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Drift properties of Monitored Drift Tubes chambers of the Atlas Muon Spectrometer
ARTICLE IN PRESS
Nuclear Instruments and Methods in Physics Research A 518 (2004) 62–64
Drift properties of Monitored Drift Tubes chambers of the Atlas Muon Spectrometer M. Cirilli Dipartimento di Fisica dell’Universita" di Roma ‘‘La Sapienza’’ e Sezione INFN di Roma, Roma I-00185, Italy
Abstract The ATLAS Muon Spectrometer relies on Monitored Drift Tube (MDT) chambers for precision tracking of muons. Variations in the properties of the gas mixture have important effects on the detector performance. The drift properties of the MDTs operating at the standard ATLAS conditions were studied as a function of temperature, composition of the gas mixture, water vapour content, applied voltage and discriminator threshold. r 2003 Elsevier B.V. All rights reserved. PACS: 29.40.Cs; 29.40.Gx Keywords: Gas detectors; Ionization chambers; ATLAS; LHC; Muon
1. Introduction
2. Analysis method
In summer 2002, 12 Monitored Drift Tube (MDT) production chambers were installed in the H8 muon beam line at CERN on full scale structures corresponding to one octant of the Endcap and one Barrel tower of the Muon Spectrometer [1]. In standard conditions, MDTs are operated with an Ar 93%–CO2 7% mixture at 3 bar absolute pressure. Systematic studies of the MDT drift properties were performed using data from the 6 barrel chambers. The MDT performance with respect to variation of the gas temperature, gas mixture, water content, applied voltage and discriminator threshold was evaluated.
2.1. Maximum drift time
E-mail address: [email protected] (M. Cirilli).
The maximum drift time tmax ; i.e. the time needed for drifting over the whole tube radius, is a simple and robust figure of the drift properties. The method to evaluate tmax consists in fitting the raw time spectrum of each tube in order to extract the rise time t0 and the fall time tf : The fit is performed using a 5-parameter function describing the shape of the signal distribution plus a constant describing the out-of-time background. The difference tf t0 is the maximum drift time. 2.2. Space–time relation The drift properties of an MDT are most adequately expressed in terms of the space–time (r–t) relation. The r–t relations for a given
0168-9002/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2003.10.024
ARTICLE IN PRESS M. Cirilli / Nuclear Instruments and Methods in Physics Research A 518 (2004) 62–64
63
chamber have been determined using the other chambers in the barrel sector for external tracking. 2.3. Simulation The response of MDTs to charged particles has been simulated using GARFIELD [2]. The program calculates field maps, electron and ion drift lines, signals induced on the wires by drifting ions and electrons, drift time tables and arrival time distributions.
3. Results 3.1. Temperature dependence In the experimental set-up, the gas pressure is kept constant at the 0.1% level. Changes in the gas temperature imply a corresponding change in the gas density and therefore in the drift velocity. The tmax has been measured in short runs taken at different times of the day, in order to follow the thermal excursion of the chamber following the atmospheric temperature. The maximum drift time decreases with increasing temperature, with an average slope on all the chambers of 2:6770:10 ns=K: The measured slope is in agreement with the Garfield prediction of 2:5870:02 ns=K: The rise time t0 is also weakly dependent on the temperature increase, with a measured slope of 0:2270:03 ns=K: A lower gas density affects the time at which the threshold is crossed, and hence t0 ; in two opposite ways: (i) the specific ionization is smaller, so that more time is needed to reach the threshold; (ii) the gas gain increases, so the threshold is reached in a shorter period. These competing mechanisms affect the r–t relation in the region close to the wire, while at larger radii the effect of the increase of drift velocity with the temperature is the dominant one (see Fig. 1). 3.2. Dependence on the gas mixture The maximum drift time exhibits a linear dependence from the Ar percentage in the mixture between 92% and 94%, with a slope of
Fig. 1. Difference in the drift time at 17:8 C and 20:7 C at any given drift radius.
Fig. 2. Difference in the drift radius for a gas mixture at Ar 94% and Ar 92% at any given drift time.
69:670:7 ns=Ar%: As expected, the higher the amount of Ar, the fastest is the gas. The value obtained is only qualitatively in agreement with the Garfield prediction of 83:5570:01 ns=Ar%: The difference in drift radius for a gas mixture at Ar 94% and Ar 92% is shown in Fig. 2. A sizeable difference is observed even for small changes in the gas composition. In addition, the change in the r–t relation is highly non linear, hence a variation in the gas mixture cannot be treated with a simple and fast algorithm (e.g. rescaling the r–t relation for the ratio of the maximum drift times). This result implies that the
ARTICLE IN PRESS 64
M. Cirilli / Nuclear Instruments and Methods in Physics Research A 518 (2004) 62–64
gas composition must be known and stable at the level of 0.5%. 3.3. Dependence on the water content In order to have a controlled amount of H2 O in the chambers, a part of the gas was flown through a vessel containing water at a given temperature. The level of water vapour in the humid flow was set by the vessel temperature, while the overall water vapour partial pressure was determined by
the ratio of humid to dry flow. Measurements were performed for water contents of 520 and 2700 ppm; observing an increase in tmax of 28 and 147 ns; respectively.
References [1] T. Alexopoulos, et al., ATL-COM-MUON-2002-007. [2] R. Veenhof, GARFIELD, CERN Program Library W5050.
Nuclear Instruments and Methods in Physics Research A 518 (2004) 62–64
Drift properties of Monitored Drift Tubes chambers of the Atlas Muon Spectrometer M. Cirilli Dipartimento di Fisica dell’Universita" di Roma ‘‘La Sapienza’’ e Sezione INFN di Roma, Roma I-00185, Italy
Abstract The ATLAS Muon Spectrometer relies on Monitored Drift Tube (MDT) chambers for precision tracking of muons. Variations in the properties of the gas mixture have important effects on the detector performance. The drift properties of the MDTs operating at the standard ATLAS conditions were studied as a function of temperature, composition of the gas mixture, water vapour content, applied voltage and discriminator threshold. r 2003 Elsevier B.V. All rights reserved. PACS: 29.40.Cs; 29.40.Gx Keywords: Gas detectors; Ionization chambers; ATLAS; LHC; Muon
1. Introduction
2. Analysis method
In summer 2002, 12 Monitored Drift Tube (MDT) production chambers were installed in the H8 muon beam line at CERN on full scale structures corresponding to one octant of the Endcap and one Barrel tower of the Muon Spectrometer [1]. In standard conditions, MDTs are operated with an Ar 93%–CO2 7% mixture at 3 bar absolute pressure. Systematic studies of the MDT drift properties were performed using data from the 6 barrel chambers. The MDT performance with respect to variation of the gas temperature, gas mixture, water content, applied voltage and discriminator threshold was evaluated.
2.1. Maximum drift time
E-mail address: [email protected] (M. Cirilli).
The maximum drift time tmax ; i.e. the time needed for drifting over the whole tube radius, is a simple and robust figure of the drift properties. The method to evaluate tmax consists in fitting the raw time spectrum of each tube in order to extract the rise time t0 and the fall time tf : The fit is performed using a 5-parameter function describing the shape of the signal distribution plus a constant describing the out-of-time background. The difference tf t0 is the maximum drift time. 2.2. Space–time relation The drift properties of an MDT are most adequately expressed in terms of the space–time (r–t) relation. The r–t relations for a given
0168-9002/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2003.10.024
ARTICLE IN PRESS M. Cirilli / Nuclear Instruments and Methods in Physics Research A 518 (2004) 62–64
63
chamber have been determined using the other chambers in the barrel sector for external tracking. 2.3. Simulation The response of MDTs to charged particles has been simulated using GARFIELD [2]. The program calculates field maps, electron and ion drift lines, signals induced on the wires by drifting ions and electrons, drift time tables and arrival time distributions.
3. Results 3.1. Temperature dependence In the experimental set-up, the gas pressure is kept constant at the 0.1% level. Changes in the gas temperature imply a corresponding change in the gas density and therefore in the drift velocity. The tmax has been measured in short runs taken at different times of the day, in order to follow the thermal excursion of the chamber following the atmospheric temperature. The maximum drift time decreases with increasing temperature, with an average slope on all the chambers of 2:6770:10 ns=K: The measured slope is in agreement with the Garfield prediction of 2:5870:02 ns=K: The rise time t0 is also weakly dependent on the temperature increase, with a measured slope of 0:2270:03 ns=K: A lower gas density affects the time at which the threshold is crossed, and hence t0 ; in two opposite ways: (i) the specific ionization is smaller, so that more time is needed to reach the threshold; (ii) the gas gain increases, so the threshold is reached in a shorter period. These competing mechanisms affect the r–t relation in the region close to the wire, while at larger radii the effect of the increase of drift velocity with the temperature is the dominant one (see Fig. 1). 3.2. Dependence on the gas mixture The maximum drift time exhibits a linear dependence from the Ar percentage in the mixture between 92% and 94%, with a slope of
Fig. 1. Difference in the drift time at 17:8 C and 20:7 C at any given drift radius.
Fig. 2. Difference in the drift radius for a gas mixture at Ar 94% and Ar 92% at any given drift time.
69:670:7 ns=Ar%: As expected, the higher the amount of Ar, the fastest is the gas. The value obtained is only qualitatively in agreement with the Garfield prediction of 83:5570:01 ns=Ar%: The difference in drift radius for a gas mixture at Ar 94% and Ar 92% is shown in Fig. 2. A sizeable difference is observed even for small changes in the gas composition. In addition, the change in the r–t relation is highly non linear, hence a variation in the gas mixture cannot be treated with a simple and fast algorithm (e.g. rescaling the r–t relation for the ratio of the maximum drift times). This result implies that the
ARTICLE IN PRESS 64
M. Cirilli / Nuclear Instruments and Methods in Physics Research A 518 (2004) 62–64
gas composition must be known and stable at the level of 0.5%. 3.3. Dependence on the water content In order to have a controlled amount of H2 O in the chambers, a part of the gas was flown through a vessel containing water at a given temperature. The level of water vapour in the humid flow was set by the vessel temperature, while the overall water vapour partial pressure was determined by
the ratio of humid to dry flow. Measurements were performed for water contents of 520 and 2700 ppm; observing an increase in tmax of 28 and 147 ns; respectively.
References [1] T. Alexopoulos, et al., ATL-COM-MUON-2002-007. [2] R. Veenhof, GARFIELD, CERN Program Library W5050.