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Radiation hardness and ageing properties of Small Gap
plus GEM Chambers
D. Bouvet, V. Chorowicz, D. Contardo, R. Haroutunian, L. Mirabito, S.
Perriès, G. Smadja
To cite this version:
of Small Gap plus GEM Chambers
D. Bouvet, V. Chorowicz, D. Contardo 1
, R. Haroutunian, L. Mirabito, S. Perries, G. Smadja,
Institut de PhysiqueNucleaire deLyon, IN2P3-CNRS et UniversiteClaude Bernard Lyon-I, France
Abstract
Large sizeSmall GapplusGEMChamberswereexposedto alowenergyhadron beamto characterize theirhardness to highly ionizingradiations.During the test, rate and e ect of discharges induced by HIPs were studied as a function of the cathode and GEM voltages. We propose an optimization of the voltages to safely operate thedetectors at thelargest signalto noiseratio.
In asecond test,theageing propertiesof thechamberswereestimatedup to a5 mC/cmintegrated charge, inducedbyan X-raybeam.
Weconcludefromthetwo measurementson thereliabilityandpossible improve-mentsoftheSmallGapplusGEMChambersforlongtermuseinaharshradiative environment.
1
As described in references [1{3] the small anode-cathode gap con guration of the MicroStrip Gas Chambers was demonstrated to avoid a substantial charging of the substrate at high particle ux. On the other hand, despite the passivation of the electrode edges, it was observed with small size SGCs thatHighlyIonizing Particlescan inducedischarges inthechambers, limiting the achievablegain [4{6] .Toimprove the radiationhardness, aGasElectron Multiplier[7] wasintroducedin the design of large size detectors ( g. 1).
Fig. 1.Geometry of theSGC+GEMchambers.
The substrates 2
were 300 m thick glasses (Desag D263) with 512 gold plated strips, 14cm long and 1 m thick. The passivation was realized with Benzo-Cyclo-Buthene. The strip pattern was wedge shaped as foreseen for the tracker end-cap detectors of the Compact Muon Solenoid experiment at LHC [9]. The pitch of the GEM holes was 200 m and their diameter was 70 m, a slightly conservative design with respect to the primary electron's transparency [8]. The drift gap was 3mm providing acollectiontime of 50ns and an eÆcient primary ionization of 36 e (with the Ne/DME 40/60 gas mixture). The transfer gap was 1.5 mm, a compromise between mechanical stability and the possibility to maintain the transfer and the drift voltages withinpracticalvalues.Toimprovethe mechanicalstability4insulating spac-ers were glued around the center of the chambers between the upper GEM and the drift plane.
2
The radiation hardness measurement was held at the Paul Scherrer Institut with the experimentalset-updescribed indetailsin reference[6]. As the data analysis was also performed according to the same procedure presented in this reference, we only remind here the main features of the experimental conditions :
-The6kHz/mm 2
beamof 350MeV pionswasused,shown by simulation[10] to produce a 1Hz/cm
2
rate of HIPs with an energy loss of more than 1 MeV inthe chambers.
-Thegas mixturewascomposedof 60%DMEand 40%Neon,found to opti-mizethe gain andthe timingpropertieswhilereducing the UV raysemission.
-The signalwasread-outattheanodesstripswithPREMUXchipsconsisting of a multiplexed array of 128 CR-RC ampli ers of 50 ns time constant and of 400 + 40/pF e equivalent noise [11]. The constant noise was checked in a measurement without chamber. Then, with the 14cm strips connected, the noise was 970 e ,showing a 1pF/cm capacitance.
Fig.2.DetectioneÆciencyofSGC+GEMasafunctionofthesignalto noiseratio.
At the PSI, the chambers were exposed to the pion beam for 22 days at Vk=V
GEM
=-360Vwithequaltransferanddrift eldsof5kV/cm.The trans-fer eld,de nedastheGEMdownminuscathodevoltageovergap,waschosen toavoidtransfer discharges. Then,the drift eld was set toan equalvalue to preserve saturation ofthe primary electron velocity while maintaininga good transparency of the GEM as it can be seen from the pulse shape shown in g. 3.
0
10
20
30
40
50
60
70
80
90
100
-40
-20
0
20
40
60
80
100
120
t (ns)
Pulse amplitude (arbitrary units)
Ed=5kV/cm Vg=360V Et=6kV/cm Vk=360V
Ed=6kV/cm Vg=360V Et=6kV/cm Vk=360V
Ed=5kV/cm Vg=360V Et=5kV/cm Vk=360V
Ed=6kV/cm Vg=360V Et=5kV/cm Vk=360V
Fig.3.Meanpulseshapesforvariousdriftandtransfer elds(seetextforde nition of thetransfer eld).
Undertheseconditions,the gain,monitoredwiththetotalpowersupplies cur-rent,wasfoundtobestableatavalueof2800300( g.4)andthemaximum probability signal to noise ratio on the strip collecting the largest fraction of the signal was 30, well above the value of 12 at the start of the eÆciency plateau( g.2).Usingthe read-out noisevalueof 970e forcalibration,itcan bechecked thatthe measured currentand theread-out signalare compatible.
In 22 days, 1 short circuit occurred out of 2410 cathode discharges and no GEM discharges were observed. The cathode dischargerate was2 10
8 cm
1 s
1
0
500
1000
1500
2000
2500
3000
3500
4000
0
2.5
5
7.5
10
12.5
15
17.5
20
22.5
Time (days)
Gain
Ne/DME (40/60)
E
T
= E
D
= 5kV/cm
V
K
= -360V,
∆V
Gem
= -360V
Fig. 4.Stabilityof themeasuredgainduringtheHIP test.
After the long stable running period, 2 voltage scans were performed : one with the cathode voltage, at a xed V
GEM
=-360V, and the other with the GEM voltage, at a xed V
k
=-360V. No GEM discharges were observed and the cathode dischargerates werefound inboth scanstofollowanexponential law with the gain, as for the SGCs without GEMs ( g. 5). The advantage with the GEM was again demonstrated by the lower discharge rate, using ampli cation at this stage rather than at the cathode level. As it can be seen in gure 5, a signal to noise ratio of 50 was reached at V
k
=-360V and V
GEM
=-380V, without increasing the cathode discharge rate as compared toV
k =V
GEM
=-360V.A cathode voltage lower than360V, compensated by an equivalent increase on the GEM, leading to a similar gain ( g. 6), should probably bean even better optimization ofthe operatingconditions.
10
-8
10
-7
10
-6
10
-5
10
-4
10
-3
S/N
Discharge rate (s
-1
cm
-1
)
Ne/DME (40/60)
20
30
40
50
2
3
4
5
6
7 8 9 10
15
Gain (x10
3
)
Cathode scan
GEM scan
Fig.5.Discharge ratesasafunctionof gainand signalto noiseratio averaged over all thechambers.
3 Long term ageing
The longterm ageingwas estimatedby thevariationof thetotal current asa function of the integrated charge per centimeter of strip. 4 chambers, 2 with GEMs and 2 without, were exposed for 2 months to an X-ray beam of 6.4 keV. The same NE/DME gas mixture as mentioned above was used and the collimationof the beam was done with 3 di erent slits of 25 mm
2
, 100 mm 2 and9cm
2
.Inallcases,asimilargainlossof25%wasobserved after5mC/cm of integrated charge, equivalentto a MIP ux of 5kHz/mm
2
700
800
900
1000
2000
310
320
330
340
350
360
370
380
390
V
Gain
Scan V
K
(
∆V
Gem
= -370V)
Scan
∆V
Gem
(V
K
= -370V)
E
T
= E
D
= 5kV/cm
Fig.6.SGC+GEMgain asa functionofcathode andGEM voltages.
0
5
10
15
20
25
30
35
40
45
0
20
40
60
80
100
Mean charge (nC)
Ne/DME (40/60)
E
T
= E
D
= 5 kV/cm
V
K
= -360 V
∆V
gem
= -360 V
〈Q 〉
C = = 1 pF/cm
8 .V
K
.L
Fig. 7.Chargedistribution ofdischarges.
4 Conclusions
Large size Small Gap chambers equipped with GEMs have been thoroughly characterized for long term use ina harsh HIP environment.
It was found that conditioning the chambers at high cathode and GEM volt-ages is a useful hardening procedure to avoid short circuits during the beam operation.
show-0
0.5
1
1.5
2
2.5
3
0
1000
2000
3000
4000
5000
6000
7000
8000
Number of sparks
Number of lost strips
SGC
SGC+GEM
Fig. 8. Number of lost stripsas a functionof the number of sparks normalized to thenumberofchambers.
0
0.2
0.4
0.6
0.8
1
1.2
0
1
2
3
4
5
6
mC/cm
gain (a.u)
SGC without GEM
Ne/DME (40/60)
V
D
= -3000V V
K
= -360V
Φ
X
# 8 kHz.mm
-2
Surface
25 mm
2
100 mm
2
0
0.2
0.4
0.6
0.8
1
1.2
0
1
2
3
4
5
6
mC/cm
gain (a.u)
SGC+GEM
Ne/DME (40/60)
V
D
= -2970V V
K
= -360V
∆
Gem
= -360V
Φ
X
# 8 kHz.mm
-2
Surface
9 cm
2
1 cm
2
Fig.9.Gainlossasafunctionofintegratedcharge.Thedatawerenotcorrectedfor temperatureand pressurevariations.
ingthat this phenomenon depends onthe cathode voltageratherthan onthe total charge collected.
valueof12atthestartoftheeÆciencyplateau,allowstoconsidershorter nal shaping times, as obtained for instance with the pulse processing discussed in[12].
Thechamberageingwasfoundtobelimitedto25%per5mC/cmofintegrated charge, which willnot a ect the detection eÆciency atthe allowed gain.
Finally, although the major parameter of the discharge rate is probably the chamber substrate print quality, it was observed that the drawback of the smallgap chambers as compared to the standard microstrip chambers liesin the strip capacitanceincrease from0.5pF/cmto1 pF/cmdue tothe smaller anode-cathode gap. It signi cantly increases the noise with the present read-out electronics and it is also a very sensitive parameter of the discharges e ect, leading to short circuits rather than cuts as observed with the 14cm stripsdetectors.A naloptimizationofthemicrostripchamberanode-cathode gap shouldtakeinto accountthe balance between this e ect andthe possible charging of the substrate at high ux.
References
[1]F.Angelinietal., Nucl.Instr.and Meth. A335(1993) 69
[2]J.F.Clergeau etal,Nucl.Instr. and Meth.A392 (1997)140
[3]V.Chorowicz etal.,Nucl.Instr.and Meth. A401(1997) 238
[4]B.Schmidt,Nucl.Instr.and Meth. A419(1998) 230
[5]V.Chorowicz etal.,Nucl.Instr.and Meth. A419(1998) 464
[6]D.Bouvetetal., Nucl.Instr.and Meth. A454(2000) 359
[7]F.Sauli, Nucl.Instr.and Meth. A386(1997) 531
[8]R.Bellazzinietal.,Nucl. Instr.and Meth.A419 (1998)429
[9]TheCMS Collaboration,Technical Design Report, CERN/LHCC98-6(1998)
[10]M. HutinenCMSNote 1997/073
[11]N.Bacchetta etal.,CMSNote 1995/164
