Bear with me, /sci/. I thought of asking /k/ but it's above their paygrade.

let's say somebody with expertise in miniature circuitry were able to get their hands on highly-enriched radioactive material.

is there a lower limit at which one could make a nuclear bomb? let's say one used an exotic radioactive material like Californium or something even more radioactive.

>Californium has acquired a reputation for having a remarkable small critical mass, usually alleged to be "in the gram range", creating much speculation about possible use in "pocket nukes" -- very small fission weapons.

https://nuclearweaponarchive.org/Nwfaq/Nfaq6.html