There are all sorts of apps available for smartphones to show atmospheric phenomena like wind speed, temperature, and rainfall, along with other observations for tides and phases of the moon. But how about something really cool like an app for measuring the amount of interstellar cosmic radiation hitting the earth? A professor of physics from the University of Wisconsin thought that would be cool as well, and created an app to turn your smartphone into a cosmic ray detector that works in a similar way to those instruments found in high-tech observatories and mega-expensive laboratories.

Thought to be created in such things as black holes and stars that go supernova, cosmic rays are high-energy subatomic particles that are spewed out across the galaxy at tremendous speeds. When they hit the atmosphere of the Earth, they are smashed apart when they collide with molecules such as oxygen and nitrogen, where they breakdown into such things as x-rays, electrons, neutrons and charged mesons.

These charged mesons then further decay into muons. Many of these muons then make it all the way to the ground and, as a result, they can then be detected. From there, extrapolations can then be made as to the amount and intensity of the cosmic rays that produced them.

"The apps basically transform the phone into a high-energy particle detector," said Professor Justin Vandenbroucke, a researcher at the Wisconsin IceCube Particle Astrophysics Center (WIPAC). "It uses the same principles as these very large experiments."

To allow this app to work, the smartphone camera is used as the muon detector. The majority of smartphone cameras today use a CMOS integrated circuit that causes a proportional voltage whenever particles of light, or photons, strike its surface and energizes its pixels.

Similarly, when a muon strikes those pixels in that same CMOS IC in a smartphone camera, it too causes an electric current flow, and the signature created by that strike can be captured, stored, and subsequently analyzed via the app.

To use a smartphone as a cosmic ray detector, you first need to download the DECO app, then you need to cover the smartphone lens with something like a piece of duct tape. This keeps out light and other stray visible electromagnetic radiation, so that the readings are much more likely to be that of muons or other invisible radiation.

After this, you set the app, and leave the phone so that it is facing screen up. You can even leave it in a cupboard, or a desk drawer, or under your bed for that matter – muons act very much like other high-energy particles and pass right through most ordinary materials.

While the smartphone is left alone, the DECO app is programmed to take an image every few seconds, and then analyzes that image. If a sufficient number of pixels are energized on the CMOS IC, the image gets recorded as an event. As a result, both cosmic rays and radioactivity detected by the phone camera can be recorded.

Excitingly, the app also includes a data logger that sends event information such as time, location, and the events observed to a database specifically designed to see if the events recorded may be matched to confirmed incoming cosmic radiation detected by scientific observatories and the experiments running therein.

Primarily designed as an educational tool, the pocket cosmic ray detector was originally a project for Professor Vandenbroucke when he was a graduate student at the University of California, Berkeley at the suggestion of another classmate – Kenny Jenson – who also wrote the original software for the first version of the app.

Testing of the smartphone and app was performed by Professor Vandenbroucke himself at high-altitude; in a commercial jet airliner where he set it up and logged data as he flew across the country. Muons are even more prevalent at high-altitude and, therefore, more easily detectable, allowing the app to be tested with greater accuracy.

Now with grant support from QuarkNet – a national program in particle physics and teacher professional development – Professor Vandenbroucke’s WIPAC group intends to work with high school teachers to create educational materials to complement the use of the smartphone cosmic ray detector.

And, though smartphone cosmic ray detectors aren't likely to put physicists and their expensive, high-tech detectors (or even astronomers doing similar things with radio telescopes) out of a job any time soon, Professor Vandenbroucke believes that if enough old or unused cellphones are employed in muon detection the project may even eventually become of considerable interest to the scientific-minded public as well.

"It would be great to get students and the public interested in gathering data and understanding the particles around them, things they ordinarily don’t get a chance to see," said Professor Vandenbroucke.

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