Geiger counters are not cheap or portable. Not the professional models anyway. But you can use the Smart Radiation Detector Pocket Geiger Type in conjunction with your iOS device to measure radiation level in your surroundings. It has a measuring range of 0.05 µSv/h to 10 mSv/h (Cs-137).
The Pocket Geiger is simple to use. Just connect it to your phone’s audio output, load its app, and use photodiode sensors to detect nearby radiation. It will cost you just a bit under $100.