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Behaviour of Small Gap plus GEM chambers in close
LHC conditions
D. Bouvet, V. Chorowicz, D. Contardo, R. Haroutunian, L. Mirabito, S.
Perriès, G. Smadja
To cite this version:
LHC conditions
D. Bouvet 1
, V. Chorowicz, D. Contardo, R. Haroutunian, L. Mirabito, S. Perries, G. Smadja
Institut de PhysiqueNucleairede Lyon,
43bd du 11 Novembre 1918 69622 Villeurbanne, France
SmallGap+GEMChambersdeveloppedfortheCMSexperiment at theLHC,were exposedto a pionbeamof350 MeV at thePaul ScherrerInstitutto establishtheirradiationhardnesswithrespect to theoperatinggain.The longtermstabilityof thegain(ageing) was thenmeasuredusingan X-raybeam.
PACS:29.40.Cs, 29.40.Gx, 51.50.+v
Keywords:MicroStrip Gas Chamber,Gas Electron Multiplier
1
Uptoend 1999, the CMS (CompactMuon Solenoid) experiment atLHCwas planingtouseMSGCsonasurfaceofabout200m
2
initsinnertracker[1].The harshexpectedenvironmentatthe10
34 cm
2 s
1
luminosityhasledtoa thor-ough studyof the radiationhardness and longterm stabilityof thedetectors. To measure the capability of SGC+GEM to whistand such an environment, 8 detectors were exposed to a 6 kHz.mm
2
pion beam of 350 MeV at the PaulScherrerInstitut.Thelong termageingtestswere performedwith a
55 Fe X-ray source of 6.4keV during2 months.
In 1996 we propose the Small Gap Chambers (SGC) as a variant of Micro-StripGapChambers[2]. Inthisdesign,theanode-cathodedistanceisreduced toavoidthe chargingofthe substrateunderparticle ux,and theedgesofthe strips are passivated to allow suÆcient gain without sparking phenomenon. We present, in the following, the results and the improvement obtained with a GEM on top of large size substrates, as compared to previous results from smallsize SGCs [3].
2 Radiation hardness
2.1 Experimental set-up
8 detectors with 512 gold strips of 14 cm long 2
were exposed to the -beam at the PSI. As compared tothe smallsize chambers, the electronic noise was increasedfrom750e to1700e duetotheadditionalcapacitance.Inorderto compensate this e ect by a gain increase, a GEM (Gas Electron Multiplier) was added to the SGC as shown in gure 1, with the GEM-cathode gap equalto1.5 mm. The GEMampli cation structure consistsof athin Kapton foil plated with copper on both sides across which a potential di erence is applied[4].
Fig.1.a) SGC+GEMmechanics,b) Electric eldina GEMhole.
Duringthetest,thedriftvolumewas ushedwithagasmixtureof60%DME and 40 % Ne. Premux chips of 128 multiplexed RC-CR ampli ers of 50 ns 2
data included 2 msADC samplings of the cathode, drift and GEM currents, a monitoringof the beam intensity and ameasurementof the anode signal.
2.2 Data analysis
The radiation hardness is evaluated by measuring the spark rate and the number ofdead channels. Accordingtothe LHC speci cations, less than 8% of the strips must be cut in 500 days of LHC running. The 8 detectors were exposed tothe 6-7kHz.mm
2
pionbeamduring22days ata cathode voltage of -360 V and V
GEM
=-360 V. The gain, continuously monitored from the current measurement, was 2000 300 ( g. 2) and corresponded to a signal to noise ratio S/N ' 30 (1.5 times the minimal signal to noise ratio for full eÆciency [5]).
0
500
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3500
0
2.5
5
7.5
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22.5
Time (days)
Gain
Fig.2.Gainstabilityat H.I.
Afterthislongstablerunning,2scanswereperformedtoincreasethedetectors gain. One was done at a xed V
GEM
= -360 V. The measured spark rates of the various chambers were contained in a cone (plain lines on gure 3) at least 1 order of magnitude lowerat the same S/N, as compared to chambers without GEM. The second scan was performed with the V
GEM
, at a xed V
K
= -360 V,andde neasecondcone(dottedlines).Takingthemeanvalues of the cones, it showed that the spark rate can be further reduce by a factor 100 at the same S/N.
10
-10
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-9
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-8
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-7
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-6
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-5
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-4
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-3
S/N
Spark rate (s
-1
cm
-1
)
Ne/DME (40/60)
15
30
40
50
1000
2000
3000
G big size
1000
2000
3000
G small size
cathode scan
without GEM
GEM scan
Fig. 3. Spark rate as a function of the signal to noise ratio and corresponding gain. Cathode scan (dashed line) of the chambers withoutGEM [3].Cathode scan with V
GEM
=-360V. GEM scan with V K =-360V (cross symbols).(At S/N=30: V K =-360V, V GEM =-360V)
pessimistic assumption if there is a saturation of the loss as observed for the SGCs butnot measured here by lackof time. Withsuch anextrapolation,we found thatdetectors willloseabout2%of theirstrips duringthe 500 daysat high intensity.
0
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Measured limit at 95%
confidence level
Number of sparks
Number of cuts
SGC
SGC+GEM
Fortheageingmeasurement,thegainlosswasestimatedbythevariationofthe total current, measured as afunction of the integrated charge percentimeter of strips. Long term tests were performed with SGCs of two sizes with and without GEM, exposed to a
55
Fe X-ray source of 6.4 keV. All the currents were registered with the dark current substracted regularly. To ensure the uniformityofthe ux, awindowsmallerthanthebeamspotwas xedinfront of the detector. Three sizes of irradiated area were used : 25 mm
2 , 100 mm 2 and 9 cm 2 .
25 mm
2
100 mm
2
Ne/DME (40/60)
V
D
= -3000 V V
K
= -360V
Φ
X
# 8 kHz.mm
-2
Surface
↑
10 years LHC
Single SGC (small size)
Fig. 5.Ageingof singleSGC(left) and SGC+GEM(right)
Ne/DME (40/60)
V
D
= -2970 V V
K
= -360 V
Φ
X
# 5 kHz.mm
-2
Surface
9 cm
2
SGC+GEM (big size)
↑
10 years LHC
Figure 5 shows the results obtained after 2 months of exposure. The same behaviour was observed with and without GEM. The loss of gain was found tobeless than 25% after5 mC/cmequivalent tothe 500 days of runningat LHC.
4 Conclusions
Theageingresults showed thatthe lossof gainshouldbeless than 25%in10 years of LHC operation,well belowthe establishedfactor 2margin.
References
[1]TheCMS Collaboration, Technical Design Report, CERN/LHCC98-6(1998) [2]J.F.Clergeau et al.,Nucl.Instr.and Meth. A 392 (1997) 140.
[3]D.Bouvetet al.,Nucl.Instr.and Meth. A 454(2000) 359. [4]F. Sauli,Nucl.Instr.and Meth. A 386 (1997) 531.
