it is stretched on the cathode
(1 kg/cm) and electrostatically attracted toward the PCB
 Possible source of gain non-uniformity.
Handling of a stretched
mesh

6. The µ-RWELL: a novel architecture (I)
The goal of this study is the development of a novel MPGD by combining in a unique approach the solutions and improvements proposed in the last years in the MPGD field (RD51).
The µ-RWELL is realized by coupling a “suitable patterned GEM foil” with the readout PCB plane coated with a resistive deposition ( M/).
The resistive coating is performed by screen printing  robust against discharges.
The WELL matrix is realized on a 50 µm thick polyimide foil, with conical channels 70µm (50 µm) top (bottom) diameter and 140µm pitch.
A cathode electrode, defining the gas conversion/drift gap, completes the detector
 compact & simple to build .
5 cm
M/

7. The µ-RWELL: a GEM-MM mixed solution
GEM detector sketch
MM detector sketch

8. The µ-RWELL: a GEM-MM mixed solution
The µ-RWELL has features in common either with GEMs or Micromegas:
from GEM it takes the amplifying scheme with the peculiarity of a “well defined amplifying gap”, thus ensuring very high gain uniformity (even better because of the absence of transfer/induction gaps).
from Micromegas it takes the resistive readout scheme that allows a strong suppression of the amplitude of the discharges.

9. µ-RWELL detector: to do list
Optimization of detector parameters (Garfield simulation + small protos):
value of surface resistivity vs gain, rate capability, charge spread, discharge rate
study & implementation of the resistive layer segmentation vs rate capability
WELL geometry (pitch, diameter, «depth») vs gain
Work to be done in 24 months (M24)

10. µ-RWELL detector: to do list
Detector engineering vs large area tracking & digital calorimetry applications:
Design of large area demonstrator
Large area readout design
Study of FEE-detector coupling
Work to be done in 24 months (M44-48)

11. SHARING OF DUTIES with the contributing partner:
INSTITUTIONS
COMMITTEMENTS
LNF-INFN
Simulation & Design of small size detector prototypes
Design of large area demonstrator
Bari-INFN
Resistive layer studies & large area readout optimization
Study of FEE-detector coupling
MILESTONES:
M18: Resistive-layer studies with built & qualified small size prototypes
M24: Segmented resistive layer studies with Small-size prototype built & qualified prototypes
M36: Detector geometrical parameters optimization (vs gain) with built & qualified prototypes
M44: Large-size prototype O(1 m2) fully engineered and validated
COMPLEMENTARY FINANCIAL RESOURCES:
From an R&D experiment – MPGD_NEXT – financed by INFN (CSN V)
∼ k€/year

12. Thanks for the attention

13. Spares slides

14. AIDA 2020: WP13.2.4 PARTECIPAZIONE AL PROGETTO (reale )
Giovanni Bencivenni – Primo Ricercatore (coordinatore WP13.2.4) : 30% FTE su 4 anni
Giulietto Felici – Dirigente Tecnologo: % FTE su 4 anni
Danilo Domenici – Ricercatore: % FTE su 4 anni
Marco Poli Lener – art 36: % FTE su 4 anni
Gianfranco Morello – art 36: % FTE su 4 anni
Patrizia De Simone – Primo Ricercatore: % FTE su 4 anni
Wenxin Wang – PHD % FTE su 2 anni
PERSONALE STAFF DA RENDICONTARE:
1 fisico(*) per 3 mesi su 4 anni  ~ 19 k€ (*P-Ric II liv, VI fascia = 88k€/y)
STRATEGIA per il completamento del programma scientifico:
Esperimento di R&D MPGD_NEXT da sottomettere alla CSN V nel 2015 con attività su un periodo di 4 anni (finanziamento in G5 su Dotazioni G5 - LNF per il 2015)
DIVISONE FINANZIAMENTO:
47200 € - personale (Art 36)
0 € - Consumi Ricerca

15. The µ-RWELL: a novel architecture (II)
The proposed detector is:
compact: one gap only (< 5mm thickness)
robust against discharges: amplification through resistive coupling
simple to build , only two components:
a single-amplification stage embedded with the readout
a cathode plane
5 cm
The first prototype of µ-RWELL has been designed at LNF and built in the at the CERN PCB Workshop
The micro-structure has some characteristics in common with two MPGDs developed by the end of last century
(J. P. III France 6 (1996) 337, NIMA 423 (1999) 125).

16. 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 then followed by a ~50 ns ion tail.
The µ-RWELL signal is similar to the MM one.
Schema di una gem …
µ-RWELL – Ar:CO2 70:30 gas mixture

17. The µ-RWELL performance (I)
The prototypes have been tested with Ar/i-C4H10 =90/10 gas mixture and characterized by measuring the gas gain, rate capability and discharge behavior in current mode.
The devices has been irradiated with a collimated flux of 5.9 keV X-rays generated by a PW2217/20 Philips Tube.
The gain has been measured vs potential applied between the top of the electrode of the amplification stage and the resistive layer.
The main difference between the two prototypes is the coupling between the top-layer of the well and the resistive-plane:
for the 100MΩ/□ is through the copper-dots;
for the 80MΩ/□ is without the copper-dots;

18. The µ-RWELL performance (III)
Discharge study: µ-RWELL vs single-GEM
Single-GEM
µ-RWELL
The max. ΔV achieved for the gain measurement is correlated with the onset of the discharge activity, that, comes out to be substantially different for the two devices:
discharges for µ-RWELL of the order of few tens of nA (<100 max gain)
for GEM discharges the order of 1µA are observed at high gas gain
Further systematic and more quantitative studies must be clearly performed

19. The µ-RWELL performance (IV)
A drawback correlated with the implementation of a resistive layer is the reduced capability to stand high particle fluxes: larger the radiation rate, higher is the current drawn through the resistive layer and, as a consequence, larger the drop of the amplifying voltage.
Ar/CO2 - GRW = 3000, GG<1000
The curves are fitted with the function:
The function allows the evaluation of the radiation flux for a given gain drop of 3%, 5% and 10% for all the collimators.
Aggiungere slide con qualche dettaglio sulla funzione di parametrizzazione
Normalized gain vs X-ray flux for GEM and µ-RWELL for irradiation at the center of the active area, with three different collimator diameters: 10 mm, 5 mm and 2.5 mm.

20. Aluminum support plate
Resistive-MicroMegas with floating mesh
(positioning rely only on the electrostics of the cell)
not to scale detector opened
Rohacell
Aluminum support plate
Stiffening panel
Drift electrode
Pillars (128 µm)
PCB1
PCB2

21. Aluminum support plate
Resistive MicroMegas with floating mesh
(positioning rely only on the electrostics of the cell)
not to scale detector closed
Stiffening panel
Rohacell
Aluminum support plate
5.00
PCB1
PCB2