Smartphones are not as accurate as conventional radiation detectors, but they are useful enough to detect radiation before it reaches dangerous levels. They can also be used for personal dose evaluations and as an alarm for the presence of high levels of radiation. As the producer of one of these applications (GammaPix, available for Android and iOS), I can confirm that smartphones and other CMOS and CCD cameras can detect radiation. Although these cameras are less sensitive than Geiger-Muller counters, specialized solid-state detectors and scintillators, they are still sensitive enough for a variety of applications.
This setting contributes to generating sometimes large background noise against which it is necessary to detect the radiation signal. For example, as a first line of detection for emergency services during an incident involving radiation. Scientists tested its radiation detection application on four smartphones and concluded that it works well enough to be a useful alert system for first responders. Of course it won't be as sensitive as a normal detector, but it's definitely enough to detect radiation before it reaches deadly levels and is useful enough as a warning system.
In theory, electronic cameras in mobile phones should be able to detect gamma rays in the same way as they detect photons of visible light. And Cogliati and company are not the first to create an application that uses smartphones to detect radiation in this way. They concluded that phones have the processing power to detect gamma radiation with their built-in cameras and measure levels on the phone. These cameras detect light when incoming photons hit electrons inside a pixel, creating a charge that an electronic circuit can detect.
The end result is that smartphones are surprisingly useful as gamma radiation detectors (the camera body is likely to stop alpha and beta radiation). In conclusion, smartphones can be used to detect radiation before it reaches dangerous levels. While they may not be as accurate as conventional detectors, they are still useful enough for a variety of applications.