Properties of streamers in streamer tubes

448

Nuclear Instruments and Methods in Physics Research 222 (1984) 448-457 North-Holland. Amsterdam

PROPERTIES OF STREAMERS IN STREAMER T U B E S Rainer BAUMGART,

Claus GRUPEN

and Ulrich SCH,~FER

Department of Physics, Siegen University, Hi~lderlinstr. 3, 5900 Siegen, German)" Received 4 October 1983

We have constructed various types of plastic streamer tubes with the purpose of investigating the properties of streamers. After having established the already known features of streamer tubes we derive results about the space and time dependent dead zone produced by streamers. We obtain dead zones of - 1 2 mm length per streamer which fully recover within 150 t~s. The streamer structure is studied by measuring the signals on narrow pads both along and perpendicular to the wire. A limit on the lateral streamer width is inferred from a simple model on the basis of our measurements. The center of the streamer charge turns out to be very. dose to the anode wire and the lateral width of the streamer is less than 0.5 ram. A two track resolution of 12 mm was obtained. The streamer coordinate along the wire can also be easily measured with a special delay line pad. The gross features of streamers obtained with the purely electronic readout were confirmed by an optical observation of streamers.

1. Introduction Particle detection in experiments where large areas or volumes have to be covered require reliable and inexpensive detector components. An excellent candidate for such applications are plastic streamer tubes [1,2]. Large areas, for example, are covered in experiments to be performed at the electron-positron collider LEP at C E R N . In this case layers of streamer tubes are used as sampling elements in hadron calorimeters or as large muon detectors outside an iron shield [3]. Large volumes need to be equipped for proton decay experiments. This decay is searched for in large volume detectors, which can be just huge water Cherenkov counters; however, stacked assemblies with the capabilities of particle tracking might be superior because they are not restricted to specific decay modes. An example for a plastic streamer tube volume detector is the proton decay search experiment in the Mont Blanc tunnel [4]. The performance of plastic streamer tubes in electron and hadron shower counters has already yielded good results [5-7]. The idea of electrodeless drift chambers [8-10] has also been applied to plastic streamer tubes and simplifies the construction of this device even further [11]. The properties of plastic streamer tubes have been investigated already quite extensively [1,2,12-14]. One of the disadvantages of plastic streamer tubes in applications for e l e c t r o n / h a d r o n calorimeters or even generally for tracking purposes is the limited two- or multi-track resolution. The streamers caused by the penetrating particles produce a temporary dead zone which leads for example to saturation effects in calorimeters already at energies as low as 15 GeV for electrons 0167-5087/84/$03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

[5] or 80 GeV for hadrons [6]. The purpose of this p a p e r is to make an extensive study of the streamer width, the time J dependence of the dead zone produced by a streamer and t h e two track resolution using streamer tubes with pad readout. The results were obtained with a purely electronic readout. We have, however, also tried to make a visual observation of streamers using image intensifiers. The cylindrical

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R. Baumgart et aL / Properties of streamers

449

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properties inferred from the optical readout are in accordance with the electronic readout results. Moreover we have investigated the possibility of determining the coordinate along the wire by a simple delay line readout. All details of the experimental setup, measurements and analysis are described in refs. 15 and 16.

2. Experimental setups The observations of limited streamer discharges were done with different experimental arrangements. For the wire readout and dead time measurement a cylindrical streamer detector was used consisting of a brass tube with end caps made from PVC. The tube dimensions are shown in fig. la. A rectangular tube (sensitive area 20 m m × 20 mm; fig. lb) was used to determine the different discharge modes, to measure the wire singles rates, pad pulses and their spatial distribution. This tube was also used for a delay line readout. The delay line layout can be seen in fig. 2. The effects of a gap reduction on the spatial resolution were studied with a wall-less streamer tube chamber (gap width 10 mm; see fig. lc).

3. Readout - schemes

Fig. 3. Typical wire (a) and pad signals (b). The scale for both figures is 20 mV/div, and 100 ns/div., respectively; the signals are measured on a 50 g2 load.

read out by inverting preamplifiers (50 $2 input impedance) and fed through main amplifiers to the above mentioned system consisting of 12 equivalent analog channels. This electronic circuitry was used for the measurements of the spatial resolution with resistive and electrodeless cover.

3. I. Wire readout 3.3. Delay line readout

The wire readout was performed with standard N I M modules. Because of the properties of the streamer pulses (about 50 mV amplitude, 100 ns duration with 50 $2 load, see fig. 3a) there was no need for sophisticated amplifiers. The signals were directly fed into leading edge discriminators with minimum 30 mV threshold or A D C s for analog readout. The data acquisition electronics consisted of a C A M A C system and a PDP 11/23 computer. 3.2. Pad readout

Positive pulses from the pick-up electrode (see fig. 3b) were taken from twelve 2 mm x 20 mm pads and

Streamer pulses injected into an "electrodeless" delay line printed circuit installed as a cover of the rectangular tube are picked up from both ends of the pad. The impedance of the delay line (100 $2) and the coaxial cable (50 $2) are matched by a series resistor. After pulse inversion and amplification the signals from both ends of the line are fed to discriminators and cable delays generating start and stop pulses for a time-to-amplitude converter (TAC). The output signal of the T A C is a measure for the time difference between left and right end of the delay line and thus for the location of the streamer development. It is digitized by a C A M A C A D C and transferred to the computer.

450

R. Baumgart et al. / Properties of streamers

4. Results

[--i 4.1. Typical wire and pad signals

Typical streamer pulses from the rectangular tube induced by a 9°Sr source and photographed from the scope are shown in fig. 3a. The wire signals are negative of about 50 mV amplitude and 100 ns duration. The pulses were obtained with a gas mixture of 60% argon and 40% isobutane at a high voltage of 4.4 kV. A stainless steel wire of 100 #m diameter was used as anode wire. The positive pad signals (fig. 3b)measured on a 20 mm x 30 mm copper strip under slightly different conditions (4.8 kV high voltage, 50% argon) still have amplitudes of about 20 mV. However, their duration is reduced to about 80 ns because of the higher concentration of the quenching agent. The pad arrangement for the signals shown in figs. 3a and b was of the electrodeless type.

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Varying the anode high voltage of the rectangular tube one can obtain different modes of discharge development with discontinuously changing characteristic pulse shapes. Fig. 4 shows small proportional pulses along with streamer pulses observed simultaneously under constant working conditions. Here a high voltage of 3.8 kV and a 60% argon/40% isobutane gas mixture was used. The entire range from the proportional region up to the end of the streamer plateau was scanned to get informations about the distribution of the signal amplitudes from the wire. The pulse height spectra of wire signals for different high voltages on the wire are shown in fig. 5. Due to the large dynamic range in streamer charges the calibration of the ADC channels had to be changed for each high voltage setting. Characteristic

charge values are indicated in the figure. For low anode voltages of 3.2 kV only proportional pulses occur. They increase in amplitude with increasing high voltage. At the same time the first streame r pulses appear at 3.4 kV. Here one can clearly see the simultaneous occurrence of proportional and streamer pulses. With increased voltage the streamer charge :spectra spread out due to the development of multiple streamers induced by a single primary ionizing particle. The collected charges corresponding to the peak values in the distributions of fig. 5 are plotted in fig. 6 as a function of the anode voltage. It is obvious that there is no continuous transition from the proportional I

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R. Baumgart et al. / Properties of streamers '

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Fig. 7. Singles rates plateau curves for different electronic dead times. to the streamer mode. The break of performance characteristics occurs for our tube at high voltages around 3.4 kV. From the charge distribution measured at the highest anode voltages one might infer that additional peaks occur. It is not clear whether this indicates another discontinuous transition region as argued by Alekseev [2]. We rather believe that the structure observed at high anode voltages is due to multiple streamer formation. 4.3. Wire singles rates Measurements of the wire singles rates in the streamer mode obtained with the rectangular tube exposed to a

9°Sr source were performed for various anode voltages. A long really flat plateau of the singles rates can be observed especially if dead time circuitries are used to suppress afterpulses generated at higher voltages. The results for the singles rates are shown in fig. 7. If no veto is generated after a streamer signal the plateau extends over approximately 100 V only. An electronic dead time after each signal of e.g. 1 /ts prevents afterpulses, which occur in this period, to be recorded• Hence the plateau length increases to more than 500 V. It was observed, that the appearance of afterpulses is highly dependent on the cathode material which is used. Because of the high first ionization potential of carbon which is relevant for the electrodeless design used for the wall-less chamber (the chamber is made of epoxy which contains essentially carbon) afterpulses are in this case suppressed• It is therefore recommended not to use aluminium as inner coating of the streamer tube but rather other materials such as carbon. This feature was already observed by Battistoni et al. [13]. The length of the singles rates plateau increases with growing quenching gas fraction and wire diameter (see figs. 8a, b). For the measurements an electronic dead time of 0.5/~s was used. 4.4. Wire and pad efficiencies According to the long singles rates plateau there exists a wide range of anode voltages where all versions

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R. Baumgart et al. /' Properttes q~ .~treamers

of our streamer tubes achieve an efficiency of essentially 100%. The wire efficiency of the rectangular tube of electrodeless design (fig. 9a) results from a measurement of cosmic ray induced streamers with cosmic ray muon,~ detected by a scintillator trigger telescope. The pad efficiency (fig. 9b) measured with the same gas mixture of 60% argon and 40% isobutane was taken with the tube irradiated by a 9°Sr source. In this case the efficiency was determined as the ratio of the wire singles rate and the coincidence rate between wire and pad signals. Therefore, fig. 9b gives only the relative pad efficiency with respect to wire efficiency. Since the wire efficiency is basically 100% the same is true for the pad efficiency. The decrease in pad efficiency starting at high vohages around 4.5 kV can be explained as follows: Here, low amplitude afterpulses appear and are recorded on the wire. However, the associated pad pulses are too 1o~ to pass the discriminator threshold for pad signals. If the high voltage is increased even further, also the pad signals associated with afterpulses are registered and increase the pad efficiency again. The drop in pad efficiency can be avoided by operating the tube at suitable high voltages or by using appropriate electronic dead times. 4.5. Dead time and its space and time structure

The local wire dead time and its dependence on the distance from the streamer position along the wire was studied by two different methods using the cylindrical tube. Fig. 10 shows the dead time as a function of the high voltage as measured by means of the two sources method. If N 1 and N 2 are the counting rates of the streamer tube in case it is irradiated separately by the two different sources and if N~2 is the rate if the tube is exposed to both sources at the same time. the local dead

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R. Baumgart et al. / Properties of streamers

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Fig. 12. Normalized charge distributions on cathode pads along (a) and perpendicular (b) to the anode wire for a rectangular streamer tube (wire diameter: 100 p~m). c o u n t i n g rate is attained. F r o m this it can be concluded that it takes the streamer tube a b o u t 150 #s to recover completely. In this period of recovery the local dead zone 6 shrinks in time as can be seen from fig. 1lb. This time dependence is directly related to the self coincidence rate as was shown by Alekseev [2]. 4. 6. Pad readout along and perpendicular to the wire The spatial resolution of the streamer tube was det e r m i n e d by use of an a r r a n g e m e n t consisting of 12 pads, as described in section 3. The pad width was 2 m m and the distance between two pads 1 mm. The m e a s u r e m e n t s of the induced signals on the pads were done along and perpendicular to the wire. F o r the m e a s u r e m e n t along the wire the pads covered a distance of 35 mm, while the distribution of pad signals perpendicular to the wire was taken over the full width of the streamer tube. The tube was operated in the electrodeless version at 4.2 kV with an a r g o n / i s o b u t a n e gas mixture of 6 0 / 4 0 a n d it was irradiated through a hole in the central pad by 5.9 keV X-rays from a 5SFe source. The spatial charge distributions o b t a i n e d are shown in figs. 12a a n d b. For all pads located within the sensitive volume of the tube there is essentially no difference in amplitude between longitudinal and transverse readout. The full width at h a l f - m a x i m u m is of the order of 13 mm. A graphite coating on the cover (see fig. l b ) leads to a b r o a d e n i n g of the distributions by a b o u t 50%. N o significant influence of the gas mixture on the width of the distribution was observed.

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4. 7. Results o f the wall-less chamber The same pad array m o u n t e d on a wall-less 3-wire epoxy c h a m b e r (electrodeless type) provides a distinct i m p r o v e m e n t in spatial resolution due to the decreased gap (see fig. 13a). The transverse readout indicates that there is essentially no cross talk between two neighbouring cells (fig. 13b). These features and the fact that the c h a m b e r can easily be constructed could make it attractive in low cost applications. 4.8. Two track resolution Cosmic rays showering in a 10 m m lead target m o u n t e d 23 cm above the rectangular tube were used to investigate its multiple track resolution. To determine this resolution the p a d array was set up to measure the signal distribution along the wire. Two typical events (figs. 14a, b) indicate that the two track resolution is only limited by the single pulse distribution (fig. 14c). Since the single pulse distribution is about 13 m m wide our pad a r r a n g e m e n t of length 35 m m permitted only two tracks to be observed in one event. To d e m o n s t r a t e how a three-track event would look like in our streamer tube we fitted two events together in fig. 14d. The fitting procedure is straightforward and u n a m b i g u o u s because the individual signal distributions are very stable a n d smooth.

R. Baumgart et aL / Properties of streamers

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POSITION [ram} Fig. 14. Examples of two track events (a) and (b) and a single track event (c) recorded in the rectangular streamer tube. A fitted three track event is shown in (d).

4. 9. Delay line readout

Finally a further simplification in the readout was tested with the rectangular electrodeless covered tube. A meandering structured delay line (fig. 2) was used as pad and integrated into the cover. The time lag between the signals from both ends was digitized (see sect. 3)

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and the streamer location was computed with a PDP 11/23. For this measurement the streamer tube was locally exposed to a 55Fe source. Fig. 15 shows the measured position of streamers for different high V01t' ages and gas mixtures. The X-rays fr0m the 55Fe source were injected into the tube at a position of 40 mm. An improvement in space resolution with increasing quenching gas fraction can clearly be seen. The slight asymmetry observed for the three distributions is proba, bly due to the fact that the X'rays were emitted not exactly perpendicular to the wire. In fig. 16 spectra of three irradiated positions are shown. The position of exposure ig indicated by the dashed vertical lines. Due to dispersions of the signals propagating along the delay line it is not perfectly linear if leading edge discriminators are used. The Calibration Of the delay line was performed by uniformly exposing the streamer tube to cosmic ray muons, recording their delay time spectrum and integrating it numerically.

5. Conclusions on streamer size and location

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5.1. Streamer width along the wire and center of dib'charge

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On the premises that streamer induced cathode pulses are produced by an electrostatic coupling of a time dependent point charge on to a conducting p l a n e the theory of electrostatics describes the charge distribution measured on pads unamhiguously.Given a point :charge q at a vertical distance a from the cathode, the surface charge density o at a distance r is given by qa 0

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can be computed according to the formula ~/

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where A is the normalized charge amplitude o ( r ) / o ( r = a). This computation was performed with data taken from our 12-pad arrangement. The results presented in fig. 17 show the calculated positions of the closest distance a of the charge center from the pad. The minimum of this distribution must b e identified with the position of the anode wire where no discharges occur. Since the wire is at a distance of 10 mm, our calculation indicates a 10% systematic shift in distance measurement, which might originate from the simplifying assumption, that the whole streamer charge is concentrated in one point. However, the distance between the two peaks of the distribution, which are produced by X-rays ionizing the gas on the near or away side of the wire with respect to the readout pads, gives information on the distance of the streamer from the wire. We find a streamer-wire distance of about 0.6 mm. The width of this distribution in streamer distance at halfmaximum is of the order of 0.5 mm. This simple model already gives reasonable results; it can thus be concluded @at the assumption of a point charge is essentially correct. The majority of the streamer charge is concentrated at a distance of 0.6 mm from the anode wire and the spread of the distance distribution of the order of 0.5 mm gives also a limit on the lateral spread of the discharge. This result is confirmed by the optical observation of streamers as described in section 6 where a lateral spread of the discharge of 0.3 mm was found. 5.2. S t r e a m e r position along the wire

The investigations on streamer discharges described above allow to determine the spatial resolution of the three different types of pad readout. If a purely digital

455

readout were chosen and the threshold for the pad signals would be half the maximum value found in the analog signal distributions, the space resolution can be read off fig. 12 to be less than 13 mm. This corresponds to a r m s resolution of o < 5.5 mm depending on the pad width. Of course the distribution of the an.?Jog signals contains more information than just a digital readout. The streamer position can be inferred from the peak of the pad distribution. For all events accumulated in fig. 12a the peaks of the pad distributions for individual events coincided with the position of the hole in the central pad through which X-rays entered the chamber. All pad distributions were completely monotonous. Therefore the streamer location can be determined at least up to the width of one pad, i.e. 2 mm by this method. The results from the wall-less chamber (fig. 13) show that the space resolutior~ can be improved by decreasing the gap width. A further segmentation of the pad array would certainly have demonstrated that more clearly. This is due to the fact that the spread of the pad signals on the cathode depends on electrostatic effects only. Using the delay line readout (fig. 15) a r m s space resolution of 2 mm can be obtained if an appropriate gas mixture is used. The result can even be improved by further decreasing the gap width. A better linearity of the delay line can be obtained by using zero cross or constant fraction rather than leading edge discriminators.

6. Optical measurement of streamers The camera used for streamer observation was originally installed in a fixed target streamer chamber experiment at the Deutsches Elektronensynchrotron, Hamburg, for the remote control of the performance of a streamer chamber. Since streamers in pulsed streamer chambers are relatively bright, it was not clear whether the light output from streamers produced in plastic streamer tubes was sufficient to be recorded with this system. The camera is of type T X K 91 originally used for the transmission of black-and-white pictures [17]. This model was modified so that it can be used as image intensifier. All electronic circuitries for picture pick up and reproduction were provided by the camera. The main element of the camera is an image telecon XQ1320 electronic pick up tube coupled to an image intensifier [181. The details of the pick up tube are described in ref. 19. An optical lense with a focal length of 24 mm of type Distagon [20] was mounted in front of the pick up tube. A hole of 15 mm diameter was drilled into the rectangular streamer tube and sealed again by a thin

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lucite window. The streamers were observed through this hole in the streamer tube. The recorded pictures could be stored in a memory MSC-1 with the help of an image storing tube [21]. A magnification of the stored picture is possible. The magnified part can be shown on a TV-monitor coupled to the system. Pictures of streamer discharges produced by electrons from a 9°Sr source were observed on the television monitor. Since the scanning frequency of the pick up system is 50 Hz, all events in a time interval of 20 ms were recorded on one frame. The streamers were photographed by an ordinary camera from the TV set. The streamer size could be inferred from the optical magnification. Fig. 18 shows an example. The discharges can be seen as light spots of (0.3 + 0.1)ram diameter. The picture shows that within the period of 20 ms two streamers can be resolved if their mutual distance is larger than 0.8 mm. The picture unfortunately does not give the minimum resolvable distance of two streamers caused by the simultaneous passage of 2 particles. It essentially only provides information on the streamer size for individual events caused by one particle. From the electronic measurements we know already that the center of the streamer is located very close to the anode wire. This is also the region where most of the light comes from. We cannot derive information about the possible faint tail of the streamer perpendicular to the wire. All we can say is that the diameter of the streamer along the wire is of the order of 0.3 ram. This finding is in agreement with the results of Atac et al. [14]. 7. Summary and outlook

The properties of basically three different types of streamer tubes have been investigated in some detail.

The signals originated by streamers can be recorded with full efficiency either on anode wires or on pads sitting outside the sensitive volume of the streamer tube. The streamer tube operation is rather uncritical as far as gas mixture and wire diameters are concerned. For our tube geometry we measure a dead zone of the order of 12 mm along the wire which fully recovers within - 150 /as. The actual lateral streamer size is less than 0.5 mm. From the optical observation we infer a streamer width of the order of 0.3 mm. The center of the streamer charge is very close to the anode wire; in our case 0.6 mm away from the wire. For practical purposes it may be useful to operate streamer tubes in an electrodeless configuration. The construction is simplified without any loss of performance characteristics. The streamer position along the wire can of course be obtained from the pad signals. The same aim is achieved by reading out the induced charge via a special meandering delay line pad. Here resolutions of o = 2 mm are obtained. An important characteristic of plastic streamer tubes is its two track resolution. This figure is rather relevant for applications of streamer tubes as sampling elements m electron and hadron calorimeters. For our large geometry tubes we measure a two track resolution of better than 13 mm. During the investigations on the characteristics of streamer tubes and properties of streamers it turned out that the operation of these particle detectors is very uncritical, their performance is rather stable and there is room for many ideas on extending their field of application. This work was initiated by possible applications of plastic streamer tubes in LEP experiments. We are indebted to G. Battistoni for valuable comments which he gave during a stay at Siegen University and also for the discussions we had with him.

R. Baumgart et al. / Properties of streamers

The image intensifier camera was provided by the Deutsches E l e k t r o n e n s y n c h r o t r o n DESY at Hamburg. W e wish to thank especially Dr. A. Ladage and Mr. K. Rehlich for their advice in setting up the optical recording system. Finally we want to t h a n k Dr. K. Stupperich for a careful reading of the manuscript.

References [1] G. Battistoni et al., Nucl. Instr. and Meth. 164 (1979) 57. [2] G.D. Alekseev, Nucl. Instr. and Meth. 177 (1980) 385. [3] See e.g. ALEPH collaboration for a LEP Experiment, Technical Report, CERN (1983). [4] G. Battistoni et al., LNF-82/81 (P), Phys. Lett. I18B (1982) 461. [5] G. Battistoni et al., Workshop on Gas sampling calorimetry, Fermilab (1982); LNF Preprint (1983).

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[6] G. Battistoni et al., Workshop on Gas sampling calorimetry, Fermilab (1982); LNF Preprint (1983). [7] M. Jonker et al., CERN EP/80 (1980). [8] J. Allison et al., Nucl. Instr. and Meth. 20l (1982) 341. [9] Chr. Becker et al., Nucl. Instr. and Meth. 200 (1982) 335. [10] A. Franz et al., Nucl. Instr. and Meth. 200 (1982) 331. [11] G. Battistoni et al., LNF-Preprint, LNF-83 (January 1983). [12] G. Battistoni et al., Nucl. Instr. and Meth. 152 (1978) 423. [13] G. Battistoni et al., LNF-Preprint LNF 83/1 (January 1983). [14] M. Atac et al., Nucl. Instr. and Meth. 200 (1982) 345. [15] R. Baumgart, Diploma-Thesis, Univ. Siegen (1983). [16] U. SchMer, Diploma-Thesis, Univ. Siegen (1983). [17] Manufactured by Fernseh GmbH (R. Bosch), Darmstadt, Germany. [18] Manufactured by AEG-Telefunken, Ulm, Germany. [19] A. Grosch, Elektronik 2 (1974) 55. [20] Manufactured by Carl Zeiss, Oberkochen, Germany. [21] Manufactured by Hughes, Oceanside, California, USA.