Yep there seems to be an app for everything these days and you can even find a few to help you prepare for the apocalypse.www.Hotray-Info.de/ have an android app that can detect radioactivity. How does it do it? Read a bit more below:

The app uses the sensor on the front or rear camera of your phone as a radiation detector. It’s not a true Geiger counter (unless your phone has a helium-filled glass tube we don’t know about), but it measures the radioactive interference that can be seen on the sensor when no light enters from the lens. The app measures non-light sources of energy from everything from a tiny and harmless radioactive chip to a full-on blast of gamma rays from a lab machine – the stuff that Stan Lee’s dreams are made of.

Get the App here: https://play.google.com/store/apps/details?id=com.rdklein.radioactivity

Judging by the reviews it actually seems to work well!