[angiosperm]

Those use strontium-90, which decays much faster than americium-241. That makes it hotter, at first, but its useful life is less. Being more than 4x less dense, you might carry enough more to make up the difference (and discard the extra heat, at first) if you have the room.

Strontium-90 decays by beta emission, ending up as stable zirconium, meaning you don't have to vent helium.