There are server cards (or were at least some time ago) with tiny bit of mildly radioactive material, well enclosed of course, and a good sensor for those isoto

There are server cards (or were at least some time ago) with tiny bit of mildly radioactive material, well enclosed of course, and a good sensor for those isotopes/particles. I've heard other approaches including that static too, ie the famous analog TV without real signal, IIRC its cosmic microwave background, or camera watching water drops fall or similar. There are many other ideas (and probably products too), the only thing is one needs to keep it 100% reliable across long time.