Potassium-40 is not an alpha emitter.

Bananas are like XML that way. If you're not getting the results you want, you're just not using enough of them.

Well, google for "DIY cloud chamber" did result in quite some entries. Apart from youtube channels, with the first entry a guide from CERN:

https://home.cern/news/news/experiments/how-make-your-own-cl...

This can be done at home with a little effort. Less effort if you can get dry ice easily.

https://hackaday.com/2019/01/13/see-the-radioactive-world-wi...

You can easily make them at home (source, I did last weekend!). - Dry ice (mine came from something shipped cold) - Dark piece of metal (I used a 3D printer hot bed) on top of dry ice to get cold - IPA vapour (I poured some on a shop towel) - Some transparent container to house it all - I found a glass display cube on the side of the road, fish tanks or Tupperware also work. - Torch or something to provide side lighting Very cool to see evidence of the particles zooming around us, can highly recommend.