Mobile neutron monitor for latitude cosmic ray monitoring

Neutron monitors are a standard tool for high-precision continuous monitoring of galactic cosmic ray flux variations arising from variations in heliospheric conditions and solar activity for space weather applications. These measurements form the basis for solving the inverse problem of determining the cosmic ray anisotropy vector beyond the magnetosphere. To support such studies, periodic latitude measurements are necessary to determine the coupling functions of primary and secondary cosmic rays variations. The aim of this work is to develop and characterize a modernized standard neutron monitor based on a CHM-15 boron thermal neutron counter and a data acquisition system designed for marine expeditionary studies of cosmic ray variations. Modern nuclear physics experimental methods and the principles of microprocessor-based data acquisition systems were used to solve this problem. The results of test trials and of continuous monitoring showed that the characteristics of the upgraded and standard neutron monitor are similar, and the ease of use, compactness, and stability allow us to conclude that the mobile neutron detector can be used in expeditionary conditions with limited access for maintenance personnel.

💡 Research Summary

The paper presents the design, construction, and field validation of a mobile neutron monitor intended for latitude‑dependent cosmic‑ray measurements, particularly in marine expeditionary settings where weight, size, and maintenance access are severely constrained. Building on the well‑established NM64 architecture, the authors replace the traditional lead rings with an array of standard lead blocks (200 × 100 × 50 mm and 180 × 90 × 50 mm) to form the neutron multiplier. Two mechanical configurations are examined: a “winged” version that closely reproduces the original NM64 geometry (total lead mass ≈ 2.2 t) and a lighter “wing‑less” version (≈ 1.3 t). GEANT4 simulations quantify the impact of the wing structures on neutron production, showing that the winged design retains a count rate of roughly 430 counts min⁻¹, comparable to a standard NM64, while the lighter version yields about 290 counts min⁻¹, a ~30 % reduction.

The detector core uses a CHM‑15 boron‑10 BF₃ proportional counter surrounded by a 25 mm polyethylene moderator and a 72 mm polyethylene reflector, mirroring the standard NM64 neutron moderation scheme. The electronic front‑end consists of a two‑stage broadband amplifier: the first stage is a current‑to‑voltage converter with a feedback resistance of 6 MΩ, delivering a +0.6 V pulse for a –100 nA input; the second stage further amplifies the signal to up to –10 V, with a selectable gain set by a potentiometer. A comparator with a 1 V reference converts the analog pulses into clean digital edges, which are fed to an Arduino‑Uno microcontroller via interrupt lines.

The data‑acquisition system records neutron counts at 1‑minute intervals and simultaneously logs atmospheric pressure, temperature, humidity (BMP280 and DHT22 sensors), GPS/GLONASS/BeiDou/Galileo position, and a DS3231 real‑time clock for time stamping. All data are stored on a 32 GB micro‑SD card, sufficient for a full year of operation. Power is supplied by a high‑voltage module (BNV‑31 or equivalents) with a temperature coefficient of 25–30 ppm °C⁻¹, ensuring that a 30 °C temperature swing changes the operating voltage by only a few volts around the nominal 2120 V.

Field tests were conducted aboard a research vessel during May–June 2025, a period close to solar maximum. The mobile monitor (designated 1NM64M) and its companion epithermal (1NM64E) and thermal (1NM64T) detectors were compared against the permanent Moscow 24NM64 station. Count‑rate stability, plateau length (>300 V) and slope (<0.05 % V⁻¹) matched the reference instrument. Both Forbush decreases (e.g., 1 June 2025) and a ground‑level enhancement (GLE C077 on 11 November 2025) were captured with amplitudes consistent across all detectors, confirming that the mobile system faithfully reproduces space‑weather signatures. Atmospheric corrections were applied using empirically derived barometric (β ≈ 0.74–0.76 % hPa⁻¹) and humidity (ε ≈ 0.034–0.056 % (g m⁻³)⁻¹) coefficients, reducing residual variability to ≈ 0.8 % per hour (≈ 0.15 % per day).

In conclusion, the authors demonstrate that a neutron monitor weighing ~2 t and occupying ~1 m² can be built with off‑the‑shelf lead blocks, a CHM‑15 counter, and a compact Arduino‑based acquisition unit, without sacrificing the performance of a full‑scale NM64. The modular design permits cascading multiple counters to improve statistical accuracy, and the integrated GPS and environmental sensors provide the necessary context for latitude‑dependent cosmic‑ray studies. The system’s robustness over >180 days of unattended operation makes it suitable for deployment on research vessels, polar expeditions, or other remote platforms, thereby extending the global neutron‑monitor network into regions previously inaccessible to traditional, heavy monitors.