Cosmic ray detector app

Turning your smartphone into a cosmic ray detector has never been easier, thanks to a new app called DECO (Distributed Electronic Cosmic-Ray Observatory).

Using a smartphone camera, the DECO app picks up muons, one of the main types of particle produced when a cosmic ray hits the atmosphere.

"Phone cameras use the photoelectric effect," says Justin Vandenbroucke, the creator of the app and professor of physics at the University of Wisconsin-Madison. "When a [light particle called a] photon hits the silicon sensor in a camera it makes [electrical] charge." This charge is then registered as a pixel signature, which is processed to produce an image.

If you place duct tape over the camera lens, light is prevented from reaching the sensor, but muons are not. Instead, any arriving muons pass straight through the duct tape and interact with the camera sensor, leaving a pixel signature behind. In the case of muons, the trace resembles a thin straight line, which the app records. Other particles can also be detected in the same way, including those that are produced by radioactive substances. 

The world around us contains trace amounts of radioactive material, which produces energetic helium nuclei, known as alpha particles, and electrons, or beta particles. DECO can detect both. Alpha particles produce straight lines on the app, which are thicker than those recorded by muons, while beta particles produce wiggly paths.

At the moment, working out which particles have been recorded is up to the user, but Vandenbroucke is looking to change this. "We'd like in the future to build in some automatic classification, both using web-based algorithms and also on the phones themselves." 

Not much is known about cosmic rays, but it is thought that they are produced in the aftermath of exploding stars. The scale of the experiments that are currently running to try to understand the rays is huge. The Pierre Auger observatory in South America has a detection area of 3,000 square kilometres. 

DECO's intended purpose is purely educational, but the DECO app brings the detection of cosmic rays into the realm of citizen science. 

Listen to the full interview with Justin Vandenbrouke here: