We describe recent experimental and theoretical work aimed at optimizing the geometry and the operation of micro-strip gas chambers in order to improve their performance and reliability. With the help of a simulation program, we have studied the mechanism of signal propagation and analyzed the effects on signal shape and size of resistivity of strips, grouping of biased strips and presence of a back-plane. Several detectors manufactured according to the results of the study and equipped with fast amplifiers have been installed in a test beam to study general operating characteristics, efficiency and localization accuracy; preliminary results of the data analysis are discussed

Bouclier, Roger

Capéans-Garrido, M

Evans, J

Garabatos, C

Manzin, G

Million, Gilbert

Ropelewski, Leszek

Sauli, Fabio

Shekhtman, L I

Temmel, T

CERN Document Server

EUROPEAN LABORATORY FOR PARTICLE PHYSICS
CERN-PPE/95-56
27 April 1995
OPTIMIZATION OF DESIGN AND BEAM TEST OF MICROSTRIP GAS
CHAMBERS
R. Bouclier, M. Capeans, J. Evans, C. Garabatos, G. Manzin, G. Million,
L. Ropelewski, F. Sauli, L.I. Shekhtman* and T. Temmel
(CERN, Geneva, Switzerland)
G. Fischer
(Institute of High Energy Physics, Vienna, Austria)
ABSTRACT
We describe recent experimental and theoretical work aimed at
optimizing the geometry and the operation of micro-strip gas chambers in order
to improve their performance and reliability. With the help of a simulation
program, we have studied the mechanism of signal propagation and analyzed
the effects on signal shape and size of resistivity of strips, grouping of biased
strips and presence of a back-plane. Several detectors manufactured according to
the results of the study and equipped with fast amplifiers have been installed in a
test beam to study general operating characteristics, efficiency and localization
accuracy; preliminary results of the data analysis are discussed.
Presented by L. Ropelewski at the Wire Chamber Conference
(Vienna, February 13-17, 1995)
Subm. to Nucl. Instrum. Methods in Phys. Research
1. INTRODUCTION
Micro-strip gas chambers (MSGCs [1]) are an attractive choice for high
resolution, high rate tracking detectors for the Large Hadron Collider, and great
efforts are being made to develop the new devices [2, 3]. This paper describes
some results of systematic studies aimed at improving the  performance of the
detectors, and in particular a signal analysis realized with an electrical model
describing MSGC structures as multi-electrode distributed transmission lines.
The current pulse simulating an avalanche propagates along the strips giving rise
to transition processes; this results in a modification of the signals read out by the
amplifiers, as well in a spread of part of the signal to neighbouring channels. The
response of the detector depends on resistance of strips, inter-strip capacitance,
capacitance to ground and to other electrodes.
Several medium-size MSGCs, manufactured according to the results
obtained from the above studies, have been tested in a minimum ionizing
charged particles beam; preliminary results on signal to noise ratio, space
resolution and detection efficiency are presented.
2.   OPTIMIZATION OF GEOMETRICAL AND OPERATING PARAMETERS
From previous work, a clear preference has emerged for the use of argon-
dimethylether (DME) mixtures for stable, long term operation of MSGCs. It was
found that a DME concentration of around 50% provides the best performance in
terms of maximum attainable gain, probably the result of a compensation
between the higher quenching properties of DME rich mixtures and the increase
of noise due to the higher potentials required [4].
We have investigated experimentally the dependence of the discharge
limit on cathode width, using a set of MSGC patterns having identical anode
width and pitch (7 m m and 200 m m, respectively), and cathode strips from 60 to
170 m m. Four patterns were realized1 in gold on the same support, electron
conducting glass with bulk resistivity of 109 W .cm. Fig. 1 shows the gain
dependence on anode voltage for the four patterns, all operated in an argon-
DME (50-50) gas mixture. Measurements stop for each geometry at the voltage at
which, due to the appearance of discharges, the current  on the power supply
exceeded a pre-set limit of 20 m A. As it can be seen, the maximum gain before
discharge increases for a decreasing anode to cathode gap, reaching a maximum
at about 50 m m, dropping then rapidly for smaller gaps; this results suggest an
optimum cathode width between 90 and 100 m m.
3.  SIGNAL SIMULATION
The basic cell of the equivalent circuit used for signal analysis is shown in
Fig. 2 [5]. Strip segments are simulated by a serial resistance with values
depending on the length, cross section and metal of the strip; sections of cathode
and anode strips are connected by an inter-strip capacitance, and each node can
be also connected to a strip-to backplane capacitance (if present). To simulate a
long chamber with distributed parameters, several cells can be added in series;
we found however only a small dependence of the results on the number of cells
if this number is greater than three. At the end of each strip, a capacitance to
ground can be introduced to represent the read-out board. The signal generated
by the avalanche process is simulated by two trapezoidal current sources (10 ns
rise and fall time and 30 ns fwhm) between one anode and two neighbouring
cathodes; the results depend only slightly on the shape of the avalanche signal. In
the simulation, we have neglected the small contribution of the signals induced
by the avalanche, in the real physical case, on the drift and backplane electrodes.
The parameters used for the equivalent circuit correspond to the values directly
measured on a MSGC plate manufactured on boro-silicate glass 300 m m thick
with 2700 Å thick chromium strips, 10 m m wide anodes, 80 m m cathodes at 100
m m distance. The strips are readout by a fast amplifier with 45 ns rise time,
emulating the characteristics of the RD-20 pre-amplifier [6].
We have computed (Fig. 3) for a chromium MSGC the signal peak pulse
height as a function of distance from the amplifier end for different total lengths
of strips, both in the case of anode (dashed lines) and cathode readout (full lines).
The points with error bars represent the results of a measurement in the case of
individual readout of cathode strips. On anodes the signal decreases for an
avalanche moving away from the amplifier towards the far end of the strip,
while for cathode readout the trend is reversed; the non-uniformity of the
response is approximately the same. This surprising result is explained by the
fact that, given a position of the avalanche along a cathode (anode) strip, the
return current has to go through the remaining length of the neighbouring
anodes (cathodes); the lower resistance of one is compensated by the higher
value of the others. The figure shows also that chambers with strips up to 10 cm
long can be realized with chromium strips, with a moderate non-uniformity of
response, around 20%, acceptable for a tracking device.
The signal non-uniformity can be reduced by lowering the strip resistance.
Fig. 4 shows the pulse height on cathodes as function of distance of the avalanche
from the amplifier end for aluminium and chromium strips, in a 30 cm long
chamber; presence of a backplane electrode has a small influence on signals for
the lower resistivity strips.
The experimental results show that channels adjacent to the strip
interested by an avalanche detect signals of reverse polarity (Fig. 5). Indeed, the
return current from the avalanche has to flow through the input impedance of
the neighbouring channels’ amplifier in reverse direction as compared to the
main strip; the number of channels detecting an inverted signals corresponds to
the strips of opposite kind connected together to apply the operating potential,
through a high value protection resistor. For the case of individual read-out of
cathode strips, we have investigated how the number of anode (biased) strips
connected in a group can affect the amplitude of the reverse signals, as well as
the main signal on the central cathode strips. As expected, the opposite polarity
signals detected on cathode strips adjacent to the main get smaller with the
increase of the number of strips in the biased group, while the main signal
increases; both reach a constant value for groups of 6 or more strips. A further
reduction of the parasitic effect can be obtained adding decoupling capacitors in
parallel to the protection resistors connecting each group to the power supply,
thus providing additional return paths for the current; one has however to weigh
the advantages of pick-up suppression with the disadvantages of an increased
stored energy in case of a discharge.
4.  TEST BEAM MEASUREMENTS
We have tested several MSGC detectors built according to the experience
gained in the optimization studies of a minimum ionizing particles beam at
CERN. The set up consists in scintillator counters for triggering, two drift
chambers for beam position monitoring and three aligned micro-strip detectors;
the central MSGC could  be rotated around the vertical axis, perpendicular to the
beam, to study inclined tracks. We have tested chambers made on different
substrates: boro-silicate glass D-2632, semi-conducting glass S-89003 and D263
glass overcoated with a thin lead silicate electron conducting layer [7-10].
Electrodes are made of 2700 Å thick chromium strips with anodes 7 m m wide,
cathodes 100 m m wide and 200 m m anode to anode pitch. The MSGCs used for
this test4 were assembled as described in Ref. [11] with glass or Vectra thin
frames and a glass 300 m m window coated with indium-tin oxide providing a 3
mm thick gas gap; the active area was 8x8 cm2. In all chambers the anodes are
connected together in groups of ten, and to HV power supply through a 500 kW
protection resistors; in each detector, 64 cathode strips are individually connected
through a pitch adapter to the input of two 32-channels RD-20 PreShape32
amplifier chip with 45 ns shaping time followed by operational amplifiers (OPA
621 KU). The pulse height digitization is done with LeCroy 2282B 48-channels
charge-sensing ADCs, with a 200 ns gate. Calibration of the electronics was
performed using the information from the data taken with special pedestal
(random) and calibration triggers; in the latter case, a known charge was injected
into the preamplifiers. We have used an argon-DME (50-50) gas mixture and
operating voltages of -1000 V and 570 V on the drift and anodes respectively, at a
gas gain of 2.103.
After subtraction of the pedestals and gain normalization using the
electronics calibration, raw data are corrected for the undershoot, as shown in
Fig. 5. The procedure for cluster finding begins with the search of the strip with
the highest signal in a chamber; the contribution from the neighbouring strips is
then considered only if it exceeds twice the standard deviation of the noise
distribution of the channel. Subsequently, hits with smaller charge are searched
to take into account multi-track events, and the final hit selection is done by
tracking on the basis of a c2 minimization. Fig. 6 provides the cluster size
distribution for tracks perpendicular to the MSGC plane; it has a mean value of
2.8, typical for cathode readout; Fig. 7 shows the total charge distribution (sum
over adjacent strips in a cluster), together with the noise spectrum obtained from
the pedestal trigger data. Signal to noise ratio, defined as the most probable value
of the signal divided by the standard deviation of the noise distribution, is 15.
After cluster finding, the space position of the hit in the chamber is
evaluated by a centre of gravity method; tracks are defined using the two outer
chambers after an iterative alignment procedure, and the  correlation between
predicted and measured track position in the central MSGC is used to compute
the localization accuracy. A scatter plot of the correlation, Fig. 8, shows
modulations with a period corresponding to the 200 m m pitch; Fig. 9 gives the
distribution of the residuals in the central chamber after correction for the
modulations using the simple sinusoidal fit shown in the figure. A space
resolution of 39 m m rms can be estimated for a single plane of measurement.
Fig. 10 shows, for tracks perpendicular to the chamber, the dependence of
the efficiency and of the noise occupancy (probability of a false hit) as a function
of threshold, expressed in number of standard deviations of the noise
distribution.
The authors acknowledge the assistance for conceiving and
manufacturing the MSGC detectors of A. Braem and O. Runolfsson, and of P.
Martucci and R. Zurbuchen for implementing the simulation programs.
REFERENCES
[1] A. Oed, Nucl. Instrum. Methods A263 (1988) 351.
[2] F. Sauli, RD-28 Status Report. CERN/DRDC/93-34 (1993).
[3] F. Sauli, Development of Microstrip Gas Chambers for Radiation
Detection and Tracking at High Rates.  CERN/DRDC/94-45 (1995).
[4] T.Beckers, R.Bouclier, C.Garabatos, G. Million, F. Sauli, and L. Shekhtman,
Nucl. Instrum. Methods A346 (1994) 95.
[5] R.Bouclier, M.Capeáns, C.Garabatos, G.Manzin, A.Peisert, L.Ropelewski,
F.Sauli, J. C.Santiard, L.I.Shekhtman, T.Temmel, and G.Fischer, Signal analysis in
micro-trip gas chambers , Proc. Int. Conf. on Microstrip Gas Chambers (Legnaro,
1994).
[6] RD-20 Status report.  CERN/DRDC 94-39 (1994).
[7] R. Bouclier, G. Million, L. Ropelewski, F. Sauli, Y. Pestov, and L.I.
Shekhtman. Nucl. Instrum. Methods A332 (1993) 100.
[8] L. Alunni, R. Bouclier, G. Fara, C. Garabatos, G. Manzin, G. Million, L.
Ropelewski, F. Sauli, L. Shekhtman, E. Daubie, O. Pingot, Y. Pestov, L. Busso,
and S. Costa, Nucl. Instrum. Methods A348 (1994) .
[9] J. Bohm, R. Bouclier, M. Capeans, C. Garabatos, G. Manzin, G. Million, F.
Sauli, T. Temmel, and L. Shekhtman, High rate operation and lifetime studies
with micro-strip gas chambers.  CERN-PPE/94-115 (1994).
[10] R. Bouclier, M. Capeáns, C. Garabatos, G. Manzin, G. Million, L.
Ropelewski, F. Sauli, E. Shefer, L. Shekhtman, T. Temmel, G. Della Mea, G.
Maggioni, and V. Rigato, Microstrip gas chambers with thin electron-conducting
layers , Proc. Int. Conf. on Microstrip Gas Chambers (Legnaro, 1994).
[11] R. Bouclier, M. Capeáns, C. Garabatos, G. Manzin, G. Million, L.
Ropelewski, F. Sauli, T. Temmel, L.I. Shekhtman, and K. Silander, Ageing studies
with microstrip gas chambers , Proc. Int. Conf. on Microstrip Gas Chambers
(Legnaro, 1994).
FIGURE CAPTIONS
Fig.  1: Gain dependence on the anode voltage measured in MSGCs with
equal anodes and pitch, and different cathode widths. The highest point in each
curve corresponds to the discharge limit.
Fig.  2: Basic cell of the MSGC equivalent circuit used in the electrical
simulation.
Fig.  3: Computed signal pulse height for cathode (full curves) and anode
(dashed) readout as a function of the avalanche distance from the amplifier, for
several lengths of chromium strips. The points with error bars correspond to
measured values in a MSGC.
Fig.  4: Pulse height for cathode readout computed as a function of the
distance of the avalanche from the amplifier end for chromium (full points) and
aluminium (open points) strips. Dashed lines correspond to the presence of a
backplane electrode.
Fig.  5: Measured signal distribution for a single avalanche (full line) and
after correction for undershoot signals (dashed line).
Fig.  6: Cluster size distribution for minimum ionizing tracks
perpendicular to the MSGC plate.
Fig.  7: Integral cluster charge distribution for minimum ionizing particles,
and noise spectrum obtained from the pedestal trigger data. The signal over
noise ratio is 15.
Fig.  8: Scatter plot of the correlation between predicted and measured
position in the MSGC with cathode readout. The line represents a sine-wave fit
through the data.
Fig.  9: Residuals distribution of the three-chambers straight line fitting; the
corresponding single MSGC position accuracy is 39 m m rms.
Fig. 10: Dependence of the detection efficiency and noise occupancy from
threshold, for minimum ionizing particles perpendicular to the chamber.

Fig. 1

Fig. 2

Fig. 3

Fig. 4

Fig. 5

Fig. 6

Fig. 7

Fig. 8

Fig. 9

Fig. 10
* Present address: Budker Institute for Nuclear Physics, Novosibirsk, Russia
1 BINP, Novosibirsk, Russia
2 DESAG, Deutsche Spezialglas AG, Grünenplan, D
3 Schott Glass Technologies, Duryea, USA
4 Manufactured by IMT, Greifensee, CH


Optimization of design and beam test of microstrip gas chambers

Optimization of design and beam test of microstrip gas chambers

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