Following in the wake of pioneering experiments carried out with photosensitive films on mountaintops, we have now a good understanding of the subatomic particles falling on the Earth. Muons, specifically, reach the Earth's surface with a fairly high and uniform flux, a feature that has now been exploited for imaging purposes.
Nuclear reactors and pyramid interiors have been inspected via muon absorption or scattering, but so far these techniques have mostly been employed for the detection of heavy elements. Now, Istvan Bikit and colleagues have demonstrated an approach for using cosmic-ray muons to image small structures made of light elements.
Their idea was to combine a gamma spectrometer with a muon tracker, and consider only particle trajectories that would lead to coincident detections. A low cutoff on the gamma spectrometer rendered it unable to detect muons directly interacting with it, meaning that it only detected secondary particles generated by interactions with the object to be imaged. A plane-by-plane reconstruction technique then enabled the team to create a tomographic image of a small copper tube — the first using particles from the sky.
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Levi, F. Pictures from the sky. Nature Phys 12, 376 (2016). https://doi.org/10.1038/nphys3765