Radioactive decay could work, but how would people feel about that?

There are many different types of radioactive decay (see table below).

That was clearly shown by the radioactive decay of certain elements.

This will have an impact on the long-term radioactive decay of the spent fuel.

Their radioactive decay produces about half of the earth's internal heat.

Most helium on Earth is a result of radioactive decay.

The weak force is responsible for one form of radioactive nuclear decay.

Its radioactive decay produces a significant amount of the Earth's internal heat.

If your goal is simply to observe radioactive decay, then you are in luck.

A specific level of radioactive decay across a uniform spectrum.