Which isotope has a critical mass under 5 lbs?
Laser ignition of deuterium/tritium mixture for a fusion explosion rather than a critical mass fission explosion.
And Wikipedia (yes, I know the warnings) lists Californium-252 as having critical mass of 2.73 kg (6.02 lbs) with a diameter of 6.9 cm.
Actually many and you would not even need a radioactive isotope. All you would need is a collapsing star before it goes super nova. Helium and Iron etcetra will do most nicely. However it is a bit difficult to carry a supernova millions of time the weight of the earth in your back pack.
This article is total Bull Sh-t!
It’s not the critical mass as such that does it, but the mass of high explosive arranged to give a spherically symmetric shock front to compress the fissile material. So if one uses another means — I thought the article mentioned lasers, though I’ve never heard of lasers reliable enough to ignite/sustain a fusion *explosion*...much of the weight requirements go away.
“Which isotope has a critical mass under 5 lbs?”
Unobtainium, of course!
Helium, iron, and just about anything. Only problem is it needs to be in a star that is collapsing before it goes super nova. Super Novas are a bit difficult to carry around. In the real world the smallest nukes will be plutonium with a critical mass of about 10 kilograms. The bomb mechanism would weigh more than the plutonium.