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Left-right ambiguity resolution in a drift chamber working in the self-quenching streamer mode
Nuclear Instruments and Methods in Physics Research A284 (1989) 439-442 North-Holland, Amsterdam
439
LEFT-RIGHT AMBIGUITY RESOLUTION IN A DRIFT CHAMBER WORKING IN THE SELF-QUENCHING STREAMER MODE L. ETIENNE, Ph. JADOT *, S.P .K . TAVERNIER * * and C. VANDER VELDE Inter-University Institute for High Energies, ULB-VUB, Brussels, Belgium
Received 10 May 1989 A method is presented of solving the left-right ambiguity in a drift chamber operated in the self-quenching streamer mode It is based on the comparison of the signals induced on two cathodes situated on the left and right sides of the anode wire. It is demonstrated that in the self-quenching streamer mode an induced charge ratio of about 1 .5 develops in less than 200 ns . This offers a practical method of removing the ambiguity even in big detectors since signals are large in this mode of operation. 1 . Introduction In drift chambers, the time taken by the primary electrons to reach the anode wire, is used to measure the distance between the ionizing particle and the anode. In configurations with an anode wire or anode wire plane positioned in the center of the drift volume there remains the ambiguity on which side of the wire the particle traversed. This is one of the drawbacks of this type of detector . Several methods have been used to solve this left-right ambiguity. Most of them require an additional layer of chambers or an additional wire . However, it has been pointed out that the ambiguity can be removed by observing the signals induced on cathodes situated on the left and right sides of the anode wire [1, 2] . This method has found few applications with chambers operated in proportional mode. There are 3 reasons for this : the induced signals are small, the charge asymmetry is a few percent and it appears only if the signal is integrated over at least ten microseconds . This is due to the fact that in the proportional mode the avalanche develops in the last few microns, close to the anode wire . A measurable asymmetry in the space charge configuration shows up only when the positive ions have drifted several millimeters away from the anode. In the self-quenching streamer mode the avalanche extends several millimeters away from the anode wire, to the direction of the incoming primary ionization [3-5] . It is clear that the asymmetry develops faster, and is larger than in the proportional mode. Breskin et al . already observed a rather large asymmetry when a drift chamber is used in the saturation region [6]. * Now at IRIS-Belgium. * * Onderzoeksleider NFWO . 0168-9002/89/$03 .50 © Elsevier Science Publishers B.V . (North-Holland Physics Publishing Division)
Wire tubes operated in the self-quenching streamer mode have become popular in recent years. It has also been demonstrated that drift chambers with a central anode wire and drift distances of 10 cm can be succesfully operated in this mode [7]. The end-cap muon identifier for the DELPHI experiment installed at the LEP e+ e- collider is built up out of such drift chambers [8]. One of these drift chambers has been modified in order to study the resolution of the left-right ambiguity by the above mentioned method . The results are reported below. 2. The experimental set-up A 90 cm long prototype drift chamber was built using elements constituting the drift cells of the end-cap muon identifier of DELPHI [8]. The field shaping electrodes are made of copper strips, co-extruded in PVC supports, parallel to the 100 p m thick anode wire . The four cathodes close to the anode wire are used to study the proposed method of solving the left-right ambiguity. They are held, two by two, by the central PVC supports facing the anode wire on each side, at a distance of 14 mm from each other. The two 5 mm wide cathodes on each support, are separated by 5 mm and arranged with a left-right symmetry around the wire. A transverse cut of the prototype is shown in fig. 1 . The signals collected at both ends of the anode wire are read through 0.01 pF blocking capacitors . The four cathodes surrounding the anode are at ground potential. The anode potential is maintained at about +5 kV. The drift field is graded from 0 kV to -7 kV at the lateral copper strip electrodes . The charge measurements, are performed using a 10 bit charge sensitive ADC (LeCroy 2249A) with a 200 ns gate, started by the anode signals.
440
L. Etienne et al / Left-right ambiguity resolution
anode wire
Fig. 1 . Transverse cut of the prototype drift chamber (dimensions m mm). In order to fit the 256 pC range of the ADC, the anode and cathode signals are amplified by a factor of about 4. Care was taken to tune the amplifiers to identical gains, in particular for those used for the cathodes . The ADC pedestals are also checked regularly. The anode drift time is measured using an 11 bit TDC (LeCroy 2228A) modified to a range of 10 g.s . The measurements are performed with cosnuc rays selected by means of a triple scintillator coincidence . The lowest scmtillator (20 x 20 cmZ ) is slightly larger than the prototype itself (19.7 cm). It has a fixed position, symmetric with respect to the anode wire and is situated below 20 cm of lead placed under the chamber. Two 10 x 10 cm Z scintillators are placed one above the other, sandwiching the prototype. Three different geometrical arrangements of the small scintillators have been used (see fig. 2) . They are either placed at 1 cm from the anode wire, selecting essentially particles crossing the "left" or "right" side of the chamber ("left configuration" and "right configuration"), or they are placed symmetric with respect to the anode, selecting equal numbers of particles traversing the "left" and "right" side of the chamber ("central configuration") . The drift chamber is filled with a 50-50 argon-isobutane gas mixture, leading to a drift velocity of about 2.5 cm/g s. With a charge threshold at about 20 pC on the amplified anode signals, the anode efficiency plateau goes from 5250 V to 5350 V. a) "left configuration" upper view
b) "central configuration" upper view
3. Results The comparison of charges induced on the left and right cathodes shows indeed an important difference correlated to the position of the track relative to the anode wire . Fig. 3 displays the distribution of the ratio of charges induced on the left cathodes to those induced on the right cathodes obtained for the "left configuration" at 5250 V. The most probable charge ratio value is about 1 .5 . As expected, the effect is important, well above the few percent asymmetry observed in the proportional mode and this, despite the short integration time of 200 ns . In order to investigate a method using this feature to solve the left-right ambiguity, a variable with left-right symmetry is best suited . Fig. 4 displays separately for the upper and lower cathodes the distribution of the difference of the charges induced on the left and right cathodes, normalized to the sum of the charges collected at both ends of the anode, obtained for the "left configuration" at 5250 V. All particles traversing the chamber on its left side should produce a positive difference if the left-right symmetry of the detector is perfect, and if the streamer always points in the primary ionisation direction. This is indeed the case for most events displayed in fig. 4. Using the sign of the difference to decide upon the left or right origin of the particle, only (2 .3 ± .3)%and (2 .6 ± .3)% of the cosmic rays selected by the "left configuration" are attributed to the right side of the chamber by the upper or lower cathodes respectively and (1 .7 ± .2)% are wrongly assigned by both pairs of cathodes . The proposed method thus leads to a correct attribution for at least 97% of the particles crossing at more than 1 cm from the anode wire. It should be noted that some of the wrongly assigned tracks could be due to showers producing particles on both sides of the anode, the right side ionization N 1IX70 "left configuration" H VA =5250 V 500
Fig. 2. Scintillator arrangement m the (a) "left configuration" and (b) "central configuration" .
Fig. 3. Distribution of the ratio of charges induced on the left cathodes to these induced on the right cathodes, with an anode potential of 5250 V, for the "left configuration" .
L. Etienne et al. / Left-right ambiguity resolution reaching the wire first . The quoted efficiency is hence a lower limit. We have checked that this result is not due to a bad tuning of the amplifier gains by permuting the left and right amplifiers. No significant difference is observed, neither in the charge distributions nor in the efficiency of the method. The efficiency is changed by less than 1%, either by changing to the "right configuration", or by varying the anode voltage to the end of the plateau. The charge distributions and the efficiency of the method also do not show significant variation with the drift distance between 1 cm and the maximum of 9 .7 cm . Up-down and left-right asymmetries are observed in the charge difference distributions displayed in fig. 4 and 5 . This last figure displays separately, for upper and lower cathodes, the charge difference distribution obtained at 5350 V with the "central configuration" . This configuration selects equal numbers of particles on either side of the anode. We note that neither the minimum between the left and right peaks is zero, nor are the distributions symmetric around this minimum ; further, the widths of both distributions are different . Since the possible contribution of the amplifier gain to this effect has been excluded by permuting the amplifiers, it can only be explained by an asymmetry of the cathode N 2000
44 1
N a) "central configuration" upper cathodes HVA=5350V
500
u
u
(OL - OR)
1000
-,
IN
6
.
_1
.2
- .3 4
b) "central configuration" lower cathodes HVA =5350 V
500
L L (OL- (D R )
-§
- .2
-1
0-1
2
.3
QA
4
Fig . 5 . Distribution of the difference of charges induced on left and right cathodes, divided by the anode charge, with an anode potential of 5350 V, for the "central configuration", separately for the (a) upper and (b) lower cathodes.
a) "left configuration" uppercathodes HVA=5250 V
1000
b) "left configuration" lower cathodes HVA =5250V
geometry around the anode wire . This indicates that great care must be taken in the mechanical construction of the cathodes. However, even with this imperfect prototype, the cut at zero charge difference adopted to solve the left-right ambiguity has already a high efficiency of at least 97% for particles with drift distances above 1 cm . In order to study the validity of the method for shorter drift distances, a hodoscope with a precise reconstruction of the track is required . Even with the present experimental set-up it is possible to show that the method most probably is still efficient nearer to the anode wire . Fig. 6 shows the left to right induced charge difference distribution for the "central configuration" . Particles with drift distances between 4 and 6 mm and below 2 mm are shown separately . The two peak shape is observed in both cases, with only a slightly better separation for the longer distances .
(aC OR)
4 . Conclusions Fig. 4 . Distribution of the difference of charges induced on left and right cathodes, divided by the anode charge, with an anode potential of 5250 V, for the "left configuration", separately for the (a) upper and (b) lower cathodes .
It is shown that it is possible to solve the left-right ambiguity in a drift chamber operated in the selfquenching streamer mode by a comparison of the
L Etienne et al / Left-right ambiguity resolution
442 N
a)
100
"central configuration" HVA=5250 V d<2mm
The high charge of a streamer (70 pC on average in
our experimental conditions) together with the well separated two peak shape of the charge difference dis-
tribution and the importance of that difference, would allow the use of rather simple electronics to perform the comparison . This feature makes the method applicable
for large size detectors . The use of such a drift cell, operated in the limited streamer mode, with a delay line
on one side of the anode wire, to measure the coordinate along the wire with a precision of - 1%o [9], and a double cathode on the other side, to solve the left-right ambiguity, b)
~ "central configuration" HVA =5250 V
allows space point reconstruction, with a
single layer of chambers .
4
Acknowledgements
I
I
-2
.Î
-~1
0
-.- .
1
We are indepted to Dr F. Udo for stimulating dis-
I
u u (Oi -QR) A 4 3 2
Fig. 6. Distribution of the difference of charges induced on left and right upper cathodes, divided by the anode charge, with an anode potential of 5250 V, for the "central configuration", for drift distances smaller than (a) 2 mm and (b) between 4 and 6 mm . charges induced on two cathodes situated to the left and right of the anode wire .
With an integration time of 200 ns the method is
shown to have an efficiency of at least 97% for drift
distances longer than 1 cm. The charge distributions for shorter drift distances indicates that the method remains effective for shorter drift distances . The impor-
tance of accurate positioning of the pick-up cathodes is also shown.
cussion concerning this work .
References [1] A H. Walenta et al ., Nucl . Instr. and Meth . 151 (1978) 461. [2] G. Charpak and F. Sauh, Nucl . Instr. and Meth. 107 (1973) 371 . [3] G.D . Alekseev et al, Nucl . Instr. and Meth . 177 (1980) 385. [4] M. Atac, A V. Tollestrup and D Potter, IEEE 29 (1982) 388. [5] FAT De Fraga et al ., Nucl . Instr . and Meth . A272 (1988) 921 . [6] A. Breskm et al ., Nucl . Instr. and Meth . 151 (1978) 473. [7] C. De Clercq et al ., Nucl . Instr and Meth. A243 (1986) 77 [8] E. Daubie et al ., Nucl . Instr. and Meth A252 (1986) 435 . [9] F. Snchelbaut et al ., Nucl . Instr. and Meth . A283 (1989) 792 (Proc 1989 Vienna Wire Chamber Conf .) .
439
LEFT-RIGHT AMBIGUITY RESOLUTION IN A DRIFT CHAMBER WORKING IN THE SELF-QUENCHING STREAMER MODE L. ETIENNE, Ph. JADOT *, S.P .K . TAVERNIER * * and C. VANDER VELDE Inter-University Institute for High Energies, ULB-VUB, Brussels, Belgium
Received 10 May 1989 A method is presented of solving the left-right ambiguity in a drift chamber operated in the self-quenching streamer mode It is based on the comparison of the signals induced on two cathodes situated on the left and right sides of the anode wire. It is demonstrated that in the self-quenching streamer mode an induced charge ratio of about 1 .5 develops in less than 200 ns . This offers a practical method of removing the ambiguity even in big detectors since signals are large in this mode of operation. 1 . Introduction In drift chambers, the time taken by the primary electrons to reach the anode wire, is used to measure the distance between the ionizing particle and the anode. In configurations with an anode wire or anode wire plane positioned in the center of the drift volume there remains the ambiguity on which side of the wire the particle traversed. This is one of the drawbacks of this type of detector . Several methods have been used to solve this left-right ambiguity. Most of them require an additional layer of chambers or an additional wire . However, it has been pointed out that the ambiguity can be removed by observing the signals induced on cathodes situated on the left and right sides of the anode wire [1, 2] . This method has found few applications with chambers operated in proportional mode. There are 3 reasons for this : the induced signals are small, the charge asymmetry is a few percent and it appears only if the signal is integrated over at least ten microseconds . This is due to the fact that in the proportional mode the avalanche develops in the last few microns, close to the anode wire . A measurable asymmetry in the space charge configuration shows up only when the positive ions have drifted several millimeters away from the anode. In the self-quenching streamer mode the avalanche extends several millimeters away from the anode wire, to the direction of the incoming primary ionization [3-5] . It is clear that the asymmetry develops faster, and is larger than in the proportional mode. Breskin et al . already observed a rather large asymmetry when a drift chamber is used in the saturation region [6]. * Now at IRIS-Belgium. * * Onderzoeksleider NFWO . 0168-9002/89/$03 .50 © Elsevier Science Publishers B.V . (North-Holland Physics Publishing Division)
Wire tubes operated in the self-quenching streamer mode have become popular in recent years. It has also been demonstrated that drift chambers with a central anode wire and drift distances of 10 cm can be succesfully operated in this mode [7]. The end-cap muon identifier for the DELPHI experiment installed at the LEP e+ e- collider is built up out of such drift chambers [8]. One of these drift chambers has been modified in order to study the resolution of the left-right ambiguity by the above mentioned method . The results are reported below. 2. The experimental set-up A 90 cm long prototype drift chamber was built using elements constituting the drift cells of the end-cap muon identifier of DELPHI [8]. The field shaping electrodes are made of copper strips, co-extruded in PVC supports, parallel to the 100 p m thick anode wire . The four cathodes close to the anode wire are used to study the proposed method of solving the left-right ambiguity. They are held, two by two, by the central PVC supports facing the anode wire on each side, at a distance of 14 mm from each other. The two 5 mm wide cathodes on each support, are separated by 5 mm and arranged with a left-right symmetry around the wire. A transverse cut of the prototype is shown in fig. 1 . The signals collected at both ends of the anode wire are read through 0.01 pF blocking capacitors . The four cathodes surrounding the anode are at ground potential. The anode potential is maintained at about +5 kV. The drift field is graded from 0 kV to -7 kV at the lateral copper strip electrodes . The charge measurements, are performed using a 10 bit charge sensitive ADC (LeCroy 2249A) with a 200 ns gate, started by the anode signals.
440
L. Etienne et al / Left-right ambiguity resolution
anode wire
Fig. 1 . Transverse cut of the prototype drift chamber (dimensions m mm). In order to fit the 256 pC range of the ADC, the anode and cathode signals are amplified by a factor of about 4. Care was taken to tune the amplifiers to identical gains, in particular for those used for the cathodes . The ADC pedestals are also checked regularly. The anode drift time is measured using an 11 bit TDC (LeCroy 2228A) modified to a range of 10 g.s . The measurements are performed with cosnuc rays selected by means of a triple scintillator coincidence . The lowest scmtillator (20 x 20 cmZ ) is slightly larger than the prototype itself (19.7 cm). It has a fixed position, symmetric with respect to the anode wire and is situated below 20 cm of lead placed under the chamber. Two 10 x 10 cm Z scintillators are placed one above the other, sandwiching the prototype. Three different geometrical arrangements of the small scintillators have been used (see fig. 2) . They are either placed at 1 cm from the anode wire, selecting essentially particles crossing the "left" or "right" side of the chamber ("left configuration" and "right configuration"), or they are placed symmetric with respect to the anode, selecting equal numbers of particles traversing the "left" and "right" side of the chamber ("central configuration") . The drift chamber is filled with a 50-50 argon-isobutane gas mixture, leading to a drift velocity of about 2.5 cm/g s. With a charge threshold at about 20 pC on the amplified anode signals, the anode efficiency plateau goes from 5250 V to 5350 V. a) "left configuration" upper view
b) "central configuration" upper view
3. Results The comparison of charges induced on the left and right cathodes shows indeed an important difference correlated to the position of the track relative to the anode wire . Fig. 3 displays the distribution of the ratio of charges induced on the left cathodes to those induced on the right cathodes obtained for the "left configuration" at 5250 V. The most probable charge ratio value is about 1 .5 . As expected, the effect is important, well above the few percent asymmetry observed in the proportional mode and this, despite the short integration time of 200 ns . In order to investigate a method using this feature to solve the left-right ambiguity, a variable with left-right symmetry is best suited . Fig. 4 displays separately for the upper and lower cathodes the distribution of the difference of the charges induced on the left and right cathodes, normalized to the sum of the charges collected at both ends of the anode, obtained for the "left configuration" at 5250 V. All particles traversing the chamber on its left side should produce a positive difference if the left-right symmetry of the detector is perfect, and if the streamer always points in the primary ionisation direction. This is indeed the case for most events displayed in fig. 4. Using the sign of the difference to decide upon the left or right origin of the particle, only (2 .3 ± .3)%and (2 .6 ± .3)% of the cosmic rays selected by the "left configuration" are attributed to the right side of the chamber by the upper or lower cathodes respectively and (1 .7 ± .2)% are wrongly assigned by both pairs of cathodes . The proposed method thus leads to a correct attribution for at least 97% of the particles crossing at more than 1 cm from the anode wire. It should be noted that some of the wrongly assigned tracks could be due to showers producing particles on both sides of the anode, the right side ionization N 1IX70 "left configuration" H VA =5250 V 500
Fig. 2. Scintillator arrangement m the (a) "left configuration" and (b) "central configuration" .
Fig. 3. Distribution of the ratio of charges induced on the left cathodes to these induced on the right cathodes, with an anode potential of 5250 V, for the "left configuration" .
L. Etienne et al. / Left-right ambiguity resolution reaching the wire first . The quoted efficiency is hence a lower limit. We have checked that this result is not due to a bad tuning of the amplifier gains by permuting the left and right amplifiers. No significant difference is observed, neither in the charge distributions nor in the efficiency of the method. The efficiency is changed by less than 1%, either by changing to the "right configuration", or by varying the anode voltage to the end of the plateau. The charge distributions and the efficiency of the method also do not show significant variation with the drift distance between 1 cm and the maximum of 9 .7 cm . Up-down and left-right asymmetries are observed in the charge difference distributions displayed in fig. 4 and 5 . This last figure displays separately, for upper and lower cathodes, the charge difference distribution obtained at 5350 V with the "central configuration" . This configuration selects equal numbers of particles on either side of the anode. We note that neither the minimum between the left and right peaks is zero, nor are the distributions symmetric around this minimum ; further, the widths of both distributions are different . Since the possible contribution of the amplifier gain to this effect has been excluded by permuting the amplifiers, it can only be explained by an asymmetry of the cathode N 2000
44 1
N a) "central configuration" upper cathodes HVA=5350V
500
u
u
(OL - OR)
1000
-,
IN
6
.
_1
.2
- .3 4
b) "central configuration" lower cathodes HVA =5350 V
500
L L (OL- (D R )
-§
- .2
-1
0-1
2
.3
QA
4
Fig . 5 . Distribution of the difference of charges induced on left and right cathodes, divided by the anode charge, with an anode potential of 5350 V, for the "central configuration", separately for the (a) upper and (b) lower cathodes.
a) "left configuration" uppercathodes HVA=5250 V
1000
b) "left configuration" lower cathodes HVA =5250V
geometry around the anode wire . This indicates that great care must be taken in the mechanical construction of the cathodes. However, even with this imperfect prototype, the cut at zero charge difference adopted to solve the left-right ambiguity has already a high efficiency of at least 97% for particles with drift distances above 1 cm . In order to study the validity of the method for shorter drift distances, a hodoscope with a precise reconstruction of the track is required . Even with the present experimental set-up it is possible to show that the method most probably is still efficient nearer to the anode wire . Fig. 6 shows the left to right induced charge difference distribution for the "central configuration" . Particles with drift distances between 4 and 6 mm and below 2 mm are shown separately . The two peak shape is observed in both cases, with only a slightly better separation for the longer distances .
(aC OR)
4 . Conclusions Fig. 4 . Distribution of the difference of charges induced on left and right cathodes, divided by the anode charge, with an anode potential of 5250 V, for the "left configuration", separately for the (a) upper and (b) lower cathodes .
It is shown that it is possible to solve the left-right ambiguity in a drift chamber operated in the selfquenching streamer mode by a comparison of the
L Etienne et al / Left-right ambiguity resolution
442 N
a)
100
"central configuration" HVA=5250 V d<2mm
The high charge of a streamer (70 pC on average in
our experimental conditions) together with the well separated two peak shape of the charge difference dis-
tribution and the importance of that difference, would allow the use of rather simple electronics to perform the comparison . This feature makes the method applicable
for large size detectors . The use of such a drift cell, operated in the limited streamer mode, with a delay line
on one side of the anode wire, to measure the coordinate along the wire with a precision of - 1%o [9], and a double cathode on the other side, to solve the left-right ambiguity, b)
~ "central configuration" HVA =5250 V
allows space point reconstruction, with a
single layer of chambers .
4
Acknowledgements
I
I
-2
.Î
-~1
0
-.- .
1
We are indepted to Dr F. Udo for stimulating dis-
I
u u (Oi -QR) A 4 3 2
Fig. 6. Distribution of the difference of charges induced on left and right upper cathodes, divided by the anode charge, with an anode potential of 5250 V, for the "central configuration", for drift distances smaller than (a) 2 mm and (b) between 4 and 6 mm . charges induced on two cathodes situated to the left and right of the anode wire .
With an integration time of 200 ns the method is
shown to have an efficiency of at least 97% for drift
distances longer than 1 cm. The charge distributions for shorter drift distances indicates that the method remains effective for shorter drift distances . The impor-
tance of accurate positioning of the pick-up cathodes is also shown.
cussion concerning this work .
References [1] A H. Walenta et al ., Nucl . Instr. and Meth . 151 (1978) 461. [2] G. Charpak and F. Sauh, Nucl . Instr. and Meth. 107 (1973) 371 . [3] G.D . Alekseev et al, Nucl . Instr. and Meth . 177 (1980) 385. [4] M. Atac, A V. Tollestrup and D Potter, IEEE 29 (1982) 388. [5] FAT De Fraga et al ., Nucl . Instr . and Meth . A272 (1988) 921 . [6] A. Breskm et al ., Nucl . Instr. and Meth . 151 (1978) 473. [7] C. De Clercq et al ., Nucl . Instr and Meth. A243 (1986) 77 [8] E. Daubie et al ., Nucl . Instr. and Meth A252 (1986) 435 . [9] F. Snchelbaut et al ., Nucl . Instr. and Meth . A283 (1989) 792 (Proc 1989 Vienna Wire Chamber Conf .) .