Got your own smartphone with you on a starry night watching the sky through your new telescope? Good! Now you can start observing ultra-high energy cosmic rays. How? A new research conducted by the scientists from the University of California reveals the real cosmic potential of smartphone cameras. “Extensive air showers generated by cosmic rays produce muons and high-energy photons, which can be detected by the CMOS sensors of smartphone cameras,” the scientists explain. “The small size and low efficiency of each sensor is compensated by the large number of active phones. We show that if user adoption targets are met, such a network will have significant observing power at the highest energies,” they write in a recently published paper.

The source of ultra-high energy cosmic rays (UHECR), remains a puzzle many decades after their discovery, as does the mechanism behind their acceleration. Their high energy leaves them less susceptible to bending by magnetic fields between their source and the Earth, making them excellent probes of the cosmic accelerators which produce them. But the mechanism and location of this enormous acceleration is still not understood.

Constructing and maintaining a new detector array to study UHECRs with a large effective area presents significant obstacles. Current arrays with large, highly-efficient devices cannot grow dramatically larger without becoming much more expensive. Thus, the researchers propose repurposing the existing network of smartphones as a ground detector array.

“A dense network of such devices has power sufficient to detect air showers from the highest energy cosmic rays,” they write in the paper. “We measure the particle-detection efficiency of several popular smartphone models, which is necessary for the reconstruction of the energy and direction of the particle initiating the shower. With sufficient user adoption, such a distributed network of devices can observe UHECRs at rates at least comparable to conventional cosmic ray observatories.”

Air showers induced by cosmic rays contain an enormous number of particles. The scientists focus on photons, which have high densities in the shower, and muons, which have excellent penetrating power and high detection efficiency. Electrons are also numerous and have high efficiency, but may be blocked by buildings, phone cases or camera lenses.

While CMOS sensors of smartphone cameras are designed to have reasonable quantum efficiency for visible light, the same principle allows the sensor to detect higher-energy photons as well. In the case of muons, the photodiode is functionally equivalent to silicon-based trackers now common in particle physics experiments, such that the charged particle will leave electron-hole pairs along its path.

The researchers acknowledge that a large network of devices would have unprecedented observing power at energies above 10^20 eV, where current ground arrays become saturated. Lack of observations of UHECRs above this energy could therefore provide powerful limits on the incident flux.

“Such a world-wide network of devices sensitive to muons and photons could also have many other potential uses, such as monitoring local radiation levels,” they conclude. “In addition, such a global network would be the first of its kind, opening a new observational window to unanticipated processes.”