Siena, 24 th May 2004 Aging Measurements on Triple-GEM detectors operated with CF4-based gas mixtures M. Alfonsi 1, G. Bencivenni 1, W. Bonivento 2, A.Cardini 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

Siena, 24 th May 2004M. Alfonsi, LNF-INFN2 OUTLINE  Introduction  Local Aging Tests (X-ray and hadrons at PSI)  Global Aging at Casaccia (1.25 MeV  )  Understanding of the aging process  Conclusions

Siena, 24 th May 2004M. Alfonsi, LNF-INFN3 Triple-GEM Detector (1) A Gas Electron Multiplier (F.Sauli, NIM A ) is made of 50  m thick kapton foil, copper clad on each side and perforated by an high surface-density of bi-conical channels. 70 µm140 µm 50 µm By applying a potential difference ( V) between the two copper sides, an electric field as high as 100 kV/cm is produced in the holes acting as multiplication channels for gaseous detectors. transfer amplification conversion and drift

Siena, 24 th May 2004M. Alfonsi, LNF-INFN4 Triple-GEM Detector (2) Conversion & Drift A Triple-GEM detector is built by inserting three GEM foils between two planar electrodes, which act as the drift cathode and the anode pad. The volume between the cathode and the first GEM (DRIFT gap) is the sensitive gap in the detector. Ionisation electrons drift through the four gaps towards the anode, they are multiplied passing through the three GEM foils and they induce signal on the readout PCB as they leave the third GEM (i.e. in the INDUCTION gap)

Siena, 24 th May 2004M. Alfonsi, LNF-INFN5 GEM detectors in LHCb In the innermost region of the first muon station (M1R1) of LHCb, the highest particle rate region of the Muon Subsystem, the experiment requires: Rate Capabilityup to 0.5 MHz/cm 2 Station Efficiency>96% in a 20 ns time window (*) Cluster Size<1.2 for a 10x25 mm 2 pad size Radiation Hardness1.8 C/cm 2 in 10 years (**) Chamber active area20x24 cm 2 (*) A station is made of two detectors “in OR”. This improves time resolution and provides someredundancy (**) Estimated with 50 e - /particle at 184 kHz/cm 2 with a gain of ~ The value quoted takes into account for both the charge integrated on the third GEM foil (1/2xQ) and the anode PADs (1/2xQ)

Siena, 24 th May 2004M. Alfonsi, LNF-INFN6 Why CF 4 -based gas mixtures? The intrinsic time spread of a GEM detector is  (t) = 1/nv drift, where n is the number of primary clusters per unit length and v drift is the electron drift velocity in the ionization gap. 9.7 ns5.3 ns 4.5 ns Single Chamber Time Spectra Considerable improvement with respect to the Ar/CO 2 =70/30 gas mixture, which exhibits a poor time resolution of about 10 ns RMS, is obtained with the new CF 4 and iso-C 4 H 10 based gas mixtures, which allow time resolutions better than 5 ns RMS Our Choice: Ar/CO 2 /CF 4 45/15/40 Fast & Non-flammable

Siena, 24 th May 2004M. Alfonsi, LNF-INFN7 Aging measurements: summary  Local Aging: performed with a high intensity 5.9 keV X-ray tube, irradiated area of about 1 mm 2 (about 50 GEM holes). Integrated charge 4 C/cm 2  25 LHCb years.  Large Area Aging: performed by means of the PSI  M1 positive hadron beam, with an intensity up to 300 MHz and an irradiated area of about 15 cm 2. Integrated charge 0.5 C/cm 2  3 LHCb years.  Global Aging: performed at Casaccia with a 25 kCi 60 Co source. Detectors were irradiated at 0.5  16 Gray/h. Integrated charge up to 2 C/cm 2  12.5 LHCb years.

Siena, 24 th May 2004M. Alfonsi, LNF-INFN8 Local Aging Measurements performed with an high intensity X-ray beam on 1 mm 2 (about 50 GEM holes) of small size (active area 10x10 cm 2 ) prototypes of the detector G = 2.5·10 4 I ~ 270 nA Ar/CO 2 /CF 4 (60/20/20) G = 6·10 3 I ~ 160nA Ar/CO 2 /CF 4 (45/15/40) G = 10 4 I ~ 160nA Ar/CF 4 /C 4 H 10 (68/25/7) 10 LHCb Years 6.6 C/cm 2  G/G < 5% 10 LHCb Years 1.8 C/cm 2  G/G < 5% Φ gas =166.6 cc/minΦ gas =200 cc/minΦ gas =154 cc/min 10 LHCb Years 3 C/cm 2  G/G ~ 10%

Siena, 24 th May 2004M. Alfonsi, LNF-INFN9 Large Area Aging with hadron beam At PSI (  M1) we exposed small size prototypes of detectors to a high intensity hadron beam (up to ~300 MHz on a spot size of about 15 cm 2 ) The beam was composed by low momentum (350 MeV/c) positive pions + ~7% proton contamination Gas mixture used: Ar/CO 2 /CF 4 (60/20/20), Ar/CO 2 /CF 4 (45/15/40), Ar/CF 4 /iso-C 4 H 10 (65/28/7), with a Φ gas =200 cc/min During the test we measured an average current on the anode of ~10 µA; With the chosen gas mixture, Ar/CO 2 /CF 4 (45/15/40), detectors integrated a charge of about 0.5 C/cm 2, equilavent to about 3 years of operation at LHCb New measurements have been performed on these detectors at T11/PS at CERN: NO LOSSES in performances (time resolution, working region width..) have been observed

Siena, 24 th May 2004M. Alfonsi, LNF-INFN10 Global Aging Test at ENEA-Casaccia: Set-up  A full size (20x24 cm 2 active area) prototype (C) in low irradiation position ~ 0.5 Gy/hour, TWO full size prototypes (A,B) and a smaller (10x10 cm 2 active area) prototype (D) in high irradiation position ~ 16 Gy/hour,  Gas mixture: Ar/CO 2 /CF 4 = 45/15/40, at reference Gain ~ 6x10 3  Monitored H 2 O (± 1ppm), T (± 0.1 ° K) in the gas line, and external P (± 0.1mbar)  Gas flows: C -> B -> A -> T/H 2 O Probe -> D -> Out initially Φ gas =200 cc/min, then Φ gas =350 cc/min  Stainless steel Gas pipes used, except for the exhaust and a short insertion to chamber inlet/outlet where polipropilene is used (proved to be radiation-hard and not hygroscopic). 4.5m Chamber A,B,D 60 Co 25 kCi Gas flow C -> B -> A -> D Chamber C

Siena, 24 th May 2004M. Alfonsi, LNF-INFN11 Gas mixture pollution B A D OFF i-UP Time (days ) Normalized Pad Current (a.u.) Gas flow During the whole test, high-irradiated chambers suffered a problem of gas mixture pollution due to the impossibility to increase the gas flow over 350cc/min. This was evident in the downstream chamber D, switching the upstream chambers B and A off. This effect causes a SYSTEMATIC DAMAGE on high-irradiated chambers!

Siena, 24 th May 2004M. Alfonsi, LNF-INFN12 Two clearly different trends! (1) 11 LHCb years 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 (%) PSI CO 2 problem H 2 O injection High-irradiated chambers exhibit a drastic current drop during the test: A  -89% after 11.5 y B  -80% after 8.5 y

Siena, 24 th May 2004M. Alfonsi, LNF-INFN13 Two clearly different trends! (2) Chamber C as well as X-ray and PSI test results show no current drops after an integrated charge of 0.15 C/cm 2  G/G ~ -10% after the same integrated charge ! Normalized Currents (%) The first 1.25 LHCb years 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 Integration time: 3  35 days

Siena, 24 th May 2004M. Alfonsi, LNF-INFN14 Preliminary Conclusions  A current drop was observed at Casaccia … but this was not observed in previous local X-ray (>10y) and PSI tests (~3y).  This systematic effect could be due to the fact that for the heavily irradiated detectors the gas flow was not increased proportionally with the irradiation rate  Nevertheless all tested chambers were still working after the irradiation  Several tests and checks to understand the aging process have been performed:  uniformity measurement  gain, charging-up, rate capability measurements with X-rays  performance of aged chambers at beam test  electron microscope analysis (SEM) of aged chambers  reproducing the low gas flow effect observed at Casaccia

Siena, 24 th May 2004M. Alfonsi, LNF-INFN15 Measurements on aged chambers: Pad current uniformity A B Uniformity 22% Uniformity 12% A remind... the chamber A was downstream in the gas line...

Siena, 24 th May 2004M. Alfonsi, LNF-INFN16 Gain and Rate Capability 1280 VAB Reference Xray LNF Casaccia MHz/cm 2 -89% (11.5 Y)-80% (8.5 Y) X-ray 1.6 MHz/cm 2 -55%-32% Gain Chamber A current drop (~89%) ≈ gain drop (~55%) + rate cap. loss at 15MHz/cm 2 (~30%) BUT no losses at the LHCb rate! ~50% LHCb XRay Casaccia B Rate Capability Casaccia A ~30% Rate (Mhz/cm 2 ) Pad current (nA)

Siena, 24 th May 2004M. Alfonsi, LNF-INFN17 Test beam results on aged chamber Before irradiation, performances of these detectors were measured at BTF- LNF Facility (500 MeV electron beam) [see P.Valente presentation] New measurements has been performed after aging, at T11-PS at CERN (*) : Chamber A (with the larger gain loss) shows only a shift of the working point of ~ 15V, without affecting working region width. This shift for chamber B is negligible, so performances of the “OR station” is practically unaffected. (*) Different T, p of the two tests has been taken into account Chamber AOR station ∑V GEM (V). ∑V GEM (V) 20ns efficiency.

Siena, 24 th May 2004M. Alfonsi, LNF-INFN18 Gem 3 down sampling SEM analysis on aged chambers (1)

Siena, 24 th May 2004M. Alfonsi, LNF-INFN19 SEM analysis on aged chambers (2) Ch A G1 down sample 5  Inner → 44 µm  Outer → 76 µm G1: No fluorine No etching - No damage on gold plated drift cathode and Pad PCB. They are perfectly clean. - Fluorine found only on the bottom side of G2 and G3; could be present as some Cu-F compound, forming a thin insulating layer (no carbon deposits observed on the surfaces). Ch A G3 down sample 11  Inner → 64÷65 µm  Outer → 77÷78 µm G3:Large fluorine etching enhanced Ch A G2 down sample 9  Inner → 47 µm  Outer → 79 µm G2:Small fluorine,etching started

Siena, 24 th May 2004M. Alfonsi, LNF-INFN20 SEM analysis on aged chambers (3) 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  m becomes  m. Cross section of the first GEM foil of the aged chamber A. Cross section of the third GEM foil of the aged chamber A.

Siena, 24 th May 2004M. Alfonsi, LNF-INFN21 3 rd GEM X-Ray Spectroscopy Fluorine (F - ions) was found only on the bottom side of the GEM foil and not on the upper side A strong Kapton etching inside the hole has been observed mainly on the 3 rd Gem. Cu-F compound layer around the hole, forming a thin insulating layer

Siena, 24 th May 2004M. Alfonsi, LNF-INFN22 Fluorine etching explains observed effects The effects of fluorine etching is twofold:  Widening of copper holes  Kapton etching, especially on the bottom part of the hole, and Cu-F compound forming an insulating layer near the hole gain reduction (*) Enhanced charging-up effects rate capability reduction (*) S.Bachmann et al., NIM A 438(1999),

Siena, 24 th May 2004M. Alfonsi, LNF-INFN23 High gas flow Low gas flow We tried to reproduce the Casaccia test results, irradiating with X-rays a 10x10 cm 2 chamber (total current ~2  A on ~ 1 cm 2 irradiation spot) flushed with a low gas flow (20 cc/min). A current drop of ~ 40% for a 0.55 C/cm 2 integrated charge (~3 LHCb years) is found on the low gas flow measurement. NO current drop is observed on the high gas flow measurement. Aging induced by low gas flow

Siena, 24 th May 2004M. Alfonsi, LNF-INFN24 Conclusions  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 occur if the gas flow is properly set, as in LHCb running condition;  Detectors, even after a severe irradiation in bad conditions, exhibit good time and efficiency performances.

Siena, 24 th May 2004M. Alfonsi, LNF-INFN25 Other slides...

Siena, 24 th May 2004M. Alfonsi, LNF-INFN26 Measurements on aged chambers: Pad current uniformity [1] Chamber A (nA/pad)Chamber B (nA/pad) Good uniformity on the chambers

Siena, 24 th May 2004M. Alfonsi, LNF-INFN27 Measurements on aged chambers: effective gain 1280 VAB Reference Xray LNF Casaccia MHz/cm 2 -89% (11.5 Y) -80% (8.5 Y) X-ray 1.6 MHz/cm 2 -55%-32% X-ray beam measurements show that aged chambers has a gain lower than the gain of a new chamber, but not so low as expected from the current drop during the test at ENEA-Casaccia But during the measurement photon rate is lower than the one of aging test. In fact..

Siena, 24 th May 2004M. Alfonsi, LNF-INFN28 Measurements on aged chambers: Rate Capability ~50% LHCb XRay Casaccia B Rate Capability Casaccia A ~30% Rate (Mhz/cm 2 ) Pad current (nA) Charging-up is a typical effect of micro-pattern detectors using insulating material as support. In a new GEM detector, charging-up of kapton inside GEM holes causes an increase of gain that last for few tens of minutes from start-up. On the contrary, aged detectors exhibit a decrease of gain (a modification of the internal kapton profile of the GEM holes?) Thus, losses in rate capability explain the current drop observed during aging test Chamber A current drop (~89%)  gain drop (~55%) + rate capability (~30%) BUT there are no losses at the LHCb rate!

Siena, 24 th May 2004M. Alfonsi, LNF-INFN29 Charging-up before and after aging Chamber A before aging.. Time (min) Time (s)..and after aging