M. ALFONSI, G. BENCIVENNI, P. DE SIMONE, F.MURTAS, M. POLI ...pinci/Proceedings/como-03-raspino.pdf · October 26, 2003 10:9 WSPC/Trim Size: 9in x 6in for Proceedings fast3gem OPERATION

October 26, 2003 10:9 WSPC/Trim Size: 9in x 6in for Proceedings fast3gem

OPERATION OF TRIPLE-GEM DETECTORS WITH FAST

GAS MIXTURES

M. ALFONSI, G. BENCIVENNI, P. DE SIMONE, F.MURTAS,

M. POLI LENER

Laboratori Nazionali di Frascati - INFN, Frascati, Italy

D. PINCI

Sezione INFN Roma1, INFN, Roma, Italy

W. BONIVENTO, A. CARDINI, C. DEPLANO, D. RASPINO, B. SAITTA

Sezione INFN Cagliari, INFN, Cagliari, Italy

S. BACCARO

ENEA-Casaccia, S.Maria di Galeria, Roma, Italy

Adding CF4 to the standard Ar/CO2 gas mixture allows to substantially improvethe time resolution of triple-GEM detectors.In this paper we discuss further improvements in time resolution obtained bothoptimising the above mentioned ternary gas mixture and using a new gas mixturewith isobutane as quencher, below the flammability limit.Moreover, measurements of discharge probability per incident charged particle witha high intensity pion/proton beam for these gas mixtures are presented.We show that, if the detector were to be used in the harsh environment of theLHCb experiment, for which it was primarily conceived, only the new gas mixturesconsidered in this paper would allow a reasonable working region around nominaloperating voltage. Some results on ageing studies are also reported.

1. Introduction

In a previous paper 1 we discussed the performance of a 10×10 cm2 triple-

GEM 2 detector for high rate charged particle triggering applications. The

experimental tests were performed in the framework of an R&D for the

inner part of the first station of the LHCb muon chambers 3. The results

contained in that paper showed that a considerable gain in time resolution

with respect to previous studies could be obtained modifying the standard

1


2

gas mixture Ar/CO2 (70/30) into Ar/CO2/CF4 (60/20/20).

These results were however only about time resolution and did not include

any quantitative study of discharge probability or ageing.

In this publication we present further improvements in time resolution ob-

tained both optimising the above mentioned ternary gas mixture and using

a new gas mixture with isobutane as quencher, below the flammability limit.

We also present measurements of discharge probability per incident charged

particle with a high intensity pion/proton beam; these measurements are

needed to know whether the detector can survive 10 years of operation in

the LHCb experiment or not.

In section 2 we remind our physics requirements and the ideas behind the

choice of gas mixtures and detector configuration. In section 3 the mea-

surement of effective gain and in section 4 efficiency and pad cluster size

maeasurements are discussed. In section 5 we present discharge probability

per incident charged particle measurements.

2. Detector and gas mixture optimisation

We remind the reader here that the efficiency requirements for detector

operation in LHCb, inner region of the first muon station are of 96 % ef-

ficiency in 20 ns time window with 1 × 2.5 cm2 pad size and an average

number of pads over threshold per incident charged particle, that we call

pad cluster size, of less than 1.25 in a 25 ns time window, corresponding to

one LHC bunch crossing time. We consider a muon station as made of two

triple-GEM detectors in OR. Moreover, the detector should survive up to

10 years with a charged particle rate of 180 kHz/cm2.

For the sake of clarity, we report here a schematic drawing of the triple-

GEM detector, Fig. 1, where the naming convention of this paper is also

introduced. The voltage differences across the various GEM foils are called

Ugem1, Ugem2 and Ugem3 and their sum U totgem.

The intrinsic time resolution of a triple-GEM detector for incident charged

particles depends on two factors 1: the resolution on the arrival time of pri-

mary clusters on the first GEM, σt, and the single electron sensitivity, i.e.

the probability of triggering on the signal corresponding to one ionisation

electron. The former is given by σt = 1/nvdrift, where n is the number of

clusters per unit length and vdrift is the electron drift velocity, both terms

depending on the choice of gas mixture; the latter depends on detector

configuration, i.e GEM fields (gas gain), GEM to GEM fields(transparency


3

E

E

E

cathode

pads

d

t

i

GEM1GEM2GEM3

d

t

g

g

gi

Figure 1. Cross-section of the triple GEM detector. Ed, Et and Ei are the drift,transfer and induction fields, respectively; gd, gt, and gi are the drift, transferand induction gaps, respectively.

of GEM foils to drifting electrons) and, with fast electronics, on detector

geometry (signal height is proportional to gi size).

The occurence of discharges, i.e. breakdown of gas rigidity, in gas detectors

is correlated with the transition from avalanche to streamer occurring when

the primary avalanche size exceeds few 107 ion-electron pairs 4, the so called

Reather limit. In GEM detectors, due to very small distance between the

two sides of the GEM foil, streamer formation can be easily followed by

a discharge. This effect can be minimised by both adding a quencher to

the gas mixture, whose quantity and type are however limited by detector

ageing, and optimising the detector configuration in order to benefit from

the diffusion effect which spreads the charge over more holes.

The above mentioned requirements lead us to select two new gas mixtures

improving both σt and quenching properties: Ar/CO2/CF4 (45/15/40)

and Ar/CF4/C4H10 (65/28/7). The isobutane content was kept below the

flammability limity of the gas mixture.

Fig. 2 shows the resolution on the arrival time of primary clusters on the

first GEM, σt, vs. electric field as calculated with Magbolz and Heed pro-

grams for the four gas mixtures we tested and shows that σt properties at

low fields are better for the new ones.

3. Measurement of effective gain

Effective gain vs. U totgem measurements were performed with a 5.9 keV X-ray

tube1. Effective gain values were obtained from the ratio of pad current

with high voltage on the GEM foils, to current on the first GEM, with no

high voltage on the GEM foils and are shown in figure 2 (b).

The gain behaviour at a given U totgem value seems to follow the Argon con-

tent of the gas mixture, with the exception of the larger gain of the gas


4

Figure 2. (a) Calculated resolution on the arrival time of primary clusters on thefirst GEM vs. electric field for the four gas mixtures tested by our group. (b)Measured effective gain vs. U tot

gem.

mixture with isobutane. The larger gain of gas mixtures with isobutane

is also observed by the authors of 4. Effective gain dependence on U totgem

was obtained from a fit, assuming an exponential behaviour. The fitted

coefficients were 0.02 V−1 for all gas mixtures.

Reliable detector operation in the experiment is only possible if all re-

quirements in terms of efficiency and detector survival can be satisfied for

a certain range of U totgem or effective gain G, in a way that voltage, pressure

or temperature variations do not bring the detector outside this range.

4. Measurements of time performance

To make a comparison of the performances of the different gas mixtures,

efficiency, cluster size and discharge probability are displayed as a function

of gain, after T and P correction.

The detector configuration used in this tests closely followed that of 1,

so did the experimental setup for efficiency measurement, and therefore

they will not be discussed again in this paper. Detectors with gap sizes

3/1/2/1 mm were tested.

Discriminator theshold was set to 2 fC delta input charge equivalent. The

first time among all pads of the chamber is considered as the time of the

event. Fig. 3(a) shows the efficiency vs. gain with two detectors in a OR

configuration after discrimination.

Therefore, the value of effective gain which corresponds to 96% efficiency in

figure 3(a) , defines the start of the operating region of the detector. This


5

Figure 3. (a) Measured efficiency vs. gain of two detectors in OR after discrim-ination in a 20 ns time window. (b) Measured pad cluster size vs. effectivegain.

value turns out to be different from gas to gas. The end of the operating

region is defined by the smallest gain value corresponding to maximum

tolerable discharge probability per incident particle, discussed in section 5,

and the maximum tolerable pad cluster size, i.e. 1.25. Pad cluster size

is defined as the number of pads per event which fired the discriminator

in a 3x3 pad region around the first triggered pad of the event and in a

time window of 25 ns. Fig. 3(b) shows the pad cluster size for the three

gas mixtures vs. effective gain. As expected, the pad cluster size depends

on signal height, and since the number of primary ionisation electrons is

similar for the three mixures (see section 2), the pad cluster size is directly

related to the effective gain.

5. Measurements of discharge probability per incident

particle

The measurement was performed at PSI, with a quasi-continuous hadron

beam (300 MHz) mainly composed of pions of 350 MeV/c momentum with

an estimated contamination of 7 % of protons. Discharge counting was

performed detecting current spikes on one of the high voltage channels

feeding the detector. Discharge probability is defined as the ratio between

the observed number of discharges and incident particle number in a given

time.

During the test, the detectors integrated about 5000 discharges each, with


6

Figure 4. Measured discharge probability per incident charged particle vs. effec-tive gain

stable performance. No dark current above 100 nA was observed on any

electrode. Taking into account the expected average charged particle rate

in the LHCb experiment, this would correspond to a discharge probability

of 3 · 10−12 in 10 years of running. The effective gain corresponding to this

value of discharge probability per incident charged particles defines the end

of the working region. However, since we did not observe any degradation in

detector performances this is a conservative estimate. Discharge probability

is shown for the three mixtures in figure 4.

6. Aging Studies

To verify if the detector is able to work 10 years at LHCb (integrating a

charge up to 1.6 C/cm2) aging studies were performed. In a local aging

test a small area (1 mm) was highly irradiated with a 5.9 keV x-ray beam

(results are reported in fig 5 (a) with green dots). In a large area test an

area of around 15 cm2 was irradiated with a 300 MHz hadron beam (red

dot in fig 5 (a) reports the normalized current at the end of the test). A

global aging test in which the full detector was irradiated by the photon

emitted by a 60Co source. In this test 3 detector were highly irradiated and

one was exposted to a lower dose.

fig. 5 (a) shows that in the local aging test and in the large area test

no aging effect was found (no decrease in the current) , while in the global

aging test in the chambers highly irradiated the current decrease with the

same slope (-15%/0.2 C). The zoom in fig.5 shows that in the chamber

exposed to a lower dose, after a period in which wasn’t working properly,


7

Figure 5. (a) Normalized current vs. integrated charge in all aging studies. (b)Normalized current vs. integrated charge in all aging studies at low integratedcharge.

the current follows the trend of local aging test with no evident aging effect.

The large and local area aging test, and the global aging test suggest

that the triple GEM detector can work 10 years at LHCb. The decrease in

current in the highly irradiated chamber of the global aging test might be

due to the gas flow that in these chmabers wasn’t increase enought respect

to the dose rate.

7. Conclusion

We discussed the performance of 10×10 cm2 triple-GEM detectors with two

new gas mixtures, Ar/CF4/C4H10 (65/28/7) and Ar/CO2/CF4 (45/15/40),

satisfying the requirements in terms of time resolution, pad cluster size and

discharge probability per incident particle corresponding to the harsh envi-

ronment of the inner part of the first station of the LHCb muon chambers.

An operating region of more than 80 V for the triple-GEM detector with

these two gas mixtures, allowing for external temperature and pressure

variation, was found. After the local aging test, large area test and global

aging test al low dose we can conclude that the triple GEM detector can

work properly 10 years at LHCb. Work is in progress to understand the

different trend in the global aging test at hih dose rate.

References

1. G. Bencivenni et al., Nucl. Instrum. Meth. A488 (2002) 493.2. F. Sauli, Nucl. Instrum. Meth. A386 (1997) 531.3. LHCb Muon System Technical Design Report, LHCb TDR 4, (2001).4. S. Bachmann et al., Nucl. Instrum. Meth. A479 (2002) 294.

Documents

M. ALFONSI, G. BENCIVENNI, P. DE SIMONE, F.MURTAS, M. POLI ...pinci/Proceedings/como-03-raspino.pdf · October 26, 2003 10:9 WSPC/Trim Size: 9in x 6in for Proceedings fast3gem OPERATION