Effect of hole pitch reduction on electron transport and diffusion: A comparative simulation study of Triple GEM detectors

TL;DR

The paper investigates how reducing GEM hole pitch in triple-GEM detectors affects electron transport, transmission, and diffusion using ANSYS for electrostatic modeling and Garfield++/Magboltz for microscopic transport. By comparing SGEM (140 μm), FGEM (90 μm), and FTGEM (60 μm) across geometric and gas variations, the study demonstrates that finer pitches can enhance electron collection efficiency and reduce transverse spread, with the 60 μm pitch showing the largest gains (Config.3) but also greater internal losses. Validation against experimental data confirms the qualitative trends, though a consistent normalization offset is observed in multi-GEM configurations due to model limitations such as lack of photon feedback and space-charge effects. The results provideGuidance on how pitch, hole geometry, metal/Kapton thickness, and gas composition influence electron loss channels and CE, informing design strategies for future high-rate GEM-based detectors, while acknowledging the need for further experimental validation and ion backflow studies.

Abstract

Advances in fabrication techniques and high-performance electronics have facilitated the development of fine-pitch Gas Electron Multipliers (GEMs). Earlier experimental and simulation findings suggest that these reduced-pitch GEMs can outperform the standard configuration in terms of effective gain, collection efficiency, and position resolution. However, a noticeable fraction of avalanche electrons is lost within the GEM systems, resulting in a degradation of charge collection efficiency. Therefore, a comprehensive simulation-based study is essential to provide deeper insights into the extent of degradation and its contributing factors. In this context, we employ ANSYS and Garfield++ to model the Triple GEM detectors with reduced pitch sizes of 90 and 60 $μ$m, and perform a comparative performance analysis with the standard configuration (pitch size: 140 $μ$m). At first, the simulation framework is validated by comparing the results of the standard configuration with available experimental data and previously reported simulation outcomes. Despite the characteristic gain offset, the framework remains physically consistent and reliable in capturing microscopic avalanche dynamics, reproducing the experimental trend. Following validation, we investigate electron losses at the metal electrodes and within the Kapton holes, electron transmission through the transfer and induction regions, electron diffusion on the induction electrode, and the overall collection efficiency. These parameters are analyzed as functions of GEM potential, outer hole diameter, inner hole diameter, Kapton thickness, metal thickness, and gas composition, thereby offering insights for designing efficient GEM detectors.

Figures (19)

• Figure 1: Schematic of a modeled triple GEM detector showing three vertically stacked GEM foils with drift, transfer, and induction regions. Config. 1 uses 3 SGEMs, Config. 2 uses 3 FGEMs, and Config. 3 uses 3 FTGEMs. The left line indicates applied potentials, while the right line shows z-axis coordinates. The left side of the dotted line represents common z-axis coordinates of Configs. 1 and 2, while the right side denotes the z-axis coordinates of Config. 3. Electrons are accelerated upward from the bottom drift region to the Tran. 3 (induction) region. LM1, LM2, and LM3 denote the lower metal layers of GEM1, GEM2, and GEM3, respectively, while UM1, UM2, and UM3 represent the corresponding upper metal layers. Tran. 1, Tran. 2, and Tran. 3 indicate the transfer regions. The illustrated figure shows the operation of GEM detectors at $\Delta V_{\mathrm{GEM}}$= 350 V.
• Figure 2: Simulated drift lines initiated by a single electron at $\Delta V_{\mathrm{GEM}}$ of 300 V, displaying 2D (XZ) distributions of avalanche electrons in (a) Config. 1 (3 SGEMs), (b) Config. 2 (3 FGEMs), and (c) Config. 3 (3 FTGEMs).
• Figure 3: Simulated peak electric field strength (E$_{z}$) inside the GEM holes across three GEM configurations : (a) Config. 1 (b) Config. 2 (c) Config. 3 at $\Delta V_{\mathrm{GEM}}$ of 300 V.
• Figure 4: Electron spread with Gaussian fits on the induction electrode for (a) Config. 1 (3 SGEMs), (b) Config. 2 (3 FGEMs), and (c) Config. 3 (3 FTGEMs), based on 1000 simulated events at $\Delta V_{\mathrm{GEM}}$ of 300 V. The standard deviations ($\sigma_{x,y}$) of the electron cloud for each configuration are also provided.
• Figure 5: Number of electrons collected in Config. 1 (3 SGEMs) as a function of $\Delta V_{\mathrm{GEM}}$. The experimental data are sourced from Ref. Bachmann:1999xc, and the simulated data are from Ref. Jung:2021cvz.