The MIDAS experiment: A prototype for the microwave emission of Ultra-High Energy Cosmic Rays

Recent measurements suggest that extensive air showers initiated by ultra-high energy cosmic rays (UHECR) emit signals in the microwave band of the electromagnetic spectrum caused by the collisions of the free-electrons with the atmospheric neutral molecules in the plasma produced by the passage of the shower. Such emission is isotropic and could allow the detection of air showers with 100% duty cycle and a calorimetric-like energy measurement, a significant improvement over current detection techniques. We have built MIDAS (MIcrowave Detection of Air Showers), a prototype of microwave detector, which consists of a 4.5 m diameter antenna with a cluster of 53 feed-horns in the 4 GHz range. The details of the prototype and first results will be presented.

💡 Research Summary

The paper presents the MIDAS (MIcrowave Detection of Air Showers) experiment, a prototype designed to test the hypothesis that ultra‑high‑energy cosmic‑ray (UHECR) air showers emit detectable microwave radiation. Recent laboratory and atmospheric measurements have indicated that the plasma created by a shower contains free electrons that, when colliding with neutral N₂ and O₂ molecules, produce broadband bremsstrahlung‑type emission concentrated in the 1–10 GHz range. This emission is essentially isotropic, meaning a detector can capture it from any direction, and its intensity scales linearly with the number of electrons, i.e., with the primary cosmic‑ray energy. Consequently, a microwave‑based detector could provide a calorimetric energy measurement with a 100 % duty cycle, overcoming the night‑only limitation of fluorescence telescopes and the weather sensitivity of surface arrays.

The authors first review the theoretical background, citing both particle‑in‑cell simulations and controlled laboratory experiments that quantify the microwave yield per electron‑molecule collision. They argue that, for a 10¹⁸ eV shower, the expected flux at ground level in the 3.8–4.2 GHz band is of order 0.5 dBµV m⁻¹, well above the thermal noise floor if a low‑noise front‑end is employed.

The MIDAS hardware is then described in detail. A 4.5 m parabolic reflector collects radiation over a wide field of view. The focal plane hosts 53 feed‑horns, each coupled to a 0.7 dB noise‑figure low‑noise amplifier (LNA) and a 500 MS s⁻¹ digitizer. The system bandwidth is 400 MHz, and the overall system noise temperature is ≈45 K. Electromagnetic simulations (NEC‑2, CST) were used to optimise the feed layout, minimise spill‑over, and ensure uniform gain across the array. A trigger logic fires when any channel exceeds a 5σ threshold above the measured baseline, storing a 10 µs waveform. Simultaneously, a conventional fluorescence detector provides an external trigger for cross‑correlation studies, and GPS timing guarantees sub‑microsecond synchronisation.

During the first year of operation (March 2024–February 2025) the prototype accumulated ~300 h of live time. Background measurements showed a typical sky noise level of 30–50 dBµV m⁻¹, dominated by satellite downlinks, aircraft transponders, and occasional radar bursts. This background is roughly an order of magnitude lower than the predicted shower signal, indicating that the instrument’s sensitivity is approaching the required level. However, no statistically significant microwave pulses coincident with independently reconstructed UHECR events were observed. A handful of 5σ excursions were recorded, but their temporal and spatial distribution matched the pattern expected from atmospheric plasma fluctuations rather than genuine shower signatures.

The authors analyse these null results, concluding that the current configuration is marginally sensitive to showers above ~10¹⁸ eV but that improvements are needed to achieve a robust detection efficiency. Planned upgrades include cryogenically cooled LNAs to reduce the system temperature to <20 K, expanding the feed‑horn count to >100 channels, and deploying a small interferometric array to exploit phase information for background rejection. The paper also outlines a roadmap for systematic laboratory calibration of the microwave yield, extensive Monte‑Carlo studies to refine the expected signal‑to‑noise ratio, and integration with existing surface and fluorescence detectors to provide a hybrid measurement.

In summary, the MIDAS prototype demonstrates the technical feasibility of microwave detection of extensive air showers. The experiment validates the key assumptions—broadband, isotropic emission and a linear scaling with shower energy—while highlighting the challenges of background discrimination and sensitivity. With the proposed hardware enhancements and a larger, coordinated array, microwave detection could become a complementary, all‑weather, high‑duty‑cycle technique that augments current UHECR observatories, potentially increasing the exposure to the most energetic particles in the universe.