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Effect of N2 contamination on drift chambers in magnetic fields
Nuclear Instruments and Methods in Physics Research A 351 (1994)
583-584
NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH
Letter to the Editor
ELSEVIER
Section A
Effect of N contamination on drift chambers in magnetic fields 2
Ulrich J. Becker, Steven C. Nahn, Jonathan P. Rodin *, Bryan R. Smith Massachusetts Institute of Technology, CambridgeMA 02139, USA
Received
15
June 1994
Abstract Since the presence of a small amount of air is unavoidable in large drift detectors, we present a measurement of the effect of N2 contamination on the electron drift velocity in a working L3 Muon Chamber placed in the MIT cyclotron magnet . The nominal gas mixture [Ar : C02 : iC,H 10 (86 :10 :4)] was varied by including as much as 1% N2 . Results at B = 0 and 0.5 T are shown. Shifts in the drift velocity curves indicate the need for a higher electric field to maintain operational accuracy . Due to their finite leak rate, drift chambers are usually subject to small amounts of air contamination . Because of the unusual shape of the nitrogen-electron cross section [1], small quantities of this gas can have a substantial effect on observed drift properties as recently shown at B = 0 [2]. This was investigated both with and without magnetic field using a Forward/ Backward muon chamber from the L3 experiment at LEP,CERN . The reader is referred to Ref. [3] for a detailed description of the F/B system, comprised of 96 such chambers . Fig. la shows the chamber in cross section, Fig. lb the leak rate measured by connecting an oil filled U-tube manometer to the exhaust of the chamber, filling to 100 mm overpressure and recording the decay. The chambers are operated at 1 atm. pressure, therefore the rate of diffusion of air into the chamber should be very close to the leak rate . We estimate the air contamination to be approximately 1% at a flow of 2 1/min (by assuming homogeneous mixing in the chamber volume of 800 1) . Note that the leak rate could increase by a factor 3-4 before reaching the manufacturing tolerance level. The chamber was placed into the MIT cyclotron magnet for measurements in a uniform B-field of 0 to 0.5 T. A UV laser [4] was directed into a chamber cell to generate photoelectrons on the I-beam, exactly 5.15 cm from the wire plane. The laser, collimated to achieve a signal comparable in magnitude to that from cosmic muons, hit the I-beam 10 cm into the chamber, well within the uniform electric field region [5]. High voltage settings for different drift fields were calculated using the Wire Chamber program [6] such that the surface field on the wires was kep at 170 kV/cm for constant gain . Drift times were
B ~100--+ -MEASURED VALUE E 80-0
TOLERANCE
60 '4020 0 0
10
20
30
40 50 60 TIME(min)
Fig 1. (a) Cross section of L3 Forward/Backward muon chamber showing I-beam structure. (b) Leak rate of F/B chamber 03 . Tolerance is exponential with 7 = 120 min.
* Corresponding author . 0168-9002/94/$07 .00 © 1994 Elsevier SSDI0168-9002(94)01079-X
measured with respect to signals from a photodiode receiving UV from a beam-splitter placed outside of the chamber. "Prepulses" from direct laser hits on the signal wires provided a t = 0 calibration to Ins precision. We used the simple formula u,1 = (5 .15 cm)/(drift time) to obtain the drift velocity parallel to the electric field; corrections were not made for the non-uniform electric field close to the signal wires. We estimate the errors to be ±0.5% experimental and ±0 .5% systematic, increasing to 1% at low E-fields due to signal degradation .
Science
B.V. All
rights reserved
584
U.J . Becker et al. /NucL Instr. and Meth . to Phys. Res. A 351 (1994) 583-584
E
_0
B=o
5
6
4 3
3
2 1
+
00
500
1000 1500 2000 E-FIELD(V/cm)
Fig 2. Drift velocity vs E-field m F/B chamber at B = 0, low flow rate. Curve is a reference measurement [7]. Fig. 2 shows the measured drift velocity in the chamber with a flow of 2 1/min L3 F/B gas [Ar :CO, :'C 4 H io (86 : 10 : 4)]. Superimposed is a (spline) curve ofmeasurements made using the test chamber described in Ref. [7]. We include this as a reference curve of the drift velocity in this gas . There is a clear discrepancy in shape between the two sets of data ; the drift velocity measured in the F/B is suppressed below 1000V/cm and enhanced above. This difference is attributed to NZ (air) contamination in the F/B chamber. In an attempt to reduce air contamination we increased the flow rate an order of magnitude and repeated the measurement at 20 .93 1/min. This corresponds to a gas turnover of approximately one chamber volume every 45 min. At this high flow rate, we expect around 0.1% N2 in the chamber and, indeed, close agreement with the reference curve is observed at both B = 0 and 0.5 T, Fig. 3. To verify the effect we repeated the experiment with the injection of a controlled 1% admixture of NZ, Fig. 4. One sees the same broadening of the plateau region and an increase in the drift velocity at high E-fields . At B = 0, this behavior agrees with the relative changes predicted by the Magboltz program [8].
Data Reference FIB
500
Flow Rate= 20 .93 Urn ArC02:lC4Hio(8610:4) 1000
E-Field(V/cm) 1500
2000
Fig 4. Drift velocity vs E-field in F/B chamber at B = 0 and 0.5 T, 1 % N, added. It is important to note that the effect of the contamination is present in both B = 0 and B = 0.5 T data and our results show it to be of a comparable magnitude in each . The presence of small quantities of dry air has a very similar effect to that of comparable amounts of nitrogen [2]. In summary, when operating large drift chambers at economical flow rates, nitrogen contamination of around 1% must be expected even with good leak rates. This has been shown here to have a significant effect on the drift velocity in a working F/B muon chamber at both B = 0 and B = 0.5 T. In particular, shifts in the drift velocity plateau suggest operating points somewhat higher than those chosen for pure gas in order to restore the operational accuracy for best physics resolution . Acknowledgements We would like to thank Darling Ren of ETHZ for calculating voltage settings for the chamber. We are grateful to the US Dept . of Energy for their support under contract no . DE-AC02-93 ERO 3069 001. References
6
vu(cm/As)
ï `I
[2] [3]
5'~ 4 3 2
B=0 r
[4] FIB Data
Reference 500
Flow Rate= 20 .93 Urn ArCOZIC4Hio(8610 4) 1000
E-Field(V/cm) 1500
2000
Fig 3. Drift velocity vs E-field in F/B chamber at B = 0 and 0.5 T, high flow rate . Reference curves from [7].
[6] [7] [8]
L. Huxley and R. Crompton, The Diffusion and Drift of Electrons in Gases (Wiley, New York, 1974). T Zhao et al , Nucl . Instr. and Meth A 340 (1994) 485. The Forward/Backward Muon Detector . L3 internal note, December 3, 1991 . Laser Science Inc. Newton, MA 02158, USA: VSL 337ND Laser. U. Becker et al ., Determination of the Fiducial Volume of the L3 Forward/ Backward Muon Chambers, L3 internal note, 1994 . Rob Veenhof, Wire Chamber Program (Garfield), 1989 . U Becker et al., Nucl . Instr. and Meth . A 306 (1991) 194. S. Biagi, Magboltz V2.2, March 1990 .
583-584
NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH
Letter to the Editor
ELSEVIER
Section A
Effect of N contamination on drift chambers in magnetic fields 2
Ulrich J. Becker, Steven C. Nahn, Jonathan P. Rodin *, Bryan R. Smith Massachusetts Institute of Technology, CambridgeMA 02139, USA
Received
15
June 1994
Abstract Since the presence of a small amount of air is unavoidable in large drift detectors, we present a measurement of the effect of N2 contamination on the electron drift velocity in a working L3 Muon Chamber placed in the MIT cyclotron magnet . The nominal gas mixture [Ar : C02 : iC,H 10 (86 :10 :4)] was varied by including as much as 1% N2 . Results at B = 0 and 0.5 T are shown. Shifts in the drift velocity curves indicate the need for a higher electric field to maintain operational accuracy . Due to their finite leak rate, drift chambers are usually subject to small amounts of air contamination . Because of the unusual shape of the nitrogen-electron cross section [1], small quantities of this gas can have a substantial effect on observed drift properties as recently shown at B = 0 [2]. This was investigated both with and without magnetic field using a Forward/ Backward muon chamber from the L3 experiment at LEP,CERN . The reader is referred to Ref. [3] for a detailed description of the F/B system, comprised of 96 such chambers . Fig. la shows the chamber in cross section, Fig. lb the leak rate measured by connecting an oil filled U-tube manometer to the exhaust of the chamber, filling to 100 mm overpressure and recording the decay. The chambers are operated at 1 atm. pressure, therefore the rate of diffusion of air into the chamber should be very close to the leak rate . We estimate the air contamination to be approximately 1% at a flow of 2 1/min (by assuming homogeneous mixing in the chamber volume of 800 1) . Note that the leak rate could increase by a factor 3-4 before reaching the manufacturing tolerance level. The chamber was placed into the MIT cyclotron magnet for measurements in a uniform B-field of 0 to 0.5 T. A UV laser [4] was directed into a chamber cell to generate photoelectrons on the I-beam, exactly 5.15 cm from the wire plane. The laser, collimated to achieve a signal comparable in magnitude to that from cosmic muons, hit the I-beam 10 cm into the chamber, well within the uniform electric field region [5]. High voltage settings for different drift fields were calculated using the Wire Chamber program [6] such that the surface field on the wires was kep at 170 kV/cm for constant gain . Drift times were
B ~100--+ -MEASURED VALUE E 80-0
TOLERANCE
60 '4020 0 0
10
20
30
40 50 60 TIME(min)
Fig 1. (a) Cross section of L3 Forward/Backward muon chamber showing I-beam structure. (b) Leak rate of F/B chamber 03 . Tolerance is exponential with 7 = 120 min.
* Corresponding author . 0168-9002/94/$07 .00 © 1994 Elsevier SSDI0168-9002(94)01079-X
measured with respect to signals from a photodiode receiving UV from a beam-splitter placed outside of the chamber. "Prepulses" from direct laser hits on the signal wires provided a t = 0 calibration to Ins precision. We used the simple formula u,1 = (5 .15 cm)/(drift time) to obtain the drift velocity parallel to the electric field; corrections were not made for the non-uniform electric field close to the signal wires. We estimate the errors to be ±0.5% experimental and ±0 .5% systematic, increasing to 1% at low E-fields due to signal degradation .
Science
B.V. All
rights reserved
584
U.J . Becker et al. /NucL Instr. and Meth . to Phys. Res. A 351 (1994) 583-584
E
_0
B=o
5
6
4 3
3
2 1
+
00
500
1000 1500 2000 E-FIELD(V/cm)
Fig 2. Drift velocity vs E-field m F/B chamber at B = 0, low flow rate. Curve is a reference measurement [7]. Fig. 2 shows the measured drift velocity in the chamber with a flow of 2 1/min L3 F/B gas [Ar :CO, :'C 4 H io (86 : 10 : 4)]. Superimposed is a (spline) curve ofmeasurements made using the test chamber described in Ref. [7]. We include this as a reference curve of the drift velocity in this gas . There is a clear discrepancy in shape between the two sets of data ; the drift velocity measured in the F/B is suppressed below 1000V/cm and enhanced above. This difference is attributed to NZ (air) contamination in the F/B chamber. In an attempt to reduce air contamination we increased the flow rate an order of magnitude and repeated the measurement at 20 .93 1/min. This corresponds to a gas turnover of approximately one chamber volume every 45 min. At this high flow rate, we expect around 0.1% N2 in the chamber and, indeed, close agreement with the reference curve is observed at both B = 0 and 0.5 T, Fig. 3. To verify the effect we repeated the experiment with the injection of a controlled 1% admixture of NZ, Fig. 4. One sees the same broadening of the plateau region and an increase in the drift velocity at high E-fields . At B = 0, this behavior agrees with the relative changes predicted by the Magboltz program [8].
Data Reference FIB
500
Flow Rate= 20 .93 Urn ArC02:lC4Hio(8610:4) 1000
E-Field(V/cm) 1500
2000
Fig 4. Drift velocity vs E-field in F/B chamber at B = 0 and 0.5 T, 1 % N, added. It is important to note that the effect of the contamination is present in both B = 0 and B = 0.5 T data and our results show it to be of a comparable magnitude in each . The presence of small quantities of dry air has a very similar effect to that of comparable amounts of nitrogen [2]. In summary, when operating large drift chambers at economical flow rates, nitrogen contamination of around 1% must be expected even with good leak rates. This has been shown here to have a significant effect on the drift velocity in a working F/B muon chamber at both B = 0 and B = 0.5 T. In particular, shifts in the drift velocity plateau suggest operating points somewhat higher than those chosen for pure gas in order to restore the operational accuracy for best physics resolution . Acknowledgements We would like to thank Darling Ren of ETHZ for calculating voltage settings for the chamber. We are grateful to the US Dept . of Energy for their support under contract no . DE-AC02-93 ERO 3069 001. References
6
vu(cm/As)
ï `I
[2] [3]
5'~ 4 3 2
B=0 r
[4] FIB Data
Reference 500
Flow Rate= 20 .93 Urn ArCOZIC4Hio(8610 4) 1000
E-Field(V/cm) 1500
2000
Fig 3. Drift velocity vs E-field in F/B chamber at B = 0 and 0.5 T, high flow rate . Reference curves from [7].
[6] [7] [8]
L. Huxley and R. Crompton, The Diffusion and Drift of Electrons in Gases (Wiley, New York, 1974). T Zhao et al , Nucl . Instr. and Meth A 340 (1994) 485. The Forward/Backward Muon Detector . L3 internal note, December 3, 1991 . Laser Science Inc. Newton, MA 02158, USA: VSL 337ND Laser. U. Becker et al ., Determination of the Fiducial Volume of the L3 Forward/ Backward Muon Chambers, L3 internal note, 1994 . Rob Veenhof, Wire Chamber Program (Garfield), 1989 . U Becker et al., Nucl . Instr. and Meth . A 306 (1991) 194. S. Biagi, Magboltz V2.2, March 1990 .