Construction, test and operation in a high intensity beam of a small system of micro-strip gas chambers

ELSEVIER

Nuclear Instruments and Methods

in

Physics Research A 403 (1998) 31-56

NUCLEAR INSTRUMENTS AMETnooS IN PHYSICS RESEARCH SectIonA

Construction, test and operation in a high intensity beam of a small system of micro-strip gas chambers A. Barra*b, S. Bachmannc, B. Boimskab, R. Bouclierb, A. Braemb, C. Camps”, M. Cape&nsb, V. Commichau”, W. Dominikb, G. Fliigge’, F. G6mezd, R. Hammarstromb, K. Hangarter”, M. Hochb, J.C. LabbCb, D. Macke”, G. Manzine, F. Meijersb, G. Millionb, K. Muhlemannb, F. Saulib-*, R. Schulte”, V. Nagaslaevf, A. Peisertb, L. Ropelewskib, 0. Runolfsson’, M. Schulzb, A. Sharmab, L. Shekhtmanf, C. Wolffb aESPCI, Paris, France b CERN, PPE Division. 121 I Genwa 23. Switzerland ’ Ph.vsikalisches Institrrt A, Aachen. German? d Unioersig oj‘santiago de Compostela. Spain ’ INFN Legnaro. Prrdolla. Itu(l f BINP. Novosibirsk. Russian Federation Received 17 July 1997

Abstract We describe the construction, test and installation procedures, and the experience gained with the operation of a small but complete system of high-rate Micro-Strip Gas Chambers, made on thin borosilicate glass with a diamond-like coating with chromium or gold strips. A set of detectors, fully equipped with read-out electronics and each with an active area of 100 x 100 mm’, was exposed during six months to a high-intensity muon beam at CERN with a peak intensity of - IO4 mm-zs-‘. Continuous monitoring of the performance of the chambers during the beam runs allowed the evaluation of detection efficiency and the monitoring of accidental rates, as well as the study of ambient induced variations and aging in realistic beam conditions. No significant difference has been found in the operation of under- and over-coated plates. Efficiencies could reach - 98% in best operating conditions, although local lower values were often observed due to missing channels (open strips, broken bonds and dead electronic channels). The long-term operation of the chambers has been more difficult than expected, with the appearance of break-downs and loss of efficiency in some detectors. possibly induced by the presence of small gas leaks, to water permeation or to residual reactivity of the quencher gas (dimethylether). ICI 1998 Elsevier Science B.V. All rights reserved.

1. Introduction

Micro-Strip Gas Chambers (MSGCs), introduced several years ago [l], have been the subject of * Corresponding author. Tel.: + 41 22 7673670; fax: + 41 21 7677100: e-mail: fabio.sauli@,cern.ch. 016X-9002/9X/$19.00 t(:’ 1998 Elsevier Science B.V. All rights reserved PII SOl68-9002(97)01095-4

intense research, mainly in view of their use as precision trackers for high-rate experiments [2,3]. The fundamental aspects of operation and the long-term behaviour under sustained irradiation have been studied by many groups; recent summaries can be found in the status reports of the RD-28 collaboration [4,5]. in the proceedings of

dedicated workshops [6,7] and in review articles [S]. The CERN Gas Detectors Development (GDD) group, in the framework of the research project RD-28, has contributed to the basic research activity, documented in numerous papers describing the high-rate and long-term behaviour of these devices [9-141. In the course of the development, a variety of MSGC plates has been manufactured and successfully operated in laboratory set-ups and beam facilities. The “20 MSGCs” project [15] was carried on by a collaboration of several groups in the CMS tracker with experience in MSGCs development: its aim was to acquire experience in the manufacturing, assembling and testing of a set of chambers of medium size (100 x 100 mm’). and to study the behaviour of a small but representative number of fully equipped detectors in a high-intensity beam for a considerable span of time (A 6 months). Due to various difficulties. the total number of operating chambers reached only ten at its peak; a description of the problems encountered and of the results achieved is provided in this paper.

2. MSGC

manufacturing

-7.1. Choice of’ the substrate MSGC plates have been manufactured by various groups on a wide range of substrates. Our experience has been mostly with thin boro-silicate and electron-conducting glass: operating characteristics of detectors realized on both types of substrates have been described in the previously quoted references. Convenient because of commercial availability and with excellent surface quality, borosilicate glass has been widely used. Its ionic type of conductivity, however, induces various kinds of instabilities and rate-dependent gain shifts in the detectors. Better stability and higher rate capability can be obtained with electron-conducting supports having resistivity from lo9 to 10” flcm. For the time being, however, these special glasses can be procured only in small quantities and sizes not exceeding 5” in diameter, with sometimes poor surface quality. In addition, being mechanically polished, their min-

imum thickness (300-500 pm, depending on size) and short radiation length. due to the high 2 of the components. are not very suited for a central tracker detector. A substantial effort has been undertaken by the BINP-Novosibirsk group in order to develop a technology capable of producing thin. continuous ribbons of electron-conducting glass with the desired specifications. This work is still in progress, with the aim of making full-size MSGCs with consistently good quality [ 163. The promising results obtained with electronconducting glass have triggered several developments aiming at modifying the surface resistivity of commercially available, low-cost borosilicate glass in order to obtain the desired surface resistivity. Among them, the Diamond-Like Carbon’ (DLC) coating introduced some time ago [ 121 has proved to be suitable for the realization of reliable, highrate MSGC detectors. The surface resistivity of DLC-coated supports, typically 5&150 nm thick, can be adjusted between 10IJ and 101hQ/U ; the homogeneity is within & 15% of the nominal value on a given plate, sufficient for proper operation r171. The conductive layer can be applied on a substrate before the deposition of the metal strips (under-coating), or alternatively above an already manufactured MSGC (over-coating). The second solution is simpler and safer to realize. because strips can be patterned on the bare glass without the risk of damaging the layer, and also because defects in the lithography resulting in imperfections of the layer are avoided. As will be discussed later, no basic operating difference has been found between over- and under-coated detectors, but doubts subsist on the long-term stability of the over-coated chambers subjected to high radiation fluxes [18]. 2.2. Micro-strips

platrs

In the course of our developments we have obtained from industry MSGC plates manufactured. with a variety of substrates, metals and technologies, from a high-accuracy master copy. All our ’ Low-pressure plasma-assisted chemical-vapour by SURMET Co. Burlington MA. USA.

deposition.

masks have been realized on 5 mm thick soda-lime glass substrates by direct laser beam writing’ with a general accuracy better than 1 pm (overall) and 0.5 pm (local). The geometry of the strips. the shape of the ends and the pattern for grouping the powered strips has also been modified repeatedly. following the progress in experimental measurements and simulation studies [10.19-211. The best performance in terms of high. stable and uniform gain has been obtained with anode and cathode strips 7 and 100 pm wide, respectively, at a 200 pm pitch (anode to anode centres). Fig. 1 shows the MS-9 layout design used for the 20 MSGC project. with an active area of 100 x 100 mm2; strips are rounded at each end to reduce the local electric field, although this appears not to be sufficient to prevent discharges (see Section 2.4). The high voltage strips (cathodes in this case) are grouped in sets of ten. and each group connected to the voltage through a high-value surface-mounting resistor (500 kR) fixed with conducting epoxy.3 Three types of plates have been used for the beam runs. All plates have been manufactured on 300 pm thick D-263 borosilicate glass.5 over- or under-coated with a thin (- 150 nm) DLC layer. with similar geometry but different implementations: two types manufactured by IMT’ with chromium and gold strips. respectively, and one made by ALENIA” with gold strips galvanically grown and provided by the INFN-Pisa group: the latter. manufactured as described in Ref. [22], was available during the last week of beam operation and a extensive study of its performance has not been possible. Chromium strips engraving by wet-etching photolithography has provided so far the highest quality product at lower cost. The insulating support is first thoroughly cleaned, metallized on one or both sides by vacuum evaporation or sputtering, and then coated with a thin layer of light-sensitive resin. A negative mask is overlaid on the plate, and the set exposed to ultraviolet light. After high-

‘TFRAPIXEL Inc.. Espvo. Finland. .’ T~PC E-204 hy EPOTECNY. Vdizy. Frwce. ’ UESAG. Dcutschc Spczialglass AG. Grtinrplan. Germany. ’ IhlT Maken and Teilungen AG, Greifensee. Switzerland. ” ALENIA. Via Tiburtina. 00131 Roma. Italy.

MS-9

(NO’I TO SCALE)

Fig. 1. MS-Y layout design l’br the micro-strip detcctw used ii,r the project. The actlvc at-ea is about I00 K 100 mm’. with anode and cathode strlpr 7 and 100 1~111 WI&. rcspectlwly. .It 700 ltrn pitch.

temperature curing, the exposed. chemically modified resin is removed by a solvent: immersion in an acid bath then etches away the open metal areas. A solvent removes the photo-resist remaining on top of the metal, and the result is a copy of the mask representing the MSGC structure. A lift-off process is generally used to obtain gold strips. Gentler to the substrate. this technique is a good choice for manufacturing MSGCs on delicate supports such as glass coated with thin electron-conducting layers that could be modified or damaged by the aggressive wet or plasma etching. The support. coated with a thin chromium

adhesion layer, is covered by photo-resist and exposed to UV light through an inverted, negative mask; after curing. the exposed resin is dissolved leaving on the support a pattern with open channels corresponding to the strips. Gold is then evaporated or sputtered uniformly over the substrate: when removing the photo-resist, the excess metal is “peeled-off’ in regions originally covered with the resin, and the adhesion layer is chemically removed. In general, this provides less well-defined edges for the strips than the etching technique, and in some cases residual of peeling process and of resin have been found generating local shorts. Various kind of cleaning, including reactive plasma etching. have been tried to remove imperfections but with only moderate results. -7.3. Ends-qf strips pmivation A known problem met with MSGCs is the appearance of discharges at the end of the strips. due to the high local electric field, at a voltage considerably lower than the operational. Several geometries have been tested to prevent discharges [23], as well as methods to generate a local-field correction by external electrodes [24]. A simpler method consists of covering the region at the end of the strips with a thin layer of insulating material (passivation). While sophisticated micro-electronics technologies, such as polyimide sputtering, can be used. we have preferred a simpler potting with epoxy, applied by silk screening. A metal mask is prepared with a thin mesh etched in an opening corresponding to the region to be passivated, typically a strip 1 mm wide. With the mask in contact with the MSGC plate, a layer of epoxy is spread over the slit; after scraping the excess, the mask is removed and the plate heated in a controlled-temperature oven for curing. As an outcome of our ageing studies [X], we have realized the passivation with a two-component epoxy of medium viscosity, certified for its negligible outgassing.’ All chromium plates have been passivated with this method. In many cases, a process of diffusion along the strips while curing the passivating polymer has

’ EPOTECNY

E-505.

been observed for the plates manufactured with gold strips: the appearance of discharges in the area. as well as difficulties for bonding the electronics to the strips have been associated with the presence of a thin diffusion layer. Of the many epoxies tested, we have found only one* satisfying the outgassing requirements. having an acceptably low level of diffusion and requiring room-temperature curing: it has been used for passivation of all the gold plates. Diffusion is not observed for a standard polyimide passivation; as this process requires. however, a very high-temperature curing (25O~‘C). a substantial change in the resistivity of the DLC layer results [lS]. The ALENIA plates. passivated following this procedure, had indeed a high surface resistivity. above 1OL6RKl. We have also developed an alternative method. consisting in sputtering an insulating material through a non-contact mask: a very good choice appears a thin ( Y 200 nm) layer of magnesium fluoride (MgF3). Several test plates were passivated in this way with excellent results, but we have preferred to use the cheaper and faster epoxy coating for the beam chambers. 2.4. Detectors

assetnbl)

The final detector assembly was made in a clean room. certified of class 200 on the working table and 2000 elsewhere. Particular care has been taken in handling the thin glass to avoid scratches or glass fragments that would degrade the quality of the coating and of the photo-lithography. While the best is to obtain the plates in the final format from the producer, when necessary we have used the following cutting procedure: raw plates are washed in de-ionized water and coarse-polished on the edges to eliminate splinters. After drying in a clean environment, they are diamond-cut. and if necessary the edges polished with optical-grade grit (cerium dioxide). Before assembly. all plates are optically and electrically inspected in order to check the quality of the diamond-like layer and the

’ STYCAST sheim. France.

Type

1266. supplied

by Zundel

Kohler.

Wittel-

strip pattern artwork. An overall measurement of the surface resistivity of each plate is performed on a probe station; voltages are applied to individual electrodes through needles connected to the voltage source and to a pica-amperometer; the surface resistivity is then estimated from the measured currents. For safety reasons, applied voltages did not exceed 200 V in air: values of resistivity lie in the range 10’4-1016 Q/U. depending on plate processing. We have developed a light and cheap frame assembly in order to satisfy the needs of particlephysics experiments, where several thousand plates may be needed, and multiple scattering and gamma conversion problems seriously constrain the material budget. In the assembly. only non-outgassing materials are used [25,26]. As shown in Fig. 2, a rectangular thin frame (1 mm thick and 3 mm high) made of VECTRA,” a glass-fibre reinforced polymer. is glued onto the plate. The material has a linear thermal expansion coefficient of 3 x lo-’ K _ ‘. close to the one of glass, and allows assembly at the rather high temperatures (80 C) required for curing the selected epoxy (EPOTECNY E-505). The polymer is injectionmoulded. and can. in principle. be obtained with the desired shape; we have used instead L-shaped profiles cut from raw plates. that can be better tailored to the desired size and pasted together. Gas input and output are realized inserting and gluing small-diameter tubes through holes in thicker reinforcements in the frame. If necessary. precision machined flanges can be added as mounting helps in the supporting structure of large detectors assembly. On the sides parallel to the strips, the frame is glued flush with the edge of the last cathode strips; we have found that the best place for gluing the sides perpendicular to the strips is just outside the passivation of the ends of strips. Attempts to glue the frame over the ends. with the aim of avoiding passivation. failed because of discharges. possibly caused by air bubbles or defects of polymerization of the epoxy. On two sides, the glass plate is larger

“VECTRA Cl30. by Hoechs by Nief Plahtic. Gcnax. France.

High Chem and manufactured

Fig. 2. Modular assembly of the detector: a rectangular. I mm thick and 3 mm high insulating frame is glued to the MSGC plate and to the drift electrode.

than the frame, to allow access to the bonding pads and the high-voltage connections. Several techniques have been used to realize the drift electrodes. A transparent, z 100 nm thick indium-tin oxide (ITO) layer vacuum-evaporated over thin borosilicate glass, with a surface resistivity of few Q/U. performs adequately and allows optical inspection of the strips after detector assembly; it has been used for the manufacturing of early prototypes. For the final detectors. however. we have preferred a u 100 nm gold layer evaporated over a chromium adhesion layer on 100 urn thick borosilicate glass, and constituting both drift electrode and gas window. Two light expanded polyurethane (Rohacell) plates. 3 mm thick, are mounted on each side of the assembled MSGC to protect the thin glass from deformations due to volume changes (pressure variation, gas flow) and from accidental damages. An assembled MSGC, complete with the read-out electronics board, is shown in the picture of Fig. 3.

3. Laboratory

tests

3.1. Complete test procedure The extensive laboratory test program follow-up of the long-term beam test a careful identification of each detector.

and the required with an

Fig. 4. Schematics of the three-lrvcl

pitch adaptor ~4

fol the

complete te\t proceLiurr.

Fly. .: An assembled

MSGC

tronich before installation

complete

with

I-cad-nul

else-

in the beam.

alphanumeric code assigned to keep track of the characteristics of the MSGC plate. We have adopted the following acronyms: l DUC: Diamond Undercoated substrate. Chromium strips; l DUG: Diamond Undercoated substrate. Gold strips: l DOC: Diamond Overcoated substrate. Chromium strips; l DOG: Diamond Overcoated Gold strips: A complete characterization of each detector is realized before its installation in the beam. For the laboratory tests. the anode strips are bonded on ;I multi-level pitch adaptor. grouping strips. that c;~n be cut to progressively finer partitions (see Fig. 4). At the first level. eight groups connecting 64 strips each. at the second level 128 groups of four strips, and finally the individual 512 anodes are available for testing. Different tests are made at each Ieve\: Level I: Eight pads of 64 strips each. A fast search for short circuits between anodes and cathodes is performed measuring, in air, the current between groups of strips at moderate voltage (< 200 V). If a short is found, the corresponding anode group is disconnected and marked for future investigation. A sample measurement of the resistivity across the plate is also obtained; Fig. 5 shows typical values for several plates. Purposely varied from plate to plate in order to study operating difierences. the resistivity is uniform across each plate within + 15%. according to specifications.

Level 2: 12X groups of four anodes strips each. The adaptor board is bonded to a PreShape multiple amplifier electronics [27] (four chips with 33 channels each). After connection to a gas distribution system. the tightness of the assembly is verified and the detector powered at progressively higher voltages. Currents are monitored as a function of applied voltages. up to ~ I kV for the drift electrode and - 600 V for the cathodes. As shown in Fig. 6. the leakage current increases slightly more than linearly with voltage. a known characteristic of thin DLC resistive layers: its v:~luc depends on the coating resistivity. If the current exceeds normal values. the defective group is identitied checking the value of the current on individual pads and the bonds to the electronics arc remo\!ed. Icavins the corresponding anodes floating. At this level. the gas gain is evaluated exposing the detector to an X-ray

Fig. 6. Lrakagc current and resistivity measured of the applied voltage for a typcal plate.

as a function

source and recording the charge signals on each channel. representing a group of four anodes (Fig. 7). After calibration of the electronics gain. the pulse-height distribution is rather uniform over the plate. as seen in Fig. 8. except on the edges where a decrease of around 20% is detected due to the proximity of the frame: the small decrease only slightly affects the detection efficiency, that remains uniform across all the active area (see later), and is beneficial to avoid edge discharges. As seen in the figure, this particular plate had two defective channels (dead or not bonded). With the help of a pair of scintillation counters. and discriminating the analogue signals on a small group of anodes, the local detection efficiency for minimum ionization particles can be done using a 90Sr electron source: Fig. Y shows a typical efficiency plateau, representative of the expected operating performances in the beam. Absolute value and length of the plateau depend from the resistivity of the plate, and from the signal-over-noise ratio. For selected plates, a rate capability test is also performed, up to _ 10’ lnrn-‘s‘: Fig. 10 gives an example. obtained with a plate at the high end of the resistivity range (- 10'" R;O). Fig. 11 shows values of relative gain measured. at a fixed cathode voltage (- 550 V), for chambers with different surface resistivity. As expected. lower values of resistivity. while intrinsically more stable at high rates. provide lower gain. implying the use of higher cathode voltages for operation; moreover, the gain is more affected by local variations of resistivity.

c

1400

.s ‘g L

1100

$ c

1000 800 600 401) 200 (I

Fig. X. Iiniformity of gain acr<>sb the plate. measured uith Nrays on 128 groups of four anodes. Groups at the edgea \hcw ;t small decrease of the signal.

A good compromise between minimal Ructuations of gain and stable response at high rates is achieved for values around 10” R,/CI;as seen in Fig. 11, a f 15% variation of resistivity around this value induces an acceptable f 7’%, change in gain. Level 3: Five hundred and twelve strips. The last cut of the pitch adaptor board allows access to individual anodes. The final electronics board (four 128-channel PreMux chips with multiplexed analogue output [IS]) is bonded to the plate: strips causing shorts in previously identified group are sorted out and left floating. Additional problems such as malfunctioning electronic channels.

38

A. Barr et al. JNucl. Instr. and Meth. in Phw. Res. A 403 (1998) 31-56

$/~,,,;q~2iL,’ b&IENCY

FOR FAST ELECTRONS

600

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Fig. 9. Efficiency plateau measured for fast electrons laboratory with a low-resistivity plate.

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650

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(n/o

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Fig. 11. Signal amplitude (relative gain) measured at fixed cathode voltage for several chambers in a range surface resistivity.

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0

1

2

3

4

5

6

7

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12

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Number of Defects

Fig. 12. Distribution of the number of shorts and dead channels for a set of ten plates tested in the laboratory. Fig. 10. Rate capability (relative gain versus flux) for a plate in the high end of the resistivity range.

interrupted strips or poor bonds are identified. Fig. 12 provides the distribution in the number of shorts and dead channels recorded in a set of twelve tested chambers; half of the plates has no defects, the average values per plate being 1.3 shorts and 3.3 defects (open strips, dead channels). Before installation in the beam, each detector was fully powered for several days at nominal voltages; however, because a trigger is required to read out the final electronics, signals could not be easily seen using sources in the laboratory. To monitor the gain, we have connected one group of cathodes, near the edge of the plate, to a charge amplifier through a decoupling capacitor; despite the inevi-

tably higher noise, representative 5.9 keV X-rays pulse-height spectra can be recorded as shown Fig. 13 for two values of the cathode voltage. This method allows also easy oscilloscope monitoring of the appearance of precursors to discharge when setting voltages, and has been used for checks in the beam set-up. 3.2. Reduced test procedure The described test protocol is very effective and fully characterizes the operating properties of each detector, but is time consuming and requires repeated manipulations. Further on in the production, we have introduced a faster method, bonding the plates to a provisional test board having 32

A. Barr et al./Nucl.

Pulse

Height

Fig. 13. Pulse height spectra for 5.9 keV X-rays a group of 10 cathodes at two values of operating

lnstr.

(ADC

and Meth.

Rrs. A 403 (1448)

31-86

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recorded voltage.

on

connectors with 16 channels each that allow access to individual anode strips. In this way, shorts could be quickly localized by group and then down to individual strips, at the same time measuring the local resistivity; defective channels are marked, and left floating at the final bonding to the PreMux read-out electronics that permits the complete high-voltage test and the recording of pulse height on one cathode group before installation in the beam. The test board is still used in many laboratory tests made recently to study, for instance, the problem of discharges generated by highly ionizing particles in the chambers [29]. 3.3. Minimum

in Phw

-

Fig. 14. Electronics test of a completed chamber: a wrc placed above the assembly is pulsed at three levels.

I+MUX test puke vc=o Wire pulsing Position A Vc=O

test procedure I

A fast laboratory test with a minimum of manipulations can be done bonding directly the chamber to the final PreMux read-out card, and using the following procedure. First, the response of the electronics is checked using the internal input test pulse facility. Then, a test procedure called “PreMux strip maps” is followed: it consists in pulsing an induction wire overlaid at three levels on the final assembled detector, on the high-voltage side, on the anode pads and on the pitch adaptor (Fig. 14). A sequencer provides the test pulse and the trigger, and a digital oscilloscope is used to visualize the output of the analogue buffers. Noisy and missing channels as well as shorts are easily identified by comparison of the strip maps, as shown in Fig. 15;

loop

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II

I

I Wire pulsing Posihon A vo 0 lklII”X test pulse voo

I

:

III

I

I I I

1;.

?

MISSING

I

t

7

SHORT

MISSING

1

Fig. 15. NOISY. missing channels and shorts are ldentlfied injecting a test pulse to the electronics and comparlnf wth the wire pulsing response.

correct performance corresponds to negative signals (dispersed by noise), and missing channels appear as peaks in the distributions. Inspection of the different signals recorded inducing a pulse at the

three levels allows the identification of defects (dead electronic channel. missing bond. open strip): a short appears as a missing channel only after the application of a moderate voltage to the MSGC. Analysis of the signals after the application of voltage allows also to find local discharges. that appear clearly even with the asynchronous read out (see Fig. 16). After removal of the bonds on defective channels. :I full high-voltage test and recording of pulse height on the special cathode group are carried out ;IS described above.

4. Ream set-up In order to study the long-term operating properties of a representative system in realistic beam conditions. a set of MSGC detectors was installed from April to September 1996 in a high-intensity beam at CERN. downstream the NA47 experiment. The nluon beam. with a nominal momentum of I90 GeV c. provided 2 x 10’ particles in spills of 2.56 s at intervals of 14.4 s with a peak rate around lOA rnm-‘s.-’ and a wide. roughly Gaussian distrihution with 4 2.1 cm rms. both in the horizontal and vertical planes: the beam profile recorded by one MSGC in the early stages of the test program is shown in Fig. 17. Fig. 18 shows the scheme of the set-up, with ;I hi$h-precision stand holding 12 MSGCs (six with

horizontal and six with vertical strips). and Fig. 19 is an actual view of the assembly. Access to the detectors for servicing was limlted to around 2 h per week. seriously constraining the interventions. As a result of various problems to be described later, the number. orientation and working conditions of detectors varied during the runs: Fig. 10 shows the evolution in time of the number of operating MSGCs. with a maximum of ten reached. The trigger for reading out the events was delined in an early stage by the colncidencc of two scintillators covering the active arca of the chambers; later on. a11 additional I cm’ sctntillator W;IS installed to facilitate data analysis bl restricting the acceptance and to study pc~sition-dL.pendcnt characteristics. All chambers were equipped with the PreMux front-end electronics (four 1X-channels chips per chamber): each channel includes a ch.lrpe-sensitive preamplifier followed by a shaper with 45 ns peaking time. and two capacitors used ;LSanaloguc buffers that can be open or closed to the shaper output through digitally controlled switches. The read out is made through an analngur shift register. transfcrring sequentially the content of the channels to aiialogue-to-digitat converters. Two strobe signals. S1 and SZ. store two values ol’ the Input charge: a double-correlated sampling of the signal is made by subtracting the two values IFI~. 31) [30]. For synchronous use at a collider. Sl would bc sent just

Fig. 19. Picture

before the event, and S2 after a delay to coincide with the peak of the signals; in a random trigger experiment, like this beam test, however. an external trigger is built with scintillation counters producing the S? strobe within a short delay

of the beam set-up.

(typically around 100 ns). After some d&y ( - 1 ps). a sequencer module generates the SI strobe used for baseline subtraction. This mode of operation has some drawbacks. particularly in a high-rate environment, as will be discussed later.

AOUT

j;,

A{

AOUT

Week

Fig. 30. Evolution in the number of active chambers during the six months test period in the high-intensity beam. The blank on weeks 34 and 35 corresponds to temporary removal of the system for repairs.

Local adaptor boards generate the fast trigger, build up the coincidences and handle the calibration signals, providing a programmable delay for S2 and distributing the power supplies and control signals up to six mother boards. Analogue outputs from the chambers are sent through coaxial cables to fast receiving amplifiers in the control room, and digitized by 6 bit resolution flash ADCs; the readout frequency is set to 1 MHz. The data acquisition system, hosted in a VME crate, recorded the events to a remote disk via an ETHERNET link; calibration and pedestal triggers were generated during the inter-spill time. The operating gas (argon-dimethylether in equal proportions), was distributed in parallel to the detectors with a flow of around 8 cm3/min per chamber. The limited gas exchange rate (15 volumes/h) coupled to undetectable gas leaks appeared to generate difficulties in the operation of some chambers. A single-wire proportional counter, connected in series to the gas outlet and irradiated with a weak 55Fe source, was used to monitor continuously the gas quality. High voltage and ambient parameters in the beam area were also recorded. 5. Operation in the high-intensity

beam

5. I. The background The data high-intensity

analysis problems generated by the operation are well illustrated in

Fig. 21. Scheme of the double-correlated sampling m the PreMuxl28 chip. The switches SI and S2 are open two time5 to sample the amplified signal: the difference between the sampled voltages is the PreMux output.

Fig. 22, showing an event with at least seven tracks detected in six parallel MSGCs. More in detail, Fig. 23 shows a typical strip map recorded on one MSGC plane; raw data have been corrected for pedestals and common mode offset with a procedure to be described in the next section. As seen, many “hits” both of positive and negative polarities are present. While positive signals correspond to real tracks (inverted anode pulses), negative signals are an artefact of the double-correlated sample measurement at high rates: if the strobe Sl has been applied with an accidental signal present, it is subtracted from the information strobed in by S3 (most often the baseline). Provided the beam has no time sub-structure. similar densities of background (negative) and real (positive) hits are expected. Fig. 24 shows distributions of cluster size and number, recorded for the two polarity signals: there are essentially identical.

.4. Barr et al.,‘Nucl. Instr. and A&h. in Ph>s. Res. ‘4 403 (1998) 31-56

43

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44

A. Barr et al. JNucl. Instr. and Meth. in Ph.vs. Res. A 403 (1998) 31-56

0

2

4

Cluster

Ev;

6

a

size (positive

signals)

Fig. 24. Cluster

4

6

size (negative

8 signals)

Eves

10 Clusters

20 (positive

size and number

30

40

10

0

Clusters

signals)

of clusters

The simultaneous presence of signals of both polarities in the events affects the evaluation of efficiency and resolution in two ways: l overshoots of negative signals, generated by the shaping constants of the amplifiers, can be mistaken as real clusters; l the probability of close, opposite polarity signals interacting destructively is not negligible. Fig. 25 provides the measured distribution of the distance between clusters; approximating with an exponential, one can deduce the probability of having overlapping clusters to be around 2%. From Fig. 24 one can also appreciate the high occupancy of the detectors: some events have more than 20 clusters in a chamber. A study of the correlation between maximum charge in two parallel chambers, Fig. 26, proves that this occupancy is due to real tracks, and not to noise or discharges in one plane. 5.2. Background

Cluster

El

0

2

0

studies and reduction

Before analysis, raw data are normalized subtracting pedestals and common mode offsets. For each channel, we define as pedestal the average value of the Gaussian distribution of the ADC counts obtained with random (off-beam) triggers; the common-mode offset, a DC shift of the whole strip map, is corrected event per event estimating the average of the pulse heights.

computed

20

30

(negative

40

signals)

for the signals of opposite

polarity

1500 a E A 1CQO

500

0

A

20

Number

Fig. 25. Experimental in one plane.

60

40

distribution

of

ships

between

of the distance

80 clusters

between

hits

The distribution of background hits cannot be computed directly from the beam rate, since it depends on the response of the particular chamber (chromium or gold strips), and on the electronics (input capacitance and impedance). The shape of the electrical signal and the way it is sampled with the Sl and S2 strobes are the key factors to determine the background distribution. Off-time background hits can exceed threshold if tracks are close enough to the sample to be detected, and if their pulse height are particularly large due to the Landau fluctuations in energy loss. Some simulation work has been necessary to better

.3 Barr et al. JNucl. Instr. and icleth. in Phts. Res. .4 403 (199X) 31 515

45

20

10 1

2

3

4

5

6

7

8

9

10

0

DLJC115 Position (cm) Fig. 26. Correlation m:~xm~um charge

between

the position

in two parallel

of clusters

with

150

200

‘lime (ns)

the

chambers. Fig. 27. Signal

understand the effect of this background on the reconstruction of tracks. For the study, we have assumed an ideal signal shape as shown in Fig. 27, corresponding to a calibration pulse with 50 ns peaking time. A Poisson distribution was taken for the hit probability on strips, taking into account the measured rates. As expected, the simulation shows that the combined pulse-height distribution, i.e. trigger plus background, is very different from the real clusters pulse-height distribution (Fig. 28). Indeed, the background pulse-height is mainly due to low charge signals, producing an important correlation between the charge threshold used for the analysis and the occupancy in the chamber. In addition. the ratio between large background over real pulses is much larger than one; using a simple cut on the cluster charge cannot separate background and real hits. Note also that an increase of the chamber gain will not help, since background and real charges will increase proportionally. To improve background rejection, we have followed u complete tracking procedure in order to reject background hits; from simulation, we estimate the probability of reconstructing a track from background hits to be three times lower than for a real one. This procedure did not seem appropriate at the beginning because of the low tracking

shape used for the simulation

studies.

Y

5

!S

180 160 140 120 100

“Real” clusters

Trigger + Background

80 60 40 20 0 -800

-400

0

Fig. 2X. Smulated pulse-height distribution ing background and trigger clusters.compared charge.

400 800 Cluster charge (a.~.) obtained comhinto the real cluster

efficiency achieved with the preliminary data. In addition, the standard definition of cluster charge is not adequate in this particular environment since it does not reproduce the real energy loss of particles.

A. Barr et al./Nucl. Instr. and Meth. in Phvs. Rex A 403 (1998) 31-56

46

x 10-Z iI

6 &

0.3

0.25 Background

0.15

0.1

0

-30

-20

-10

0

10

20

30

Clusters charge

Fig. 29. Experimental

noise and signal charge

distributions.

A second, more efficient method to study background was implemented reducing the trigger area by hardware with a small additional counter; this does not modify the charge distributions, but gives a higher probability to real track, thus enhancing the signal-over-background ratio in the delimited region. Fig. 29 represents the cluster charge distribution with a small counter (1 cm2) in coincidence placed at 5 mm from the detector edge; it represents the overlap of the contributions from electronic noise, real tracks and background due both to positive and negative polarity signals. Clearly, due to the combined effect of high rates and of the response of the electronics, even for the described selection the signal-to-noise ratio achievable is not very good. Track multiplicity can be reduced operating the PreMux128 chip as recently reported in Ref. [31], where the Sl strobe is sampled 50 ns before S2. At the time of the test reported here, this mode of operation was not possible. 5.3. Timing studies We have used the small-size counter in coincidence to study the position dependence of the chamber’s response. Fig. 30 shows the cluster distribution recorded in six MSGCs with the narrow

trigger at the edge of the detector’s sensitive area. Under these conditions, the dependence of the recorded pulse height from the S2 delay for tracks was measured, in the range between 50 and 1.50 ns allowed by the adaptor board; Fig. 31 shows the distribution of total charge in DUC 116 for different delays with the small scintillator located 6 cm from the amplifier end; Figs. 32 and 33 show the mean value of the total cluster charge for a gold and a chromium chamber, for different delays and three positions of the small trigger counter with respect to the amplifier end (1, 6 and 9 cm). In agreement with the results of electrical models developed to study the mechanism of signal propagation [21], chambers made with low-resistivity gold strips do not require a significant change of the trigger delay with position, while for chromium electrodes, due to the higher resistance, a clear dependence is seen. Notice also in this case the slower rise of the curves, due to the deterioration of the signals’ rise time. The signal attenuation is obviously a drawback of the use of resistive strips.

6. Problems encountered during the operation The mechanical support for the beam set-up allowed for the mounting of 12 detectors; as indicated above, we reached ten simultaneously operating devices, but towards the end of the six months running period only six of the chambers were still operational. Four of these (DUC 118, DUC 116, DUC 115 and DUG 222) were actually active during the entire running period, confirming the fundamental role of the original quality in determining the survival probability. We describe in this chapter the evolution of operation, and speculate on the reasons for the failures encountered with some devices. The major problem appeared to be a slow but persistent decrease of the safe operating voltage. certified from the laboratory tests, after installation in the beam set-up. Fig. 34 shows the evolution with time of the maximum voltage in a good and a bad MSGCs, the other plates being between both extremes. Several factors may explain this slow degradation, although a clear understanding of the observations has not emerged yet. Some MSGCs

A. Barr et al. lNuc1. Instr. and Meth. in Phw. Res. A 403 (1998) 31-56

Do

160

DK

318

120

I

vd-2

DUC 107

80

v,-590v

140

70

kV

SO

100

50

00

40

20

30

40

20

20

10

0

200

0

400

Cluster position

Cluster position 200

120

150

I

100

125 100

20

75

50

50

40

25

DUG 222 vc -540v

140

DUC 118 vc -5lOV

175

20

I

0

Cluster position

Cluster position

180 150 140

DUC 115 v, -58OV

200 175

Vd -2 kV

160

120 100

125

50

100

60

75

DUC 116 vc -570v Vd -2 kV

50 25 0

ir!

200

Cluster position Fig. 30. Strips map for six MSGCs

400

Cluster position obtained

appeared to be affected by microscopic gas leaks, developing after several months of operation; this is seen also in Fig. 34, where an increase of the gas flow (in week number 30) results in a moderate recovery of the chambers performance. The gas leaks could have been a result of a slow reaction between the epoxy used to glue the frames with DME, combined with the mechanical stress be-

triggering

with a

I cm’ scintillator

tween frame and glass due to variations in atmospheric pressure. The pulse-height analysis on the monitor counter at the gas output line did not provide evidence for gas pollution due to leaks; as shown in Figs. 35 and 36, the 5.9 keV peak in the pulse-height distributions at the beginning of the test run and two months later are in the same position, demonstrating that the oxygen pollution

3x

DUC

116

Vc -54OV, Vd -2kV

2

I--

2

Mean RMS

F W 40

20.32 14.94

9

W 40 130 ns 20

20

0 0

20

40

60 Cluster

80

0

100

20

40

60 Cluster

charge

80

100

charge

E

3

40

20

0 20

40

60 Cluster

80

100

0

20

40

60

80

100

0

20

40

60

80

100

charge

25

40

60 Cluster

Fig. 31. Total charge measured the plate.

in a chromium

chamber

80 charge

100

Cluster

for different trigger delays. with the small counter

(if any) was contained at ppm levels. Towards the end of the run, however, we initiated an emergency recovery program removing all ten chambers from the beam; a thin kapton foil was added on the drift

charge located

ar-ound rhe middle 01

window side, and the frame edges potted with epoxy in order to eliminate the leaks. Before re-installation, the detectors were baked at 80’ C in vacuum for 12 h; this procedure was allowed by the choice

‘4. Barr et al.~Nucl. Instr. and Meth. in Ph>,s. Res. .4 403 1199X) 31 -56

E

Xl

60

80

100

120

140

650

/I,

I, /

I, I

MAY

APRII.

49 / ‘,

-,-i,--,-r-prrr

JUNE

JULY

1

AUGUST

160

Week

Delay (ns)

Fig. 32. Mean of the cluster charge distribution in a MSGC with gold strips for different delays and three positions of the small trigger counter with respect to the amplifier end.

SEPT

Fig. 34. Evolution in time of the maximum operating voltage for two MSGCs. representative of the general behaviour of the system during the test.

40

n :

2

-r--7

~~-

1-

]

---.T7-T-r--r.

[ BEAM PROPORTIONAL 35 E GAS OUTPUT

_~

I

1

~~

COUNTER

I

LINE

30 25 ;

11

5 CHROMIUM

STRIPS 0

5 JO

60

80

100

120

140 160 Delay (ns)

Pulse Heqht

(ADC

Channel)

FIN. 33. Mean of the cluster charge distribution measured in a MSGC with chromium strips for different delays and at three positions of the small counter.

Fig. 35. X-rays pulse-height spectrum recorded with the monitor counter placed at the gas output lone of the MSGCs. at the beginning of the test run.

of assembly materials and epoxy. After this intervention, six chambers fully recovered to their original performance, although many bonds at the pitch adaptor level were lost due to manipulations and thermal stresses; the recovery to the original operating voltage for one chamber is seen in Fig. 34. This is suggestive of a slow degradation of the plate surface, possibly due to water adsorption and/or reaction with DME, eliminated by the baking procedure. A second, more fundamental problem has been the increase of shorts with time, in many cases appearing suddenly at the occurrence of the highintensity beam spill. Fig. 37 shows the evolution in

the number of missing channels (including shorts or missing bonds but not dead electronic channels) for three chambers; the initial numbers correspond to the defective channels found in the laboratory tests. The most likely cause for the deterioration are discharges triggered by the beam pulse, perhaps due to the presence of heavily ionizing tracks or showers. In addition. it has been observed that the probability of failure increases in the neighbourhood of strips left floating to recovery from a short; nalysis of the cluster distributions shows indeed that the detected charge is larger and close to a non-connected anode, certainly due to the local distortion of the electric field (Fig. 38). This effect is

50

A. Barr et al. /Nucl. Instr. and Meth. in Ph.vs. Rex A 403 (1998) 31-56

1 125

1000 Pulse Height

Fig. 36. As in the previous The 5.9 keV peak position

1500 (ADC

Channel)

figure. after two months of running. is substantially unchanged. I

160

180

200

220

I

I

240

260

280

300

& 320

Cluster position DOC 118 Fig. 38. Reconstructed cluster position in the central section of a plate; several missing groups are seen. with an increase of counts at the edges.

;-_I’~~~ 15

17

28

32

33

37

38 Week

Fig. 37. Evolution in time of the number of missing channels (including shorts and missing bonds but not dead electronic channels) for three chambers.

also observed in case of dead electronic channels, and is a potentially very dangerous feature of the detector. The evolution of performances and the modifications made in the system hindered the long-term evaluation of detection efficiency, as well as the comparison of performance between chambers with different characteristics, and only some general trends could be deduced.

7. Analysis procedure and general results A software events reconstruction was realized off-line on recorded data. In each record, after the

0

2

4

6

8

0

2

4

Cluster size

0

2

4

6

8

Clustersize Fig. 39. Cluster dicular tracks).

size distribution

6

8

Cluster size

0

2

4

6

8

Cluster size

for several chambers

(perpen-

pedestal and common-mode corrections mentioned in Section 5.2, a cluster finding algorithm was used to find rows of consecutive strips with detected charge above the threshold set at a value

.A.

Barr

et al.!Nucl.

Ins!r.

rtnti Mrth.

in Ph,vs

Meall

Y

RMS

22.57 15.66

51

A 403 (I ‘)9X) 31-56

ponding distributions of total charge (sum within the cluster) for several chambers in tnoderately good operating conditions, with a signalinoise ratio around 17. In this case, the rms of the noise is around 1600 electrons. Fig. 41 shows the cluster pulse height for a run with a better performing detector, and a signal-to-noise ratio around 30. The position of the tracks is calculated applying the centre of gravity method to the charge distribution. The limited angular spread ( < 0.1 ‘) allows to use simple alignment of hits in consecutive planes as tracking algorithm. After an iterative precise alignment of the chambers, one plane is taken as

corresponding to twice the rms of the noise. We define as noise on each strip the rms of the (Gaussian) distribution measured on the corresponding channel in the absence of radiation (random triggers between spills); signal over noise is then defined as the ratio between the most probable value of the detected charge and the rms of the noise distribution. If a hole (strip with charge under the threshold) is found, the search for a new cluster is started. Fig. 39 shows the cluster size distribution in several chambers for a representative run, with a mean value of 1.4 typical for anode read out and perpendicular tracks; Fig. 40 provides the corres-

5 &lo0

Rcs.

3

5 s150 w

DOC 318

DUG 221 HVd -2kV

60

80

100

60

80

MeaIl RMS

20.56 14.20

20, 8. &

DUC 118

100

Pulse height

Pulse height

60 40

60

80

60

100

80

Pulse height

100

Pulse height

MtWI RMS

25.85 16.31

DUC 116

20

40

60

80

100

60

Fig. 40. Total cluster

charge

for several chambers,

80

100

Pulse height

Pulse height recorded

in a range of operating

conditions

52

A. Barr et al. /Nucl. Instr. and Meth. in Ph.vs. Res. A 403 (1998) 31-56

DUC 115 vc -590 v V;2kV

ou t 0

~-I-.L.L-_LJ

A

20

40

60

80

Pulse height

Fig. 41. Optimal cluster charge distribution measured in a well performing chamber.

100

(ADC units)

and noise spectrum

reference, and for each cluster found there tracks are searched within an acceptance window in the other planes; we have fixed its width to five times the strip’s spacing (i.e. one mm). All possible combinations are tried with the candidate hits fitting a straight line, allowing each cluster to belong to different but very close tracks. The reconstructed track is then selected on the basis of a chi-square minimization. For the reasons described, the evaluation of efficiency and spatial resolution is done using the data recorded with the small trigger counter in coincidence. Fig. 42 shows the distribution of residuals for each chamber, difference between the predicted and measured position. After taking into account the geometrical factor, the spatial resolution for tracks perpendicular to the detectors varies between 40 and 55 pm rms depending on the chamber performances. The efficiency for each MSGC plane is evaluated as the ratio between the number of tracks reconstructed by clusters found in the other planes, and the number of clusters found in the chamber, with an acceptance equal to + 3 times the standard deviation of its residuals. Efficiencies reach - 98% for chambers in optimum operating conditions and in different gas mixtures (Ar-DME [50-501, Ne-DME [50-503, DME), although lower values are often obtained due to forced operation at lower gains or to a large number of missing channels; the

data analysis evaluates 12% lower efficiency in case of 10% of dead channels, typical value at the end of the beam test. Fig. 43 shows examples of efficiency as a function of cathode voltage for the plates DOC 318 and DUC 115, and the small trigger counter close to the amplifier end. For the higher voltages, the detection efficiency is in the range 95-98%; the different behaviour of the plateaux has to do more with time-dependent variations of performance than with intrinsic properties of the plates (overcoat and undercoat, with similar values of resistivity). Fig. 44 shows the cluster charge as a function of the strip number for DUC 115; it is clear that the out-of-time tracks have lower pulse height; the signals detected in the small counter region have 15% higher amplitude. Consequently, the tracking program tends to reconstruct more efficiently hits coming from the trigger area than hits from the out-of-time background, as shown in the example of Fig. 45. Nevertheless, out of the counter region the efficiency is rather constant independent of the exposure rate. Fig. 46 shows the number of reconstructed tracks per event in the small counter region as a function of the applied threshold (given in multiples of the noise rms); a maximum is found at three times the rms of noise. Fig. 47 shows for several chambers the overall efficiency value and the corresponding noise occupancy, i.e. the ratio between the number of strips with signal over the threshold registered with a random trigger in the absence of the particle beam, and the total number of strips; both values are calculated outside the small counter region and therefore they are more pessimistic than the real noise and efficiency of the chambers. Between 2 and 3 standard deviations, the efficiency does not decrease more than 1% keeping a reasonable noise occupancy ( < 5%).

8. Summary and conclusions A 6 months long, high-rate beam test was carried out to study and gain experience on various aspects of assembling and operation of several 100 x 100 mm2 MSGCs with anodes read out by PreMux128 chips.

Mean -.890&02

0 -0.4

-0.2

0

0.2

-0.7.

-0.4

0.4

0

Mean

Sigma

-.3DoE-02 .49E-01

$3002

w

200 -

0

0.2

0.2

,

Residuals (mm)

Residuals (mm)

DUG 222

v, -500v

’ r

Mean -.368&02 Sigma .53E-01

Vd -2kV

0.4

Residuals (mm)

Residuals (mm)

Mean

2

-l.O5E-0:

G &

200

0

-0.4

-0.2

0

0.2

0.4

Residuals (mm) Fig. 41. Distribution

of residuals

Residuals (mm)

for reconstructed

We have described in detail the construction and test procedures of the chambers, using the light and cheap mechanical assembly developed in order to satisfy the material budget of particle-physics experiments. A dozen MSGCs were admitted in the set-up at several stages of the test program, with a maximum number of ten simultaneously operating devices. 5000 being the number of read-out channels; at the end of the six months continuous operation six

tracks

in a set of SIX chamber>

chambers were still operational, four of them active for all the running period. Operation of the chambers in the high-intensity beam appeared more arduous than expected. A slow but continuing decrease in the maximum voltage reached during the laboratory tests has been observed after installation in the beam. Factors that might have affected the operation are the complete but stressing test procedure with many manipulations, the reduced gas exchange rate

54

A. Barr et al. /Nucl. Instr.

Meth. in Phys. Rex A 403 (1998) 31-56

and

85

80

90

75: 70

I11 540

550

560

570

580

590

600 -v<

--m-V

85

AI-DME (SO-501 Drift -6.7 kV/cm

-590V,Vd-2kV

610

200

400

(V)

Strip

Fig. 43. Efficiency as a function of cathode voltage for two detectors.

500 number

Fig. 45. Efficiency as function of strip number for DUC I15.

100 E .a .z 1 z

90 80 70 60

B E

50

0.8

i O-6

40

0.4

30

Threshold

20 10 0 0

I

I

I

I

100

200

300

400 Strip

1

Fig. 46. Reconstructed tracks per event in the small counter region. as a function of the threshold value used by the cluster finding program.

500 number

Fig. 44. Total cluster charge as a function of coordinate corded in a chamber.

re-

with the presence of microscopic gas leaks in the beam set-up, and the possible deterioration of the assembling materials with prolonged exposure to DME. A second, more fundamental problem has been the regular increase in the appearance of high current spikes, in many cases coincidental with the occurrence of the high-intensity beam spill; the most likely cause for this deterioration is a discharge triggered by the beam, perhaps due to the presence of heavily ionizing tracks or showers.

combined

(rms of noise)

Use of the PreMux128 circuit, an intermediate stage in the development for the electronics of the CMS tracker, in a random trigger experiment posed several difficulties to evaluate the appropriate signal-to-noise ratio, efficiency and spatial resolution of the detectors under study. Together and indistinguishable from the real, a large number of out-of-time tracks are detected due to the memory of system, complicating the tracking procedure. With the help of a 1 cm2 trigger counter in coincidence, and with detectors operating in optimal conditions, efficiencies can reach 95-98% with a signal-to-noise ratio around 30 and noise occupancies below 5%. With lowering gas gains, and

0

A. Barr et ul. /Nucl.

Insrr. and A&h

1

6 7 SO (rms-noise)

2

3

4 5 Threshold

4 5 6 7 Threshold (mu-noise)

irr Phrs. Rw. .4 403 (IWX) 31.-56

0

I

2

3

4 5 6 7 8’ Threshold (rms-noise)

o

I

2

3

4

SO 75 70 65

60u80 0

,

2

3

0

1

2

3

Fig. 47. Overall

I,,,,,,,,,,,,,,,,,,, 4 5 6 7 R” Thrffhald (rms-noise)

efficiency

and

5 6 7 8’ Threshold (rms-Norse)

occupancy as 21function of threshold

due to the progressive increase in the number of missing channels during the operation, the detection efficiency did not exceed in some chambers 90-91%,. Some of these results may suffer for our attempt to minimize the frame and drift window materials. A more conservative choice could help. For the reasons mentioned above, an evaluation of detection efficiency as a function of time as well as a comparison of performances between chambers with different metals, substrate resistivity or diverse gas mixtures is not conclusive.

for six chambers.

Acknowledgements The authors would like to acknowledge A. Magnon and the NA47 group for their help in the beam area.

References [I] A. Oed. Nucl. Instr. and Meth. A 263 (19%X) 351. [2] CMS Technical Proposal, CERN/LHCC 94-M. December 1994.

IS

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[9]

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[13]

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HERA-B, DESY-PRC 94/02, 1994. F. Sauli, CERN-DRDC/94-45, 1995. F. Sauli, CERN/LHCC 96-18. 1996. Proc. Int. Workshop on Micro-Strip Gas Chambers, Legnaro, Italy, 1994. Proc. Int. Workshop on Micro-Strip Gas Chambers, Lyon, France, 1995. F. Sauli, CERN-PPE/97-19, Proc. 5th Int. conf. on advanced technology and particle physiscs, Como. Italy. 1996. J. Bohm, R. Bouclier. M. Cape&. C. Garabatos. Cl. Manzin, G. Million, F. Sauli. T. Temmel, L. Shekhtman, Nucl. Instr. and Meth. A 360 (1995) 34. R. Bouclier, M. Capeans. C. Garabatos. Cl. Manzin. G. Million. L. Ropelewski, F. Sauli, T. Temmel, L. Shekhtman, V. Nagaslaev, Yu. Pestov. A. Kuleshov, Nucl. Instr. and Meth. A 365 (1995) 65. R. Bouclier. M. Capiens. C. Garabatos. G. Manzin, G. Million, L. Ropelewski, F. Sauli. T. Temmel, L. Shekhtman. G. Della Mea, Cl. Maggioni, V. Rigato, Nucl. Instr. and Meth. A 367 (1995) 168. R. Bouclier, M. Capeans. G. Million, L. Ropelewski, F. Sauli, T. Temmel. R.A. Cooke, S. Donnel. S.A. Sastri, N. Sonderer. Nucl. Instr. and Meth. A 369 (1996) 328. B. Boimska. R. Bouclier, M. Capeins, W. Dominik. M. Hoch, G. Million, L. Ropelewski, F. Sauli, A. Sharma, CERN-CMS-Tracking Note 1996/016. R. Bouclier, M. Capeans. C. Garabatos. G. Manzin, G. Million, L. Ropelewski, F. Sauli, L. Shekhtman. K. Silander, T. Ropelewski-Temmel, Nucl. Instr. and Meth. A 381 ( 1996) 289. A. Peisert, F. Sauli, Proposal for the Construction Installation and long-term beam test of a system of 20 medium-size micro-strip gas chambers, Internal Note CERN-PPE/GDD 8.8.95. L. Shekhtman, private communication. R. Bouclier, M. Capeans. M. Hoch. G. Million. L. Ropelewski, F. Sauli. T. Ropelewski-Temmel, CERNPPE/95-167. A. Barr, B. Boimska, R. Bouclier. M. Capeans. W. Dominik, M. Hoch. G. Manzin. G. Million, L. Ropelewski,

[19] [20]

[21]

[22]

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[27] 1283 1291

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