Performance improvement of streamer drift chambers by the addition of organic vapours to the gas mixture

Performance improvement of streamer drift chambers by the addition of organic vapours to the gas mixture

Nuclear Instruments and Methods in Physics Research A273 (1988) 553-558 North-Holland, Amsterdam 553 PERFORMANCE IMPROVEMENT OF STREAMER DRIFT CHAMB...

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Nuclear Instruments and Methods in Physics Research A273 (1988) 553-558 North-Holland, Amsterdam

553

PERFORMANCE IMPROVEMENT OF STREAMER DRIFT CHAMBERS BY THE ADDITION OF ORGANIC VAPOURS TO THE GAS MIXTURE Evelyne DAUBIE *, Francis DEFONTAINES, Fernand GRARD, José KESTEMAN * *, Maxime CHEVRY, Omer PINGOT and Chantal POIRET Université de l'Etat, Mons, Belgium

Walter VAN DONINCK + Inter-University Institute for High Energies, ULB-VUB, Brussels, Belgium

The stability of operation and the detection efficiency of drift chambers running in streamer mode are substantially improved when adding a few percent of organic vapour to gas mixtures composed of Ar, C02 and iC4Hlo . 1. Introduction In connection with the construction of the forward muon identifier of the DELPHI detector (LEP-CERN) [1], systematic tests are conducted in order to find safe gas mixtures ensuring efficient and stable operation of drift chambers in the limited streamer mode with drift distances up to 10 cm [2]. This investigation is made by exposing drift chamber prototypes to radioactive sources and to cosmic muons, in conditions ensuring low oxygen and water contaminations . Tests have shown that the chambers can be operated successfully using gas mixtures composed of Ar, C0 2 and iC4 Hlo in which the concentration of hydrocarbon quencher is as low as 15% [3]. In order to further improve the drift chamber performance, the addition of a few percent of organic vapours to the above gas mixture will be shown to have a decisive influence on the stability of operation of chambers running in the self-quenching streamer mode as already observed by several authors [4]. Moreover, the low ionization potential of the organic vapours is well known to substantially reduce ageing problems [5]. 2. Prototypes and experimental setup The two prototypes used for the measurements are described in ref. [3]. Essentially, they are 20 cm long drift chambers with about 10 cm maximum drift distance: * Aspirant FNRS. ** Chercheur qualifié FNRS . + Bevoegdverklaard navorser . 0168-9002/88/$03 .50 © Elsevier Science Publishers B.V . (North-Holland Physics Publishing Division)

- a section of a genuine DELPHI forward muon chamber [3] which is exposed to minimum ionizing cosmic rays in a cosmic hodoscope; - a thin wall prototype which is irradiated by radioactive sources and used mainly to determine the loss of electrons during drift (SS Fe 5.9 keV X-rays) and to measure their drift velocity (9° Sr beta particles) . During measurements, special attention is given to the oxygen and water vapour impurities since a large fraction of C0 2 is present in the gas composition . The gas mixture is monitored by measuring the content of Oz (Oxymeter, Orbisphere) and the content of H2O (Hygrometer, Shaw). With a typical gas flush rate through the chambers of the order of 5 1/h, the content of oxygen and water is less than 10 ppm and 20 ppm respectively. To achieve the given percentage of vapour either Ar or C02 (a part or all) is bubbled through the organic liquid . The temperature of the bubbler is maintained around 20 ° C for the alcohols and at 0 ° C for the methylal . The drift time and the charge distributions are investigated by means of a LeCroy 2250Q ADC and a LTD (LEP time digitizer) prototype [6] which also allows a study of the afterpulsing. Data were taken with drift fields of 750 and 850 V/cm in the DELPHI and the thin wall prototype respectively . 3. Results A summary of the results obtained with the different gas mixtures that have been tested is presented in table 1 . From the left to the right, the features of the gas mixtures become more and more attractive for running drift chambers in streamer mode especially through the II . GASEOUS DETECTORS

Drift velocity [cm/!us] for 850 V/cm drift field

2

39

- 0 .75 '~

400

Ar-C02-'C 4 H , o 15-70-15 +0 .6% isop

33

not measured

500

Ar-C0 2 -'C4 H to 15-70-15 +1 .8% eth

3

not measured

- 0 .75 '~

700

Ar-C0 2 -iC4 H , o 15-70-15 +1 .8% isop

4

1 .9 71

29 23 16 32

( " aft / % Afterpulsing

% Events with

b)

51

Charge [pC]

91

40 29 16 15

1 .2 60

56

180 ? 15 91

not measured

not measured

not measured

200 50 20

Measured from LTD anode time distributions. Loss from 8 .4 to 1 .2 em drift distance, for a drift field of 850 V/cm .

0 aft . 1 aft . 2 aft . 3 aft .

170 60 20

Pulse height [mV] Singles rate [Hz] Dark current [nA]

51 29 12 8

0 .84 49

76

200 45 116

Properties for 5 .0 kV anode potential, 750 V/em drift field recorded in the final configuration prototype

12

0 .85+0 .20

Plateau length [V]

b) [%]

150

Gas mixture (Concentration) Vapour included

Loss of drifting e-

1

Ar-C02-'C 4 H IO 15-70-15

Column number

Table 1 Summary of results for different gas mixtures

not measured

not measured

not measured

200 32 15

80

not measured

700

Ar-C02-iC4 H , o 15-70-15 +3 .5% eth

5

73 20 5 3

0 .41 27

60

160 30 10

71

98

0 .74+0 .20

800

Ar-C0 2 -'C4 H fo 15-70-15 2 .7% isop

6

76 17 4 3

0 .38 24

40

200 26 10

16

62

0 .79+0 .20

800

Ar-C0 2 -iC 4 H ,(, 15-70-15 +2 .3% meth

7

93 4 2 1

0 .11 7

28

120 20 5

14

51

2 .9+0.20

1000

Ar-iC4 H j o 1-2 +1 .5% isop

8

s a o-

m

n

0

b

R

c

b

A

55 5

E. Daubie et al / Performance improvement of streamer drift chambers

longer efficiency plateaux and the lower level of afterpulsing. 3.1 . Data taking

=

Nto Nmain Nmain

where Ntot is the total number of anode pulses (main pulses + afterpulses) and Nmain is the number of main pulses or primary pulses, i.e . the number of ionizing events recorded . (2) Afterpulsing percentage %(AFTER) = where

Nmain

Nmain - Nwithout Nmain

is defined as above and

N~itho t

is the

100 80

+

80 It

û c w

70 0

60 50 40

0

30

0

20

-

10

_~

0

4 .2

4 .4

190 180 170

,.

The results presented here have been obtained in the following conditions : - The efficiency measurements for the plateaux at increasing voltages were stopped when the singles rate and the dark current became too high (> 150 Hz and > 200 nA respectively). This is valid for all mixtures containing vapours except for the Ar-C02-iC,H10 (15-70-15) mixture for which the efficiency measurements were stopped when a permanent streamer appeared in the chamber, corresponding to a dark current larger than 1 ILA. - The values of the streamer pulse height given in table 1 are in fact estimates read from the oscilloscope into 50 Sl while the dark current values are average values read on an electrometer . - The amount of afterpulses is defined in two ways : (1) The average number of afterpulses recorded per ionizing event Naft %

200

4 .6

4.8

5

5 .2

5 .4

5 .5

5 .8

Anode potential ( kV )

Fig. 1. Efficiency plateaux recorded in the final configuration prototype exposed to minimum ionizing cosmic muons, for the gas mixtures : 0 : Ar-CO2-'C4 H, 0 (15-70-15) and + : Ar-C02 -'C4Ht0 (15-70-15)+2.7% isopropanol.

i

v ó

160

-

150

-

140 0

-

120

-

710 100

-

ó ao ~n

70

-

60

-

0

40 30

-

20

-

10

-

0

0 4 .2

4 .4

4 .6

4.8

5

5.2

5.4

5 .6

5 .6

Anode potential ( kV )

Fig. 2. Singles rate as a function of the anode potential measured in the same conditions and for the same gas mixtures as in fig. 1. number of main pulses without afterpulses, i.e . among the total number of ionizing events the ones detected properly . 3.2 . Properties improved by the addition of organic vapours 3.2.1 . Efficiency plateaux for minimum ionizing particles Fig. 1 shows that the efficiency plateaux begin practically for the same anode potential (±4.9 kV) whatever the gas mixture while the length of the plateaux depends strongly on the nature of the mixtures (see table 1) especially on the presence or absence of organic vapours. 3.2.2. Singles rate and dark current Both the dark current and the singles rate increase with the anode voltage but much more rapidly in the absence of vapour. The singles rate variation as a function of the anode potential for the Ar-CO,-iC,H t0 (15-70-15) and the Ar-CO,-iC4 Ht0 (15-70-15) + 2.7% isopropanol mixtures is illustrated in fig. 2. It clearly appears that the addition of vapour to the gas composition increases considerably the stability of operation of the detector . 3.2 .3. Afterpulsing From table 1, it is seen that the afterpulsing level decreases strongly as a function of the vapour concentration in the gas mixture. The most likely explanation of this effect is that the tested vapours absorb a large amount of the photons emitted in the streamer avalanches [7]. 3.3. Properties quasi-unchanged 3.3.1 . Drift velocity For the gas mixtures presented here, the drift velocity lies between 0.75 and 0.85 cm/Ls for a drift field of 11 . GASEOUS DETECTORS

E. Daubie et a1 / Performance improvement of streamer drift chambers

55 6

850 V/cm . These rather low values are due to the large amount of CO, present in the gas mixtures. 3 .3.2. Streamer pulse height and charge

The streamer pulse height and charge increase with the applied anode voltage but no systematic dependence as a function of the gas composition has been observed . Fig. 3 shows charge spectra for some gas fillings corresponding to an anode potential of 5 kV . One notices a double peak structure with average charges lying between 40 and 75 pC for the first peak and between 62 and 116 pC for the second peak . Streamer charge distributions for six different anode potentials are given in fig. 4 for the gas filling Ar-C02iC4 H, o (15-70-15) with 2.3% methyldl . The relative population of the second peak grows with the

6

anode voltage to become the same as in the first peak . At 5 kV anode potential, the first peak contains 86% of hits and the second one 14% of hits ; at 5.4 kV, the repartition of the hits is 57/43 . 3 .4. Loss of electrons during drift

As can be seen from table 1, the loss of electrons for a drift field of 850 V/cm is particularly important for the mixtures containing a large amount of CO, together with a few percent of isopropanol or ethanol. Moreover, it grows with the alcohol concentration . It is interesting to notice that the value of the recorded streamer charge is about the same whether the gas contains vapours or not. This indicates either that the charge developed in the streamer avalanche does not depend on the number

Charge (PC)

Charge (PC?

600

500

200

100

0 Charge (PC)

60

120 Charq. ( pC )

Fig. 3. Streamer charge distributions recorded in the final configuration prototype exposed to minimum ionizing cosmic muons, for the gas mixtures : (a) Ar-CO2 -'C4H10 (15-70-15), (b) Ar-CO2-'C4H10 (15-70-15)+1 .8% isopropanol, (c) Ar-C02 -'C4H~ 0 (15-70-15)+2 .7% isopropanol, (d) Ar-CO2-'C4H10 (15-70-15)+2 .3% methylal .

E. Daubie et al. / Performance improvement of streamer drift chambers

557

0.9 0.8 0.7 0.6 .5 0 200

0.4 0.3 0.2

100

7 40

I -

r

80

II 120

I

I

160

1-~ 200

0

0

40

80

120

160

20C

Charge ( PC )

Charge ( PC )

710,

600

600

500 -~

500

400

1

400 J 300 200 -

100

100 -

100

0

40

80

120 Charge ( PC )

160

0 200

0

40

80

120

160

200

Charge ( pC )

300 280 260 240 220 200 180

9 160 4 \ 140 120 100

so 60 40 20

0

200

Fig. 4 . Streamer charge distributions recorded with the gas mixture Ar-C02 -'C4 H 1o (15-70-15)+2.3% methylal for different anode potentials : (a) 4.9 kV, (b) 5 .0 kV, (c) 5 .1 kV, (d) 5 .2 kV, (e) 5 .3 kV, (f) 5 .4 kV . II . GASEOUS DETECTORS

558

E. Daubie et aL / Performance improvement of streamer drift chambers

of electrons reaching the anode, or that in the presence of isopropanol the gas gain is larger, probably due to the Penning effect . 4. Conclusions From all our measurements, it is found that the

addition of a few percent of organic vapour to the 3-component mixture of Ar, CO, and iC4 Hlo , with a

rather low iC4Hlo content (±15%) improves substantially

the

streamer

operation

of

drift

chambers :

lengthening of the efficiency plateaux by several hundred volts and reduction of the rate of afterpulsing by factors

as large as 3 .

The use of alcohols, however, leads to a loss of

drifting electrons due to their electronegative properties, but at a degree which does not affect the dectection efficiency when operating in the SQS mode. References

[1] DELPHI Progress Report, CERN/LEPC/84-16, LEPC/ PR6 (September 1984).

[2] C. De Clercq, L. Etienne, B. Goorens, J. Lemonne, S. Tavernier, G. Van Beek, C. Vander Velde, W. Van Doninck, L. Van Lanker, J. Wickens, E. Daubie, F. Defontaines, F. Grard, J. Kesteman, O. Pingot and C. Poiret, Nucl . Instr. and Meth . A243 (1986) 77 . E. Daubie, F. Defontaines, F. Grard, J. Kesteman, O. Pingot, C. Poiret, C. De Clercq, L. Etienne, B. Goorens, J. Lemonne, S. Tavernier, G. Van Beek, C. Vander Velde, W. Van Doninck, L. Van Lanker and J. Wickens, Nucl . Instr. and Meth. A252 (1986) 435. [4] E.P . de Lima, IEEE Trans. Nucl. Sci. NS31 (1984) ; M. Atac, IEEE Trans. Nucl . Sci. NS-31 (1984) . [5] G. Charpak, H.G . Fisher, C.R . Gruhn, A. finten, F. Sauli, G. Plch and G. Flügge, Nucl . Instr. and Meth . 99 (1972) 279; J. Va'vra, SLAC-PUB-3882 (1986), presented at the Wire Chamber Ageing Workshop Berkeley, California (1986) . [6] G. Delavallade and J.P. Vannuxem, Nucl . Instr. and Meth . A252 (1986) 596. E. Daubie, M. Chevry, F. Defontaines, F. Grard, J. Kesteman, O. Pingot and C. Poiret, DELPHI Note, DELPHI 86-107 TRACK-41 .