While not built-in to a smart phone…still pretty relavent to this challenge. Just thought the community should be aware of it. This is an add-on board for the Raspberry Pi Zero which allows the DIY enthusiast to easily build a networked radiation monitor with a rich web-based UI. This is a fully open hardware and open source design. The tube provided with the board is an STS-5 (or SBM-20) which is quite sensitive.
Patrick Schippers Changed status to publish
Here is another whole category of gas sensors.
Benjamin Boyette Answered question