Logo
ScienceStack
Table of Contents
Fetching ...
Sign In

The Use of O2 in Gas Mixtures for Drift Chambers

A. M. Baldini, L. Bianco, H. Benmansour, G. Cavoto, F. Cei, M. Chiappini, A. Corvaglia, M. Francesconi, E. Gabbrielli, L. Galli, G. Gallucci, F. Grancagnolo, E. G. Grandoni, M. Grassi, F. Leonetti, D. Nicolo', M. Panareo, D. Pasciuto, A. Papa, F. Renga, S. Scarpellini, A. Venturini, C. Voena

TL;DR

This work challenges the conventional caution against Oxygen in drift-gas mixtures by showing that a helium-based He:Isobutane mixture with 0.5% Oxygen and 1.5% Isopropanol can operate stably in the MEG II drift chamber. The authors combine in-situ operation, a dedicated low-drift Time Projection Chamber, and Garfield++ simulations to quantify electron attachment, demonstrating that measured attachment is negligible for their operating conditions when three-body processes are properly modeled. Key findings include a measured attachment coefficient consistent with Garfield++ predictions using a small Alcohol fraction (≈$0.4\%$) and a drift-velocity in close agreement with simulations, supporting a practical tolerance for permille-level Oxygen in small-drift chambers. The results offer guidance for stabilization strategies in current and future drift chambers (e.g., IDEA at FCC-ee) and highlight the need to customize Magboltz/Garfield++ implementations for complex ternary/quaternary mixtures.

Abstract

The use of Oxygen in gas mixtures for drift chambers is highly discouraged because Oxygen, being strongly electronegative, is generally believed to lead, even in very small quantities, to extremely reduced drift electron attachment values, thus preventing the detector's operation.The drift chamber of the MEG II experiment at PSI has been operating for several years with a gas mixture that mainly contains He:Isobutane in relative proportions of 90:10% by molar concentration, in addition to 1.5% Isopropanol and 0.5% Oxygen. Oxygen and Isopropanol are essential for the proper functioning of the chamber. The electron attachment in the mixture used has proven negligible for the proper operation of the chamber and agrees well with the Garfield++ simulation after correctly accounting for the three-body attachment simulation.

The Use of O2 in Gas Mixtures for Drift Chambers

TL;DR

This work challenges the conventional caution against Oxygen in drift-gas mixtures by showing that a helium-based He:Isobutane mixture with 0.5% Oxygen and 1.5% Isopropanol can operate stably in the MEG II drift chamber. The authors combine in-situ operation, a dedicated low-drift Time Projection Chamber, and Garfield++ simulations to quantify electron attachment, demonstrating that measured attachment is negligible for their operating conditions when three-body processes are properly modeled. Key findings include a measured attachment coefficient consistent with Garfield++ predictions using a small Alcohol fraction (≈) and a drift-velocity in close agreement with simulations, supporting a practical tolerance for permille-level Oxygen in small-drift chambers. The results offer guidance for stabilization strategies in current and future drift chambers (e.g., IDEA at FCC-ee) and highlight the need to customize Magboltz/Garfield++ implementations for complex ternary/quaternary mixtures.

Abstract

The use of Oxygen in gas mixtures for drift chambers is highly discouraged because Oxygen, being strongly electronegative, is generally believed to lead, even in very small quantities, to extremely reduced drift electron attachment values, thus preventing the detector's operation.The drift chamber of the MEG II experiment at PSI has been operating for several years with a gas mixture that mainly contains He:Isobutane in relative proportions of 90:10% by molar concentration, in addition to 1.5% Isopropanol and 0.5% Oxygen. Oxygen and Isopropanol are essential for the proper functioning of the chamber. The electron attachment in the mixture used has proven negligible for the proper operation of the chamber and agrees well with the Garfield++ simulation after correctly accounting for the three-body attachment simulation.

Paper Structure

This paper contains 15 sections, 9 equations, 15 figures.

Table of Contents

  1. Introduction
  2. The MEG II Drift Chamber and Its Operation
  3. The MEG II Drift Chamber
  4. Gas system upgrade for the addition of Oxygen and Isopropyl Alcohol
  5. Chamber's Operation and Drift Electron Attachment
  6. A Dedicated Experiment to Measure the Attachment Coefficient in the MEG II Mixture
  7. Time Projection Chamber
  8. Data samples and analysis
  9. Results
  10. Predictions from Garfield++ and Comparison with MEG II Measurements and Previous Measurements
  11. Garfield++ Simulation of Attachment on Oxygen
  12. Comparison of Garfield++ Attachment Estimates with Experimental Measurements
  13. Comparison of Simulation with Existing Literature Data for Isobutane and Oxygen Mixtures
  14. Conclusions
  15. Acknowledgements

Figures (15)

  • Figure 1: The internal structures of the MEG II drift chamber
  • Figure 2: Currents in one sector of the drift chamber when reducing Oxygen content from $2~\% \rightarrow 1~\% \rightarrow 0.5~\% \rightarrow 0.1~\%$. The abrupt reductions of the current were due to instabilities of the PSI particle beam
  • Figure 3: Simplified schematic of the setup for the inclusion of Alcohol in the gas mixture, with bottles of Helium and Isobutane, mass flow controllers (MFC) to regulate the flows, and the bubbler filled with alcohol (in light blue) with its temperature probe (T) controlling the supply voltage (V) to the heating pads (in red). Gases are finally mixed in the mixing buffer as described in Baldini:2018ing. An additional MFC (not shown in this schematic) is used to add Oxygen to the Helium line before it is mixed with Isobutane.
  • Figure 4: An example of the waveforms recorded from both sides of one anode
  • Figure 5: Number of hits reconstructed per track (left) and positron momentum resolution (right) obtained by fitting the Michel spectrum
  • ...and 10 more figures