Scintillation muon telescope module with fiber-optic light collection

A scintillation muon telescope module with fiber-optic light collection using silicon photomultipliers was developed, tested, and installed for continuous monitoring to study cosmic ray variations. The aim of this study was to create a scintillation muon telescope module, continuously monitor the muon component in test mode, and study the long-term stability of the detector parameters. Methods for processing the obtained data were developed. To assess the stability of the module parameters, an internal control technique was used, involving data from other detectors. The results of testing and long-term continuous monitoring showed that the stability of the developed muon detector is better than 0.1%/year, without the need for its operation in a thermostatic chamber. The study concludes that ease of operation, cost, compactness, low power consumption and stability are factors that determine the advantages of the developed module, which is an essential element for constructing a multidirectional muon telescope.

💡 Research Summary

The paper presents the design, construction, testing, and long‑term operation of a scintillation muon telescope module that employs fiber‑optic light collection and silicon photomultipliers (SiPMs). The authors aim to create a compact, low‑cost, low‑power detector suitable as a building block for a multidirectional muon telescope used in space‑weather monitoring and heliospheric research.

The detector consists of two layers of 0.5 m × 0.5 m × 5 cm plastic scintillator plates, separated by 0.5 m and interleaved with a 5 cm thick lead absorber to suppress the soft component of cosmic rays. Each plate is equipped with two 2 m‑long wavelength‑shifting optical fibers that collect the scintillation light (peak emission at 425 nm) and re‑emit it at 476 nm, matching the peak sensitivity of the SiPM. The SiPM used is a SensL MicroFC‑30035‑SMT device, offering a gain of ~3 × 10⁶ at a bias of 29 V. The SiPM’s temperature‑dependent gain is compensated by a precision power supply with built‑in temperature correction, achieving voltage stability better than 0.1 V (≈0.01 %/mV).

The analog front‑end amplifies the SiPM signal by a factor of 3 × 10⁶, shapes it to a 400 ns pulse, and applies a 1 V discrimination threshold. The resulting digital pulses from the upper and lower layers, as well as their coincidence, are fed to an STM32F103 microcontroller. The firmware counts pulses in 60‑second intervals, timestamps each count using a DS3231 real‑time clock synchronized via NTP (≈±5 s per month), and records auxiliary data: atmospheric pressure and temperature (BMP280), local temperature (DS1631, DS18B20), and SiPM bias voltage (12‑bit ADC). Data are transmitted every minute over Ethernet (W5500) to a remote server; in case of network failure, up to ten days of data are stored in a 32 Mbit flash memory (AT25DF321).

On the server side, a three‑tier software architecture processes incoming data. The backend, written in Python 3.9, applies barometric and temperature corrections using meteorological models retrieved on demand, stores raw and corrected data in PostgreSQL, and serves results through a JavaScript‑based web client. Users can visualize muon count rates, correction coefficients, and environmental parameters via a browser interface (e.g., https://tools.izmiran.ru/w/muon).

Calibration of the SiPM involves setting the breakdown voltage for each channel and determining the counting efficiency plateau. The counting curve slope is 0.01 %/mV for single channels and 0.003 %/mV for the coincidence channel, implying that a bias stability of 0.1 V yields a counting stability better than 0.3 % for hourly averages. The detector efficiency, measured with auxiliary scintillators in a triple‑coincidence setup, is 98.7 % ± 0.2 %.

A key concern for scintillation detectors is long‑term aging, typically a 1–2 % loss in light yield per year due to radiation damage, temperature, and humidity. The authors address this by employing an internal control technique: data from the module are continuously cross‑checked against a reference proportional counter, which exhibits far superior long‑term stability. By applying a correction derived from the reference detector, the effective drift of the muon count rate is reduced to less than 0.1 % per year, even without housing the system in a thermostatically controlled chamber.

The geometric configuration yields a solid angle of 54.7° and an angular resolution of roughly 15°, suitable for monitoring the vertical muon flux and its modest directional variations caused by heliospheric disturbances. Scaling the design to a 4 × 4 array of identical modules provides a total detection area exceeding 4 m², achieving the target statistical precision of 0.1 % per hour for the vertical direction. Power consumption remains modest due to the low‑voltage SiPMs and efficient front‑end electronics, enabling autonomous operation in remote locations.

In summary, the study demonstrates that a fiber‑optic light‑collected scintillation module with SiPM readout can be built at low cost, with compact dimensions, minimal power requirements, and excellent long‑term stability (≤0.1 %/year). The integrated data acquisition, real‑time atmospheric correction, and robust networking make the system ready for deployment in a larger multidirectional muon telescope network aimed at continuous space‑weather monitoring and heliospheric research.