THGEM-based photon detectors for the upgrade of COMPASS RICH-1

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29 July 2021

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THGEM-based photon detectors for the upgrade of COMPASS RICH-1

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DOI:10.1016/j.nima.2013.08.020

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THGEM-based photon detectors for the upgrade of COMPASS RICH-1

M. Alexeev

a

, R. Birsa

a

, F. Bradamante

b

, A. Bressan

b

, M. Büchele

f

, M. Chiosso

c

, P. Ciliberti

b

,

S. Dalla Torre

a

, S. Dasgupta

a,1

, O. Denisov

d

, V. Duic

b

, M. Finger

e

, M. Finger Jr.

e

, H. Fischer

f

,

M. Giorgi

b

, B. Gobbo

a

, M. Gregori

a

, F. Herrmann

f

, K. Königsmann

f

, S. Levorato

b

,

A. Maggiora

d

_{, A. Martin}

b

_{, G. Menon}

a

_{, F. Nerling}

f

_{, K. Novakova}

a,g

_{, J. Novy}

e

_{, D. Panzieri}

h

_,

F.A. Pereira

i

, C.A. Santos

i

, G. Sbrizzai

b

, P. Schiavon

b

, C. Schill

f

, S. Schopferer

f

, M. Slunecka

e

,

F. Sozzi

a

, L. Steiger

b,g

, M. Sulc

g

, S. Takekawa

b

, F. Tessarotto

a,n

, J.F.C.A. Veloso

i

a

INFN, Sezione di Trieste, Trieste, Italy

b

INFN, Sezione di Trieste and University of Trieste, Trieste, Italy

c_{INFN, Sezione di Torino and University of Torino, Torino, Italy} d_{INFN, Sezione di Torino, Torino, Italy}

e_{Charles University, Prague, Czech Republic and JINR, Dubna, Russia} f

Universität Freiburg, Physikalisches Institut, Freiburg, Germany

g

Technical University of Liberec, Liberec, Czech Republic

h

INFN, Sezione di Torino and University of East Piemonte, Alessandria, Italy

i

I3N - Physics Department, University of Aveiro, Aveiro, Portugal

a r t i c l e i n f o

Available online 29 August 2013 Keywords:

RICH

Gaseous photon detectors MPGD

THGEM PID CsI

a b s t r a c t

New Cherenkov photon detectors are being developed for the upgrade of COMPASS RICH-1. The detectors are based on THGEMs, arranged in a three layer architecture, with a CsIfilm on the first layer acting as a reflective photocathode. The response of THGEMs with various geometries under different conditions has been studied and photon detector prototypes have been built, tested in laboratory and operated during test beam runs providing a typical gain of 105_{and a time resolution of better than 10 ns. A photon}

detector prototype with 300 300 mm2_{active area, operated at the CERN PS T10 test beam in November}

2012, has conﬁrmed the validity of this novel technology and has allowed further studies of the detector response.

1. Introduction

The physics program of the COMPASS Experiment [1] at the CERN SPS has recently been enlarged with the approval of a new series of measurements to be performed with an upgraded version of the apparatus. In the Particle Identi_{ﬁcation (PID) sector} increased rate capability, greater stability and higher efﬁciency need to be guaranteed.

Present hadron identi_{ﬁcation requirements (}

π

-K separation from 3 to 55 GeV/c over7200 mrad angular acceptance, at beam rates of 40 MHz and trigger rates up to 50 kHz) are fulﬁlled by the COMPASS RICH-1 detector[2], a Ring Imaging Cherenkov counter with a 3 m long gaseous C4F10radiator, a 21 m2large focusing VUV

mirror surface and Photon Detectors (PDs) covering a total surface of 5.5 m2.

COMPASS RICH-1 is in operation since 2001, and in its original version it used as PDs MWPCs having as a cathode plane a Printed Circuit Board (PCB) segmented in pads and coated with a CsIﬁlm. In spite of their good performance, MWPC-based PDs present intrinsic limitations: aging (decrease of quantum efﬁciency) after a few mC/cm2 _{charge collection, feedback pulses with a rate}

increasing at large gain values, long recovery time after occasional discharge in the detector and long signal formation time (due to the slow ion motion); the MWPCs have to be operated at low gain and present a non-negligible detector memory and dead time.

Since 2006 the central region of the PDs (25% of the surface) is instrumented with matrices of Multi-Anode PMTs coupled to individual fused silica lens telescopes and read out via sensitive front-end digital electronics and high resolution TDCs.

In view of the challenges to be faced by RICH-1 for the future measurements, the COMPASS THGEM R&D project[3]was started, aimed to develop a large size gaseous detector of single photons, able Contents lists available atScienceDirect

journal homepage:www.elsevier.com/locate/nima

Nuclear Instruments and Methods in

Physics Research A

n_{Corresponding author. Tel.:}_{þ39 0403756228.}

E-mail address:

fulvio.tessarotto@ts.infn.it (F. Tessarotto).

1

On leave from Matrivani Institute of Experimental Research and Education, Kolkata, India.

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to stably operate at large gain and high rate, to provide fast response, good time resolution and insensitivity to the magneticﬁeld.

This article reports on the recent progress of this project.

2. THGEM-based PDs

Among the photon detection technologies (PMTs in standard or hybrid version, Micro Channel Plates, Silicon Photomultipliers, Gas-eous PDs) offering interesting perspectives for large scale applica-tions, Gaseous PDs are still the only option for instrumenting large surfaces in Cherenkov Imaging Counters at affordable costs.

Gaseous PDs for future RICH applications should have a small signal formation time, a closed geometry to avoid photon feedback and a reduced ion back-ﬂow to the CsI photocathode: these requirements suggest the use of Micro-Pattern Gaseous Detectors (MPGDs). Gas Electron Multipliers (GEMs)[4]have been success-fully used as PD elements in the Hadron Blind Detector[5]of the PHENIX Experiment at RHIC, although not as detectors of single photons. Theﬁne space resolution and the low amount of material offered by GEMs are not essential for RICH applications, while the possibility of higher robustness against electrical discharges and larger operational gains provided by Thick-GEMs (THGEMs) are very appealing.

THGEMs, introduced about 10 years ago[6], are robust gaseous electron multipliers derived from the GEM design, scaling the geometrical parameters and changing the production technology: standard PCBs are used instead of the Cu-coated polyimide foils and the holes are obtained by drilling. Typical values of THGEM geometrical parameters are: PCB thickness from 0.2 to 1 mm, hole diameter from 0.2 to 1 mm and hole pitch from 0.4 to 1.5 mm. A metal-free clearance ring around the hole, the rim, has a width ranging from 0 (no rim) to 0.2 mm.

For the COMPASS THGEM R&D project more than 50 different samples of small (30 30 mm2 _{active area) THGEMs have been}

characterized using soft X-ray and UV light sources to study the effect of different production procedures, the role of each geometrical parameter, in particular the rim, and tofind the optimal configura-tion for this applicaconfigura-tion. Electrostatic calculaconfigura-tions played an essential role for reaching a qualitative understanding of the observed THGEM behavior. Photoelectron extraction and collection efficiencies have been studied under various conditions, together with the gain stability, leading to the choice of the_{final parameters.}

Chambers hosting multilayer THGEM arrangements (Fig. 1) with CsI coating on the top of theﬁrst THGEM have been built and operated under various conditions: the signal amplitude distributions have exponential shape and the gain is typically in the range of 105_–106_{. The measured time resolutions are between}

6 and 12 ns.

An extensive study of the Ion Backﬂow (IBF) ratio[7]has been performed and geometrical arrangements providing reduced IBF have been found: they consist in mounting three THGEMs (having the same hole array) in a staggered hole alignment conﬁguration

[8] and in operating the PD with a large difference in the ﬁeld values of the regions above and below the middle THGEM (for instance 1 kV/cm and 4 kV/cm, respectively), in order to direct most of the positive ions to the intermediate electrodes. This solution can guarantee that the new PDs provide a signiﬁcantly lower ion bombardment rate at the photocathode with respect to the presently used MWPC-based ones, despite of the increase in the electron multiplication gain.

3. The 300 300 mm2_{active area PD Prototype}

The production of large area THGEMs of high quality and uniformity of response is challenging: a strict quality control procedure has been defined, including systematic optical inspec-tion, electrostatic tests and validation in stand-alone configuration inside a detector. Specific treatments (polishing, micro-etching or polyurethane coating) have sometimes been applied after the industrial production to achieve the desired result. Large area samples of good quality provide the same response as the small size THGEMs.

Several THGEMs with 300_{300 mm}2 _{active area have been}

produced for COMPASS by an industrial PCB producer2 _using

standard FR-4 Halogen-free Glass Epoxy Laminate3 _{with 35}

_μ

_m

thick Cu on both sides. The raw material is cut and selected on the basis of thickness uniformity with strict tolerances (7 3%) to guarantee gain uniformity.

All electrodes are segmented into 6 parallel sectors of 48 mm width and a 0.8 mm spacing is left between sectors to minimize their coupling and the discharge propagation probability. A set of holes placed between sectors and on the PCB border are used for precise and stable positioning. The high voltage is individually provided to each sector.

The PD Prototype (Figs. 2and3) consists of a chamber hosting two planes of wires (needed to de_{ﬁne a uniform electric ﬁeld in} the region above the photocathode) and of three THGEMs, all having a hole diameter ¼ 0.40 mm, hole pitch ¼ 0.80 mm and a rim ofr5

μ

m; the CsI coated THGEM has a thickness of 0.40 mm, the other two are 0.80 mm thick.

Precise positioning is provided by a set of inner columns, border blocks and spacers, made of PEEK (Polyether-ether-ketone) and ﬁxed by screws to guarantee the THGEM planarity and to allow easy mounting (and dismounting) operations.

The anode plane is provided by a PCB segmented into a square array of 576 pads, each with an area of 12 12 mm2_{. The spacings}

between the electrodes are: 2.50 mm between the anode and THGEM3, 3.00 mm between THGEMs, 5.20 mm between CsI and the drift wire plane, 30.0 mm between wire planes.

In November 2012 the 300 300 mm2_{PD prototype has been}

operated at the CERN PS T10 test beam of 6 Gev/c

π

þ.

A truncated cone fused silica radiator (Fig. 4) was placed on the beam axis and equipped with a cylindrical interceptor having a remotely controlled movement. A trigger system based on a 5-fold scintillator coincidence allowed selecting beam particles traveling along the radiator axis.

An Ar/CH4(60/40) gas mixture was used, and the high voltage

was provided to each sector via individual resistive dividers

Fig. 1. Scheme of a THGEM-based photon detector.

2_{ELTOS S.p.A. Strada E, 44}_{– San Zeno – 52100 Arezzo, Italy; eltos@eltos.it;}

〈http://www.eltos.com〉

3

R-1566W by Panasonic Corporation or DURAVER-E-Cu 156 ML by Isola GmbH. M. Alexeev et al. / Nuclear Instruments and Methods in Physics Research A 732 (2013) 264–268 265

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(6 channels from a CAEN A1526N board on an SY1527 mainframe were used).

The detector was equipped with a digital electronic readout based on the CMAD [9] front-end chip and on the F1 TDC[10]

during the whole run, except for an initial test where the anode

Fig. 3. CAD view of the components of the THGEM-based PD.

Fig. 4. CAD images of the movable interceptor, including the corona of the Cherenkov photons. Fig. 2. Picture of a 300 300 mm2

THGEM mounted in a PD.

Fig. 5. Superimposed events including the beam region.

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pads have been grouped and connected to an analog readout for signal amplitude studies.

The detector response to charged particles and to Cherenkov photons has been studied at different voltages, corresponding to gains between 104 _{and 10}5_{, keeping the drift wires and the}

photocathode at the same potential.

Typical images of superimposed events and of single events are shown inFigs. 5,6and7.

A set of measurements was performed at various interceptor positions: the measured multiplicity decreases linearly with the closing of the interceptor (corresponding to a shorter unscreened radiator thickness) as can be seen inFig. 8, where the measured number of hits per event in one sector of the detector is presented as a function of the interceptor position.

In spite of the challenges related to both the production of large area THGEMs and to the engineering problems of large area PDs, the performance of the 300_{300 mm}2 _{PD prototype has}

con-ﬁrmed the validity of this technology for RICH applications. 4. Hybrid THGEM-micromegas photon detector

An alternative approach has in parallel been investigated by combining two MPGD technologies: THGEM and Micromegas. A hybrid detector prototype has been built using small active area (30 30 mm2_{) THGEMs as photoconverter and as}_ﬁrst

multiplica-tion stages, and a standard small bulk Micromegas, provided by COMPASS CEA Saclay colleagues for an exploratory test. Encoura-ging results have been directly obtained from theﬁrst measure-ments, performed without any optimization, using a UV LED as light source: the hybrid THGEM-Micromegas prototype has been

stably operated at gains close to 2 106

and, as expected, it showed a low IBF ratio of r5%; this feature makes the hybrid THGEM-Micromegas technology extremely promising for many future applications.

5. Conclusions

For the future upgrade of COMPASS RICH-1, a set of 12 new, high performing photon detectors with an active area of 600 600 mm2

has to be provided.

The COMPASS THGEM R&D team has performed a systematic study of the THGEM response and of the optimization of the main parameters and architecture of the THGEM-based PDs to be used for the upgrade.

Small PD prototypes providing gains in the range of 105_–106

and time resolutions between 6 and 12 ns have been produced. An important progress has recently been made by building and successfully operating at the CERN PS T10 test beam a 300 300 mm2_{PD prototype, which con}_{ﬁrmed the validity of the basic}

technological choices.

The main problems related to the production of high-quality large area THGEMs and to the engineering of THGEM-based PDs with stable and uniform response and minimal dead areas are being actively investigated and solutions are being found.

A parallel exploration of an alternative technology, based on the coupling of THGEM and Micromegas has started.

COMPASS RICH-1 will most likely be theﬁrst large RICH equipped with THGEM-based photon detectors.

Acknowledgments

This work has partly been performed in the framework of the RD51 collaboration. It is supported in part by the European Community - Research Infrastructure Activity under the FP7 program (HadronPhysics3), by PTDC/FIS/110925/2009 and by CERN/FP/123597/2011 through COMPETE, FEDER and FCT (Lisbon). One author (S. Dasgupta) is supported by the ICTP UNESCO TRIL fellowship program. Two authors (C.A. Santos and F.A. Pereira) are supported by doctoral grants from FCT, reference SFRH/BD/80985/ 2011 and SFRH/BD/81259/2011.

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