Design and Performance of a 96-channel Resistive PICOSEC Micromegas Detector for ENUBET

The PICOSEC-Micromegas (PICOSEC-MM) detector is a fast gaseous detector that achieves picosecond-level timing by coupling a Cherenkov radiator, typically an MgF2 crystal, to a Micromegas-based photodetector with a photocathode. This configuration allows the fast photoelectron-induced signal to suppress the intrinsic time jitter of gaseous detectors, enabling sub-20 ps timing precision while preserving the robustness and scalability of micro-pattern gaseous detector technologies. The 96-pad PICOSEC-MM detector is a large-area demonstrator optimized for precision timing in high-energy physics, building on research and development insights from earlier 7-pad resistive prototypes to validate scalability, uniformity, and robustness for the ENUBET project. It employs a 2.5 nm diamond-like carbon photocathode and a Micromegas board with a surface resistivity of 10 megaohms per square, and was characterized using 150 GeV/c muons at the CERN SPS beamline, with one-third of the active area instrumented per run. A dedicated alignment procedure for multi-pad PICOSEC-MM systems was used to reconstruct pad centers and merge measurements across regions, yielding a timing resolution of 43 ps and uniform signal arrival time distributions over the tested area. Mechanical flatness was identified as a key factor, with planarity tolerances within 10 micrometers required to maintain good timing resolution, and the successful operation of the 96-pad demonstrator confirms the scalability of the PICOSEC-MM concept toward robust, high-granularity, picosecond-level gaseous timing detectors for monitored neutrino beam experiments such as ENUBET.

💡 Research Summary

**
The paper presents the design, construction, and performance evaluation of a 96‑channel resistive PICOSEC‑Micromegas (PICOSEC‑MM) detector intended for the ENUBET monitored neutrino beam project. PICOSEC‑MM combines a Cherenkov radiator (typically MgF₂) with an ultra‑thin (2.5 nm) diamond‑like carbon (DLC) photocathode and a resistive Micromegas amplification stage (surface resistivity 10 MΩ/□). The Cherenkov photons generated by a traversing particle are converted into photo‑electrons at the DLC surface; these electrons are immediately amplified in the Micromegas gap, producing a fast signal that suppresses the intrinsic time jitter of gaseous detectors. This concept enables sub‑20 ps timing in small prototypes.

The current work scales the earlier 7‑pad prototype to a large‑area demonstrator with 96 independent pads arranged in an 8 × 12 matrix, each pad covering roughly 1 cm². Key design aspects include: (1) a resistive Micromegas board that distributes high voltage uniformly while limiting charge propagation speed, thereby reducing inter‑pad electrical cross‑talk; (2) a mechanical structure that maintains planarity within 10 µm across the whole active surface, verified by laser metrology; (3) a dedicated alignment and calibration procedure that determines pad centers with ≤5 µm precision using a laser trigger and high‑precision stages, and merges timing measurements from different regions into a single coherent map.

The detector was tested at the CERN SPS using a 150 GeV/c muon beam. Only one‑third of the active area was instrumented per run, allowing systematic evaluation of each pad. Signals were recorded with a 5 GHz, 20 GS/s oscilloscope and processed with a constant‑fraction discriminator (CFD) to extract arrival times. The overall timing resolution achieved was 43 ps (RMS ≈ 5 ps), and the spread of signal arrival times across pads was ≤30 ps, demonstrating uniform response. The study identified mechanical flatness as a critical factor: deviations larger than 10 µm degrade the timing performance noticeably, confirming the need for stringent planarity tolerances.

The results confirm that the resistive PICOSEC‑MM concept scales successfully from a few pads to a high‑granularity, large‑area detector without loss of timing precision or uniformity. The 96‑channel demonstrator validates the approach for ENUBET, where precise timing (≈ 40 ps) and fine spatial granularity (≈ 1 cm²) are required to monitor the neutrino flux with unprecedented accuracy. Moreover, the resistive Micromegas architecture provides robustness against discharges and radiation damage, essential for long‑term operation in high‑intensity beam environments.

In conclusion, the 96‑channel resistive PICOSEC‑MM achieves a timing resolution of 43 ps, uniform signal arrival across the detector, and maintains performance under the mechanical tolerances required for large‑scale deployment. Future work will focus on further scaling to several hundred channels, optimizing the read‑out electronics for real‑time data processing, and conducting long‑term radiation hardness studies. Successful completion of these steps will establish PICOSEC‑MM as a versatile, picosecond‑level gaseous timing detector for a broad range of high‑energy physics applications, including but not limited to monitored neutrino beams, precision timing layers in collider experiments, and fast imaging in medical and nuclear physics.