Meet the Pocket Geiger Counter: a compact device that detects Gamma and Beta radiation. The Type 5 Radiation Sensor has a pulse output that can be used with your favorite microcontroller. Arduino code examples are provided to help you get started.
These sensors have a measurement range of “0.05uSv/h to 10mSv/h at 0.01cpm to 300Kcpm.” The sensor comes an optional enclosure and cable to connect it to your mobile devices. Costs under $70 at this time.