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Can we see a cosmic muon with the naked eye?
Can we see a cosmic muon with the naked eye?
I am currently running an experiment to see whether a person can detect the Cherenkov radiation that is emitted when a cosmic muon passes through our eyeball. This is based on a course I ran at the Kashiwa science camp 2026, University of Tokyo.
Are you a muon see-er? If you visit Kavli IPMU, please volunteer to put on some 'muon shades' and take part in the experiment! It will take up 50 minutes of your time: 30 minutes for dark-adjusting and for training - where the muon shades tell you when a muon has passed - followed by a silent 20 minutes where you press a button when you think you see a muon. Your button presses during the final 20 minutes are then compared with the muon shade timestamps, and a p-value score is assigned.
The muon shades are built out of four CosmicWatch detectors - a fantastic outreach and educational project developed by Prof Spencer Axani at the University of Delaware.
Below details some of the theory and the set-up for the experiment.
A muon passing through the vitreous humor (mainly water) emits Cherenkov radiation
A muon passing through the vitreous humor (mainly water) emits Cherenkov radiation
Cosmic muons pass through each of our eyeballs at a rate of around one per minute at ground-level. A back of the envelope calculation estimates around 200 photons in the visible spectrum are emitted per passage. Since the 1940s we have known that the human eye can see order 5-7 photons. So the number of photons produced is definitely above the threshold for sensitivity...
A Geant4 simulation
A Geant4 simulation
The figure on the left shows the result of a simulation where a muon with an energy of 1 GeV is injected downwards along the z-axis into an eyeball-sized sphere of water. The blue dots are where resulting photons exit the sphere. On average around 1000 photons are produced, most of which exit from the southern hemisphere of the sphere. This is a higher yield than the simple estimate above, likely due to further Cherenkov light from ionized electrons produced in the realistic simulation. The retina covers roughly a hemisphere of the eyeball, so if we face the heavens we can expect muon passages where a good fraction of these photons are collected by our rods and cones. But the shape of their distribution is rather uniform, and this could pose a problem for our brains to identify the signal...
Muon shades
Muon shades
The shades consist of a coincidence muon telescope in front of each eyeball. Each telescope consists of two layers of plastic scintillator blocks that each have a silicon photomultiplier sensor (SiPM) attached to them. This SiPM picks up the scintillation light that a muon creates when it passes through the plastic block. A coincident signal between the two layers indicates a cosmic muon has passed through them and gone on into the eyeball.
The shades are worn over an eye-mask, in a dark-room.
Looking for muon flashes
Looking for muon flashes
The experiment then proceeds in three stages.
Stage 1 (nominal 15 mins): Dark adaption, and 'muon flash training'. The shades play audio to let you know when they detect a muon passing through the left or right part of the glasses.
Stage 2 (nominal 15 mins): Continued dark adaption, and 'delayed muon flash training'. As in stage 1, but with a 2 second delay on the audio.
Stage 3 (nominal 20 mins): Data taking. No audio is played by the shades, and the user presses a clicker when they think they see a muon passage. The timing of the clicks is compared with the timestamped data from the shades. See analysis for more details on constructing a p-value.
References
References
A number of papers have studied the visual effects of radiation. Astronauts frequently report seeing flashes of light. Included below is a reference (D'Ary, Porter, Nature 196, 1962) that has a similar setup to the above, which claims a small-sample, very mild evidence for muon detection.
D'ARCY, F., PORTER, N. Detection of Cosmic Ray µ-Mesons by the Human Eye. Nature 196, 1013–1014 (1962). https://doi.org/10.1038/1961013a0
TOBIAS, C., BUDINGER, T. & LYMAN, J. Radiation-induced Light Flashes observed by Human Subjects in Fast Neutron, X-ray and Positive Pion Beams. Nature 230, 596–598 (1971). https://doi.org/10.1038/230596a0
MCAULAY, I. Cosmic Ray Flashes in the Eye. Nature 232, 421–422 (1971). https://doi.org/10.1038/232421a0
CHARMAN, W., ROWLANDS, C. Visual Sensations Produced by Cosmic Ray Muons. Nature 232, 574–575 (1971). https://doi.org/10.1038/232574a0
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