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Also: An alpha emitter IS the best source of "usuable" heat for a SNAP-like generator. (The heat difference is the source of power to the satellite.)

But an alpha-emitter won't emit enough gamma or beta (neither of which would penetrate even a sheet of paper!) to kill the bearer within hours.

ONLY massive amounts of gamma rays could kill like that, no matter how much shielding was removed from the outside of the can-sized package.

The Soviets HAD small SNAP-like generators that powered their ocean-covering RORSAT radar satellites.....but those were much, much more powereful than a US SNAP generator. Had to be .... since they were trying to bounce radar signals of ships back from the ocean to the satellite in orbit, and then relay the signal back to Moscow.

But that much gamma?

At the Sandia Nuclear Museum they have (had?) mockups of several space power Snap-like RTGs. One used Co60 in a gadget with variable fins to maximize radiant cooling. The source for this would have been pretty hot, I think it had over 4 grams of cobalt (4 KCi), which even after twenty years (a little less than 4 half-lives) would have been lethal in a few hours, at 500 curies the dose rate at a meter would be well over 100 REM/hr. If Sr90 had been the source, the half life is 27 years, and the source strength would be higher than for Co.

I saw a Snap-7 with a charge of Sr90 and its shielding was pretty big, the whole thing was like a fireplug. It put out about 2KW of electricity. I think the shielding was U238.

If the dose rate was hundreds of REM/hr (seems likely from the facts in the story) the stuff probably was not Pu. We played around with thulium RTGs for a while, they'd be hot.

Another possibility is that the cans were Ir192 field radiography sources, removed from their shielding for theft. Half life is only 70 days.

Sometimes Cs137 was used in RTGs, and sometimes used in radiography. Pretty penetrating stuff, between Co and Ir in photon energy, and the halflife is 33 years.

I doubt we will know what the capsules are until they are found. Should be pretty easy if they are that hot!