Characterisation of Thick Gaseous Electron Multipliers as charge readout operated in pure argon

TL;DR

The work addresses achieving high, stable charge amplification in Thick Gaseous Electron Multipliers (ThGEMs) operated in pure argon under cryogenic-like densities. By combining COMSOL Multiphysics field simulations with novel embedded-electrode ThGEM designs, the study demonstrates that two embedded internal electrodes can mitigate edge-field instabilities and allow operation at higher gains. Using Am-241 alpha scans at $3.3\,\text{bar}$, the total gain is extracted as $G_{total} = Q_{measured} / Q_{initial} = \tau \cdot \epsilon_{collection} \cdot G_{ThGEM}$, with $G_{ThGEM}(E, ho,t) \propto e^{\alpha(\rho,E) x} C(t)$ and $\alpha(\rho,E) = A\rho e^{-B\rho/E}$, achieving gains up to about 100 at fields approaching $40\,\mathrm{kV/cm}$ and sparking rates below $4$ per minute. Purification reduces observed gains, likely due to fewer easily ionisable impurities. These results support scalable ThGEM readouts for large-area dual-phase argon TPCs, balancing stability and sensitivity in pure Ar environments.

Abstract

The gain measurements of several 1 mm-thick, 10$\times$10 cm$^2$ Thick Gaseous Electron Multipliers (ThGEMs), operated in pure argon at 3.3 bar and room temperature are presented. Electrostatic simulations, performed with the COMSOL MultiPhysics software, were employed to guide the design of the detectors, and the field configurations are discussed. A modified ThGEM design, incorporating two embedded internal electrodes in addition to the conventional top and bottom electrodes, was developed to mitigate discharge-induced instabilities and to enable operation at higher gains. Several such structures were produced and compared to the standard 1 mm-thick two-electrode ThGEM. Gain measurements were conducted for every design described here over a wide range of applied electric fields and up to large values. The experimental setup and measurement methodology are described, alongside a comparative analysis of the performance of the different detector geometries.

Figures (20)

• Figure 1: Left: Operating electrostatic conditions of the ThGEM. The typical operating potentials are indicated on the left. Top Right: Design of the anode reading out the ThGEM, with the two readout views. Bottom right: Microscopic image of the holes drilled through the ThGEM highlighting their dimensions.
• Figure 2: Production process using "global etching" for the rims around ThGEMs holes. In yellow, the FR4 base material surrounded by the orange copper-clad electrodes deoliveira. On the rightmost sketch, the location of the rims is visible as the area without copper on the edge of the holes.
• Figure 3: Pictures of the various versions of ThGEMs developed at IRFU. Left: 10$\times$10 cm$^2$ prototype, Middle: CFR-34 50$\times$50 cm$^2$ detector, Right: CFR-35 50$\times$50 cm$^2$ detector.
• Figure 4: Picture of the sparks observed in the pressurised vessel on a CFR-34 ThGEM taken by a camera. The image is obtained by stacking several images.
• Figure 5: Illustration of the main differences between the CFR-34 and CFR-35 ThGEMs.