Optical effects in Gaseous Electron Multipliers (GEMs)

TL;DR

This study investigates optical broadening effects in GEM-based optical time projection chambers, demonstrating that scintillation light propagation causes increased track width and intensity, especially in glass GEMs, affecting particle track measurements.

Contribution

It provides experimental measurements and simulations of optical broadening in GEMs, explaining discrepancies in optical readout data and highlighting effects in glass GEMs.

Findings

Optical broadening is strongest in glass GEMs.

Simulations reproduce observed optical effects.

Track intensity and width increase by up to 26% and 31%.

Abstract

Optical time projection chambers (OTPCs) are well suited for applications that require the highest spatial resolution for particle track reconstruction. The MIGDAL experiment uses a glass GEM-based OTPC and observes a systematic excess in both the intensity and width of particle tracks in its optical readout, when compared with charge readout simulations. One hypothesis is that scintillation light produced inside a GEM hole during the avalanche propagates through the GEM substrate and exits neighboring holes. We present lab measurements testing this hypothesized optical broadening effect in three types of GEM substrates: glass, ceramic, and FR4. Our observations quantify this optical broadening and demonstrate it to be strongest in glass GEMs. Additionally, we use Geant4 simulations to both reproduce our observations and quantify optical broadening effects in realistic charge avalanches. Applying our glass GEM effects to simulated particle tracks yields increases of track intensity and widths by up to around 26% and 31%, respectively. This may explain the larger than expected intensity and track widths observed in the MIGDAL OTPC and is expected to be an observed effect in all GEM-based OTPCs.