Ioannis Giomataris

The PICOSEC-Micromegas (PICOSEC-MM) detector is a fast gaseous detector concept that achieves picosecond-level timing. It couples a Cherenkov radiator, typically an MgF ₂ crystal, to a Micromegas-based photodetector equipped with a photocathode, allowing the fast photoelectron-induced signal to suppress the intrinsic time jitter characteristic of gaseous detectors. This design enables sub-20 ps timing precision while preserving the robustness and scalability of Micro-Pattern Gaseous Detector (MPGD) technologies. The 96-pad PICOSEC-MM detector represents the latest advancement in this development, optimized for precision timing in high-energy physics. Building upon the R&D insights obtained with earlier 7-pad resistive prototypes, this large-area demonstrator was developed to validate the scalability, uniformity, and robustness of the technology for integration into the ENUBET project. The detector employs a 2.5 nm Diamond-Like Carbon (DLC) photocathode with a Micromegas board equipped with surface resistivity of 10 MΩ/□, providing an excellent timing performance. The prototype was characterized using 150 GeV/ c muons at the CERN SPS beamline, with one-third of the active area instrumented during each run. A dedicated alignment procedure, developed for multi-pad PICOSEC-MM systems, was used to reconstruct the pad centers and combine measurements across different detector regions. The measured timing resolution was 43 ps across the instrumented pads, while the Signal Arrival Time (SAT) distributions exhibited a good uniformity among the detector area that was tested. Mechanical flatness was identified as a key factor influencing timing precision. Maintaining a planarity tolerance within 10 μm is therefore critical to preserve a good timing resolution over large active areas. The successful operation of the 96-pad demonstrator confirms the scalability of the PICOSEC-MM concept marking a significant step toward implementing robust, high-granularity, picosecond-level gaseous timing detectors in monitored neutrino beam experiments such as ENUBET.

Micromegas (MICRO-MEsh GAseous Structure) detectors have found common use in different applications since their development in 1996 by the group of I. Giomataris and G. Charpak. In this review article, we present implementations of Micromegas sub-detectors in different physics experiments and highlight the current state of development for innovative detection concepts with Micromegas.

A novel type of pulsed X-ray generator especially suitable for rare event searches, as dark matter and neutrino experiments, has been developed. Being compact and built with selected radio-pure materials, it is appropriate for low-background experiments. UV light generated by a pulsed lamp produces photoelectrons on a metallic cathode, which are subsequently accelerated to produce X-rays. The X-rays are emitted in bunches with a time resolution better than 1 ns and the system provides a fast trigger for event tagging. We present the idea of the new X-ray generator as well as experimental results showing the proof of concept of the device. Data are in good agreement with simulation results and future optimisations of the system are also discussed.

Due to the so-called ³He shortage crisis, many detection techniques for thermal neutrons are currently based on alternative converters. There are several possible ways of increasing the detection efficiency for thermal neutrons using the solid neutron-to-charge converters ¹⁰B or ¹⁰B₄C. Here, we present an investigation of the Micromegas technology. The micro-pattern gaseous detector Micromegas was developed in the past years at Saclay and is now used in a wide variety of neutron experiments due to its combination of high accuracy, high rate capability, excellent timing properties, and robustness. A large high-efficiency Micromegas-based neutron detector is proposed for thermal neutron detection, containing several layers of ¹⁰B₄C coatings that are mounted inside the gas volume. The principle and the fabrication of a single detector unit prototype with overall dimension of ~15 × 15 cm² and its possibility to modify the number of ¹⁰B₄C neutron converter layers are described. We also report results from measurements that are verified by simulations, demonstrating that typically five ¹⁰B₄C layers of 1–2 μm thickness would lead to a detection efficiency of 20% for thermal neutrons and a spatial resolution of sub-mm. The high potential of this novel technique is given by the design being easily adapted to large sizes by constructing a mosaic of several such detector units, resulting in a large area coverage and high detection efficiencies. An alternative way of achieving this is to use a multi-layered Micromegas that is equipped with two-side ¹⁰B₄C-coated gas electron multiplier (GEM)-type meshes, resulting in a robust and large surface detector. Another innovative and very promising concept for cost-effective, high-efficiency, large-scale neutron detectors is by stacking ¹⁰B₄C-coated microbulk Micromegas. A prototype was designed and built, and the tests so far look very encouraging.

We report on the design, construction and operation of a low background x-ray detection line composed of a shielded Micromegas (micromesh gaseous structure) detector of the microbulk technique. The detector is made from radiopure materials and is placed at the focal point of a $\sim$~5 cm diameter, 1.3 m focal-length, cone-approximation Wolter I x-ray telescope (XRT) comprised of thermally-formed (or "slumped") glass substrates deposited with multilayer coatings. The system has been conceived as a technological pathfinder for the future International Axion Observatory (IAXO), as it combines two of the techniques (optic and detector) proposed in the conceptual design of the project. It is innovative for two reasons: it is the first time an x-ray optic has been designed and fabricated specifically for axion research, and the first time a Micromegas detector has been operated with an x-ray optic. The line has been installed at one end of the CERN Axion Solar Telescope (CAST) magnet and is currently looking for solar axions. The combination of the XRT and Micromegas detector provides the best signal-to-noise ratio obtained so far by any detection system of the CAST experiment with a background rate of 5.4$\times$10$^{-3}\;$counts per hour in the energy region-of-interest and signal spot area.

The dark matter directional detection opens a new field in cosmology bringing the possibility to build a map of nuclear recoils that would be able to explore the galactic dark matter halo giving access to a particle characterization of such matter and the shape of the halo. The MIMAC (MIcro-tpc MAtrix of Chambers) collaboration has developed in the last years an original prototype detector based on the direct coupling of large pixelized micromegas with a devoted fast self-triggered electronics showing the feasibility of a new generation of directional detectors. The discovery potential of this search strategy is discussed and illustrated. In June 2012, the first bi-chamber prototype has been installed at Modane Underground Laboratory (LSM) and the first underground background events, the gain stability and calibration are shown.