First tests of a RICH detector consisting for a matrix of CsI coated Thick GEMs G. Bencze 1,2, A. Di Mauro 1, P. Martinengo 1, L. Molnar 2, D. Mayani Paras 3, E. Nappi 4, G. Paic 1,3, V. Peskov 1,3 1 CERN, Geneva, Switzerland 2 Particle and Nuclear Physics Institute, Wigner PC, Budapest, Hungary 3 Instituto de ciencias Nucleares, UNAM, Mexico city, Mexico 4 INFN Bari, Bari, Italy

Outline The Very High Particle Identification in ALICE for the upgrade (VHMPID) Construction of a triple TGEM with a CsI photocathode Tests with beam in a RICH configuration Optimisation of the TGEM geometry Elba2

VHMPID in ALICE The VHMPID should be able to identify, on a track-by- track basis, protons enabling to study the leading particles composition in jets (correlated with the π0 and /or γ energies deposited in the electromagnetic calorimeter).. (HMPID) In the framework of the ALICE upgrade program we are investigating the possibility to build a new RICH detector allowing to extend the particle identification for hadrons up to 30GeV/c.It is called VHMPID

Competing technology The TGEM approach is a possible alternative to the well established RICH technology which consists of MWPC with a pad cathode covered with a CsI layer This technology is satisfactorily functioning in the HMPID, but having a TGEM alternative would have some advantages : – It operates in badly quenched gases as well as in gases in which are strong UV emitters. This allows to achieve high gains without feedback problems. – It can be used them in unflammable gases or if necessary using windowless detectors (as in PHENIX) –It has generally a lower positive ion bombardment of the CsI – they can operate in “hadron blind mode” with zero and even reversed electric field in the drift region which strongly suppress the ionization signal from charged particles (PHENIX) Elba4

purpose To build a CsI-TGEM based RICH prototype, demonstrate the single photon capability in conjunction with ionizing particles Compare the performance with a similar “conventional” MWPC beam test Elba5

MWPC results summary Elba6

Some basics about TGEM photocatodes Elba7

The challenges of the CsI coated GEM/TGEM CsI layer Important: we are collecting the electrons from the surface and not from the drift space! Elba8

Schematic of a TGEM photon detector Elba9

TGEM 100mm Thickness: 0.45 mm Hole d: 0.4 mm Rims: <10 μm Pitch: 0.8 mm Active area: 77% TGEM is a hole-type gaseous electron multiplier based on standard printed circuit boards featuring a combination of mechanical drilling (by a CNC drilling machine) and etching techniques Elba10

Design of the CsI-TGEM based RICH prototype

Schematic of the beam test setup

top view of the RICH prototype (from the electronics side)

View from the back plane Elba14

CsI side

Drift meshes ( three independent grids)

Voltage dividers There was a possibility to independently observe analog signals from any of electrodes of any TGEM and if necessary individually optimize voltages on any TGEM

Six triple TGEMs were assembled using a glow box inside the RICH prototypes gas chamber.

Extra windows Front view The RICH prototype has windows in front of each triple TGEM allowing to irradiate the detectors ether with the radioactive sources such as 55 Fe or 90 Sr or with he UV light from a Hg lamp

Beam test

proximity focusing TGEM-based RICH prototype installed at CERN T10 beam test facility (mostly ~6 GeV/c pions) Scintillators Liquid radiator

Electronics side

Beam test results 6 GeV/c PS

MIP Single event displays Elba 24

Ne+10%CH 4 (o verlapping events, radiator thickness 10mm) November 2010 beam test. Noise was removed offline

Ne+10%CF 4 ( overlapping events, rad. thickness 15 mm) May 2011 beam test. Raw data, no noise removal

Photoelectron statistics Main conclusion : ~1p.e. per TGEM

Four triple TGEMs together After corrections on geometry and nonuniformity of the detector response the estimated mean total number of photoelectrons per event is about Elba28

Comparison to the MWPC photocathode The present results correspond, taking into account the different radiator thicknesses, to about 60% of the MWPC performance. The inferior performance may be attributed to the following factors: –Uneven gains across the TGEM surface –Hole topology Elba29

DETAILED STUDY OF THE PHOTOELECTRON YIELD IN A TGEM Elba30

Studies of the topologies of holes. Scanning optical table with a UV led focussed on the TGEM Precision 2.5 micro/step Elba31 The light is focussed on the TGEM (Ni-Au plated and the photoelectrons are detected on the wire plane (the resolution of the readout does not play a role!

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Photoelectron yield map Clearly visible a high yield around the edge of the hole (with a rim in that case) However the yield is not uniform around the hole and a large part of the CsI surface is almost inactive! Elba33

Main conclusions of the measurements with scanning Elba34

The response is very nonuniform –Probably due to the surface treatment –A uniform response might give 2x more! The highest yields are concentrated in a rim closest to the hole (not taking into account the proper rim) Elba35

Conclusions for the optimisation The surface ratio CsI/hole is not the primary criterion. A wider hole (preliminary results) seems to favor a higher yield – a larger rim thickness, filling a larger part of the active CsI surface. Hence one might envisage holes of 500 micrometers to be tested in near future. The scanning of the p.e yield is a powerful tool! Elba36

General conclusion A beam test on triple TGEMs has been tested for the first time in a regime of single photon and ionising particles. The results are comforting for a first try. The scanning of the yield with a UVLED on the Au surface of the TGEM indicates clearly the way to go for farther tests. Improvements possible! Elba37

backup Elba38

How much p.e one can expect in “ideal conditions”: full surface (without holes) and CH 4 gas: Corrections: 0.9 (extraction)x0.75= p.e/0.68~ 15pe