The detector is a Geiger tube. There are more details on the GitHub page [0], including this note:
> Disclaimer: the radioactivity from the bananas doesn't quite make the difference, the background radiation is more than enough to make the generator work and afaik the banana's is not even detectable.
> Disclaimer: the radioactivity from the bananas doesn't quite make the difference, the background radiation is more than enough to make the generator work and afaik the banana's is not even detectable.
[0] https://github.com/valerionew/Banana-Random-Number-Generator