1. M.Alfonsi 1, G. Bencivenni 1, W. Bonivento 2,A.Cardini 2,C. Deplano 2, P. de Simone 1, F.Murtas 1, D.Pinci 3, M. Poli-Lener 1, D. Raspino 2 and B.Saitta 2 1 - Laboratori Nazionali di Frascati - INFN, Frascati, Italy 2 - Sezione INFN di Cagliari – Cagliari, Italy 3 – Sezione INFN di Roma 1, Roma, Italy Aging Test of Triple-GEM at Casaccia

2. PC 60 Co BOTTLES HV CONTROL ROOM 4.5 m Casaccia Aging Test set-up GEM A,B,D P  Gas mixture:Ar/CO 2 /CF 4 = 45/15/40;  Large chamber A,B + small D in test position ~ 16 Gray/hour;  Large chamber C in monitor position ~ 0.5 Gray/hour;  Large chamber E in low irradiation position ~ 0.1-0.2 Gray/hour;  HV(“reference”) = 1280 V, G ~ 6x10 3 ;  H 2 O/T/ P = ± 1ppm / ± 0.1 ° K / ± 0.1mb (Panametrics/MKS probe systems) GEM-C GEM-E

3. Triple-GEM detectors “A, B”: 20x24 cm 2 detectors (module-0) tested in may at BTF-LNF; 100 k  embedded resistors mounted on sectors of the up-side of each GEM foil; test in current mode. “C” : 20x24 cm 2 detector (module-0) tested in october at T11-CERN; 1M  embedded resistors mounted on sectors of the up-side of each GEM foil; test in current mode. “D” : 10x10 cm 2 detector; 100 k  external resistors mounted on each electrode; test in current mode. “E” : 20x24 cm 2 detector (old-style) tested at PSI-2002; efficiency measurement under global irradiation ( ~ 0.1-0.2 MHz/cm 2 ) with ASDQ-FEE.

4. Gas mixture supply & P,T,RH monitoring The gas mixture, Ar/CO 2 /CF 4 = 45/15/40, was supplied to chambers in series/open mode; the gas flow was initially set at 200 cc/min (0<t(days)<19.5) then increased at 340 cc/min (from t(days)>19.5); Gas pipes: stainless-steel from MKS-mixing system to chambers C, B, A, D, then we used polypropilene tube from D to E and from E to exhaust (polypropilene is proved to be rad-hard and it is not hygroscopic); The order of the chambers in the gas line was: C – B – A – D - E A humidity+temperature probe (Panametrics) is placed on the gas line between A and D chambers; the probe allows a direct measurement of the humidity and temperature of the gas (at a level of a ± 1ppm / ± 0.1 ° K) at the output of the High Irradiated chambers; the probe has been placed in a safe position and shielded with 10 cm thick lead bricks; A pressure probe is placed near the MKS system to monitor the atmospheric pressure

5. exhaust MKS mixer Ar CO 2 CF 4 C B A D E CO 60 not in scale T, RH probe P atm probe Polypropilene-gas pipes Stainless steel-gas pipes Gas mixture supply & P,T,RH monitoring

6. We used the knowledge of dependence of the gain on V(t)*T(t)/P(t) to correct and normalized the detector current T ( o K) P (mb) T/P ( o K mb -1 ) Time (days) Temperature and Pressure in the gas mixture

7. Time (days) Water content (ppm) Water in the gas mixture intrinsic humidity humidity artificially injected

8. Gas mixture pollution under strong irradiation B A D Time (days) Currents (  A) Gas flow OFF i-UP

9. Currents vs Time (I) 1280V1225V 1280V 1320V 1225V 200 cc/min 340 cc/min B A Currents vs Time for chamber A and B, normalized in VT/P CO 2 crisis & successive gas bottle change observed also on MWPC High rate

10. Currents vs Time for chamber C and D, normalized in VT/P 1280V 1225V 200 cc/min 340 cc/min C D Currents vs Time (II) CO 2 crisis & successive gas bottle change observed also on MWPC High rate Low rate

11. B A H2OH2O Currents behaviour becomes flat when water concentration drops down to few ppm Currents & Humidity (I) Currents (  A) Water (ppm) Time (days) B A

12. Currents (  A) Water (ppm) Currents & Humidity (II) NO effect of humidity is observed in the low irradiated chamber ( ~ 1MHz/cm 2 ) …

13. Current drop: what might be the mechanism? To evaluate the gain we measure the anode current, which, for uniform irradiation, is proportional to the gain averaged all over the detector active area  How can the current decrease?  Global gain reduction  Local detector modification  What might be the cause? Very likely HF production due to H 2 0 presence and probably strongly accelerated by the very high chambers irradiation (no effects in low irradiated chambers) … but what is the maximum amount of water one can have? 100 ppm? 10 ppm? 1 ppm?

14. Global gain reduction ? R GEM decrease: it was observed, after PSI discharge test, that each GEM had decreased its resistance by a factor as large as 100 (without affecting detector performances)– we believe due to a “carbonization” of the GEM holes. A small GEM resistance could change the GEM working point due to the HV resistor on the GEM supply. This effect was NOT observed at Casaccia, where GEM resistance had varied by ~5 in about 3 LHCb years, so this effect seems to be related only with sparks… R HV increase due to radiation: this also would have changed the GEM working point, but this effect also was NOT observed, HV resistances, as expected, were always @ 100 k  ; GEM holes modifications: if HF is produced and copper is etched away near the holes this would change the electric fields and by consequence the gain. One parameter easily measurable – the electron extraction efficiency of the last GEM – does not depend on gain, but should be affected by geometry modification. However this parameter does NOT vary much (max. ± 2%) during all the period, so it is difficult to believe that GEM geometrical (global) properties might have changed substantially

15. Other “local” aging test on GEM We have performed, in the past years, other aging tests with triple- GEM detectors: NO STRONG aging effects were ever found. X-ray test @ LNF: we integrated, in a region of ~ 1 cm 2, 10 LHCb year with a  G/G < 5% (If you don’t believe in local aging, please note that aging test of gaseous detectors is usually done in this way, and aging effects are FOUND!) PSI 2002 spark test: the hadron beam intensity was 300 MHz in a ~15 cm 2 region at the detector center. This means that the central region integrated, in 10 days, about 400 LHCb days. NO gain drop was found, and detector time performances were measured to be the expected ones in a subsequent test performed with the same detector at CERN PS.

16. Integrated Charge At the end of the test (july 14) we estimate the following integrated charge: CHAMBER A: ~ 2 C/cm 2 equivalent to ~ 5.4 LHCb years (+ ~1.5 y)* CHAMBER B: ~ 1.5 C/cm 2 equivalent to ~ 4.0 LHCb years (+ ~0.5 y)* CHAMBER D: ~ 0.6 C/cm 2 equivalent to ~ 1.6 LHCb years (+ ~0.5 y)* CHAMBER C: ~ 0.1 C/cm 2 equivalent to ~ 0.3 LHCb years * = 1 additional week of irradiation (july 31-august 7)

17. Some preliminary conclusions  Current drop was observed at Casaccia … but this was not observed in previous local X-ray (>10y) and PSI tests (~4y);  Current decrease seems to be much reduced (or even stopped) when water content in gas is below a few ppm;  Current drop mechanism is not yet understood;  Further investigations are needed:  to confirm the observed current stability at low H 2 O concentration, we would like to continue the irradiation test for another week; the addition of a new “dry” chamber could confirm that the current drop is due to the presence of water in the gas mixture;  open one of the Casaccia detectors and see what GEM foils look like;  measure absolute gain of detectors;  measure efficiency of detectors;

18. Local detector modification ? We had observed, on the GEM foils which were used for the PSI discharge test, that an area close to the gas input changed color, and the copper surface looked like “heated metal”. At the optical microscope we saw this The left part of the GEM was not in front of the pads due to a small shift, and looks like a new GEM. The right part looks “strange”…, but the detector performances are not affected by this. We do not have an explanation of this. Proton Induced X-ray Emission test we performed at INFN Firenze suggest that the change in color is not due to fluorine-based deposit. However this is a local GEM modification that we should keep in mind…

27. Large Area Aging test at Casaccia

28. After T/P normalization the GAIN of large chamber A, B show a flat behaviour. Residual shifts seem to be correlated with humidity in the gas mixture (especially at the start of the measurement). Normalized gain vs day for chamber A and B Normalized gain vs equivalent LHCb year (in M1R1) for chamber A and B Very Preliminary