The observation of Ground Level Enhancement GLE 77 by the neutron detectors of the Experimental Complex NEVOD

A Ground Level Enhancement event was observed by neutron detectors designed for the registration of extensive air showers at the Experimental Complex NEVOD. The potential for that was unlocked by a recent modernization of the experimental setup that included implementation of additional channels for measuring neutron flux variation. At 10:15 UT on November 11, 2025, a sudden and significant increase in the neutron flux was detected by two installations: PRISMA-36 and URAN arrays. For the first time, a GLE has been recorded using a set of neutron detectors oriented at the extensive air shower studies. We present the measured EC NEVOD data and the results of the preliminary analysis of the observed GLE.

💡 Research Summary

The paper reports the first observation of a Ground Level Enhancement (GLE 77) using neutron detectors originally designed for extensive air‑shower (EAS) studies at the Experimental Complex NEVOD (EC NEVOD) in Moscow. A recent modernization of the facility added dedicated channels for monitoring neutron flux variations, enabling the two detector arrays—PRISMA‑36 and URAN—to record the event on 11 November 2025 at 10:15 UT.

PRISMA‑36 consists of 36 cylindrical polyethylene tanks each equipped with a ZnS(Ag)+⁶LiF scintillator, a 35 g cm⁻² concrete absorber, and a photomultiplier tube (PMT), providing an effective area of ~13 m². URAN comprises 72 similar detectors using ZnS(Ag)+B₂O₃ scintillators, mounted on building rooftops without an absorber, contributing ~26 m². Together the two arrays contain 108 detectors with a total effective area of 39 m². The 2024 upgrade introduced separate electronics that record PMT signals independently of the standard EAS trigger, allowing continuous neutron‑flux monitoring.

Both arrays share identical GPS‑based timing with ≤10 ns synchronization, and raw waveforms are stored for offline analysis. Pulse‑shape discrimination (PSD) exploits the distinct decay times of scintillation light generated by charged particles versus thermal neutrons, enabling reliable extraction of neutron‑induced pulses. Background counting statistics are evaluated using Poisson errors, and the data are averaged over 5‑minute and 30‑minute intervals to assess the significance of any enhancement.

During the GLE, both arrays exhibited a near‑simultaneous rise in counting rates. PRISMA‑36 reached a maximum increase of 20.1 % ± 1.1 %, while URAN peaked at 24.8 % ± 1.3 %. The peak occurred around 11:45 UT, consistent with the timing reported by the global neutron‑monitor network (e.g., MOSC, SOPO). MOSC recorded a 20.9 % ± 0.5 % rise, whereas the high‑latitude SOPO monitor saw an unprecedented 102.5 % ± 0.1 % increase. The shape, amplitude, and duration of the NEVOD signals match those of the conventional monitors, confirming that the NEVOD neutron‑detector system can reliably detect GLEs.

GLE 77 was associated with an X5.1 solar flare observed by the GOES X‑ray satellite, which began at 09:49 UT and peaked at 10:04 UT on 15 November 2025 (note: the flare date appears to be a typographical error in the manuscript; the GLE occurred on 11 November). The authors argue that the successful detection demonstrates the capability of high‑sensitivity thermal‑neutron detectors to complement traditional neutron monitors, especially for lower‑energy solar energetic particles that produce secondary neutrons in the atmosphere.

The discussion highlights several advantages: (1) the large number of detectors and substantial total area provide statistical power comparable to a single standard monitor; (2) the thermal‑neutron sensitivity offers a different energy response, potentially enriching space‑weather diagnostics; (3) the existing infrastructure can be integrated into the Global Neutron Monitor Network with modest upgrades.

Limitations include the reliance on 5‑minute averaged data, which smooths out finer temporal structures, and the lack of a fully calibrated energy response for the detectors. Atmospheric pressure and temperature corrections, standard in NM analyses, were not applied at the same precision, limiting long‑term trend studies. Future work should aim at higher time resolution (≤1 min), detailed efficiency calibration across neutron energies, and automated cross‑validation with the worldwide NM database.

In conclusion, the NEVOD complex’s neutron‑detector arrays have proven capable of recording a GLE event with accuracy comparable to dedicated neutron monitors. This opens the possibility of using EAS‑oriented detector installations as auxiliary nodes in the global space‑weather monitoring network, thereby enhancing the spatial coverage and redundancy of solar‑particle observations.