Production of GEM-like structures for cryogenic applications, using laser-cutting techniques

A novel concept for electroluminescence (EL) structures was recently proposed. In it, a wavelength-shifting material is deposited inside the holes of GEM-like structures which, after suitable optical treatment of its electrodes, improves the light collection and detection efficiency in noble gas TPCs. This new development directly addresses problems related with the scalability of future dual-phase TPCs for rare-event searches, matching (and potentially exceeding) the performance of conventional EL techniques. We report the newest developments on the production of such structures using laser-based techniques, namely the manufacture of a first batch of the so-called FAT-GEMs. This process allows low-cost and reproducible manufacturing of a high volume of such structures. In addition to the detailed description of the production, we present a performance assessment in pure argon, at a gas density close to the one expected in LAr conditions. An energy resolution of 23.5$\pm$1~% (FWHM) at 5.9~keV was obtained, indicating a consistent improvement over previous batch. The optical treatment of the electrode surfaces has been greatly simplified and modestly improved, while charging-up effects arising from the use of laminates eliminated.

💡 Research Summary

The paper presents a comprehensive study on the fabrication and performance of Field‑Assisted Transparent Gaseous Electroluminescence Multipliers (FAT‑GEMs), a novel electroluminescence (EL) device designed for dual‑phase noble‑gas time projection chambers (TPCs). The authors address the scalability challenges of conventional mesh‑based EL regions—mechanical sagging, electric‑field non‑uniformities, and poor VUV photon collection—by developing a thick (5 mm) PMMA substrate with laser‑drilled holes that are internally coated with a wavelength‑shifting material (tetraphenyl‑butadiene, TPB).

Key innovations include: (1) a low‑cost, reproducible manufacturing flow based on CO₂ laser cutting, vacuum aluminum deposition, annealing, and TPB evaporation; (2) a revised electrode architecture that replaces the previous ITO‑ESR sandwich with a single‑sided indium‑tin‑oxide (ITO) layer on one face and a sputtered aluminum layer on the opposite face, which serves simultaneously as an electrode and a visible‑light reflector. This redesign reduces the number of dielectric interfaces, eliminates the long‑term charging‑up observed in earlier batches, and improves optical transmission by roughly 10 %. (3) Precise control of hole geometry (cylindrical or conical, 2–3 mm diameter, 4–5 mm pitch) and the Δ parameter (difference between entrance and exit diameters) to tailor the EL gap and light‑collection efficiency.

The production line, carried out at the Astrocent facility in Warsaw and at CEZAMA‑T in Poland, begins with 28 × 28 cm² hard‑coated PMMA plates. Six samples (7 × 7 cm² or 5 × 5 cm²) are laser‑cut, ultrasonically cleaned, and edge‑masked for electrical isolation. Aluminum is vacuum‑evaporated to a target thickness of 400 nm under <2 × 10⁻⁵ mbar. Laser drilling creates the hole pattern in a single pass (1–10 % power, 25 mm s⁻¹ speed). Post‑drilling annealing at 80 °C for 5 h (20 °C h⁻¹ heating, 15 °C h⁻¹ cooling) relieves residual stress. TPB is then thermally evaporated in two steps—first coating the hole walls, then the top surface—to achieve a uniform 1.5 µm layer, with chamber pressure kept below 5 × 10⁻⁴ mbar and boat temperature stabilized at ~200 °C. Final quality checks include surface resistivity measurements, microscopic inspection, and vacuum‑sealed packaging.

Performance was evaluated in a dedicated high‑pressure argon vessel (purity 6.0, 4 bar, corresponding to liquid‑argon density). A ⁵⁵Fe 5.9 keV X‑ray source illuminated the detector while a transparent mesh served as a grounded field‑shaping electrode. The EL field was scanned from 2 to 4.25 kV cm⁻¹ bar⁻¹, and the scintillation signal was read out by a photomultiplier tube (PMT) connected to standard ORTEC pre‑amplifier, amplifier, and MCA chain. Spectra were background‑subtracted, rebinned, and fitted with Gaussians to extract peak position and full‑width at half‑maximum (FWHM).

The upgraded FAT‑GEMs demonstrated a breakdown voltage of 10–12 kV, with no catastrophic damage to the ITO coating thanks to a series 100 MΩ resistor limiting discharge currents. Structures featuring 2 mm holes (sample C) achieved a 21 % increase in light yield compared with the previous generation and delivered an energy resolution of 23.5 ± 1 % (FWHM) at 5.9 keV for an EL field of 4.0 kV cm⁻¹ bar⁻¹. Larger 3 mm holes yielded lower light output, confirming the importance of hole geometry. No charging‑up effects were observed over the measurement period, and intentional discharge tests showed that the TPB coating remained intact, indicating robust optical performance.

The authors conclude that the laser‑cut FAT‑GEM platform offers a scalable, low‑radioactivity, and cryogenically compatible solution for large‑area EL amplification in noble‑gas TPCs. By simplifying the electrode stack and eliminating dielectric charging, the devices achieve superior optical transmission and stable operation at pressures equivalent to liquid argon. The demonstrated reproducibility and cost‑effectiveness of the manufacturing chain suggest that multi‑square‑meter modules could be produced for next‑generation dark‑matter and neutrino experiments. Future work will explore systematic optimization of hole size, pitch, and electrode materials to further enhance S1 (primary scintillation) and S2 (secondary EL) detection, as well as integration with large‑area photosensor arrays.