Couldn’t we have a lead box lined with these radiation to electricity converters with a small amount of radioactive material in the center, and have an energy generating device that would last for thousands or even millions of years? Imagine putting the sun in a box lined with solar cells, but on a much smaller scale.
Is there a reason this wouldn’t work?
The problem you face is that there is several types of radiation and what is emitted differs with each nuclear isotope.
Alpha particles: Basically a helium atom stripped of its electrons.
Beta particles: basically electrons
Gamma radiation: High energy electromagnetic radiation.
Protons and neutrons that are emitted at various levels of energy.
While it might be possible, I don’t know if it could/would be viable.