Cosmic Ray Muon Physics
Measuring the invisible across Nepal's altitude gradient
Cosmic rays — high-energy protons and nuclei from supernova remnants and active galactic nuclei — strike the upper atmosphere continuously. At about 15 km altitude they collide with atmospheric nuclei and produce cascades of secondary particles. Among these are muons: heavy, unstable cousins of the electron that rain down on every square centimetre of Nepal, roughly one per minute at sea level — and measurably more with altitude.
HICS is designing, locally manufacturing, and field-deploying Nepal's first cosmic ray muon detector, to generate the country's first altitude-gradient muon flux dataset. Nepal spans one of the world's greatest accessible altitude ranges, from the Terai lowlands near sea level to peaks above 8,000 m — a natural laboratory for altitude-dependent particle physics that has never been systematically used.
Why muons reach the ground
A muon at rest decays in about 2.2 microseconds — far too quickly to travel the ~15 km from where it is born to the ground, if Newtonian physics held. Yet muons arrive at the surface in abundance. The reason is relativistic time dilation: muons travel at nearly the speed of light, so in our frame their internal clock runs slow and their effective lifetime is stretched by the Lorentz factor.
The fraction of muons that survive the journey down through the atmosphere to a detector follows an exponential decay in the dilated lifetime:
Muon flux increases with altitude because there is less atmosphere overhead to absorb the cascade — the muons are detected closer to where they are produced. The amount of atmosphere above a point is its atmospheric depth, which falls off roughly exponentially with height:
At mid-latitudes the flux roughly doubles per 1,500 m of altitude gain. In Eastern Nepal, background radiation has been observed to rise about 16% per 1,000 m using Geiger–Müller counters (Neupane, 2024) — but no dedicated muon detector has ever characterised this relationship for the country.
Theoretical atmospheric-depth model. Highlighted points mark HICS measurement sites; the real curve for Nepal has never been measured.
An open-source instrument, hardened for the Himalaya
The detector is based on the open-source CosmicWatch v3X architecture developed at MIT, adapted for Nepal's environmental conditions. It implements a four-stage detection chain, and two units operate in coincidence mode — only logging an event when both register a pulse within a nanosecond-scale window — to suppress background radiation and electronic noise and isolate genuine muons.
1 · Scintillator ▸
A plastic scintillator converts the energy a passing muon deposits into a brief flash of photons.
2 · Silicon photomultiplier (SiPM) ▸
The SiPM converts that photon burst into an electrical avalanche current.
3 · Analog front-end ▸
An analog amplifier shapes the resulting pulse for digitisation.
4 · RP2040 microcontroller ▸
A Raspberry Pi Pico (RP2040) digitises the pulse amplitude via a 12-bit ADC, timestamps the event, and logs it to microSD alongside onboard pressure, temperature, and humidity. An OLED shows the live event rate in the field.
Coincidence mode ▸
Two stacked detectors logging only simultaneous hits eliminates uncorrelated gamma radiation, electronic noise, and low-energy background — substantially improving muon purity over a single unit.
Environmental hardening ▸
Silicone conformal coating against monsoon humidity; LiFePO4 batteries for sub-zero power reliability; silicone optical grease stable across large temperature swings; ASA-filament enclosures for UV resistance. The baseline CosmicWatch assumes a climate-controlled lab — Nepal's field is not one.
An altitude transect from the Terai to a Himalayan pass
At each site both detectors are deployed vertically in coincidence and log continuously for at least eight hours — enough for statistical uncertainty below 2% on the measured flux. GPS coordinates, altitude, and atmospheric conditions are recorded at every location, and the complete raw dataset is published openly.
| Site | Approx. altitude | Duration | Notes |
| Nepalgunj | ~150 m | 1–2 days | Terai baseline |
| Kathmandu | ~1,400 m | Planned | First detector site — base of the altitude transect |
| Nagarkot / Phulchowki | ~2,100–2,700 m | 1 day | Valley rim, road-accessible |
| Dunai, Dolpo | ~2,140 m | 1–2 days | En route to Dho Tarap |
| Dho Tarap, Dolpo | ~3,900 m | 2–3 days | Crystal Mountain School base |
| High pass (Numa La) | ~5,100 m+ | Several hours | Maximum-altitude measurement |
From raw counts to a citable flux dataset
The measured coincidence rate at each site is corrected for live-time, dead-time, and geometric acceptance, then expressed as a flux in particles per square centimetre per minute. Corrected flux is compared against atmospheric depth — computed from the onboard pressure via the barometric formula — and against the established global relationship. Pulse-amplitude (energy) spectra from the 12-bit ADC are recorded at each altitude as a secondary dataset. All analysis is in Python, published openly under the MIT licence alongside the raw data.
Detector design complete; component sourcing underway. The programme is proposed to the Nepal Academy of Science and Technology (NAST). No muon data has been collected yet — this page describes the design and plan, not a finished result.
Particle physics, built around Nepal's own data
Three curriculum-aligned open modules are developed alongside the detector, in English and Nepali: What Falls from the Sky? (Grades 6–8), Measuring the Invisible (Grades 9–10), and Nepal's Cosmic Ray Observatory (Grades 11–12). Each is built around printable handouts and real, locally collected data.
This programme builds on the principal investigator's peer-reviewed work in particle detection — Dhakal et al. (2023), New Constraints on Macroscopic Dark Matter Using Radar Meteor Detectors, Physical Review D 107, 043026 — and on the open CosmicWatch detector (Axani et al., 2018, 2025) and stacked-coincidence work (Banerjee et al., 2026).
