Most craft stores carry a glass etchant like Armor Etch [0] as an over the counter product. The MSDS [1] indicates that it's an aqueous solution containing ammonium bifluoride, which exists in equilibrium with hydrofluoric acid.
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.
[0] https://www.michaels.com/product/armour-etch-glass-etching-c... [1] https://www.jacksonsart.com/media/pdf/armour_etch_sds.pdf