Potassium Chloride is sold as a sodium-free table salt substitute and will give you far more decay events per second than a banana. I'd have to double check the decay rate in a kilo of KCl vs background radiation, but even if the decay rate is too low for random number generation you can still use it to salt your hashes[0].
[0] https://www.allrecipes.com/gallery/best-hash-recipes/