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Fast gaseous detectors in high magnetic fields
Nuclear Instruments and Methods in Physics Research A 335 (1993) 439-442 North-Holland
NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SectionA
Fast gaseous detectors in high magnetic fields Ulrich J . Becker, Jonathan P. Rodin and Bryan R. Smith Massachusetts Institute of Technology, Cambridge, MA 02139, USA
Received 24 May 1993 To cope with the high bunch crossing rates of future particle accelerators, new detectors must have short recovery times. We measured the transit time of drifting photoelectrons in straw tubes of 2.5 mm radius for magnetic fields ranging from 0 to 8 T. Six different gas mixtures, primarily CF4 based, were studied. Long transit times and signal degradation indicate the need for careful choices in detector design . 1. Introduction Straw tubes have been proposed as a part of various detectors at future high energy, high luminosity colliders [1] where typical bunch crossing times of 15-25 ns are intended . Experiments will operate with magnetic fields of up to 4 T [2]. Recovery time of the detectors in such fields will limit the usable number of bunch crossings . We measured the electron transit time as a function of a magnetic field applied parallel to a straw tube containing various gas mixtures . The gases tested were primarily CF4 based and had a range of transport properties . Our data indicate that B = 0 measurements of drift velocities can be misleading when selecting a
Ga In Hypodermic I Needle_, -------:
5mm Aluminized
Straw Tube
Signal
gas to operate in high magnetic fields. In fact, some proposals appear unrealistic in view of our measurements. 2. Setup Two straw tube detectors of 5 mm diameter were constructed out of aluminized 25 ltm thick mylar foil tubing, fig. la. Such tubes may be used in low mass central detectors. One tube contained a 30 ~Lm signal wire, and the other a 75 wm signal wire . Clear plastic end-pieces allowed for the passage of laser light. The signal wires were held by spring loaded hypodermic needles of 110 wm internal diameter inserted accuratly
GasfOut
End Plug
Fig. 1. (a) Straw tube . Note how the UV enters through the transparent end plug to create ionization at the inner tube wall . (b) Schematic of the experimental setup. 0168-9002/93/$06 .00 © 1993 - Elsevier Science Publishers B.V . All rights reserved
440
U.J. Becker et al. / Fast gaseous detectors in high magnetic fields
57
Irn 0 ô
rme
m
30 um wire
Time ->
m_ LO â0
B.0 B-3
N
0
50
100
Drift Time After Trigger [nsl
150
Fig. 2. Principle of measurement illustrated for Ar :CF4 :iC4 Hill (40 :40 :20) using the 30 wm wire . Signals recorded at B=Oand B=3T .
into the center of the end-pieces . The offset of the wires was less than 75 wm from the center of each tube . Gas was injected directly into the tubes through the end plugs. The gases were premixed using the method of partial pressures. Concentrations were checked by measuring electron drift velocity at various electric fields using the setup described in ref. [3]. The drift velocities of our gas mixtures had previously been measured using the same apparatus [3,4] and were reproduced to within 1% . A hermetic chamber containing the tubes was placed into the uniform field region of a 10 cm diameter solenoidal magnet located at the Bitter Magnet laboratory [5], fig. lb. The magnet was set up for a maximum current of 20000 A and generated fields of up to 8 T with an uncertainty of +I% .
1 2 3 4 5 6
CF4 :CO2 50:50_V-2600 CF4 :C02 :iC4H10 6020:20_V-2150 V.2200 CF4 :iC4H10 80:20 Ar:CF4 :IC4H16 40 :4020 V .1850 V .1700 Ar :C2H6 50:50 V.1450 P10
Light pulses of 3 ns width from a 337 nm N2 laser [6] were reflected into the chamber . Photoelectrons were liberated from the aluminized inner surface of the straw tube and drifted to the signal wire . The signal was amplified, averaged and stored to disk for analysis using a digital oscilloscope [7]. The scope was triggered by a phototube, which received laser light from a beam splitter placed outside of the chamber . This trigger was constant to within 0.4 ns . An 55 Fe source was placed in the chamber next to the straw tubes. The tubes were operated at a voltage which gave an amplified SS Fe signal of approximately 80 and 30 mV in the 30 and 75 wm tubes, respectively . The gain of the 75 wm wire could not be increased beyond this value without the tube sparking . The intensity of the laser beam was adjusted so that the signals from the laser pulses were comparable in magnitude to the 55 Fe signals, i.e ., approximately 200 photoelectrons . For each gas and tube the signal voltage was kept constant while the magnetic field was varied . We measured the delay between the trigger and the point where the signal pulse reached one half of its maximum value, fig. 2. The 50% level was chosen in order to limit the effects of laser noise and wire eccentricity . The time jitter of the averaged rising edge was ±0.8 ns at B = 0, but larger at higher magnetic fields . Absolute drift time was obtained by measuring the delay due to light propagation, the cable delays, and all instrumental delays . In order to check for systematic effects a duplicate detector was later built and the experiment repeated with newly mixed gases. The largest effect was caused by replacing the CF, filter with a nominally identical model [8]. The sensitivity of CF, based admixtures to trace impurities is well known. At low ß-fields, we estimate from numeric integration the absolute time
1 3 4 6
CF4 :C02 V.3150 50:50 CF4 :IC4H10 80 :20 V.2650 Ar :CF4:IC4H10 40 :40:20 V.2300 P10 V.1700
6 8 6 0 2 B lte es]al ial Fig. 3. (a) Drift times in r = 2.5 mm drift tube with 30 wm signal wire as a function of magnetic field. (b) Results for 75 wm wire drift tube . Note that the numbering code of gases traces to fig. 3a . 4
B [t
441
U.J. Becker et al. / Fast gaseous detectors in high magnetic fields
Time i3=0 ->
=0
N
a
0 N '
30um wire
0
100
200
300
Drift Time After Trigger [ns]
400
Fig. 4. Deterioration of pulse height in P10 gas. Signals shown at B=0 and B=3T.
measurements to be 5 ns less, but the relative error between gases is smaller.
Fig. 3b shows the corresponding measurements for the 75 wm tube . Note that the drift times are shorter than for the 30 lL wire due to the higher signal voltage required for adequate gain. The increase of the drift times in rising magnetic field is suppressed, but the relative behavior of the gases is reproduced . Although higher E-fields appear to be advantageous, the tube had to be operated at the limit of streamer mode which could adversely affect the lifetime of the detector . This will be investigated in the future . Fig. 4 shows signal pulse shapes in P10 at B = 0 and B = 3 T. Note how the pulses decrease in magnitude and increase in width with increasing B field . The small oscillation of the baseline is rf noise from the laser. Fig. 5a shows normalized pulse height vs magnetic field for all gases in the 30 [Lm tube . Comparison with fig. 5b shows that, as the field is raised, gases with slowly increasing drift times also display relatively constant pulse heights. Indeed, there is a one to one correspondence between the ordering of the gases on the two plots for B > 3 T.
3. Data
4. Interpretation and discussion
The electron drift time vs applied magnetic field is shown in fig. 3a for all gases tested in the 30 p,m tube . Note that, at B = 0, freon based gases have significantly lower drift times than those based on argon. As expected [9] the gas with the highest freon concentration shows the shortest time at B = 0. As the magnetic field is increased the transit time in most gases rises sharply. However, the increase is more modest in gases containing C0 2. Above 4 T CF4 : CO 2 (50 :50) emerges as the fastest gas with drift times a factor of 3 smaller than CF4 : iC,H 1 , (80 : 20), the fastest gas at low fields.
The extremely long drift times shown in fig. 5b can be interpreted as being due to highly curved electron trajectories . For illustrative purposes fig. 6 shows a simplified path in P10 at 4 T with no diffusion. Our data clearly demonstrate that a gas chosen for its high drift velocity at low (< 1 T) magnetic fields is not necessarily fast at the fields proposed for the next generation of particle detectors. Conversely, the emergence of CF4 : CO 2 (50 : 50) as the fastest gas at B > 4 T would not be predicted by naive extrapolation of its B = 0 behavior.
1 2 3 4 5 8
CF4:CO2 50 :50 V .2800 CF4:C02 :IC4HlO 802020_V .2150 CF4:IC4H10 8020 V.2200 Ar:CF4:IC4H10_40:4020_V.1850 Ar:C2H8 50 :50_V.1700 P10 V .1450
1 2 3 4 5 8
0 0
CF4 :C02 50:50 V.2600 CF4 :C02 :1C4H10 6020:20V:2150 CF4AC4H10 8020 V.2200 Ar:CF4AC4H10_40 :40:20_V .1850 Ar:C2H8 50:50_V.1700 P10 V.1450
5
ç C:) m CO E ~ 0 0 ô
30um
0 0
N 0
0
6
wire
MWM
8 6 a [tésia] Fig. 5. (a) Relative pulse heights in all gases, normalized to 1 at B = 0, 30 wm tube . Curves guide the eye only. (b) Drift times in expanded scale compared with fig. 3a . Note the extremely long times at high B-fields . 2
U.J. Becker et al. / Fast gaseous detectors in high magnetic fields
442
Acknowledgements We would like
to thank the staff at the Bitter
Magnet laboratory for the use of their facilities, in
particular, Dr. L.G . Rubin, division head . Also, we thank Mr . M. Grossman for machining the detector
end pieces . This work was supported in part by US
Straw Tube ->
Department of Energy contract no . DE-AC02-93 ERO 3069 001 .
References -4
-2
0
Distance[mm]
2
Fig. 6. Illustration of underlying electron trajectories. It should be pointed out that this discussion also applies to gas microstrip chambers which have similar dimensions to our tubes with a more inhomogeneous electric field configuration .
5. Conclusion If high magnetic fields are to be used in future particle detectors, careful consideration must be given to the performance of the proposed drift gases. Our measurements show that a gas chosen without explicit investigation of its high ß-field drift velocity could result in long detector recovery times and impose severe restrictions on the useful bunch crossing rate . We propose that a mixture of CF4 and CO Z has the required properties and will pursue further research into this promising area .
[1] L.R . Cormell et al ., Accelerator Parameters for Detectors design, Report SSCL-264 (1990) . Design Study of the Large Hadron Collider (LHC), CERN 91-03 (1991) . [2] Letter of Intent by SDC Nov. 1991, SSC-L01 0001 . ATLAS CERN/LHCC/92-4, CMS CERN/LHCC/92-3, UP CERN/LHCC/92-5 . [3] U.J . Becker et al ., Nucl . Instr. and Meth . A 306 (1991) 194. [4] U.J . Becker et al ., Nucl . Instr. and Meth . A 315 (1992) 14 ; Y.H . Chang et al ., Nucl . Instr. and Meth . A 311 (1992) 490. See also ref. [9]. [5] Francis Bitter National Magnet Laboratory, MIT, Cambridge, MA 02139. [6] Laser Science Inc. Newton, MA 02158: VSL 337ND Laser. [7] LeCroy Research Corporation : 612A amplifier, 7200 digital oscilloscope . [8] Semi-Gas Systems, San José, CA : Nanochem 1600 Purifi-
cation System . [9] L.G . Christophorou et al ., Nucl . Instr. and Meth . 163 (1979) 141 ; T. Yamashita et al ., Nucl . Instr. and Meth . A 283 (1989) 709 : R. Openshaw, IEEE Trans. 36 (1989) 567.
NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SectionA
Fast gaseous detectors in high magnetic fields Ulrich J . Becker, Jonathan P. Rodin and Bryan R. Smith Massachusetts Institute of Technology, Cambridge, MA 02139, USA
Received 24 May 1993 To cope with the high bunch crossing rates of future particle accelerators, new detectors must have short recovery times. We measured the transit time of drifting photoelectrons in straw tubes of 2.5 mm radius for magnetic fields ranging from 0 to 8 T. Six different gas mixtures, primarily CF4 based, were studied. Long transit times and signal degradation indicate the need for careful choices in detector design . 1. Introduction Straw tubes have been proposed as a part of various detectors at future high energy, high luminosity colliders [1] where typical bunch crossing times of 15-25 ns are intended . Experiments will operate with magnetic fields of up to 4 T [2]. Recovery time of the detectors in such fields will limit the usable number of bunch crossings . We measured the electron transit time as a function of a magnetic field applied parallel to a straw tube containing various gas mixtures . The gases tested were primarily CF4 based and had a range of transport properties . Our data indicate that B = 0 measurements of drift velocities can be misleading when selecting a
Ga In Hypodermic I Needle_, -------:
5mm Aluminized
Straw Tube
Signal
gas to operate in high magnetic fields. In fact, some proposals appear unrealistic in view of our measurements. 2. Setup Two straw tube detectors of 5 mm diameter were constructed out of aluminized 25 ltm thick mylar foil tubing, fig. la. Such tubes may be used in low mass central detectors. One tube contained a 30 ~Lm signal wire, and the other a 75 wm signal wire . Clear plastic end-pieces allowed for the passage of laser light. The signal wires were held by spring loaded hypodermic needles of 110 wm internal diameter inserted accuratly
GasfOut
End Plug
Fig. 1. (a) Straw tube . Note how the UV enters through the transparent end plug to create ionization at the inner tube wall . (b) Schematic of the experimental setup. 0168-9002/93/$06 .00 © 1993 - Elsevier Science Publishers B.V . All rights reserved
440
U.J. Becker et al. / Fast gaseous detectors in high magnetic fields
57
Irn 0 ô
rme
m
30 um wire
Time ->
m_ LO â0
B.0 B-3
N
0
50
100
Drift Time After Trigger [nsl
150
Fig. 2. Principle of measurement illustrated for Ar :CF4 :iC4 Hill (40 :40 :20) using the 30 wm wire . Signals recorded at B=Oand B=3T .
into the center of the end-pieces . The offset of the wires was less than 75 wm from the center of each tube . Gas was injected directly into the tubes through the end plugs. The gases were premixed using the method of partial pressures. Concentrations were checked by measuring electron drift velocity at various electric fields using the setup described in ref. [3]. The drift velocities of our gas mixtures had previously been measured using the same apparatus [3,4] and were reproduced to within 1% . A hermetic chamber containing the tubes was placed into the uniform field region of a 10 cm diameter solenoidal magnet located at the Bitter Magnet laboratory [5], fig. lb. The magnet was set up for a maximum current of 20000 A and generated fields of up to 8 T with an uncertainty of +I% .
1 2 3 4 5 6
CF4 :CO2 50:50_V-2600 CF4 :C02 :iC4H10 6020:20_V-2150 V.2200 CF4 :iC4H10 80:20 Ar:CF4 :IC4H16 40 :4020 V .1850 V .1700 Ar :C2H6 50:50 V.1450 P10
Light pulses of 3 ns width from a 337 nm N2 laser [6] were reflected into the chamber . Photoelectrons were liberated from the aluminized inner surface of the straw tube and drifted to the signal wire . The signal was amplified, averaged and stored to disk for analysis using a digital oscilloscope [7]. The scope was triggered by a phototube, which received laser light from a beam splitter placed outside of the chamber . This trigger was constant to within 0.4 ns . An 55 Fe source was placed in the chamber next to the straw tubes. The tubes were operated at a voltage which gave an amplified SS Fe signal of approximately 80 and 30 mV in the 30 and 75 wm tubes, respectively . The gain of the 75 wm wire could not be increased beyond this value without the tube sparking . The intensity of the laser beam was adjusted so that the signals from the laser pulses were comparable in magnitude to the 55 Fe signals, i.e ., approximately 200 photoelectrons . For each gas and tube the signal voltage was kept constant while the magnetic field was varied . We measured the delay between the trigger and the point where the signal pulse reached one half of its maximum value, fig. 2. The 50% level was chosen in order to limit the effects of laser noise and wire eccentricity . The time jitter of the averaged rising edge was ±0.8 ns at B = 0, but larger at higher magnetic fields . Absolute drift time was obtained by measuring the delay due to light propagation, the cable delays, and all instrumental delays . In order to check for systematic effects a duplicate detector was later built and the experiment repeated with newly mixed gases. The largest effect was caused by replacing the CF, filter with a nominally identical model [8]. The sensitivity of CF, based admixtures to trace impurities is well known. At low ß-fields, we estimate from numeric integration the absolute time
1 3 4 6
CF4 :C02 V.3150 50:50 CF4 :IC4H10 80 :20 V.2650 Ar :CF4:IC4H10 40 :40:20 V.2300 P10 V.1700
6 8 6 0 2 B lte es]al ial Fig. 3. (a) Drift times in r = 2.5 mm drift tube with 30 wm signal wire as a function of magnetic field. (b) Results for 75 wm wire drift tube . Note that the numbering code of gases traces to fig. 3a . 4
B [t
441
U.J. Becker et al. / Fast gaseous detectors in high magnetic fields
Time i3=0 ->
=0
N
a
0 N '
30um wire
0
100
200
300
Drift Time After Trigger [ns]
400
Fig. 4. Deterioration of pulse height in P10 gas. Signals shown at B=0 and B=3T.
measurements to be 5 ns less, but the relative error between gases is smaller.
Fig. 3b shows the corresponding measurements for the 75 wm tube . Note that the drift times are shorter than for the 30 lL wire due to the higher signal voltage required for adequate gain. The increase of the drift times in rising magnetic field is suppressed, but the relative behavior of the gases is reproduced . Although higher E-fields appear to be advantageous, the tube had to be operated at the limit of streamer mode which could adversely affect the lifetime of the detector . This will be investigated in the future . Fig. 4 shows signal pulse shapes in P10 at B = 0 and B = 3 T. Note how the pulses decrease in magnitude and increase in width with increasing B field . The small oscillation of the baseline is rf noise from the laser. Fig. 5a shows normalized pulse height vs magnetic field for all gases in the 30 [Lm tube . Comparison with fig. 5b shows that, as the field is raised, gases with slowly increasing drift times also display relatively constant pulse heights. Indeed, there is a one to one correspondence between the ordering of the gases on the two plots for B > 3 T.
3. Data
4. Interpretation and discussion
The electron drift time vs applied magnetic field is shown in fig. 3a for all gases tested in the 30 p,m tube . Note that, at B = 0, freon based gases have significantly lower drift times than those based on argon. As expected [9] the gas with the highest freon concentration shows the shortest time at B = 0. As the magnetic field is increased the transit time in most gases rises sharply. However, the increase is more modest in gases containing C0 2. Above 4 T CF4 : CO 2 (50 :50) emerges as the fastest gas with drift times a factor of 3 smaller than CF4 : iC,H 1 , (80 : 20), the fastest gas at low fields.
The extremely long drift times shown in fig. 5b can be interpreted as being due to highly curved electron trajectories . For illustrative purposes fig. 6 shows a simplified path in P10 at 4 T with no diffusion. Our data clearly demonstrate that a gas chosen for its high drift velocity at low (< 1 T) magnetic fields is not necessarily fast at the fields proposed for the next generation of particle detectors. Conversely, the emergence of CF4 : CO 2 (50 : 50) as the fastest gas at B > 4 T would not be predicted by naive extrapolation of its B = 0 behavior.
1 2 3 4 5 8
CF4:CO2 50 :50 V .2800 CF4:C02 :IC4HlO 802020_V .2150 CF4:IC4H10 8020 V.2200 Ar:CF4:IC4H10_40:4020_V.1850 Ar:C2H8 50 :50_V.1700 P10 V .1450
1 2 3 4 5 8
0 0
CF4 :C02 50:50 V.2600 CF4 :C02 :1C4H10 6020:20V:2150 CF4AC4H10 8020 V.2200 Ar:CF4AC4H10_40 :40:20_V .1850 Ar:C2H8 50:50_V.1700 P10 V.1450
5
ç C:) m CO E ~ 0 0 ô
30um
0 0
N 0
0
6
wire
MWM
8 6 a [tésia] Fig. 5. (a) Relative pulse heights in all gases, normalized to 1 at B = 0, 30 wm tube . Curves guide the eye only. (b) Drift times in expanded scale compared with fig. 3a . Note the extremely long times at high B-fields . 2
U.J. Becker et al. / Fast gaseous detectors in high magnetic fields
442
Acknowledgements We would like
to thank the staff at the Bitter
Magnet laboratory for the use of their facilities, in
particular, Dr. L.G . Rubin, division head . Also, we thank Mr . M. Grossman for machining the detector
end pieces . This work was supported in part by US
Straw Tube ->
Department of Energy contract no . DE-AC02-93 ERO 3069 001 .
References -4
-2
0
Distance[mm]
2
Fig. 6. Illustration of underlying electron trajectories. It should be pointed out that this discussion also applies to gas microstrip chambers which have similar dimensions to our tubes with a more inhomogeneous electric field configuration .
5. Conclusion If high magnetic fields are to be used in future particle detectors, careful consideration must be given to the performance of the proposed drift gases. Our measurements show that a gas chosen without explicit investigation of its high ß-field drift velocity could result in long detector recovery times and impose severe restrictions on the useful bunch crossing rate . We propose that a mixture of CF4 and CO Z has the required properties and will pursue further research into this promising area .
[1] L.R . Cormell et al ., Accelerator Parameters for Detectors design, Report SSCL-264 (1990) . Design Study of the Large Hadron Collider (LHC), CERN 91-03 (1991) . [2] Letter of Intent by SDC Nov. 1991, SSC-L01 0001 . ATLAS CERN/LHCC/92-4, CMS CERN/LHCC/92-3, UP CERN/LHCC/92-5 . [3] U.J . Becker et al ., Nucl . Instr. and Meth . A 306 (1991) 194. [4] U.J . Becker et al ., Nucl . Instr. and Meth . A 315 (1992) 14 ; Y.H . Chang et al ., Nucl . Instr. and Meth . A 311 (1992) 490. See also ref. [9]. [5] Francis Bitter National Magnet Laboratory, MIT, Cambridge, MA 02139. [6] Laser Science Inc. Newton, MA 02158: VSL 337ND Laser. [7] LeCroy Research Corporation : 612A amplifier, 7200 digital oscilloscope . [8] Semi-Gas Systems, San José, CA : Nanochem 1600 Purifi-
cation System . [9] L.G . Christophorou et al ., Nucl . Instr. and Meth . 163 (1979) 141 ; T. Yamashita et al ., Nucl . Instr. and Meth . A 283 (1989) 709 : R. Openshaw, IEEE Trans. 36 (1989) 567.