Cosmic Rays and the Sky
How the particles from space change with height
Cosmic rays strike the top of the atmosphere and make showers of particles. The ones that reach the ground are mostly muons, and there are more of them the higher you go, because there is less air overhead to stop them. Nepal climbs nearly 8,800 m in under 200 km, which makes it the place to measure how the muon rate changes along the climb.
A muon lives about 2.2 microseconds. Even at nearly the speed of light that is too short to cross 15 km of air, yet muons arrive in large numbers: their clocks run slow, which is relativistic time dilation.
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How much air is overhead, the atmospheric depth, falls off roughly exponentially with height, and the muon rate follows it:
A model: the curve these measurements are meant to test. Marked points are planned sites.
Lower pressure means less air overhead and more muons. These pressures are weather-model estimates for each site.
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Source: Open-Meteo weather forecast models · CC BY 4.0.
Source: Open-Meteo air quality, from the Copernicus Atmosphere Monitoring Service (CAMS) · CC BY 4.0; contains modified Copernicus Atmosphere Monitoring Service information.
Borders: Natural Earth, public domain.
An open design, adapted for the Himalaya
The detector will be based on CosmicWatch, an open design from MIT: a plastic scintillator, a silicon photomultiplier and a small microcontroller, which we are adapting for Himalayan deployment: cold, damp and long runs on solar power. Two units stacked in coincidence count only the particles that pass through both, which rejects most of the background. Each count is logged with the air pressure, so readings at different heights can be compared. The plan is a pair beside each IESH hub along the transect, from Nepalgunj at about 150 m to Dho Tarap at about 4,000 m.
The all-sky camera, then radar
The full IESH station is designed to carry an all-sky camera, which would record meteors as well as cloud. Radar comes after it: a meteor's trail of ionised air reflects radio waves, so radar sees meteors by day and through cloud. HICS's founder has published work using meteor-radar data (Physical Review D, 2023).
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Planned. The muon detector will be based on CosmicWatch, adapted for Himalayan deployment, with one beside each IESH hub on the transect. The all-sky camera is part of the full IESH station design.
