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Gain stability of microstrip gas chambers with high resistivity substrates
__ !B
&g c
Nuclear
Instruments and Methods in Physics Research A 37X ( 1996) 439-442
-
__
ELSEVIER
NUCLEAR INSTRUMENTS a METHODS IN PHVW RESEARCM SectionA
Gain stability of microstrip gas chambers with high resistivity substrates R. Fang*, W. Geist, J.M. Brom, A. Bergdolt, J.L. Riester Cmtre
de Ruchrrches
Nuclktirrs.
INZP_?-CNRSIUnil’ersifP
Louis Pasteur.
BP 2X. F-670.77 Strashourg
Cedex 2, Frmcr
Received I December 1995; revised form received 25 April 1996 Abstract Microstrip gas chambers made from high resistivity substrates are often plagued by gain losses. A model of surface charging coupled to precise electrostatic simulations, which reveal details of electric fields close to the substrate surface, suggests how stable gains can be achieved nevertheless. Measurements support these considerations.
1. Introduction
2. Simulation
of electric fields
Recently a microstrip gas chamber (MSGC) made from D263 glass was used operated at rather stable gain for 8 keV X-ray rates up to 4 X IO5 counts/(s mm’) [I]. An explanation [2] for these features is presented here, based upon a precise simulation of electric fields close to the substrate surface. One finds that the position of the point between anode and cathode, where the field component normal to the substrate surface changes sign plays a key role; it can be displaced in a predictable way by changing the potential of the back plane or the drift voltage. New measurement support these arguments.
Two-dimensional simulations of electric fields are performed in the -Y-Y plane orthogonal to anode and cathode strips (Fig. I ); they are based on the method of finite differences [3]. A multi-grid with up to 2 X IO’ pixels of varying sizes down to 0.1 X 0.1 pm’ is used. A new algorithm for speeding up the convergence has been introduced to obtain solutions of the potentials with a relative uncertainty of about IO-’ after about 1400 iterations 14). Between an anode and the neighbouring cathode of a MSGC a point F exists (Fig. I and 2) where the component of the electric field normal to the substrate surface Et, changes sign. Es < 0 on the surface near anode: El > 0 on the surface near cathode. The presence of back plane and drift plane cannot change this basic feature except displacing point F. For the MSGC geometry of Ref. [I], and that of the present measurement, detailed in Table I. electric fields were simulated for various voltage settings. Some of them are displayed in Figs. 2 through 5. Increasing the drift field (Fig. 3) and/or raising the potential of the back plane (Figs. 4 and 5) reduce the distance 6 of point F from the anode edge D. Distances 6 < 4 p.m can be achieved (Table I. Fig. 5). A higher back plane potential is needed to obtain the same value of S for a thicker substrate.
3. Surface charging Fig.
I.
Schematic representation of a MSGC
the strips.
cut perpendicular to
According to the Rule of Charge Accumulation of Ref. [5], the surface charge density at equilibrium ay. is given by:
* Corresponding author.
0168.9002/96/$15.00 PI/
Copyright 0
SO168-9002(96)00478-O
1996 Elsevier Science B.V. All rights reserved
440
R. Fang et al.
Fig. 2. The electric fields near the substrate cv,,, = V<,,v,,, = - 1000 V).
I Nucl. Instr. and Meth.
surface of MSGC
1
in Phys. Res. A 378 (1996)
Fig. 3. The electric
439-442
fields near the anode
edge
of MSGC
I
(r/,,> = y,,. V& = -2000 V).
where E, and I’, are the permittivity and resistivity in the vicinity of the surface of dielectric i = g(as), s(ubstrate), respectively. E,! is the value of EV at gas side. One arrives at the following relations for the surface charging: uq = 0
for
.srrB =
qr,
;
a, >O
for
.zgrp < qr,
,
g
for
cgrg>E,r,
at the surface between anode edge D and point F (DF); cq
for
cgrg < &,r,
uq > 0
for
Errs > qr,
,
at the surface between
The numerical
point F and cathode edge Q (FQ). value of rB is, in the vicinity of DF,
Fig. 4. The electric fields near the substrate (Vhd= VJ”, vdr, = - I200V).
surface of MSGC 2
Table I The distances
(6) between point F and the anode edge for different
voltage settings of the two MSGCs made on glass D263
MSGC [p,m]
Ycl,,VI
MSGC 1
410
0
-100
0
410
0
-2000
0
9.77
3
410
0
-2000
410
3.52
_
0
7.42
4
Pitch = 200, Gap = 3000 Anode width = 7 Cathode width = 80 Sub. thickness = 300
YAWI
b,, [VI
v,., [VI
6 [Pm1
Fig.
27.1
2
Strip thickness = 0.5
MSGC 2 Pitch = 2CO,Gap = 3000
0
-420
-1200
Anode width = 9
0
-420
-1200
650
3.71
_
0
-420
-1200
800
3.32
5
Cathode width = 70 Sub. thickness = 5OQ Strip thickness = 0.95
441
R. Fang et a/. I Nucl. Imtr. urrd Meth. in Php. Rex. A 378 (1996) 439-442
Fig. 5. The electric fields near the substrate (Vba= y,,, + x00 v, v,,, = - 1200 V).
surface of MSGC 2
determined by ion conductance as electrons can hardly reach this part of the surface; on the other hand. r$ is, in the vicinity of FQ, dominated by electron conductance and much higher than that in the vicinity of DF. Positive charging reduces the gain seriously and negative charging increases it somewhat 151.
0
According to the empirical formula [6] gas amplification is expected to develop at about 20-25 kV/cm, i.e. at a distance of about 30-40 km from the anodes. However, the main contribution to avalanches comes from within a typical distance (Y= 4-6 Frn from the anodes. Some of the drift electrons arrive at the substrate surface adjacent to the anode edges due to diffusion [7] such that avalanche formation will occur there. Based upon Ref. [5] a gas resistivity rg < IO” Rem in the vicinity of DF is estimated for a rate of ionizing particles larger than about IO5 counts/(s mm’). Therefore rg -=x r, for high resistivity substrates, and thus Us > 0 at DF. The FQ area is slightly charged in this case (negatively or positively) because of the much higher r,. This implies gain reduction if 6 is large enough, at least S > LY.For such a configuration (Fig. 2) only substrates with low resistivities, e.g. r, = 10’ f IO I’ 0 cm, can support high rates at stable gain. Obviously, reducing S (Figs. 3-5) would reduce the area with aq > 0. For the case of point F lying in the strong avalanche region (S < a) a part of FQ is affected by avalanches such that it becomes negatively (may be seriously) charged. The positive charges at DF, and the negative charges at FQ form a dipole tending to stabilize the gain. Hence substrates with high resistivities can support high rates provided S < (Y.
2
3
RATE(106C/(smmZ)
Fig. 6. The gain variations as function of counting different back plane voltages of MSGC 2 (D363 IO” fl cm).
5. Experimental 4. Gain variations
1 COUNTING
rates for substrate.
results
The data from Ref. [I] (MSGC 1 of Table I ) do show a pronounced gain drop as a function of rate beyond IO’ counts/(smm’) for equal back plane and cathode potentials at a low drift field as expected from the values of S from Table I (Fig. 2). For higher drift fields (Fig. 3) and/or equal back plane and anode potentials the gain remains rather stable beyond this rate. Relative gains as function of counting rates have been measured for MSGC 2 of Table I operated with an ArlDME (90110) gas mixture at the selected voltage settings given in the table. The field configurations corresponding to some voltage settings are shown in Figs. 4 and 5. Measured relative gains are displayed in Fig. 6. A gain reduction is only observed for equal back plane and anode potentials.
6. Conclusion By increasing the drift voltage and/or the back plane potential one is able to decrease in a predictable way the area of the positively charged (at high rates) surface. Therefore better gain stability is achieved. Raising the potential of the back plane to be higher than that of the anode can move F into the strong avalanche region. This makes MSGCs made from high resistivity substrates
R. Fang et al. I Nucl. Instr. and Meth. in Phys. Rex. A 378 (1996)
442 support
high
rates.
The thinner
the substrate,
the stronger
the influence of the back plane potential. However such a gain stability is based on a compromise between the positively charged area and the negatively charged area, so the gain uniformity would not be as perfect as that obtained for MSGCs made from low resistivity substrates (r, = IO’- IO’ ’ Cl cm) and operated at equal back plane and cathode potentials (Fig. 2).
References [I] R. Bouclier, M. Capeans, C. Garabatos, G. Manzin, G. Million, L. Ropelewski, F. Sauli. L.I. Shekhtman and T. Temmel, Contribution to the Vienna Wire Chamber Conf., February 13-17, 1995.
439-442
[2] For qualitative arguments see: R. Bellazzini and M.A. Speaziga, La Rivista de1 Nuovo Cimento della Socied italiania di
Fisica 12 (1994) p. 21. [3] F.J. Mulligan, Eur. J. Phys. I3 (1992) 57. [4] R. Fang. Ph.D. thesis (1996). [5] R. Fang, R. Blaes, J.M. Brom, W. Geist, A. Michalon, J.L. Riester and C. Voltolini, Nucl. Instr. and Meth. A 365 (1995)
59. [6] M.W. Charles, J. Phys. E5 (1972) 95. [7] R. Fang, R. Blaes. J.M. Brom, W. Geist, A. Michalon, J.L. Riester, R. Seitz and C.Voholini, Nucl. Instr. and Meth. A 361
(1995) 85.
&g c
Nuclear
Instruments and Methods in Physics Research A 37X ( 1996) 439-442
-
__
ELSEVIER
NUCLEAR INSTRUMENTS a METHODS IN PHVW RESEARCM SectionA
Gain stability of microstrip gas chambers with high resistivity substrates R. Fang*, W. Geist, J.M. Brom, A. Bergdolt, J.L. Riester Cmtre
de Ruchrrches
Nuclktirrs.
INZP_?-CNRSIUnil’ersifP
Louis Pasteur.
BP 2X. F-670.77 Strashourg
Cedex 2, Frmcr
Received I December 1995; revised form received 25 April 1996 Abstract Microstrip gas chambers made from high resistivity substrates are often plagued by gain losses. A model of surface charging coupled to precise electrostatic simulations, which reveal details of electric fields close to the substrate surface, suggests how stable gains can be achieved nevertheless. Measurements support these considerations.
1. Introduction
2. Simulation
of electric fields
Recently a microstrip gas chamber (MSGC) made from D263 glass was used operated at rather stable gain for 8 keV X-ray rates up to 4 X IO5 counts/(s mm’) [I]. An explanation [2] for these features is presented here, based upon a precise simulation of electric fields close to the substrate surface. One finds that the position of the point between anode and cathode, where the field component normal to the substrate surface changes sign plays a key role; it can be displaced in a predictable way by changing the potential of the back plane or the drift voltage. New measurement support these arguments.
Two-dimensional simulations of electric fields are performed in the -Y-Y plane orthogonal to anode and cathode strips (Fig. I ); they are based on the method of finite differences [3]. A multi-grid with up to 2 X IO’ pixels of varying sizes down to 0.1 X 0.1 pm’ is used. A new algorithm for speeding up the convergence has been introduced to obtain solutions of the potentials with a relative uncertainty of about IO-’ after about 1400 iterations 14). Between an anode and the neighbouring cathode of a MSGC a point F exists (Fig. I and 2) where the component of the electric field normal to the substrate surface Et, changes sign. Es < 0 on the surface near anode: El > 0 on the surface near cathode. The presence of back plane and drift plane cannot change this basic feature except displacing point F. For the MSGC geometry of Ref. [I], and that of the present measurement, detailed in Table I. electric fields were simulated for various voltage settings. Some of them are displayed in Figs. 2 through 5. Increasing the drift field (Fig. 3) and/or raising the potential of the back plane (Figs. 4 and 5) reduce the distance 6 of point F from the anode edge D. Distances 6 < 4 p.m can be achieved (Table I. Fig. 5). A higher back plane potential is needed to obtain the same value of S for a thicker substrate.
3. Surface charging Fig.
I.
Schematic representation of a MSGC
the strips.
cut perpendicular to
According to the Rule of Charge Accumulation of Ref. [5], the surface charge density at equilibrium ay. is given by:
* Corresponding author.
0168.9002/96/$15.00 PI/
Copyright 0
SO168-9002(96)00478-O
1996 Elsevier Science B.V. All rights reserved
440
R. Fang et al.
Fig. 2. The electric fields near the substrate cv,,, = V<,,v,,, = - 1000 V).
I Nucl. Instr. and Meth.
surface of MSGC
1
in Phys. Res. A 378 (1996)
Fig. 3. The electric
439-442
fields near the anode
edge
of MSGC
I
(r/,,> = y,,. V& = -2000 V).
where E, and I’, are the permittivity and resistivity in the vicinity of the surface of dielectric i = g(as), s(ubstrate), respectively. E,! is the value of EV at gas side. One arrives at the following relations for the surface charging: uq = 0
for
.srrB =
qr,
;
a, >O
for
.zgrp < qr,
,
g
for
cgrg>E,r,
at the surface between anode edge D and point F (DF); cq
for
cgrg < &,r,
uq > 0
for
Errs > qr,
,
at the surface between
The numerical
point F and cathode edge Q (FQ). value of rB is, in the vicinity of DF,
Fig. 4. The electric fields near the substrate (Vhd= VJ”, vdr, = - I200V).
surface of MSGC 2
Table I The distances
(6) between point F and the anode edge for different
voltage settings of the two MSGCs made on glass D263
MSGC [p,m]
Ycl,,VI
MSGC 1
410
0
-100
0
410
0
-2000
0
9.77
3
410
0
-2000
410
3.52
_
0
7.42
4
Pitch = 200, Gap = 3000 Anode width = 7 Cathode width = 80 Sub. thickness = 300
YAWI
b,, [VI
v,., [VI
6 [Pm1
Fig.
27.1
2
Strip thickness = 0.5
MSGC 2 Pitch = 2CO,Gap = 3000
0
-420
-1200
Anode width = 9
0
-420
-1200
650
3.71
_
0
-420
-1200
800
3.32
5
Cathode width = 70 Sub. thickness = 5OQ Strip thickness = 0.95
441
R. Fang et a/. I Nucl. Imtr. urrd Meth. in Php. Rex. A 378 (1996) 439-442
Fig. 5. The electric fields near the substrate (Vba= y,,, + x00 v, v,,, = - 1200 V).
surface of MSGC 2
determined by ion conductance as electrons can hardly reach this part of the surface; on the other hand. r$ is, in the vicinity of FQ, dominated by electron conductance and much higher than that in the vicinity of DF. Positive charging reduces the gain seriously and negative charging increases it somewhat 151.
0
According to the empirical formula [6] gas amplification is expected to develop at about 20-25 kV/cm, i.e. at a distance of about 30-40 km from the anodes. However, the main contribution to avalanches comes from within a typical distance (Y= 4-6 Frn from the anodes. Some of the drift electrons arrive at the substrate surface adjacent to the anode edges due to diffusion [7] such that avalanche formation will occur there. Based upon Ref. [5] a gas resistivity rg < IO” Rem in the vicinity of DF is estimated for a rate of ionizing particles larger than about IO5 counts/(s mm’). Therefore rg -=x r, for high resistivity substrates, and thus Us > 0 at DF. The FQ area is slightly charged in this case (negatively or positively) because of the much higher r,. This implies gain reduction if 6 is large enough, at least S > LY.For such a configuration (Fig. 2) only substrates with low resistivities, e.g. r, = 10’ f IO I’ 0 cm, can support high rates at stable gain. Obviously, reducing S (Figs. 3-5) would reduce the area with aq > 0. For the case of point F lying in the strong avalanche region (S < a) a part of FQ is affected by avalanches such that it becomes negatively (may be seriously) charged. The positive charges at DF, and the negative charges at FQ form a dipole tending to stabilize the gain. Hence substrates with high resistivities can support high rates provided S < (Y.
2
3
RATE(106C/(smmZ)
Fig. 6. The gain variations as function of counting different back plane voltages of MSGC 2 (D363 IO” fl cm).
5. Experimental 4. Gain variations
1 COUNTING
rates for substrate.
results
The data from Ref. [I] (MSGC 1 of Table I ) do show a pronounced gain drop as a function of rate beyond IO’ counts/(smm’) for equal back plane and cathode potentials at a low drift field as expected from the values of S from Table I (Fig. 2). For higher drift fields (Fig. 3) and/or equal back plane and anode potentials the gain remains rather stable beyond this rate. Relative gains as function of counting rates have been measured for MSGC 2 of Table I operated with an ArlDME (90110) gas mixture at the selected voltage settings given in the table. The field configurations corresponding to some voltage settings are shown in Figs. 4 and 5. Measured relative gains are displayed in Fig. 6. A gain reduction is only observed for equal back plane and anode potentials.
6. Conclusion By increasing the drift voltage and/or the back plane potential one is able to decrease in a predictable way the area of the positively charged (at high rates) surface. Therefore better gain stability is achieved. Raising the potential of the back plane to be higher than that of the anode can move F into the strong avalanche region. This makes MSGCs made from high resistivity substrates
R. Fang et al. I Nucl. Instr. and Meth. in Phys. Rex. A 378 (1996)
442 support
high
rates.
The thinner
the substrate,
the stronger
the influence of the back plane potential. However such a gain stability is based on a compromise between the positively charged area and the negatively charged area, so the gain uniformity would not be as perfect as that obtained for MSGCs made from low resistivity substrates (r, = IO’- IO’ ’ Cl cm) and operated at equal back plane and cathode potentials (Fig. 2).
References [I] R. Bouclier, M. Capeans, C. Garabatos, G. Manzin, G. Million, L. Ropelewski, F. Sauli. L.I. Shekhtman and T. Temmel, Contribution to the Vienna Wire Chamber Conf., February 13-17, 1995.
439-442
[2] For qualitative arguments see: R. Bellazzini and M.A. Speaziga, La Rivista de1 Nuovo Cimento della Socied italiania di
Fisica 12 (1994) p. 21. [3] F.J. Mulligan, Eur. J. Phys. I3 (1992) 57. [4] R. Fang. Ph.D. thesis (1996). [5] R. Fang, R. Blaes, J.M. Brom, W. Geist, A. Michalon, J.L. Riester and C. Voltolini, Nucl. Instr. and Meth. A 365 (1995)
59. [6] M.W. Charles, J. Phys. E5 (1972) 95. [7] R. Fang, R. Blaes. J.M. Brom, W. Geist, A. Michalon, J.L. Riester, R. Seitz and C.Voholini, Nucl. Instr. and Meth. A 361
(1995) 85.