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Magnetic field influence on the self-quenching streamer discharge
Nuclear Instruments and Methods in Physics Research A268 (1988) 151-154 North-Holland, Amsterdam
MAGNETIC FIELD INFLUENCE ON THE SELF-QUENCHING STREAMER DISCHARGE
G.D . ALEKSEEV and A.V. KORYTOV Joint Institute for Nuclear Research, Laboratory of Nuclear Problems, Dubna, USSR
Received 14 May 1987 and in revised form 19 November 1987 The influence of the magnetic field on the self-quenching streamer discharge characteristics was investigated. The effect is several percent in a field of about 10 kG . This work was performed within the framework of methodical investigations connected with the elaboration of the hadron calorimeter of the detector DELPHI [1] for LEP. The DELPHI hadron calorimeter consists of iron layers in the gaps of which there are plastic streamer tubes [2] operating in the self-quenching streamer mode. The calorimeter is a yoke of the superconducting solenoid and hence the magnetic field can influence its work for at least two reasons: (1) influence of the field on the development of showers in the calorimeter; (2) influence of the field on the characteristics of the streamer mode itself.
Fig. 1. Schematic view of the plastic streamer tube : 1 envelope, 2 - cover, 3 - anode wire, 4 - profile .
The first aspect was investigated in ref. [3]. With the help of a Monte Carlo simulation it was shown that due to the distortion of particle tracks in a shower the response of the calorimeter increases by 20-30% in a magnetic field of about 12 kG and the value of the effect depends on both the field magnitude and its direction. The aim of the present work was to investigate the influence of a magnetic field on the selfquenching streamer discharge characteristics . The measurements were performed with a short (about 30 cm) plastic streamer tube (fig . 1) . The diameter of the anode wires is 75 ftm, the inner dimensions of the tube cells are 9 x 9 mmz. All four sides of the cells are covered with a resistive graphite paint with typical resistivity in the range 50-2000 kg/square. To screen, the tube was wrapped in a thin aluminium foil. To increase the difference in the charge characteristics between the limited-proportional and streamer modes we have chosen a gas mixture with a relatively small organic component (the mixture was argon + isobutane = 1 + 1) . This allows one to investigate the influence of the magnetic field on the limited-proportional, transition
Opeak IPC) 100
10
Fig. 2. The characteristics of the streamer tube in the absence of a magnetic field: (a) the charge vs high voltage (the three lines correspond to peaks of proportional, streamer and double-streamer modes in the spectra obtained with the X-ray source); (b) the singles rate curve (X-ray source, 150 jA threshold). 0168-9002/88/$03 .50 © Elsevier Science Publishers B.V . (North-Holland Physics Publishing Division)
15 2
G. D. Alekseev, A. V. Korytov / Magnetic field influence on self-quenching streamer discharge
and streamer modes in detail . The tube was irradiated with X-rays of about 8 keV (K-series in Cu). Giving point-like ionization in a gas and penetrating into a magnetic field without interaction, X-rays provide good
N 2000 1000
N U=2,7kV 5000
0
50
ADC channels
100
Fig. 4. Control charge spectra with the magnet on and off; the tube is out of the field.
6000 3000 2000 1000 0
50
100
50
100
N 3000 2000 1000 0 N U-3.2kV
3000 2000 1000 0
50
100
N 3000 2000 1000 0
Fig. 3. The charge spectra of the signals with and without the magnetic field at different high voltages (dots - no field; open circles - magnetic field of 12 .2 kG perpendicular to the anode wires) : (a) limited-proportional mode, 0.02 pC/channel; (b) transition region, 0.3 pC/channel ; (c) streamer mode, 0.5 pC/channel ; (d) double-streamer mode, 1 .3 pC/channel .
conditions for measurements . All eight anode wires of the tube were connected together to give one output signal . The shaping time for measurements of singles rate curves was 700 ns, the integration time for charge measurements was 1 lis. The DELPHI hadron calorimeter consists of a barrel and end-caps . In the barrel the streamer tubes are placed in gaps between iron layers in parallel to a magnetic field so the magnetic field in the tubes is small. Such a small field does not perturb the streamer discharge. But in the end-caps there are large regions where the magnetic field has a large component which is perpendicular to the streamer tubes so the magnetic field in the tubes may be quite large - up to 12 kG . Thus, attention was focussed on the case of a perpendicular field. Fig. 2 represents the singles rate and charge characteristic of the tube without a magnetic field. The influence of the magnetic field on the collected charge spectra for different voltages applied to the tube is shown in fig. 3. The magnetic field is perpendicular to the anode wires. Fig. 3a shows the charge spectrum modification in the region of the limited-proportional mode . The modification in the region of the self-quenching streamer mode is shown in figs . 3c and d. It is clear from these spectra that the magnetic field decreases the total charge both in an avalanche and in a streamer . The interesting fact is that in spite of a decreasing avalanche charge in the magnetic field the probability of an avalanche transition to a streamer (under the high voltage of the transition region) is greater than without the magnetic field (fig. 3b). As a consequence the mean charge in the transition region is considerably greater in the magnetic field. The observed effect being small, measures were taken to minimize possible systematic errors . To exclude apparatus drift, the time intervals between measurements with and without field under a certain high voltage were not larger than 10-15 min (two spectra measured under equal conditions with a time interval of 15 min differed from each other within statistical errors). Agreement of the results from different sets of measurements was also
G.D. Alekseev, A . V. Korytov / Magnetic field influence on self-quenching streamer discharge
Fig. 5. The singles rate curves with and without the field at different registration thresholds (crosses - no field, circles magnetic field of 12 .2 kG and perpendicular to the anode wires) . checked. To check if there was any influence of the large magnetic field on the apparatus and the X-ray source which were near the magnet, the tube was taken out of the field and the charge spectra were measured with the magnet on and off while all components of the apparatus were in their places . Fig. 4 shows the results of these measurements . The shape of the spectrum did not change and the mean values of the collected charge coincided within the statistical error. Thus, the observed effect is completely due to the influence of the magnetic field on the development of an avalanche and a streamer. The singles rates measured in the magnetic field and without it with different thresholds (fig . 5) show the following. When the threshold level is lower than amplitudes of streamer signals but higher than amplitudes of limitedproportional signals, the rise of a singles rate curve begins earlier in the magnetic field. This corresponds to an earlier beginning of the streamer mode in the magnetic field. When the threshold level is not in the region between two modes and, consequently, when only sufficiently large pulses are registered, there is a reverse effect. This corresponds to smaller amplitudes of pulses in the region of the streamer mode in the magnetic field. Thus, one may conclude that the streamer charge decrease in the field is due to the decreasing amplitude of the current signals rather than to any change in time duration of the signals. Photographs of the pulses (fig . 6) confirm this . It is also seen that the time dynamics of the streamer formation (shape of the pulses) at the different field values and directions does not practically change . Concerning the calorimeter operation it is interesting to know the magnetic field influence on the mean value of the collected charge. Fig. 7 represents the charge mean value as a function of the magnetic field tension for the case of a perpendicular field and for the case of an inclined field (the angle between the direction of the
15 3
magnetic field and the anode wires is a= 48') . The high voltage was chosen in the beginning of the plateau of the singles rate curve which corresponds to the single streamer mode . In both cases a sufficiently linear dependence upon the field was observed . It should be noted that in the case of an inclined field the effect is smaller than could be expected if only a perpendicular component of the field affected the discharge (the dotted line in fig. 7b which is obtained from the line in fig. 7a by multiplying by sin 48'). Fig. 8 represents the change in the mean value of the collected charge in a large high voltage range. It is seen that the effect is quite small in the region of the limited-proportional and streamer modes. In the transition region the effect is considerable (10-15%) . Thus
Fig. 6. Photographs of the streamer signals (U=3 .5 kV ; 400 [LA/div. ; 50 ns/div.) : (a) B = 0 ; (b) B =12 .2 kG ; a = 90 ° (the angle between the direction of the magnetic field and the anode wires) ; (c) B =12 .2 kG ; a = 48 ° .
154
G.D. Alekseev, A . V. Korytoo / Magnetic field influence on self-quenching streamer discharge
10 5 0 -5
b
-10 Fig. 8. The shift of the charge mean value vs the high voltage for perpendicular magnetic field (12.2 kG). Open and closed points represent the different sets of measurements . Fig. 7. The shift of the charge mean value vs the value of the magnetic field and its direction: (a) magnetic field perpendicular to the anode wires; (b) the angle between the direction of the magnetic field and the anode wires is a = 48 ° (solid line). the preferable operating high voltage for the streamer tubes of the hadron calorimeter should be the voltage for the single streamer mode for the majority of the shower particles, i .e. in the vicinity of the beginning of the plateau of a counting rate curve. In such a case the systematic shift in responses of the barrel and the end-caps of the calorimeter will be negligible . To avoid confining ourselves to the chosen gas mixture it was checked if there is any effect in another mixture (argon + isobutane =1 + 3) in the streamer mode. Similar results were observed. Thus the magnetic field influences the development of an avalanche and a streamer, and the effect is about several percent in a field of about 10 kG. This effect seems to be negligible for the hadron calorimeter, but it
is certainly of interest for understanding the mechanism of the avalanche and streamer development . Acknowledgements The authors are grateful to C. Bosio for providing components to assemble the plastic streamer tubes, to L.M . Onishchenko, N.L. Zaplatin and Yu .G . Alenitskij for lending the magnet, to G.V. Micelmacher for his interest in the work and to Yu.V . Bonyushkin for his help in performing the measurements . References [1] DELPHI Technical Proposal, CERN/LEPC/ 83-66, Geneva (1983). [2] E. Iarocci, Nucl . Instr. and Meth. 217 (1983) 30 . [3] G.D. Alekseev and L.G. Tkachev, JINR E13-84-640, Dubna (1984) .
MAGNETIC FIELD INFLUENCE ON THE SELF-QUENCHING STREAMER DISCHARGE
G.D . ALEKSEEV and A.V. KORYTOV Joint Institute for Nuclear Research, Laboratory of Nuclear Problems, Dubna, USSR
Received 14 May 1987 and in revised form 19 November 1987 The influence of the magnetic field on the self-quenching streamer discharge characteristics was investigated. The effect is several percent in a field of about 10 kG . This work was performed within the framework of methodical investigations connected with the elaboration of the hadron calorimeter of the detector DELPHI [1] for LEP. The DELPHI hadron calorimeter consists of iron layers in the gaps of which there are plastic streamer tubes [2] operating in the self-quenching streamer mode. The calorimeter is a yoke of the superconducting solenoid and hence the magnetic field can influence its work for at least two reasons: (1) influence of the field on the development of showers in the calorimeter; (2) influence of the field on the characteristics of the streamer mode itself.
Fig. 1. Schematic view of the plastic streamer tube : 1 envelope, 2 - cover, 3 - anode wire, 4 - profile .
The first aspect was investigated in ref. [3]. With the help of a Monte Carlo simulation it was shown that due to the distortion of particle tracks in a shower the response of the calorimeter increases by 20-30% in a magnetic field of about 12 kG and the value of the effect depends on both the field magnitude and its direction. The aim of the present work was to investigate the influence of a magnetic field on the selfquenching streamer discharge characteristics . The measurements were performed with a short (about 30 cm) plastic streamer tube (fig . 1) . The diameter of the anode wires is 75 ftm, the inner dimensions of the tube cells are 9 x 9 mmz. All four sides of the cells are covered with a resistive graphite paint with typical resistivity in the range 50-2000 kg/square. To screen, the tube was wrapped in a thin aluminium foil. To increase the difference in the charge characteristics between the limited-proportional and streamer modes we have chosen a gas mixture with a relatively small organic component (the mixture was argon + isobutane = 1 + 1) . This allows one to investigate the influence of the magnetic field on the limited-proportional, transition
Opeak IPC) 100
10
Fig. 2. The characteristics of the streamer tube in the absence of a magnetic field: (a) the charge vs high voltage (the three lines correspond to peaks of proportional, streamer and double-streamer modes in the spectra obtained with the X-ray source); (b) the singles rate curve (X-ray source, 150 jA threshold). 0168-9002/88/$03 .50 © Elsevier Science Publishers B.V . (North-Holland Physics Publishing Division)
15 2
G. D. Alekseev, A. V. Korytov / Magnetic field influence on self-quenching streamer discharge
and streamer modes in detail . The tube was irradiated with X-rays of about 8 keV (K-series in Cu). Giving point-like ionization in a gas and penetrating into a magnetic field without interaction, X-rays provide good
N 2000 1000
N U=2,7kV 5000
0
50
ADC channels
100
Fig. 4. Control charge spectra with the magnet on and off; the tube is out of the field.
6000 3000 2000 1000 0
50
100
50
100
N 3000 2000 1000 0 N U-3.2kV
3000 2000 1000 0
50
100
N 3000 2000 1000 0
Fig. 3. The charge spectra of the signals with and without the magnetic field at different high voltages (dots - no field; open circles - magnetic field of 12 .2 kG perpendicular to the anode wires) : (a) limited-proportional mode, 0.02 pC/channel; (b) transition region, 0.3 pC/channel ; (c) streamer mode, 0.5 pC/channel ; (d) double-streamer mode, 1 .3 pC/channel .
conditions for measurements . All eight anode wires of the tube were connected together to give one output signal . The shaping time for measurements of singles rate curves was 700 ns, the integration time for charge measurements was 1 lis. The DELPHI hadron calorimeter consists of a barrel and end-caps . In the barrel the streamer tubes are placed in gaps between iron layers in parallel to a magnetic field so the magnetic field in the tubes is small. Such a small field does not perturb the streamer discharge. But in the end-caps there are large regions where the magnetic field has a large component which is perpendicular to the streamer tubes so the magnetic field in the tubes may be quite large - up to 12 kG . Thus, attention was focussed on the case of a perpendicular field. Fig. 2 represents the singles rate and charge characteristic of the tube without a magnetic field. The influence of the magnetic field on the collected charge spectra for different voltages applied to the tube is shown in fig. 3. The magnetic field is perpendicular to the anode wires. Fig. 3a shows the charge spectrum modification in the region of the limited-proportional mode . The modification in the region of the self-quenching streamer mode is shown in figs . 3c and d. It is clear from these spectra that the magnetic field decreases the total charge both in an avalanche and in a streamer . The interesting fact is that in spite of a decreasing avalanche charge in the magnetic field the probability of an avalanche transition to a streamer (under the high voltage of the transition region) is greater than without the magnetic field (fig. 3b). As a consequence the mean charge in the transition region is considerably greater in the magnetic field. The observed effect being small, measures were taken to minimize possible systematic errors . To exclude apparatus drift, the time intervals between measurements with and without field under a certain high voltage were not larger than 10-15 min (two spectra measured under equal conditions with a time interval of 15 min differed from each other within statistical errors). Agreement of the results from different sets of measurements was also
G.D. Alekseev, A . V. Korytov / Magnetic field influence on self-quenching streamer discharge
Fig. 5. The singles rate curves with and without the field at different registration thresholds (crosses - no field, circles magnetic field of 12 .2 kG and perpendicular to the anode wires) . checked. To check if there was any influence of the large magnetic field on the apparatus and the X-ray source which were near the magnet, the tube was taken out of the field and the charge spectra were measured with the magnet on and off while all components of the apparatus were in their places . Fig. 4 shows the results of these measurements . The shape of the spectrum did not change and the mean values of the collected charge coincided within the statistical error. Thus, the observed effect is completely due to the influence of the magnetic field on the development of an avalanche and a streamer. The singles rates measured in the magnetic field and without it with different thresholds (fig . 5) show the following. When the threshold level is lower than amplitudes of streamer signals but higher than amplitudes of limitedproportional signals, the rise of a singles rate curve begins earlier in the magnetic field. This corresponds to an earlier beginning of the streamer mode in the magnetic field. When the threshold level is not in the region between two modes and, consequently, when only sufficiently large pulses are registered, there is a reverse effect. This corresponds to smaller amplitudes of pulses in the region of the streamer mode in the magnetic field. Thus, one may conclude that the streamer charge decrease in the field is due to the decreasing amplitude of the current signals rather than to any change in time duration of the signals. Photographs of the pulses (fig . 6) confirm this . It is also seen that the time dynamics of the streamer formation (shape of the pulses) at the different field values and directions does not practically change . Concerning the calorimeter operation it is interesting to know the magnetic field influence on the mean value of the collected charge. Fig. 7 represents the charge mean value as a function of the magnetic field tension for the case of a perpendicular field and for the case of an inclined field (the angle between the direction of the
15 3
magnetic field and the anode wires is a= 48') . The high voltage was chosen in the beginning of the plateau of the singles rate curve which corresponds to the single streamer mode . In both cases a sufficiently linear dependence upon the field was observed . It should be noted that in the case of an inclined field the effect is smaller than could be expected if only a perpendicular component of the field affected the discharge (the dotted line in fig. 7b which is obtained from the line in fig. 7a by multiplying by sin 48'). Fig. 8 represents the change in the mean value of the collected charge in a large high voltage range. It is seen that the effect is quite small in the region of the limited-proportional and streamer modes. In the transition region the effect is considerable (10-15%) . Thus
Fig. 6. Photographs of the streamer signals (U=3 .5 kV ; 400 [LA/div. ; 50 ns/div.) : (a) B = 0 ; (b) B =12 .2 kG ; a = 90 ° (the angle between the direction of the magnetic field and the anode wires) ; (c) B =12 .2 kG ; a = 48 ° .
154
G.D. Alekseev, A . V. Korytoo / Magnetic field influence on self-quenching streamer discharge
10 5 0 -5
b
-10 Fig. 8. The shift of the charge mean value vs the high voltage for perpendicular magnetic field (12.2 kG). Open and closed points represent the different sets of measurements . Fig. 7. The shift of the charge mean value vs the value of the magnetic field and its direction: (a) magnetic field perpendicular to the anode wires; (b) the angle between the direction of the magnetic field and the anode wires is a = 48 ° (solid line). the preferable operating high voltage for the streamer tubes of the hadron calorimeter should be the voltage for the single streamer mode for the majority of the shower particles, i .e. in the vicinity of the beginning of the plateau of a counting rate curve. In such a case the systematic shift in responses of the barrel and the end-caps of the calorimeter will be negligible . To avoid confining ourselves to the chosen gas mixture it was checked if there is any effect in another mixture (argon + isobutane =1 + 3) in the streamer mode. Similar results were observed. Thus the magnetic field influences the development of an avalanche and a streamer, and the effect is about several percent in a field of about 10 kG. This effect seems to be negligible for the hadron calorimeter, but it
is certainly of interest for understanding the mechanism of the avalanche and streamer development . Acknowledgements The authors are grateful to C. Bosio for providing components to assemble the plastic streamer tubes, to L.M . Onishchenko, N.L. Zaplatin and Yu .G . Alenitskij for lending the magnet, to G.V. Micelmacher for his interest in the work and to Yu.V . Bonyushkin for his help in performing the measurements . References [1] DELPHI Technical Proposal, CERN/LEPC/ 83-66, Geneva (1983). [2] E. Iarocci, Nucl . Instr. and Meth. 217 (1983) 30 . [3] G.D. Alekseev and L.G. Tkachev, JINR E13-84-640, Dubna (1984) .