A few years ago, the first lump of plutonium scientists ever made on Earth disappeared from display in Berkeley’s Lawrence Hall of Science. Berkeley physicists think they have finally found it again — thankfully before it got thrown out as radioactive waste.
This precious lump of plutonium dates back to 1941. Plutonium doesn’t exist naturally on Earth, except in trace amounts. So to study plutonium, scientists first had to make it. Berkeley physicist Glenn Seaborg got access to a newly built cyclotron, where he and his collaborators bombarded uranium with neutrons. The material then decays into the new element of plutonium.
After a year, they had enough plutonium for the first sample large enough to weigh. It was all of 2.77 micrograms.
Seaborg would go on to win a Nobel Prize for his discovery of plutonium and other transuranium elements. The room where he did his work in Gilman Hall has since become a US National Historic Landmark. That historic sample was converted into plutonium oxide and placed in a glass tube, where it was put on display in Berkeley’s Lawrence Hall of Science.
In the late 2000s, however, Berkeley began to worry about the dangers putting the plutonium on display. The sample was removed and put away — except no one really knew where. Some time later, a box labelled “First sample of Pu weighed” was found at the Berkeley’s Hazardous Material Facility, a waystation for hazardous waste. Thankfully, a knowledgable eye saw it and discerned its historical value.
The label’s claim was promising, but how could we prove that this was actually Seaborg’s sample? With science, of course. The Physics ArXiv Blog explains:
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.
Eric Norman’s lab at Berkeley’s Department of Nuclear Engineering monitored the sample for gamma rays with an energy of 59 kiloelectron volts. They didn’t find any, meaning the sample was most likely created in a cyclotron like Seaborg’s. In addition, the mass matches up. The evidence all points toward this being the missing plutonium.
Now that Seaborg’s sample has been recovered, there’s talk of putting it back on display in his former office in Gilman Hall — perhaps a more fitting place than the trash bin. [The Physics ArXiv Blog]
Picture: Norman et al.