Speaker
Description
Resistive Plate Chambers (RPCs) are vital gaseous detectors used for precise particle detection and fast triggering in experiments like the Compact Muon Solenoid (CMS) at the LHC. Under high voltage (10 kV) and intense radiation, the standard gas mixture (95.2% $C_{2}H_{2}F_{4}$, 4.5% $i-C_{4}H_{10}$, 0.3% $SF_{6}$) undergoes molecular dissociation under electron avalanches. This cascade produces reactive fluorine radicals and ionic species, forming hydrogen fluoride (HF)—a corrosive neutral byproduct. Unlike ions, HF transport is decoupled from the electric field and behaves as a passive scalar driven by convective fluid dynamics and molecular diffusion within the thin-gap geometry.
Predicting HF accumulation is crucial for the upcoming LHC Phase-2 upgrade. This study presents a 3D physicochemical transport model of steady-state HF dispersion within an RPC segment. The fluid flow is resolved via incompressible Navier-Stokes equations, while chemical transport is governed by the advection-diffusion equation, using molecular diffusivities from Chapman-Enskog binary collision theory. The system is discretized via a mixed Taylor-Hood (P2-P1) Finite Element Method (FEM) for the flow and second-order Lagrangian elements (P2) for the scalar, implemented using the Gridap library in Julia.
Simulations at an operational regime (Re = 11.41) reveal a monotonic concentration gradient, identifying localized stagnation regions prone to degradation. Mass balance analysis shows that most HF is removed via diffusive fluxes to the walls due to the high surface-to-volume ratio. These results demonstrate order-of-magnitude consistency with experimental data from the Gamma Irradiation Facility (GIF++), validating the model for geometric optimization of next-generation RPCs.