Thick Gas Electron Multiplication (THGEM) Boards: Modeling the Hole Story

This summary describes a method for simulating the impact of mechanical tolerances on signal amplitude and collection efficiency for Thick Gas Electron Multiplication (THGEM) boards. Modeling THGEM board performance has a well defined workflow for the idealized case. This standard workflow defines a unit cell to simplify the model and reduce the computational requirements for the simulating the electron transport and ionizing collisions in Garfield++. However, the standard unit cell does not include an important aspect of THGEM manufacturing, the mechanical tolerances. Simulating how mechanical tolerances affect electron propagation in the gas is time intensive. An initial electron under our sensor’s ideal conditions creates more than 10,000 electrons at readout. Tracking that many electrons takes hours to days in Garfield++ when the electric field is complex. Since Garfield++ scales almost 1:1 with the number of available CPUs, we develop an architecture using Amazon Web Services (AWS) Elastic Compute Cloud (EC2). Using this architecture, we model a variety of configurations with high statistical fidelity. The results from 1000 simulations per configuration show concentricity of the insulator and metallized THGEM hole has a substantial impact on the simulated performance of a THGEM’s gain and efficiency. Both the gain and efficiency of the THGEM reduce from their optimal value in concentric holes as the distance from nominal increases. A greater than 40% loss in simulated gain and a greater than 16% loss in simulated efficiency materialize by 0.03mm from nominal. In addition, the range of simulated gain broadens, which implies a degradation in sensor energy resolution. As such, experimental analysis of THGEM gain and efficiencies should define the allowed or measured tolerances of hole concentricity when matching experimental and simulated gain values.