1. R&D Experiment - INFN PD - GR. V
Development of “Large gas Electron Multiplier” (LEM) detectors for high gain operation in ultra-pure noble gasses
B. Baibussinov, G. Mannocchi, G. Meng, F. Pietropaolo, P. Picchi, S. Centro.
R&D Experiment - INFN PD - GR. V

2. Ideas and Goals (from 2006 proposal)
The basic idea:
Macroscopic version of hole Gas Electron Multipliers (GEM’s)
~mm hole size in standard double-face Cu-clad Printed Circuit Board (PCB)
Characteristics & performance under investigation by several collaborations
Goals:
Stable High gain (> 104 up to streamer regime) in pure noble gasses
quenching gas replaced by UV photon absorption in hole walls
Good energy resolution
negligible charge loss due to electron diffusion and avalanche size (small wrt hole size)
Direct readout of LEM electrodes
X-Y segmentation
Possible applications:
Cryogenic double phase TPC’s
low energy (~keV) event localization (Dark Matter, Solar Neutrinos)
High pressure TPC for medical imaging
R&D activity fully funded in PD by PRIN 2005
Photosensitive large area detectors, RICH
coupling with radiation conversion detectors (CsI photocathodes)

3. What is a LEM Standard GEM LEM
A thick GEM-like gaseous electron multipliers made of standard printed-circuit board perforated with sub-millimeter diameter holes, chemically etched at their rims
In-house fabrication using automatic micromachining
Self-supporting
Extremely resistant to discharges (low capacitance)
First introduced within the ICARUS R&D group
for double phase noble gasses TPC’s in the keV region
H. Wang, PhD Thesis, UCLA, 1999
L. Periale et al, 2000.
Developed also as GEM alternative
Coarser resolution
Low rate physics (slower signals)
A. Rubbia et al.
Photo conversion detectors
Breskin et al.
Policarpo et al.
Standard GEM
LEM

4. LEM: principle of operation
Upon application of a voltage difference across the LEM, a strong dipole field Ehole is established within the holes.
Electrons deposited by ionizing radiation in a conversion region above the LEM, or produced on a solid radiation converter, are drifting towards the LEM under Edrift and are focused into the LEM holes by a strong electric field inside the holes.
Electrons are multiplied within the holes under the high electric field (~25-50 kV/cm)
Avalanche electrons are collected on the LEM bottom electrode (a fraction could also be further transferred to a collecting anode or to a second, possibly similar, multiplier element).
Each hole acts as an independent multiplier.
A more favorable hole aspect ratio allows better avalanche confinement, reducing photon-mediated secondary effects.
This leads to higher gains in LEM wrt GEM with similar gas mixtures and to high-gain operation in a large variety of gases, including highly scintillating ones like pure noble gasses or CF4.

5. Open problems for further R&D
Residual charging-up of holes walls due ions/electrons diffusion especially at high rate and residual photon feed-back in pure noble gasses, affecting:
Maximum Gain
Energy resolution
Time stability
Possible fields of investigation:
LEM geometry (including multi-step)
To reduce diffusion effects
Electrodes oxidation
To minimize photon feed-back and electron extraction
Resistive electrodes
To improve “quenching” effect (RPC-like) and reach streamer mode gain >>> In the First 6 Months of 2007 We Concentrated on This Promising Development <<<
Needle-LEM
To avoid discharges and carbonization of LEM hole walls

6. Resistive electrodes +++++++++++++++ Before _ _ _ _ _ _ _ _ _ _ _ _
(oxided) electrode
Hybrid RPC concept:
Resistive layer “quenches” the electron avalanche
Vetronite holes “limit” the photon propagation and after pulses
Goal
It Should allow gains up to streamer mode (maybe limited by photon feed-back through hole input)
Disadvantages
Choice of resistive material critically depending on rate and gain (resistive materials from Quadrant Technology, ranging from 105 to 1015 Ω-cm, under investigation)
Vetronite HV
Resistive plates
Signal pickup
Before
_ _ _ _ _ _ _ _ _ _ _ _
A charged particle entering the hole induces an avalanche, which develops into a spark. The discharge is quenched when all of the locally (~1 hole) available charge is consumed. Photons are blocked by vetronite walls.
_ _ _ _ _ _ _ _ _ _
After
Preliminary results:
Gain >> 104 easily reached
The discharged area recharges slowly through the high-resistivity plates.

7. LEM with resistive coating
P.Fonte et al.
Large area UV photosensitive detectors
Single LEM structure
UV Window
Hybrid RPC
CsI photocathode
G-10 with
resistive
coating
Readout plate
Readout plate
Blue: Cu coated G-10
Red: G-10 with resistive coating.
Gas: Ar+5% isob.
Red: G-10 with resistive coating.
Green: same plate combined with CsI photocathode.
Gas :Ar+5% isob.
Preliminary results
higher gains are possible with resistive coating

8. Laboratory equipment Temperature controlled Cryogenic vessel
UHV chamber with VUV window

9. LEM with resistive coating
Examples CrO and CuO resistive coatings
Resistive GEM/ LEM coated with CsI photocathode installed inside
the LAr test chamber
A full report will be presented at the IEEE-2006

10. Thick GEM-like multipliers: THGEM
L. Periale at al., NIM,A478,2002,377; J. Ostling et al.,IEEE Nicl Sci.,50,2003,809
Manufactured by standard PCB techniques of
precise drilling in G-10 (+other materials) and Cu etching.
ECONOMIC & ROBUST !
Hole diameter d= mm
Dist. Bet. holes a= mm
Plate thickness t= mm
A small THGEM costs ~3$ /unit.
With minimum order of 400$  ~120 THGEMs.
~10 times cheaper than standard GEM.
TGEM was further developed by Breskin group : R. Chechik et al. NIM A535, 2004,

11. J. Ostling et al., IEEE Nucl. Sci., 50, 2003, 809
Thick GEM-TGEM
GEM
TGEM
2 mm thick
J. Ostling et al., IEEE Nucl. Sci., 50, 2003, 809

12. Resistive Electrode GEM-RETGEM
Holes
High resistivity layer +V -V
Dielectric - +

13. Surface resistivity 200 - 800 kΩ/□ (100XC10E5)
CNC drilling
Glue
PCB a) mm
Cu foil
Resistive kapton 50 μm b)
Contact wires
Diameter of holes: mm, pitch mm
Active area 30x30 and 70x70 mm2
Important feature: for the first time the resistive electrodes have not any metallic substrate
Surface resistivity kΩ/□ (100XC10E5)

14. Resistive Electrode GEM-RETGEM
Active area 30x30 and 70x70 mm2
Diameter of holes: mm, pitch mm

15. Exp. Setup Hg lamp PMs for monitoring discharges Radioactive Window
source
Window A
GEMs
RETGEMs
Charge- sensitive or current amplifiers

16. RETGEM gains

17. Energy resolution study
Energy resolution of ~33% FWHM was achieved for uncollimated 55Fe
at gains of At higher gains the detector may lose the proportionality and
sometimes even works in “Geiger” mode

18. Robustness against sparks
In several cases, we initiated continuous glow discharges in the RETGEM
for a more than 10 minutes. After the discharge was stopped (by reducing
the voltage on the detector’s electrodes), the RETGEMs continued to operate without any change in their characteristics, including that of the maximum achievable gain.
Results:
In Ar spark current in kapton RETGEM is almost 1000 times less that metallic Thich GEM or standard GEM
In kapton RETGEM initial “sparks”/’streamers” with further increase of the voltage may transit to glow discharge
Neither sparks or streamers damage the detector or electronics
Continuous discharge in RETGEM hole

19. Gains of single (solid symbols) and double (open symbols) kapton RETGEMs
Holes 0.3 mm in diameter
on a 0.7 mm pitch.
Holes 0.8 mm in diameter
with a 1.2 mm pitch
With double RETGEMs Raether limit for “macroscopic” detectors was reached : An0~108 electrons

20. Recent preprints and publications
R. Oliveira, V. Peskov, F. Pietropaolo and P. Picchi, First tests of thick GEMs with electrodes made of a resistive kapton, NIM-A 576, (June 2007) Pages
L. Periale, V. Peskov, C. Iacobaeus, B. Lund-Jensen, P. Picchi, F. Pietropaolo and I. Rodionov, Photosensitive gaseous detectors for cryogenic temperature applications, NIM-A 573, (April 2007) Pages
L. Periale, V. Peskov, A. Braem, Di Mauro, P. Martinengo, P. Picchi, F. Pietropaolo and H. Sipila, Development of new sealed UV sensitive gaseous detectors and their applications, NIM-A 572, (March 2007), Pages
L. Periale, V. Peskov, C. Iacobaeus, B. Lund-Jensen, P. Pavlopoulos, P. Picchi and F. Pietropaolo, A study of the operation of especially designed photosensitive gaseous detectors at cryogenic temperatures, NIM-A 567, (November 2006) Pages
A. Di Mauro et al., Development of innovative micropattern gaseous detectors with resistive electrodes and first results of their applications, arXiv: ; 04 Jun 2007
V. Peskov, R. de Oliveira, F. Pietropaolo, P. Picchi, First Tests of Thick GEMs with Electrodes Made of a Resistive Kapton, arXiv:physics/ ; 13 Jan 2007
Di Mauro, A, Lund-Jensen, B., Martinengo, P., Nappi, E., Peskov, V. Periale, L.,Picchi, P.; Pietropaolo, F.; Rodionov, I., A New GEM-like Imaging Detector with Electrodes Coated with Resistive Layers, arXiv:physics/ ; 17 Dec 2006, Nuclear Science Symposium Conference record 2006, IEEE Volume 6, (Oct. 2006) Page(s):
V. Peskov, B. Baibussinov, S. Centro, A. Di Mauro, B. Lund-Jensen, P. Martinengo, E. Nappi, R. de Oliveira, F. Pietropaolo, P. Picchi, L. Periale, I. Rodionov, S. Ventura, Development and first tests of GEM-like Detectors with Resistive Electrodes, Nuclear Science, IEEE Accepted for publication (July 2007)

21. Partecipanti, tempi, richieste 2008
Padova (Tot: 1 FTE)
B. Baiboussinov (15%)
S. Centro (10%)
F.Pietropaolo (25%, Resp. Naz.)
G. Meng (25%)
LNF (associati a PD)
G. Mannocchi (20%)
P. Picchi (30%)
CERN
R. De Oliveira
A. Di Mauro
P. Martinengo
V. Peskov
Nessuna Richieste ai servizi PD per il 2008
Trasferte:
Interne: 3 mesi uomo (metabolismo + tests a LNL) : 2000 €
Estere: 1 mese uomo (progettazione PCB e deposizioni CsI): 3000 €
Durata: 24 Mesi ( )
Milestones:
Primo anno: Prototipi piccola scala (10x10 cm2):
ottimizzazione layout LEM, LEM+needles, LEM resistive
Guadagno
Risoluzione
Stabilita temporale
Accoppiamento con fotoconvertitori per VUV
Secondo anno: LEM di medie dimesioni (30x30 cm2):
Readout segmentato per:
Imaging medicale in Xenon ad alta pressione (CARDIS)
Fotorivelatori a grande area
LAr-TPC doppia fase
Previsioni di spesa (2008):
Consumo (totale ~19000 € su 2 anni):
Forfait workshop PCB CERN (materiale + lavorazione) ~3000 €
Fornitura Argon e Xenon gas per test ~ 3000 €
Fornitura materiale resistivo ~2000 €
Deposizione CsI al CERN (materiale + lavorazione) ~2000 €
Per il 2007 stiamo seguendo il programma previsto senza intoppi