Un Rivelatore a Tripla-GEM per le Camere a Muoni di LHCb M. Alfonsi1, G. Bencivenni1, W. Bonivento2, A. Cardini2 , P. de Simone1, F. Murtas1, D. Pinci3, M. Poli-Lener1, D. Raspino2 and B. Saitta2 Laboratori Nazionali di Frascati - INFN, Frascati , Italy Sezione INFN di Cagliari – Cagliari, Italy Sezione INFN di Roma 1, Roma, Italy

A. Cardini / INFN Cagliari M1R1 Rate Capability up to 0.5 MHz/cm2 Station Efficiency >96% in a 20 ns time window (*) Cluster Size <1.2 for a 10x25 mm2 pad size Radiation Hardness 1.6 C/cm2 in 10 years (**) Chamber active area 20x24 cm2 (*) A station is made of two detectors “in OR”. This improves time resolution and provides some redundancy (**) Estimated with 50 e-/particle at 184 kHz/cm2 with a gain of ~ 6000 CSN1, 5 aprile 2004 A. Cardini / INFN Cagliari

A. Cardini / INFN Cagliari A Triple-GEM detector is built by inserting three GEM foils between two planar electrodes, which act as the cathode and the anode. The volume between the cathode and the first GEM is usually referred to as the DRIFT gap. This is also the sensitive gap in the detector. Conversion & The TRANSFER gaps are the volumes between the GEMs. The volume between the last GEM and the anode is usually called the INDUCTION gap. The anode is segmented in pads to provide the spatial information and these electrodes are readout with charge/current preamplifiers. CSN1, 5 aprile 2004 A. Cardini / INFN Cagliari

A. Cardini / INFN Cagliari Large (20x24 cm2) GEM foils, divided in 6 sectors, are stretched with the tool shown above and then glued on frames. CSN1, 5 aprile 2004 A. Cardini / INFN Cagliari

A. Cardini / INFN Cagliari M1R1 Full Size Prototype ASDQ FEE Boards Sensitive gaps CSN1, 5 aprile 2004 A. Cardini / INFN Cagliari

A. Cardini / INFN Cagliari 9.7 ns 5.3 ns 4.5 ns Considerable improvement with respect to the Ar/CO2=70/30 gas mixture, which exhibits a poor time resolution of about 10 ns RMS, is obtained with the new CF4 and iso-C4H10 based gas mixtures, which allow to reach time resolutions better than 5 ns RMS Our Choice: Ar/CO2/CF4 45/15/40 Fast & Non-flammable Single Chamber Time Spectra CSN1, 5 aprile 2004 A. Cardini / INFN Cagliari

A. Cardini / INFN Cagliari Working region, upper limited by Cluster Size = 1.2, is found to be 70-80 V wide, a large plateau for a micro-pattern gaseous detector! OR Efficiency in 20 ns Working region 2.0 fC Cluster Size 3.0 fC G~4000 G~20000 CSN1, 5 aprile 2004 A. Cardini / INFN Cagliari

Discharge probability per incident particle Recenti misure suggeriscono che un rivelatore possa tollerare fino a circa 1000 scariche/cm2 P < 5.4 10-11 G < 2.7 104 CSN1, 5 aprile 2004 A. Cardini / INFN Cagliari

A. Cardini / INFN Cagliari Local Aging Test high intensity X-rays detector active area 10х10 cm2 irradiated area ~1mm2 Ar/CO2/CF4 (60/20/20) Ar/CO2/CF4 (45/15/40) Ar/CF4/C4H10 (68/25/7) ΣVGEM= 1230 V ΣVGEM= 1280 V ΣVGEM= 1030 V G=2.5·104 G=6·103 G=104 IG3_Dw+IPad ~270 nA IG3_Dw+IPad ~160 nA IG3_Dw+IPad ~160 nA Φgas=166.6 cc/min Φgas=200 cc/min Φgas=154 cc/min ΔG/G < 5% ΔG/G < 5% ΔG/G ~10% 10 LHCb Years 6.6 C/cm2 10 LHCb Years 1.8 C/cm2 10 LHCb Years 3 C/cm2 CSN1, 5 aprile 2004 A. Cardini / INFN Cagliari

Aging set-up at Casaccia PC 60Co 25 kCi GAS BOTTLES HV 4.5m GEM A,B,D GEM-C GEM-E Gas mixture:Ar/CO2/CF4 = 45/15/40; Large chamber A, B + small D in test position ~ 16 Gy/hour; Large chamber C in monitor position ~ 0.5 Gy/hour; Large chamber E in low irradiation position ~ 0.1-0.2 Gy/hour; HV(“reference”) = 1280 V, G ~ 6x103; H2O/T/ P = ± 1ppm / ± 0.1°K / ± 0.1mb (Panametrics/MKS probe systems) CSN1, 5 aprile 2004 A. Cardini / INFN Cagliari

A. Cardini / INFN Cagliari PSI Normalized Currents (%) Casaccia / Big Detector A / 16 Gy/h Casaccia / Big Detector B / 16 Gy/h Casaccia / Big Detector C / 0.5 Gy/h Casaccia / Small Detector D / 16 Gy/h X-Ray / Small Detector / Local Aging H2O injection CO2 problem 2 clearly different trends ! 11 LHCb years CSN1, 5 aprile 2004 A. Cardini / INFN Cagliari

A. Cardini / INFN Cagliari Casaccia / Big Detector A / 16 Gy/h Casaccia / Big Detector B / 16 Gy/h Casaccia / Big Detector C / 0.5 Gy/h Casaccia / Small Detector D / 16 Gy/h X-Ray / Small Detector / Local Aging Normalized Currents (%) G/G ~ 0 G/G ~ -10% for 0.15 C/cm2 Integration time: 3  35 days 1.2 LHCb years 2 clearly different trends ! CSN1, 5 aprile 2004 A. Cardini / INFN Cagliari

A. Cardini / INFN Cagliari @ Settembre 2003 Clarify aging mechanism: Physical/Chemical analysis of irradiated detectors Understanding different chamber behavior: Investigation on the Gas-flow effect Absolute Gain Measurement Gain Uniformity Gain/Efficiency Recovery by voltage increase: A gain recovery by a factor 2 has already been performed during the Casaccia test CSN1, 5 aprile 2004 A. Cardini / INFN Cagliari

W.Witzeling (Tech. Coord.) in the Zürich Sept. ’03 LHCb-week: “Ageing is a delicate issue in this region and recent ageing results on the GEM require further clarification before a final decision can be taken.” “The TB congratulates the GEM team to the results achieved and encourages them to pursue the work with the aim of coming to a positive decision in March 04.” CSN1, 5 aprile 2004 A. Cardini / INFN Cagliari

X-ray test : Uniformity (Pad Current) Chamber A (nA/pad) Chamber B (nA/pad) Good uniformity on the chambers CSN1, 5 aprile 2004 A. Cardini / INFN Cagliari

Aging induced by low gas flow We tried to reproduce the Casaccia test results, irradiating with X-rays a 10x10 cm2 chamber (total current ~2 mA on ~ 1 cm2 irradiation spot) flushed with a low gas flow (20 cc/min). A current drop of ~ 40% for a 0.55 C/cm2 integrated charge (~3 LHCb years) is found on the low gas flow measurement. NO current drop is observed on the high gas flow measurement. High gas flow Low gas flow CSN1, 5 aprile 2004 A. Cardini / INFN Cagliari

SEM analysis on Casaccia chambers No damage on gold-plated drift cathode and Pad PCB. They are perfectly clean. Fluorine found on G2 and G3 could be present as Cu-F compound, forming a thin insulating layer (no carbon deposits observed on the surfaces). G1: No fluorine No etching G2:Small fluorine,etching started G3:Large fluorine etching Ch A G1 down sample 5 Inner → 44 µm  Outer → 76 µm Ch A G2 down sample 9 Inner → 47 µm  Outer → 79 µm Ch A G3 down sample 11 Inner → 64÷65 µm  Outer → 77÷78 µm CSN1, 5 aprile 2004 A. Cardini / INFN Cagliari

SEM analysis on Casaccia chambers The fluorine strongly etched the third GEM, not only widening the copper holes but also etching the kapton inside hole (from the bottom to the top), changing the hole shape. The effective inner hole diameter, from the standard 45-50 mm becomes 60-65 mm. Cross section of the first GEM foil of the aged chamber A. Cross section of the third GEM foil of the aged chamber A. CSN1, 5 aprile 2004 A. Cardini / INFN Cagliari

Gain and rate capability on aged chambers Gain @ 1280 V A B Reference Xray LNF 6.103 Casaccia drop @ 15-20 MHz/cm2 -89% (11.5 Y) -80% (8.5 Y) X-Ray 1.6 MHz/cm2 -55% -32% Gain Rate Capability Casaccia B Casaccia A ~50% ~30% X-Ray LHCb Rate capability reduced due to charging down Chamber A current drop (~89%) ~ gain drop(~55%) + rate capability (~30%) CSN1, 5 aprile 2004 A. Cardini / INFN Cagliari

Beam test results on aged chambers After Casaccia, chambers A and B were tested at CERN PS-T11. Time and efficiency performances are similar to those found before Casaccia aging test, except for a 20-25 V shift in the working point, and practically unaffected working regions. 96% old new old new CSN1, 5 aprile 2004 A. Cardini / INFN Cagliari

A. Cardini / INFN Cagliari Conclusioni Casaccia results seem to be understood : the etching observed is correlated with bad gas flow rate condition; No corrosion effect observed on the cathode and anode; Etching effects mainly on the third gem with fluorine deposits near the copper holes edges; No aging occurs if the gas flow is properly set, as in the LHCb running condition; Detectors, even after a severe irradiation in bad conditions, exhibit good time and efficiency performances. CSN1, 5 aprile 2004 A. Cardini / INFN Cagliari

Conclusion of the Muon Group on the M1R1 Technology (Feb. ’04) “We are satisfied with the understanding of the ageing process in GEM detectors and recommend them to be adopted for M1R1.” Addendum-2 to the Muon TDR is under preparation CSN1, 5 aprile 2004 A. Cardini / INFN Cagliari

Technical Board on 25 February ’04 Final approval of the GEM chambers for Region 1 of the Muon Station 1 (Cagliari+LNF) “After the preliminary decision in September 03, more studies on ageing were performed, the effects seen in the Casaccia tests are now understood (gas flow). TB supported proposal by Muon group to adopt GEM chambers for the M1 R1 region.” (W. Witzeling) CSN1, 5 aprile 2004 A. Cardini / INFN Cagliari

A. Cardini / INFN Cagliari Cosa viene ora? Maggio 2004: programma dettagliato sulla costruzione e costi M1R2: ci è stato chiesto di studiare la fattibilità Ci sono indicazioni che sia una scelta “saggia” Problemi di manpower: necessari almeno 2 tecnici a tempo pieno Costi da valutare Sistema GAS da studiare … CSN1, 5 aprile 2004 A. Cardini / INFN Cagliari