Glenn Seaborg won the 1951 Nobel Prize for chemistry for his discovery of the transuranium elements, in particular, plutonium. Seaborg created it by bombarding uranium-238 with deuterons to create plutonium-239.

During 1941, Seaborg and his collaborators began to characterise the properties of this new element using only the trace amounts they could make. But to study it in its pure form required a macroscopic lump of the stuff.

So the team began an extraordinary effort to create a lump of plutonium by bombarding uranium with accelerated neutrons in cyclotron’s at Berkeley and at Washington University. (This process had only recently become possible thanks to the development of the cyclotron at Berkeley, by the physicist Ernest Lawrence, who went on himself to win a Nobel Prize for his work.)

After more than a year’s bombardment, the team eventually produced a sample weighing 2.77 micrograms.

This was a huge achievement — the first sample of plutonium ever weighed. Because of its historical importance, the sample was evaporated, allowed to react with oxygen to form plutonium oxide and then condensed on a platinum base. This was then preserved inside a glass tube and eventually displayed at the Lawrence Hall of Science in Berkeley where anybody could view it.

Then something strange happened. Various concerns arose over radiation safety so staff at the Lawrence Hall of Science removed the sample from display. What happened next is not known but somehow the sample was lost.

Today, Eric Norman and colleagues at the Department of Nuclear Engineering at Berkeley, say they have found the sample and explain how they were able to identify it with high certainty as the one Seaborg created over 70 years ago.

The story begins at Berkeley’s Hazardous Material Facility that researchers use to dispose radioactive samples that are no longer needed. In 2008, staff there found a box labelled “First sample of Pu weighed. 2.7 micrograms”. This was duly labelled as sample S338 for administrative purposes.

“There it could have been discarded as radioactive waste were it not for the knowledgeable eye of Phil Broughton, a health physicist in UC Berkeley’s Environmental Health and Safety Department,” say Norman and co.

Broughton suspected that S338 was Seaborg’s famous sample but since the paper trail documenting its history had been lost, some other way had to be found to establish its authenticity.

In July last year, Broughton approached Norman and co in the nuclear engineering department to see whether they could help. All those involved immediately agreed that to preserve the historical significance of the sample, it should not be opened and that only non-destructive testing should be performed on it.

So how to identify the sample without touching it? It turns out that plutonium created in a cyclotron is very different from most plutonium, which is created inside nuclear reactors and then separated from spent nuclear fuel. That’s because this stuff always contains another isotope, plutonium-241.

This is a half-life of just over 14 years and decays into americium-241. So samples of plutonium from nuclear reactors, always contain americium-241 in amounts that grow over time. What’s more, Am-241 in turn decays producing gamma rays with an energy of 59 kiloelectron volts.

That was the clue that Norman and co needed. All the team had to do was to monitor the radiation from the sample to see whether it emitted anything characteristic of americium at 59 KeV. If not, then the sample could not have been created in a nuclear reactor and would be fundamentally different to almost all of the artificially-created plutonium on the planet.

And that is exactly how things turned out. Norman and co measured the radiation produced by the sample with unambiguous results. “No evidence of the 59-keV gamma ray produced by the decay of Am-241 was observed,” they say. “All of the gamma rays we observed can be attributed to the decay of Pu-239.”

That’s important evidence about the samples likely origin. “The plutonium contained in sample S338 was most likely not produced in a reactor, but in a low neutron fluence environment,” they conclude.

The team went on to calculate the mass of plutonium-239 which agrees more or less exactly with Seaborg’s original data.

“While not 100% conclusive, these observations are consistent with the identification of this object being the 2.77-microgram PuO2 sample described by Glenn Seaborg and his collaborators as the first sample of 239Pu that was large enough to be weighed,” say Norman and co.

The team now plan to set up a commemorative display in Seaborg’s old office at Berkeley where the sample will be displayed — hopefully with a little more care this time.

Ref: http://arxiv.org/abs/1412.7590 : Seaborg’s Plutonium?