[evan_au]

xenon 124

I assume that Xenon has been studied quite extensively, since it is at the heart of some Dark Matter experiments.
- Increasingly large vats of liquid xenon studded with photomultiplier tubes have been studied carefully to find the tiniest glimmer of the merest brush with a hypothetical Dark Matter particle.
- The largest current experiment at Gran Sasso in Italy has over 3 tonnes of liquid Xenon, and larger ones are planned
- The decay of Xenon would be one form of "false positive" that they would need to detect (and find some way to distinguish from a potential Dark Matter particle).

https://en.wikipedia.org/wiki/XENON

[evan_au]

From the link provided by Bored Chemist:

Bureaucratium. A commonly heard description describes it as "having a negative half-life". In other words, the more time passes, the more massive "Bureaucratium" becomes; it only grows larger and more sluggish.

I guess a negative half-life beats a half-life of 10^-6 seconds?

https://en.wikipedia.org/wiki/Administratium