Do Androids Search For Cosmic Rays?

We always like citizen science projects, so we were very interested in DECO, the Distributed Electronic Cosmic-ray Observatory. That sounds like a physical location, but it is actually a network of cell phones that can detect cosmic rays using an ordinary Android phone’s camera sensor.

There may be some privacy concerns as the phone camera will take a picture and upload it every so often, and it probably also taxes the battery a bit. However, if you really want to do citizen science, maybe dedicate an old phone, put electrical tape over the lens and keep it plugged in. In fact, they encourage you to cover the lens to reduce background light and keep the phone plugged in.

According to the project website:

Cosmic rays are energetic subatomic particles produced by powerful cosmic accelerators, such as black holes and exploding stars. When they hit the top of the atmosphere, they produce showers of secondary particles, including electrons, photons, neutrinos, and muons. Many of these muons, which are similar to electrons but heavier, reach the ground at sea level and are a great tool for studying cosmic rays because they are easy to detect.

In practice, the app takes a photo every 1-2 seconds and looks for bright pixels. If it finds enough, the image becomes a candidate and receives further processing. Only a small number of frames are candidates and even fewer are actual events. They mention that it typically takes about 24 hours to get a few events on your phone. We aren’t sure how much data is sent to the server for processing and how much is done locally, but we suspect almost all the analysis is on the server. The app records your location but does degrade it somewhat for privacy.

You can see your results and the results of others on a public data page. There’s a map and you can narrow data by location, altitude, time, or even check in on a specific device or model.

Citizen and crowd-sourced science is all the rage lately, even NASA’s in. Citizen science even located a lost moon lander.

20 thoughts on “Do Androids Search For Cosmic Rays?

  1. “Many of these muons, which are similar to electrons but heavier, reach the ground at sea level and are a great tool for studying cosmic rays because they are easy to detect.”

    Wouldn’t it do better looking at the sky?

    1. Muons are so energetic that they typically pass through several meters of rock before absorbed. It’s been used to probe for hidden rooms in the pyramids: put a muon detector in the middle, more muons in a particular direction might indicate a void in the rock that could be a room.

      Whether the camera faces down or up makes only a tiny bit of difference.

      The camera has an aperture, and if the phone is vertical the aperture is a very thin slit while a horizontal camera will have a square aperture with considerably more area. The probability of detection is proportional to the aperture area.

      (Full disclosure: I saw the tops off of metal can power transistors and use the B-E junction to detect alpha particles, and this also detects the occasional muon.)

        1. Then said: “Like a to3? Any hints at building a alpha detector from such a device?”

          Large surface area photodiodes are better than exposed power transistor PN junctons as alpha detectors. Here’s an alpha detector design from CERN that you can easily build. The design is open source and there’s even a Kitspace page for it. The thing uses an OSRAM BPX-61 photodiode in a windowed TO-39 package. The BPX-61 costs $12.93 USD in unit quantity from Mouser and at write time there are 454 in stock.

          You can use the alpha detector to monitor the environment or buy a radiation source from Ebay for less than five dollars. Search for terms like Geiger Counter Source, Monazite, and Thorium.

      1. I’ve often wondered if garden variety silicon solar cells could be used to detect alpha particles. Lots of surface area, but annoyingly sensitive to ordinary light. Never thought of hacking a power transistor, that’s awesome. Do you have to bias it? Does PNP or NPN work better?

        1. Solar cells can be used as detectors, but the relatively enormous size results in an enormous capacitance, which reduces the signal quality quite a bit. I haven’t done this, but there are some papers that talk about it.

          I use an unbiased 2n3055 with collector to V+ and 4K resistor to ground on the emitter, and measure voltage on the emitter. The collector is therefore signal ground, and acts like an E/M shield for the detector junction.

      2. Saaaaay .. hang on there a minute, how do you *know* you detect the occasional muon? Can you distinguish the events? If so, how, is it just more energetic? What about beta rays?

        1. Strictly speaking, I don’t know that they are muons – I’m only assuming. I measure alpha particles at a rate of a thousand per second, with well defined voltage and timings, and occasionally will see an enormous sudden high-frequency spike. The detector is enclosed in metal at signal ground, so it’s very likely not an artifact of the circuit or stray RF. Beta particles tend to have low energy and are absorbed by just about everything, so it’s probably not beta. The detector is largely insensitive to gamma.

          The Alphas I’m using are around 5 MeV, and muons at ground level are around 1000x that energy, so it’s a good match for the data I’m seeing.

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