Ultra high energy cosmic rays create a shower of particles when they encounter the Earth's atmosphere. The shower is large enough that distributed detector arrays are used to detect them. You need very large arrays and/or a lot of time to obtain a meaningful sample of events.
Greg noted an interesting approach that would use smartphones. The camera sensor can detect some types of shower particles if they happen to hit it. Of course the probability is small and you couldn't tell much from single events, but smartphones know where they are and what time it is and have a link to the Internet. There are also a lot of them. The scheme is to use them when they're idle, but plugged into a power source (so they don't drain the battery). When they detect energy being deposited in the camera sensor they send a report with the type of phone, time, place and rough amount of energy. The individual points are mapped by the experimentors who look for time and space correlated shower footprints.
A neat idea, but it would take a lot of cooperation to build a meaningful array. The problem is more social than technical in nature at this point.
The paper (pdf) is on arXiv
Observing Ultra-High Energy Cosmic Rays with Smartphones
Daniel Whiteson,1 Michael Mulhearn,2 Chase Shimmin,1 Kyle Brodie,1 and Dustin Burns2
1Department of Physics and Astronomy, University of California, Irvine, CA 92697
2Department of Physics, University of California, Davis, CA
We propose a novel approach for observing cosmic rays at ultra-high energy (> 1018 eV) by re- purposing the existing network of smartphones as a ground detector array. 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 small size and low efficiency of each sensor is compen- sated 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.
The source of ultra-high energy cosmic rays (UHECR), those with energy above 1018 eV, remains a puzzle even many decades after their discovery, as does the mecha- nism behind their acceleration. Their high energy leaves them less susceptible to bending by magnetic fields be- tween their source and the Earth, making them excel- lent probes of the cosmic accelerators which produce them [1, 2]. But the mechanism and location of this enor- mous acceleration is still not understood, despite many theoretical conjectures [3–6].
When incident on the Earth’s atmosphere, UHECRs produce extensive air showers, which can be detected via the particle flux on the ground, the flourescence in the air, or the radio and acoustic signatures. A series of dedicated detectors [7–9] have detected cosmic rays at successively higher energies, culminating in observation up to 3 · 1020 eV. The flux of particles drops precipitously above 1018 GeV, due to the suppression via interaction with the cosmic microwave background [10, 11], making observation of these particles challenging.
To accumulate a sufficient number of observed showers requires either a very long run or a very large area. Con- structing and maintaining a new detector array with a large effective area presents significant obstacles. Current arrays with large, highly-efficient devices (Auger , AGASA ) cannot grow dramatically larger without becoming much more expensive. Distributed detector ar- rays with small, cheaper devices (ERGO , etc) have the potential to grow very large, but have not achieved the size and density required to probe air showers, poten- tially due to the organizational obstacles of production, distribution and maintenance of their custom-built de- vices.
It has been previously shown that smartphones can de- tect ionizing radiation [15, 16]. In this paper, we demon- strate that a dense network of such devices has power sufficient to detect air showers from the highest energy cosmic rays. 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 adop- tion, such a distributed network of devices can observe UHECRs at rates at least comparable to conventional cosmic ray observatories. Finally, we describe the oper- ating principles, technical design and expected sensitivity of the CRAYFIS (Cosmic RAYs Found In Smartphones) detector array. Preliminary applications for Android and iOS platforms are available for testing .