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Some properties of thin gap gas chambers
NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH
Nuclear Instruments and Methods in Physics Research A309 (1991) 422-426 North-Holland
Section A
Some properties of thin gap gas chambers H. Hofer a, P . Le Coitltre a, H.T. Li R .J. Wu b and K.S. Yang b b
b,
X .G . Li
b,
Y .S . Lu
b,
X.W. Tang
b,
J.H . Wang
b,
Eidgen6stsche Technische Hochschule, CH8093 Zurich, Switzerland. Institute of High Energy Physics, Beijing, China.
Received 20 November 1990 and m revised form 15 April 1991 The operation of a thin gap multiwire chamber with a CO Z /n-pentane mixture is described for different wire diameters, gap distances and cathode material . 1. Introduction
2. Chamber construction and gas filling
In collider experiments calorimeters play an increasingly important role . The physics research at high energy colliders of the next generation (LHC, SSC, . . . ) needs the development of new types of calorimeters . The main requirements for them are: compact volume, fine granularity, fast response, large output signals and stable run conditions [1,2]. The recently reported thin gap gas chambers filled with strong quenching gas and operating in the high gain mode [3,4] are possible candidates as sampling detectors in calorimeters for the future . Chambers working in the limited streamer mode give similar energy resolutions and time response as scintillators [5]. They also give the same energy resolution as chambers working in the proportional mode [6]. The purpose of this article is to report our results of measurements on the properties of thin gap chambers .
The chamber structure is shown in fig. 1 . The anodes are gold-plated tungsten wires with diameters of 80, 60, 55, 30 and 20 wm . They have an effective length of 100 mm, both ends are soldered on printed boards . The wire signals are read out through 1 nF capacitors . The cathodes of the chambers are made of brass plates or graphited Mylar sheets . The high voltage is put to the anode wires and the cathodes are grounded . For some chambers Mylar foils (60 mu thickness) are glued to a brass plate with layers double sticky tape and then painted with a graphite solution and dried. The resistivities of the graphite are about 20 to 100 k fl/cm` . The distances between the wire planes and the cathodes are 1, 1 .5, 2 mm respectively for different chambers and the wire spacing is 2 mm for all chambers . The chamber frames were glued with epoxy. There are holes with thin windows on the cathodes to allow for Anode - Wires
(0 = 20, 30, 50, 60, 80 ~tm)
20 mm 1 .5 mm 1 0 mm
Fig
1.
Output Chamber structure.
0168-9002/91/$03 .50 C 1991 - Elsevier Science Publishers B.V. All rights reserved
\
Cathodes
H. Hofer et al. / Properties of thin gap gas chambers
tests with X-rays or ß-rays . The operating gas is a mixture of CO, and n-pentane at 1 atm, obtained by flowing the CO, gas through the n-pentane liquid . In order to keep stable temperature of the 71-pentane liquid and maintain the constant saturated vapour pressure of the n-pentane, the n-pentane liquid container is put in a water bath with temperature control. The variation of the temperature of the container was kept stable within 0.1 ° C.
U iN
l-
C O
U
3. Properties of the different chambers 3.1 . Charge and timing characteristics
We have measured the output signals of the chambers as a function of the high voltage. For a chamber with brass cathodes, anode wires of 55 mu diameter, a distance of 1.5 mm between cathodes and wires and for incident 55 Fe X-rays, the amplitude of the direct output signals (with 50 fl load) is 80 mV at 3.5 kV . In fig. 2a the distribution of the pulse amplitude is shown. A dispersion of 60% FWHM is seen . The relationship between the charge of pulses and the chamber operating voltage is measured with a charge sensitive amplifier . The result is shown in fig. 3 for a temperature of 14 .5 ° C of the n-pentane. It can be seen that for the thin gap chamber with strong quenching gas filling the amplitude of the pulse increases slowly as the high voltage increases and there is no discontinuous pulse height transition in this region . The charge characteristics for 55 Fe X-rays with pure CO Z filling and CO, + npentane filling is shown for comparison . At 3.6 kV the charge of the signal drops by approximately 30% if the
423
Channel No.
O N
1 0 ns/div.
Fig. 2.(a) Distribution of pulse amplitudes for 55 Fe X-rays . (b) Direct output signals of thin gap chambers . temperature of the n-pentane is raised from 10 to 18 ° C. The pulse shape and amplitude of the direct output signals from the anode wires are studied with a fast
100
U
m m
10
Cu t
U
2 .5
30
Fig. 3. Charge characteristics for 55 Fe X-rays and
3 .5 106
4 .0
HV (kV)
Ru ß-rays with pure CO Z filling and CO z +n-Pentane filling.
H. Hofer et al. / Properties of thin gap gas chambers
424
100.
Û fl. v~ 10 . Lro U
1 .5
2.0
3.0
2.5
3.5
HV (kV)
Fig. 4. Charge characteristics of chambers with different anode wires. oscilloscope . Fig. 2b shows the direct signal pulses of "Fe X-rays on a 50 St load obtained e.g . for the chamber with graphited cathodes (similar pictures are obtained with the brass cathodes). The diameters of the anode wires are 55 Wm, the distance between the wire plane and the cathode is 1 .5 mm and the high voltage is put to 4.0 kV . The horizontal scale is 10 ns/div . and the vertical scale is 20 mV/div. in this picture . It is seen that the signals have a very fast rise time and a narrow width. The rise times of the pulses
are smaller than 5 ns and they have a base width of about 10 ns . There is no noticeable change in the timing characteristics of the chamber with different operating voltages . 3 .2 .Influence of anode wire diameter, gap distance and cathode material
Fig. 4 shows the pulse charge vs chamber voltage for various anode wire diameters. The chamber with brass
100 .
m uD
L
U
2.0
3.0
4.0
5.0 HV (kV)
Fig. 5. Charge characteristics for different wire-cathode distances .
6.0
H Hofer et al. / Properties of thin gap gas chambers
425
O
O
O
O
0
0
0
O
O
O
_J
O U
55 Fe
O
C0 2 + n - Pentane (17° C)
0
04 0
3 .5
4.0
4.5
HV (kV)
Fig. 6. Plateau of the counting rate.
cathodes and the 55 Fe source is used for this test . The distance between the wire plane and the cathode is 1 mm, the temperature of the n-pentane liquid is 18 ° C. As the diameter of the wire decreases the curves of the charge characteristics are shifted to lower voltage, but the shapes of the curves are similar . For the same chamber, source and gas fillings the relative pulse charge as a function of the chamber voltage for various wire-cathode distances is shown in fig. 5 (the wire diameter is 55 wm). For smaller gap distances the curves of the charge characteristics shift to the region of low voltage, but the shape of the curves does not change . We have compared the characteristics of the brass cathode chamber with the same for the graphited cathode chamber of the same geometry and gas filling . No difference is observed . 3.3. Response for different primary ionzation Using collimated 55 Fe X-rays (E (Y) = 5.9 keV) and R-rays (E(mm) = 3 .54 MeV)) we measured the curves of pulse charge characteristics for two different primary ionization . The results from the chamber with brass cathode are shown in fig. 3 (anode wire diameter 55 wm wire-cathode distance 1 .5 mm). The ratio of the chamber response to X-rays and ß-rays shows that the pulse charges are proportional to primary ionization at low operation voltages (= 2.7kV) and reach saturation at higher voltages . 1o6Ru
3.4. Counting rate plateau We have measured the plateau of the counting rate of the chambers for 55 Fe X-rays . One typical plateau curve for a chamber with graphited mylar cathodes, 55 mu wires and a wire-cathode distance of 1.5 mm, is shown in fig. 6 (measured at 17 ° C). The length of the plateau is about 0.7 kV .
4. Conclusions We have measured some properties of thin gap gas chambers operating in the high gain mode . The small volume of the sensitive cells improves the multitrack resolution of calorimeters . The large output signals (= 100 mV on 50 SZ load) simplifies the readout electronics. The fast time response of the signals ( < 5 ns rise time, = 10 ns width) is useful for the operation of calorimeters with high counting rates . Plateau lengths of about 700 V assure easy run conditions . Further measurements of thin gap gas chambers with different construction and geometries (especially integrating the absorber into the chamber design) and different gas fillings are in progress . Special attention will be given to the ageing problem with CO Z/n-pentane mixtures . A first information can be found in ref. [4], where it has been demonstrated that no deteriora-
426
H. Hofer et al. / Properties of thin gap gas chambers
tion of the pulse height has been observed after the passage of 10 9 particles/mm z in a thin gap chamber flushed with the same mixture. This would correspond
to a lifetime of at least one year at a mean distance of 100 cm from the interaction point at the SSC [7].
Acknowledgement We thank C. Chen for the preparation of the measurements in the early stage of these tests.
References [1] H. Williams, Ann. Rev. Nucl . Sci. 36 (1986) 361. [2] C. Fabian and T. Ludlam, Ann Rev. Nucl . Part . Sci. 32 (1982) 355. [3] G. Mikenberg, Nucl . Instr. and Meth . A265 (1988) 223 ; J.S . Majewski et al ., Nucl . Instr. and Meth . 217 (1983) 265. [4] G. Bella et al ., Nucl . Instr. and Meth . A252 (1986) 503. [5] F. Cellet et al ., Nucl . Instr. and Meth . 225 (1984) 493. [6] B. Bleichert et al ., Nucl . Instr . and Meth . A241 (1985) 43 and Nucl . Instr. and Meth . A254 (1987) 529. [71 SSC-SR-1021 (1986) .
Nuclear Instruments and Methods in Physics Research A309 (1991) 422-426 North-Holland
Section A
Some properties of thin gap gas chambers H. Hofer a, P . Le Coitltre a, H.T. Li R .J. Wu b and K.S. Yang b b
b,
X .G . Li
b,
Y .S . Lu
b,
X.W. Tang
b,
J.H . Wang
b,
Eidgen6stsche Technische Hochschule, CH8093 Zurich, Switzerland. Institute of High Energy Physics, Beijing, China.
Received 20 November 1990 and m revised form 15 April 1991 The operation of a thin gap multiwire chamber with a CO Z /n-pentane mixture is described for different wire diameters, gap distances and cathode material . 1. Introduction
2. Chamber construction and gas filling
In collider experiments calorimeters play an increasingly important role . The physics research at high energy colliders of the next generation (LHC, SSC, . . . ) needs the development of new types of calorimeters . The main requirements for them are: compact volume, fine granularity, fast response, large output signals and stable run conditions [1,2]. The recently reported thin gap gas chambers filled with strong quenching gas and operating in the high gain mode [3,4] are possible candidates as sampling detectors in calorimeters for the future . Chambers working in the limited streamer mode give similar energy resolutions and time response as scintillators [5]. They also give the same energy resolution as chambers working in the proportional mode [6]. The purpose of this article is to report our results of measurements on the properties of thin gap chambers .
The chamber structure is shown in fig. 1 . The anodes are gold-plated tungsten wires with diameters of 80, 60, 55, 30 and 20 wm . They have an effective length of 100 mm, both ends are soldered on printed boards . The wire signals are read out through 1 nF capacitors . The cathodes of the chambers are made of brass plates or graphited Mylar sheets . The high voltage is put to the anode wires and the cathodes are grounded . For some chambers Mylar foils (60 mu thickness) are glued to a brass plate with layers double sticky tape and then painted with a graphite solution and dried. The resistivities of the graphite are about 20 to 100 k fl/cm` . The distances between the wire planes and the cathodes are 1, 1 .5, 2 mm respectively for different chambers and the wire spacing is 2 mm for all chambers . The chamber frames were glued with epoxy. There are holes with thin windows on the cathodes to allow for Anode - Wires
(0 = 20, 30, 50, 60, 80 ~tm)
20 mm 1 .5 mm 1 0 mm
Fig
1.
Output Chamber structure.
0168-9002/91/$03 .50 C 1991 - Elsevier Science Publishers B.V. All rights reserved
\
Cathodes
H. Hofer et al. / Properties of thin gap gas chambers
tests with X-rays or ß-rays . The operating gas is a mixture of CO, and n-pentane at 1 atm, obtained by flowing the CO, gas through the n-pentane liquid . In order to keep stable temperature of the 71-pentane liquid and maintain the constant saturated vapour pressure of the n-pentane, the n-pentane liquid container is put in a water bath with temperature control. The variation of the temperature of the container was kept stable within 0.1 ° C.
U iN
l-
C O
U
3. Properties of the different chambers 3.1 . Charge and timing characteristics
We have measured the output signals of the chambers as a function of the high voltage. For a chamber with brass cathodes, anode wires of 55 mu diameter, a distance of 1.5 mm between cathodes and wires and for incident 55 Fe X-rays, the amplitude of the direct output signals (with 50 fl load) is 80 mV at 3.5 kV . In fig. 2a the distribution of the pulse amplitude is shown. A dispersion of 60% FWHM is seen . The relationship between the charge of pulses and the chamber operating voltage is measured with a charge sensitive amplifier . The result is shown in fig. 3 for a temperature of 14 .5 ° C of the n-pentane. It can be seen that for the thin gap chamber with strong quenching gas filling the amplitude of the pulse increases slowly as the high voltage increases and there is no discontinuous pulse height transition in this region . The charge characteristics for 55 Fe X-rays with pure CO Z filling and CO, + npentane filling is shown for comparison . At 3.6 kV the charge of the signal drops by approximately 30% if the
423
Channel No.
O N
1 0 ns/div.
Fig. 2.(a) Distribution of pulse amplitudes for 55 Fe X-rays . (b) Direct output signals of thin gap chambers . temperature of the n-pentane is raised from 10 to 18 ° C. The pulse shape and amplitude of the direct output signals from the anode wires are studied with a fast
100
U
m m
10
Cu t
U
2 .5
30
Fig. 3. Charge characteristics for 55 Fe X-rays and
3 .5 106
4 .0
HV (kV)
Ru ß-rays with pure CO Z filling and CO z +n-Pentane filling.
H. Hofer et al. / Properties of thin gap gas chambers
424
100.
Û fl. v~ 10 . Lro U
1 .5
2.0
3.0
2.5
3.5
HV (kV)
Fig. 4. Charge characteristics of chambers with different anode wires. oscilloscope . Fig. 2b shows the direct signal pulses of "Fe X-rays on a 50 St load obtained e.g . for the chamber with graphited cathodes (similar pictures are obtained with the brass cathodes). The diameters of the anode wires are 55 Wm, the distance between the wire plane and the cathode is 1 .5 mm and the high voltage is put to 4.0 kV . The horizontal scale is 10 ns/div . and the vertical scale is 20 mV/div. in this picture . It is seen that the signals have a very fast rise time and a narrow width. The rise times of the pulses
are smaller than 5 ns and they have a base width of about 10 ns . There is no noticeable change in the timing characteristics of the chamber with different operating voltages . 3 .2 .Influence of anode wire diameter, gap distance and cathode material
Fig. 4 shows the pulse charge vs chamber voltage for various anode wire diameters. The chamber with brass
100 .
m uD
L
U
2.0
3.0
4.0
5.0 HV (kV)
Fig. 5. Charge characteristics for different wire-cathode distances .
6.0
H Hofer et al. / Properties of thin gap gas chambers
425
O
O
O
O
0
0
0
O
O
O
_J
O U
55 Fe
O
C0 2 + n - Pentane (17° C)
0
04 0
3 .5
4.0
4.5
HV (kV)
Fig. 6. Plateau of the counting rate.
cathodes and the 55 Fe source is used for this test . The distance between the wire plane and the cathode is 1 mm, the temperature of the n-pentane liquid is 18 ° C. As the diameter of the wire decreases the curves of the charge characteristics are shifted to lower voltage, but the shapes of the curves are similar . For the same chamber, source and gas fillings the relative pulse charge as a function of the chamber voltage for various wire-cathode distances is shown in fig. 5 (the wire diameter is 55 wm). For smaller gap distances the curves of the charge characteristics shift to the region of low voltage, but the shape of the curves does not change . We have compared the characteristics of the brass cathode chamber with the same for the graphited cathode chamber of the same geometry and gas filling . No difference is observed . 3.3. Response for different primary ionzation Using collimated 55 Fe X-rays (E (Y) = 5.9 keV) and R-rays (E(mm) = 3 .54 MeV)) we measured the curves of pulse charge characteristics for two different primary ionization . The results from the chamber with brass cathode are shown in fig. 3 (anode wire diameter 55 wm wire-cathode distance 1 .5 mm). The ratio of the chamber response to X-rays and ß-rays shows that the pulse charges are proportional to primary ionization at low operation voltages (= 2.7kV) and reach saturation at higher voltages . 1o6Ru
3.4. Counting rate plateau We have measured the plateau of the counting rate of the chambers for 55 Fe X-rays . One typical plateau curve for a chamber with graphited mylar cathodes, 55 mu wires and a wire-cathode distance of 1.5 mm, is shown in fig. 6 (measured at 17 ° C). The length of the plateau is about 0.7 kV .
4. Conclusions We have measured some properties of thin gap gas chambers operating in the high gain mode . The small volume of the sensitive cells improves the multitrack resolution of calorimeters . The large output signals (= 100 mV on 50 SZ load) simplifies the readout electronics. The fast time response of the signals ( < 5 ns rise time, = 10 ns width) is useful for the operation of calorimeters with high counting rates . Plateau lengths of about 700 V assure easy run conditions . Further measurements of thin gap gas chambers with different construction and geometries (especially integrating the absorber into the chamber design) and different gas fillings are in progress . Special attention will be given to the ageing problem with CO Z/n-pentane mixtures . A first information can be found in ref. [4], where it has been demonstrated that no deteriora-
426
H. Hofer et al. / Properties of thin gap gas chambers
tion of the pulse height has been observed after the passage of 10 9 particles/mm z in a thin gap chamber flushed with the same mixture. This would correspond
to a lifetime of at least one year at a mean distance of 100 cm from the interaction point at the SSC [7].
Acknowledgement We thank C. Chen for the preparation of the measurements in the early stage of these tests.
References [1] H. Williams, Ann. Rev. Nucl . Sci. 36 (1986) 361. [2] C. Fabian and T. Ludlam, Ann Rev. Nucl . Part . Sci. 32 (1982) 355. [3] G. Mikenberg, Nucl . Instr. and Meth . A265 (1988) 223 ; J.S . Majewski et al ., Nucl . Instr. and Meth . 217 (1983) 265. [4] G. Bella et al ., Nucl . Instr. and Meth . A252 (1986) 503. [5] F. Cellet et al ., Nucl . Instr. and Meth . 225 (1984) 493. [6] B. Bleichert et al ., Nucl . Instr . and Meth . A241 (1985) 43 and Nucl . Instr. and Meth . A254 (1987) 529. [71 SSC-SR-1021 (1986) .