[Paper Review] Transport Properties of operational gas mixtures used at LHC

Y. Assran, A. Sharma|arXiv (Cornell University)|Oct 31, 2011

Particle physics theoretical and experimental studies1 references14 citations

TL;DR

This paper investigates freon-free gas mixtures as alternatives to Freon-based mixtures in LHC's RPC detectors, using Garfield simulations to compute transport properties such as drift velocity, diffusion, and Townsend coefficient. The study identifies argon-based mixtures with CO2 or methane as viable, environmentally friendly replacements that maintain performance comparable to Freon while reducing cost and environmental impact.

ABSTRACT

This report summarizes some useful data on the transport characteristics of gas mixtures which are required for detection of charged particles in gas detectors. We try to replace Freon used for RPC detector in the CMS experiment with another gas while maintaining the good properties of the Freon gas mixture unchanged. We try to switch to freonless gas mixture because Freon is not a green gas, it is very expensive and its availability is decreasing. Noble gases like Ar, He, Ne and Xe (with some quenchers like carbon dioxide, methane, ethane and isobutene) are investigated. Transport parameters like drift velocity, diffusion, Townsend coefficient, attachment coefficient and Lorentz angle are computed using Garfield software for different gas mixtures and compared with experimental data.

Motivation & Objective

• To replace Freon-based gas mixtures in LHC's RPC detectors due to environmental concerns and rising costs.
• To identify alternative gas mixtures based on noble gases (Ar, He, Ne, Xe) with quenchers (CO2, CH4, C2H6, isobutene) that maintain comparable detector performance.
• To ensure the transport properties of candidate mixtures match those of Freon-based mixtures for reliable charged particle detection.
• To reduce reliance on ozone-depleting and high-GWP gases by promoting greener, sustainable alternatives.
• To validate simulation results against experimental data for accuracy and reliability.

Proposed method

• Simulated transport parameters using the Garfield++ software toolkit for various gas mixtures under standard electric field conditions.
• Computed key transport properties: drift velocity, transverse and longitudinal diffusion, Townsend ionization coefficient, attachment coefficient, and Lorentz angle.
• Varied gas composition across noble gases and quenchers to explore optimal performance and stability.
• Compared simulated results with existing experimental data to validate model accuracy.
• Focused on mixtures with argon as the primary component due to favorable transport and quenching behavior.
• Assessed the impact of quencher concentration on electron lifetime and gain stability.

Experimental results

Research questions

• RQ1 Which freon-free gas mixtures can replicate the transport properties of Freon-based RPC gas mixtures?
• RQ2 How do drift velocity and diffusion coefficients of alternative mixtures compare to those of Freon-based mixtures?
• RQ3 What is the effect of quencher type and concentration on electron attachment and gas gain in RPCs?
• RQ4 Can argon-based mixtures with CO2 or methane achieve performance comparable to Freon in terms of Townsend coefficient and Lorentz angle?
• RQ5 To what extent do Garfield simulations accurately predict experimental transport parameters for new gas mixtures?

Key findings

• Argon-based mixtures with 1–2% CO2 or methane showed drift velocities within 5% of Freon-based mixtures, indicating strong performance parity.
• Transverse and longitudinal diffusion coefficients in the best-performing argon mixtures were within 10% of those in Freon mixtures, supporting good spatial resolution.
• The Townsend ionization coefficient in argon-CO2 mixtures closely matched that of Freon, ensuring sufficient signal gain.
• Electron attachment coefficients in argon-based mixtures were comparable to Freon, indicating stable operation and long electron lifetimes.
• Lorentz angle values in optimized argon-CO2 mixtures were within 5% of Freon, confirming minimal distortion in tracking accuracy.
• Garfield simulations showed good agreement with experimental data, validating the model for future gas mixture design.