Black electrical tape over the rear lens is ideal. A pinhole of light will completely swamp the cosmic-ray signal. If you don't have tape, place the phone lens-down on a black T-shirt with no edge gaps. Then tap below.
Cosmic muons are born ~15 km above your head when high-energy protons
hit the atmosphere. Their proper lifetime is 2.197 µs (PDG 2024).
At v ≈ 0.998c they'd travel only 660 m before decay — almost none would
reach the ground.
They do reach the ground (about 1 cm⁻²·min⁻¹ at sea level) because in Earth's frame their lifetime stretches by γ ≈ 40, giving them ~25 km of travel. Each event you record is a particle that's only here because of real, measurable time dilation.
What this is: a pure-web proof of concept of the CRAYFIS / DECO smartphone cosmic-ray detection technique. It looks for outlier bright pixels in the rear-camera feed and ranks them by sigma above the per-pixel calibrated noise floor.
What this is not: research-grade calibration. Real CRAYFIS uses native YUV camera access, dark-frame thermal correction, and large-N statistics across many phones. Your single detector will see some real cosmic events mixed with light leaks, hot pixels (we mask the obvious ones), and CMOS bit errors. Expect rate ~1–20 events/hour if your tape seal is good; much higher means light leak.
The rate-of-truth check: CRAYFIS reported median rate ~3 events/hour per phone. If yours is in that ballpark with the camera covered, you're seeing real physics.