IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 45, NO. 3, JUNE 1998 249

Study of the Backside Signal of Micro-Strip Gas Counters on Electronic Conducting Glass

G. Cicognani, D. Feltin, B. Guerard and A.Oed lnstitut Laue Langevin, B.P. 156X, F-38042 Grenoble Cedex, France

Abstract

Microstrip gas counters (MSGC) on electronic conducting glass such as Schott S8900 have a very stable long-time behaviour. However, this glass is available in relatively thick plates only. A thick substrate limits the performances of two-dimensional detectors by attenuating the signal of the backside electrode which cames the second position coordinate. A structure with "open cathodes", where the central area of each cathode is non-metallized, reduces the screening effect. By increasing the cathode- backside potential difference, the backside signal increases considerably. Its amplitude becomes even equal to the anode signal when the cathodes are not connected to an external potential. As in the present mode the cathode strips do not contribute to the amplification process, this naturally leads to a structure where the cathodes are removed. Another advantage of this new structure design is the very high gas ampllfication which can be achieved. However, as the ions are dtscharged by a cwrent through the substrate, the voltage drop caused by this current gives rise to counting rate dependence of the gas amplification .

I. INTRODUCTION Microstrip Gas Counters realised on a substrate with

electronic conductivity, such as Schott S8900, exhibit a very stable behaviour in long-term operation [ 11.

counts I I I -

2000

1 S M

0 100 200 3w 400 500

pulse height (chn)

Fig. 1 : Anode and backside signal of the 5.9keV X-rays of a "Fe source (300c/s mm') in 900mbar (Ar+1O%C&). The weak drift- field in the detection volume (1 5mm in depth), which is produced by the positive anodes only, is sufficient for the collection of the total amount of the charges generated by the ionisation.

There is, however, a more technical reason which hinders the use of this glass in two-dimensional detectors: the relative thchess of the available plates. The amplitude of the backside signal which delivers the second position coordinate decreases with increasing glass thickness. The metallic strips on the front side, and especially the wider cathodes, screen the signal induced by the motion of the charges onto the backside.

A microstrip plate, called ILL6C, (engraved on a l O O n m chromium layer with a lmm pitch, 8pm wide anodes and 4 0 0 p wide cathodes) realised on S8900 glass of 0.45 mm in thickness, delivers a backside signal of 30% of the anode signal, as shown in the energy spectra of Fig. 1 for the 5.9keV X-rays of a *' Fe source (100 c/s mm2) measured in 9OOmbar

This value drops to only 10% when the dimensions of the structure are reduced to one third (pitch 3 0 0 ~ cathode width 1 3 0 ~ ) . The measured backside signal as a function of the ratio of the anode-cathode distance (gap) to the substrate thickness is shown in Fig.2.

Ar +lo% CH,

0 2 4 6 8

thicknesdgap Fig 2: Dependence of the relative backside signal as function of the ratio gap (anode-cathode distance) to substrate thickness.

With the available substrate thickness the only way to get a higher backside signal is to reduce the screening caused mainly by the large cathodes which cover 40 % of the front surface.

11. OPEN -CATHODE STRUCTURE A new structure, called ILL6A, was therefore designed

where the central part of the cathodes has been removed, leaving only their borders which are 8pm wide. The metallized part then amounts to only 2.4% of the total

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surface. With this structure, the backside signal reaches 55% of the anode signal, as shown in Fig. 3.

Fig.3: Anode and rear side signal spectra of the 5 9 keV X-rays of a 55 Fe source ( 1 OOc/s mm2 ) measured with a structure with open cathodes ( LL6A on SS900) in 900 mbar (Ar +lo% C&). Anodes 795V, cathodes 200V, backside and window OV.

Compared to structures with large closed cathodes, a strong dependence of the gas amplification on the potential Merence between cathodes and rear side has been measured. In Fig.4 the electric field map for the structure ILL6A is shown for a cathode - rear side potential difference of 250V, as computed by the program ELF1 [2]. The predominant part of the electric field lines issued from the anodes into the gas volume ends on the substrate surface and not on the cathode .

window __.__---

--7-1---- I

ca!&ode ‘ I ’ ,e’‘ anode borders ‘\

Fig.4: Electm field map for the structure ILL6A with open

entrance window on OV. cathodes Anodes on 1000 V, cathodes on 250 V, backside and

By increasing this potential difference, the influence of the cathodes on the field distribution is strongly reduced, whereas the field strength near the anodes and hence the gas gain increase considerably.

It should be mentioned that microstrip plates with this structure on a substrate with high resistivity Ohmcm), like the glass D263, work very poorly. Their surface is immediately charged up because most of the avalanche ions

are discharged onto the substrate surface, as it appears in the field plot of Fig.4, whereas the electronic conducting substrate with a resistivity of about 2 to 10. 10” Ohm ‘cm is able to evacuate the collected ions .

111. THE3 ASYMMETRIC MS PLATE As the cathode is only of minor importance for the field

Qstribution, the behaviour of this MS-plate was investigated without connection of the cathodes to an external potential [3]. In tlvs case the electric field for the gas amplification is exclusively generated by the potential Merence between the anodes on the front side and the electrodes on the backside. Such an arrangement corresponds to a very asymmetric multiwire proportional counter and has already been tested [4], but the resistivity of the plastic substrate used there was too high for a stable operation. This is not the case with the electronic conducting S8900 substrate which delivers very stable signals as shown by the rather good energy resolution in the measured energy spectra of Fig. 5, In this configuration, the anode and backside signal have the same amplitude. Recently, this particular operation mode has also been studied by the CERN group [ 5 ] .

counts 1 I I I I I (

0 100 200 300 400 500

pulse height (chn 1 Fig.5: Anode and backside signal spectra for the aspmetric MS arrangement with not-connected cathodes of the structure ILL 6A on S8900 for the 5.9 keV X-rays of a 55Fe source ( 300c/s mm2 ) in 740 mbar (Ar + 10% CH4). Anodes 780 V, cathodes not connected, backside and window 0 V.

Nearly all the avalanche ions are neutralised on the substrate surface by a current which is delivered from the backside through the substrate. At higher counting rates and / or higher gas amplification a larger number of ions must be Qscharged by this current. Due to the voltage drop caused by &IS current in the resistive substrate, a reduction of the surface potential occurs with a corresponding reduction of the gas gain. For the 5.9 keV X-rays at a rate of 5000 c/s mm2 and a gas amplification factor of 600, a gain loss of 15% was measured compared to a rate of 50 c/s mm’. For the rate of 5000 c/s mm2, the current density was 9 lO-”A /mm2 which led to a voltage drop of 27V for a resistivity of 3 lolo Ohm cm. A gain loss of 70% was measured with an amplification factor

of 2700. If available substrates with lower resistivity are used, these gain reductions should be correspondingly smaller .

By their capacitive coupling to the back electrodes the floating metallic cathode strips also induce part of the backside signal. Th~s part is independent of the position because the frontside and the backside structures are orthogonally oriented [6]. Therefore a new structure without cathodes, named "bidimbis", was designed for a two- dimensional X-ray detector. The surprising feature of the "bidimbis" structure is its very high gas gain. A gas amplification factor of lo5 was reached without any sparking or discharge problems. A higher gain is certainly possible, but as no spare plate was available, its limits were not checked. These high gains at low rates will suffer an important reduction at higher rates, but such a detector is well adapted for X- rays with low energy and weak intensity . Moreover, this asymmetric MSGC resists to large pulses produced by heavy ionizing particles which may damage standard MSGCs by a streamer discharge [7]. The high resistance of the substrate automatically limits the discharge current and thus avoids any damage.

IV. CONCLUSION It is possible to increase the backside signal of an MSGC

on an electronic conducting substrate by using a structure with open cathodes. The asymmetric codiguration of the electrodes leads to a new kind of MSGC. The following arrangement will be tried : a substrate with the engraved rear side structure will be covered with a layer of several microns of highly resistive but electronic conducting material on the surface of which the anodes are fixed. This should be a very reliable solution for two-dimensional MS gas detectors.

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Submitted to Nucl. 1nstr.and Meth. N. Vellettaz et al., Proceedings of the International Workshop on MSGC,Lyon,Nov. 30-Dec. 2( 1995). Editors: D.Contardo, FSauli B.D. Ramsey et al, TEEE Trans.Nucl.Science 44 (1997) 640,

PPE/97-6 1 April 1997.