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
1.
Laboratori Nazionali di Frascati - INFN, Frascati , Italy
2.
Sezione INFN di Cagliari – Cagliari, Italy
3.
Sezione INFN di Roma 1, Roma, Italy
M1R1
 Rate Capability
 Station Efficiency
 Cluster Size
 Radiation Hardness
 Chamber active area
up to 0.5 MHz/cm2
>96% in a 20 ns time window (*)
<1.2 for a 10x25 mm2 pad size
1.6 C/cm2 in 10 years (**)
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
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A. Cardini / INFN Cagliari
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A Triple-GEM detector is
built by inserting three GEM
foils between two planar
electrodes, which act as the
cathode and the anode.
Conversion &
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.
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.
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Large (20x24 cm2) GEM foils, divided in 6
sectors, are stretched with the tool shown
above and then glued on frames.
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M1R1 Full Size Prototype
ASDQ FEE
Boards
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Sensitive gaps
A. Cardini / INFN Cagliari
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9.7 ns
4.5 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
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OR Efficiency in 20 ns
Working region, upper limited by Cluster Size = 1.2, is
found to be 70-80 V wide, a large plateau for a micropattern gaseous detector!
Working
region
Cluster Size
2.0 fC
3.0 fC
G~4000
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G~20000
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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
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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)
Φgas=166.6 cc/min
ΔG/G < 5%
Φgas=200 cc/min
ΔG/G < 5%
Φgas=154 cc/min
ΔG/G ~10%
10 LHCb
Years
6.6 C/cm2
10 LHCb
Years
1.8 C/cm2
ΣVGEM= 1230 V
G=2.5·104
IG3_Dw+IPad ~270 nA
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ΣVGEM= 1280 V
G=6·103
IG3_Dw+IPad ~160 nA
A. Cardini / INFN Cagliari
ΣVGEM= 1030 V
G=104
IG3_Dw+IPad ~160 nA
10 LHCb
Years
3 C/cm2
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 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;
GEM-E
GEM
A,B,D
PC
 HV(“reference”) = 1280 V, G ~ 6x103;
 H2O/T/ P = ± 1ppm / ± 0.1°K / ± 0.1mb
(Panametrics/MKS probe systems)
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GEM-C
A. Cardini / INFN Cagliari
HV
4.5m
 Gas mixture:Ar/CO2/CF4 = 45/15/40;
GAS BOTTLES
Aging set-up at Casaccia
60Co
25 kCi
10
Normalized Currents (%)
PSI
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 !
CSN1, 5 aprile 2004
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11 LHCb years
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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
G/G ~ 0
G/G ~ -10%
for 0.15 C/cm2
Integration time: 3  35 days
2 clearly different trends !
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A. Cardini / INFN Cagliari
1.2 LHCb years
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@ 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
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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.”
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X-ray test : Uniformity (Pad Current)
Chamber A (nA/pad)
Chamber B (nA/pad)
Good uniformity on the chambers
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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.
High gas flow
Low gas flow
NO current drop is observed
on the high gas flow
measurement.
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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
Ch A G1 down sample 5
 Inner → 44 µm
 Outer → 76 µm
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G2:Small fluorine,etching started
Ch A G2 down sample 9
 Inner → 47 µm
 Outer → 79 µm
A. Cardini / INFN Cagliari
G3:Large fluorine etching
Ch A G3 down sample 11
 Inner → 64÷65 µm
 Outer → 77÷78 µm
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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.
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Cross section of the third GEM foil
of the aged chamber A.
A. Cardini / INFN Cagliari
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Gain and rate capability on aged chambers
Gain @ 1280 V
A
B
6.103
6.103
-89% (11.5 Y)
-80% (8.5 Y)
-55%
-32%
Reference Xray LNF
Gain
Casaccia drop @ 15-20 MHz/cm2
X-Ray
1.6 MHz/cm2
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%)
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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.
old new
old new
96%
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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.
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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
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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)
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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
– …
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