Neptunium

Traces of neptunium have been found in uranium minerals like this sample, but not enough that you could ever see it.  Neptunium is highly radioactive and has only a few exotic applications in nuclear research.
Scroll down to see examples of Neptunium.Smoke detector element.
The main listing for this sample is under americium, because that is the active ingredient in these small radioactive buttons used in ionization type smoke detectors.  This sample had been in my collection for five years before reader Michael Okun pointed out an interesting fact: Americium-241 decays (with a half-life of 452 years) into Neptunium-237, which has a much longer half-life of 2.1 million years.  That means neptunium is building up in my existing americium sample, and has been for years.

Michael calculates you get about a trillion new atoms of neptunium every year in a typical smoke detector, and very few of them decay.  Much like fine wine, the sample gets better the longer you keep it!  (Actually I hate wine, why not drink your grape juice before it goes sour, that's what I say.  But anyway, it makes a good analogy, for those people who do think wine tastes better the older it gets.)

If someone has a really old smoke detector, like 20 or 30 years old, I'd love to get it.  You know you're supposed to replace those things, they don't last forever, so I'd be doing you a service by taking it off your hands.

Unfortunately even 50 trillion atoms of neptunium still isn't much, only about 20 nanograms.  On the other hand, the americium in the button only weighs about 260 nanograms to start with, so after only 50 years it would already be almost 10% neptunium, not too bad for such an exotic element.  And when my collection has been gathering dust in a museum for about 500 years, this button will reach a milestone: It will be more than half neptunium.  They'll have to move it from the americium shelf to the neptunium shelf and get a new, fresher smoke detector button to replace it.  Gosh I hope someone remembers to do that, maybe I can get the Clock of the Long Nowpeople to set an alarm or something.

Well if Neptunium is in detector it is it should make, detector work faster or something like that.